29. An introduction to A Level organic chemistry
Formulas, Fn groups, Naming
Functional groups and formulae
Functional group: atom/group of atoms in an organic molecule that determines its characteristic chemical and physical properties
Arenes: aromatic compounds that contain a benzene ring
chemical properties
Compound can undergo Electrophilic attack: is e- rich due to π system of delocalised e-
Resistant to addition reactions: delocalised e- ring system makes benzene stable
physical properties
b.p 80 C: Benzene has id-id forces of attraction between the molecules
Insoluble in water: non-polar hydrocarbons in arene (not energetically favourable for benzene to break many H bonds between water molecules to be soluble)
Halogenoarenes: aromatic compounds containing halogen bonded to benzene ring
chemical properties
Prone to Electrophilic attack
can take part in substitution reactions
physical properties
Chloro, broom, iodo benzene liquid at r.t with oily texture
b.p. Increases down group: larger halogen = more e- = stronger id-id forces = more energy to separate
insoluble in water: non-polar hydrocarbon ring
Phenols: aromatic compounds containing a hydroxide bonded to benzene ring
chemical properties
acidic: O in -OH donates a lone pair into ring (more acidic than alcohols) = increased e- density in ring = more reactive = easier for H to be donated
react with reactive metals (e.g. Na) to form phenoloxide ions and H gas
physical properties
White, crystallline solid
disinfectant-like smell
soluble in water: can form H bonds with water due to -OH group
Acyl chlorides: carboxylic acid derivatives containing a chlorine atom, and an acyl group attached to C=O
chemical properties
colourless
extremely reactive and readily takes part in substitution (Cl substituted )
fuming liquids with strong smell: react with water vapor in air
Physical properties
react violently with water (cannot determine its solubility with water )
Amines: compounds with -NH2 (primary), -NH (2 °) , or -N (3 ° ) amine group
chemical properties
basic compounds: due to lone pair of e- on nitrogen
physical properties
lone pair on N can form H bonds
soluble in water: form H bond with water
solubility ↓ as nonpolar hydrocarbon chain gets longer / molecule gets bigger
fishy smell
Amides: compounds containing
amine group (-NH2)
carbonyl group (C=O)
amide group -CONH2
chemical properties
less basic than amides: lone pair of e- on N is delocalised
physical properties
soluble in water: form H bonds with water
solubility ↓ as nonpolar hydrocarbon chain gets longer / molecule gets bigger
Amino Acids: building blocks of proteins, consisting amine group (-NH2) and carboxylic group (-COOH)
chemical properties
react with bases to form salts
react with alcohols to form esters
react with amines to form amides
physical properties
most are soluble in water but insoluble in organic solvents
most have chiral centres, exhibit optical isomers
Formulae of Organic Compounds
Nomenclature of Compounds
Naming organic compounds
Identify longest C chain containing the functional group
Identify fn group on the chain to determine the suffix or prefix on the compound
Count along the carbon chain such that the functional group has the lowest number
Add any side chains or lower priority functional groups as prefixes to the beginning of the name in alphabetic order
Use the prefixes di-, tri- and tetra- if there are two or more identical functional groups or side chains
Characteristic organic reactions
Electrophilic substitution
Electrophile: e- loving species
they are e- deficient and acts as e- pair acceptor
Substitution reactions: reactions replacing 1 atom / group by another
Electrophilic substitution: group / atom replaced by an electrophile
e.g. bromine electrophile attacks e- - rich benzene ring
Addition-elimination (nucleophilic)
nucleophilic addition of small molecule followed by elimination of small molecule
includes
hydrolysis
reaction of alcohols to form esters
reaction with ammonia and 1° amines to form amides
e.g. Hydrolysis of acyl chlorides
Hydrolysis - acyl chloride to carboxylic acid
water molecule adds across C=O bond
HCL eliminated
Ester formation - acyl chlorides + alcohol
Alcohol adds across the C=O bond
HCl molecule is eliminated
Amide formation - acyl chloride + 1° amines + conc ammonia
amine or ammonia adds across C=O bond (lone pair on N attacks carbonyl C atom in acyl chloride)
HCl eliminated
Shapes of aromatic organic molecules
Aromatic molecules consist of one or more rings with conjugated π systems
Conjugated π systems arise from alternating double and single bonds in which the electrons are delocalised
Shape of benzene and aromatic compounds - why are they more stable?
Benzene and other aromatic compounds are planar and regular because each carbon atom is sp² hybridised. Each C forms three σ bonds in a trigonal planar arrangement with bond angles of 120°.
The remaining unhybridised p orbital on each carbon overlaps sideways with neighbouring p orbitals to form a continuous, delocalised π system above and below the ring.
Because the π electrons are delocalised over the whole ring, all the C–C bonds are identical and have intermediate character between a single and double bond. This is why benzene is unusually stable.
Why are benzene more stable?
delocalised π electrons above and below the ring
Optical isomerism, Chirality and drug production
Stereoisomers: same structural formula, diff arrangement in space (geometrical, optical)
Optical
4 diff groups of / atoms
central chiral carbon
chiral molecules exist as 2 optical isomers / enantiomers
non-superimposable mirror images
same physical / chemical properties
rotate in opposite directions in plane polarised light (1 clockwise 1 anticlockwise )
Similarities of enantiomers
differences in enantiomers
same physical and chemical properties
differ from ability to rotate plane polarised light and biological activity
nonsuperimposable mirror images
Biological activity of enzymes
enzymes are chiral proteins, bond to specific active site for biological response
mirror image of enantiomer will not bind to same active site as it is not the same shape, no biological response
Optical activity and Racemic mixtures
reaction mixture is optically active if there is an uneven mixture of each enantiomer (e.g. 20% rotates clockwise, 80% rotates anticlockwise, net effect is plane of polarised light rotates anticlockwise)
racemic mixture: equal amounts of enantiomers present in soln
optically inactive as enantiomers cancel out each others effect, plane polarised light not changed
the rotation of plane polarised light can be used to determine the identity of an optical isomer
e.g. pass plane polarised light through a sample containing one of the two optical isomers of a single substance
Depending on which isomer sample contains, the plane polarised light will be rotated clockwise / anti-clockwise
clockwise rotation: R enantiomer
anticlockwise rotation: S enantiomer
No effect will be observed when the sample is a racemic mixture
Chirality and Drug production
most drugs to treat disease have 1+ chiral centres and can ∴exist as enantiomers
diff in biological activity in enantiomers means difference in effectiveness as medicine
drugs should be prepared so that only 1 optical isomer produced, to ↑ effectiveness
Potential different biological activity of enantiomers
e.g. drug naproxen treats pain from arthritis
1 enantiomer eases pain
1 enantiomer causes liver damage
separate racemic mixture into 2 pure enantiomers
due to difference in biological activities
reduce side effects
↓ dosage by half, reduces production cost
chiral catalysts to produce 1 pure optical isomer
only small amount chiral catalysis needed + reused
e.g. enzyme
30. Hydrocarbons - Arenes
Reactions of arenes
substitution
halogenation
nitration
Friedel-Craft's alkylation
Friedel-Craft's acylation
other reaction
Complete Oxidation
Hydrogenation
Electrophilic substitution
electrophile: e- pair acceptor, attracted to delocalised e-
substitution: 1+ H replaced by a fn group
generating electrophile+ using catalyst
Electrophilic attack on benzene ring (electrophile accepts 2 π e- , +ve charge remains)
regenerating aromaticity by heterolytically cleavage of C-H bond (bonding pair foes into benzene π system)
Addition reactions of arenes
arenes undergo substitution due to delocalisation of e-, aromaticity restored by heterolytic cleavage of C-H bond
in addition, aromaticity / stability not restored (e.g. hydrogenation),
Hydrogenation of benzene is less exothermic than expected for three isolated C=C bonds, because the aromatic stabilisation of benzene is lost
Halogenation (in the aromatic ring)
reactants: Cl2 / Br2 (Cl+ / Br+ electrophile) , arene
conditions: rt and pressure
catalyst: anhydrous AlCl3 / AlBr3 (catalyst needed due to stability of benzene structure)
formation: AlCl3 + Cl2 ⇒ AlCl4- + Cl+
reformation: H+ + AlCl4- ⇒ AlCl4 + HCl
product: halogenoarenes / aryl halide
In halogenation of Alkylarenes, e- donating alkyl group activates positions 2, 4 (as well as OH-, NH2). forms 2 products
multiple substitutions when excess halogen used
Halogenation (of alkylarenes) in side chain
free radical substitution reaction
reactants: halogen, boiling alkyl arene
conditions: UV light
if excess halogen: all H on alkyl side chain will be substitution by X atoms
Nitration
reactants: conc HNO3 (NO2+ electrophile), arene
conditions: 20-60 ° C (higher = more NO2+ substituted)
catalyst: conc H2SO4
formation: HNO3 + H2SO4 ⇒ NO2+ + 2HSO4- + H3O+
reformation: H+ + HSO4- ⇒ H2SO4
In nitration of Alkylarenes, e- donating alkyl group activates positions 2, 4
Friedel-Crafts alkylation
substitution of alkyl chain
reactant: chloroalkane (CH3CH2+ electrophile), arene
condition: heat under reflux
catalyst: anhydrous AlCl3
formation: AlCl3 + CH3CH2Cl ⇒ CH3CH2+ + AlCl4-
reformation: AlCl4- + H+ ⇒ AlCl3 + HCl
Friedel-Crafts acylation
substitution of an acyl group (group containing carbonyl, C=O )
reactant: acyl chloride (CH3CH2CO+ electrophile)
conditions: heat under reflux
catalyst: anhydrous AlCl3
formation: AlCl3 + CH3CH2COCl ⇒ CH3CH2C=O+ + AlCl4-
reformation: AlCl4- + H+ ⇒ AlCl3 + HCl
Complete oxidation of side chains
alkyl side-chains in alkyl arenes are oxidised to carboxylic acids when refluxed with alkaline KMnO4 and then acidified with dilute H2SO4
reactants: hot alkaline KMnO4, dilute H2SO4, alkyl arenes
conditions: heat under reflux
Hydrogenation
addition reaction to form cyclohexane
reactants: H2 gas
conditions: heat (200 ° C at 30 atm )
catalyst: Pt/Ni
Directing effects of substituents on arenes
substituents: already on arene that direct substitution reactions to diff. ring positions
E- withdrawing groups / substituents
e- donating groups / substituents
remove e- density from π system in benzene ring
deactivating groups (slow attack by e+)
directs e+ to positions 3, 5
e.g. COOH, X, NO2
donate e- density into π system of benzene ring
activating groups (allow e+ to react faster)
directs e+ to positions 2, 4, 6
e.g R, NH2, OH
31. Halogen compounds
Halogenoarenes: arenes bonded to X atoms
Producing halogenoarenes
substitution reactions of arenes with chlorine or bromine in presence of anhydrous AlCl3 catalyst
reactants: Cl2 / Br2 (Cl+ / Br+ electrophile) , arene
conditions: rt and pressure
catalyst: anhydrous AlCl3 / AlBr3 (catalyst needed due to stability of benzene structure)
formation: AlCl3 + Cl2 ⇒ AlCl4- + Cl+
reformation: H+ + AlCl4- ⇒ AlCl4 + HCl
product: halogenoarenes / aryl halide
Substitution of benzene
Substitution of methylbenzene
substitution of 6 position only occurs in excess chlorine
Reactivity of halogenoarenes
Why are aryl halides less reactive than haloalkanes?
C-H bond in aryl halides is stronger
due to the partial overlap of lone pairs on halogen atom with the π system in the benzene ring
C-H bond ∴ has a partial double bond character
32. Hydroxy compounds
Reactions of alcohols
Formation of esters
reaction: addition-elimination (esterification)
reactants: alcohols/phenols, acyl chlorides
nucleophile: OH- (attacks e- deficient carbonyl (COCl) C )
COCl bond breaks, Cl is substituted, HCl fumes formed
Why are esters formed from acyl chlorides rather than from C. acids?
more effective to form esters from acyl chlorides
acyl chlorides are more reactive (produce esters faster)
acyl chloride reactions go to completion (more ester formed)
Esterification using phenols
additional reactants: base (e.g. NaOH)
base: deprotonate phenol to form phenoxide ion
phenoxide better nucleophile than phenol, attack on carbonyl C occurs more readily
Phenol
phenol: benzene ring with -OH group attached
Production of Phenol
From reaction of phenylamine with nitrous acid (HNO3)
reactants: NaNO2, dilute acid (to form HNO2), phenylamine
conditions: ice (under 10 ° C) step 1 and 2, heat step 3
forming nitrous acid
NaNO2 + HCl ⇒ (ice) HNO2 + NaCl
forming unstable diazonium salt
forming phenol via thermal decomposition (due to instability)
reactions of phenol
Many reactions due to e- rich benzene ring and polar -OH group
with bases
with reactive metals
with diazonium salts
nitration
bromination
Acid-base reactions
Acid-base reaction of phenols with base
-OH: slightly acidic (weak acids), act as acid in acid-base reaction
dissolve in alkaline soln forming phenoxide salt and water
Acid-base reaction of molten phenols with reactive metals
forms soluble phenoxide salt and H2 gas
Reaction with diazonium ions
diazonium ions very reactive due to -N2+ group
acid-base reaction
conditions: ice for cold diazonium ion added to phenoxide ion
Reactions of the aromatic ring
phenols react more readily with e+ than benzene due to one lone pair of e- on O in -OH overlaps with π bonding system, ↑ e- density of phenol’s benzene ring and making it more susceptible to electrophilic attack
-OH activates ring, directs e+ to 2, 4, 6 positions
Nitration of phenols
electrophilic substitution reaction
reactant: dilute HNO3, phenol
when conc HNO3 used, product is 2, 4, 6- trinitrophenol
conditions: rt
milder conditions as phenol is more reactive
Bromination of phenols
electrophilic substitution reaction
reactants: bromine water, phenol
conditions: rt
phenol decocolourises orange bromine soln to form 2, 4, 6 - tribromophenol (white ppt)
as phenol is more reactive, no need for anhydrous AlBr3 catalyst, can react with bromine water
Reactions of other phenolic compounds (contain phenol functional group, e,g, 1-naphthol)
-OH group is also e- donating, activates benzene ring to electrophilic substitution reactions, e+ directed to 2, 4 positions (on phenol)
C in position 6 shared by both rings, no H atom = no substitution
can react in similar way as phenol
Acidity of phenols
Phenols are weak acids
phenols react more readily with e+ than benzene due to one lone pair of e- on O in -OH overlaps with π bonding system, ↑ e- density of phenol’s benzene ring and making it more susceptible to electrophilic attack
Phenol’s conjugate base is the phenoxide ion.
-ve charge is delocalised over the entire ion as a lone pair on O overlaps with the ring’s delocalised π system, which reduces charge density on oxygen
H⁺ ions are ∴ less strongly attracted to phenoxide ion, so phenol is less likely to reform
Therefore, phenol is more likely to lose H⁺ and act as an acid
Stability of conjugate base
phenoxide more stable as -ve charge spread out, so phenol more likely undergo ionisation
eqm shifts right, more phenol donate H+ (act as acid)
Relative acidities of water, phenol, ethanol
smaller pKa = stronger acid
order of acidity explained by conjugate bases
Delocalisation of charge density
phenoxide (conjugate base of phenol):
charge density of O is spread out over ion,
O less available for bond formation with H+
ethoxide (conjugate base of ethanol):
ethyl group is e- donating, donates e- density to O atom,
O more available for bond formation with H+
hydroxide (conjugate base of water):
charge density of O cannot become delocalised over a ring,
OH- more readily accepts H+ than phenoxide
no e- donating alkyl groups to O, less -ve O, O less readily accepts H+ compared to ethoxide
Overall
conjugate base more readily accepts H+ = stronger conjugate base = weaker acid
strong acid eqm shifts right favouring dissociated ions and H+
33. Carboxylic acids and derivatives
Benzoic acid: C6H5COOH
can be used to synthesize esters
Carboxylic acids
Production of benzoic acid
formation of benzoic acid from Oxidation of alkylbenzenes
alkylbenzene heated under reflux with hot alkaline KMnO4, then acidified with dilute acid to protonate carboxylate salt and form benzoic acid
reactants: hot alkaline KMnO4 (oxidising agent), dilute acid (e.g. HCl), alkylbenzene
purple Mn2+ turns colourless as reduced to Mn4+
brown ppt MnO2 formed
conditions: heat under reflux
Reactions of Carboxylic acids
Reactions of C, acids to form acyl chlorides
reactants
PCl5 (s)
PCl3 (l) + heat
SOCl2 (l)
Forming carboxylic acids from oxidation of 1 ° alcohols
alcohol ⇒ aldehyde ⇒ c. acid
Further oxidation of c. acids such as methanoic and ethanedioic acid
methanoic acid (strong reducing agent) gets further oxidised to CO2 by
warming with mild oxidising agents like Fehling’s or Tollen’s reagent
Fehlings: Cu2+ reduced to Cu+, form red ppt Cu2O
Tollens: Ag+ reduced to Ag
using stronger oxidising agents like acidified KMnO4 or acidified K2Cr2O7
KMnO4: purple soln turns colourless as Mn7+ reduced to Mn2+
K2Cr2O7: orange soln turns green as the Cr6+ reduced to Cr3+ ions
Ethanedioic acid gets further oxidised to CO2
Conditions: Strong oxidised agent - warm acidified KMnO4 needed
Relative acidities of C. acids, Phenols, and alcohols
C acids can lose H+ in aq soln to form carboxylate salts and water
Acidities of C. acids
longer C chain = weaker acid
inductive effect ↑ -ve charge on COO- group
More substituted Cl atoms = stronger acid
Cl atoms withdraw e- density from the –COOH group, weakening the O–H bond and stabilise carboxylate ion (reducing the -ve charge density on it)
The more stable the carboxylate ion, the less likely it is to attract H⁺ and reform the O–H bond
Acidities of c. acids, phenols, alcohols
acidities explained by strength of O-H bond and stability of conjugate bases of acids
Strength of O-H bond
carboxylic acids
e- in O-H bond drawn towards C-O bond, e- in C-O bond drawn to C=O bond
O-H bond weakened due to C=O withdrawing e- density from it for itself
easier to act as acid and lose H+ from OH
alcohols
lack e- withdrawing C=O group,
O-H bond not weakened
harder to act as acid and lose H+
Stability of conjugate bases
carboxylate ion (c base of carboxylic acid)
charge density on O spread over ion because charge is delocalised on an electronegative carbonyl O atom
O less available for bond formation with H+ to reform -COOH
eqm shifts right, C base more stable than carboxylic acid
alkoxide ion (c base of alcohol)
alkyl is e- donating group, donates e- density to O atom
O more available for bond formation with H+ ion to reform OH
lack ability to delocalised charge density on ion, c bases less stable than alcohols, weaker acids than c. acid and phenols
eqm lies left
phenoxide ion (c base of phenol)
charge density of O spread out over ion due to delocalisation of e- (stabilises ion)
O less available for bond formation with H+
Phenoxide more stable than phenol
Phenoxide less stable than carboxylate since delocalisation of charge density is on C atoms, not electrogenative O atoms
eqm lies more right than alcohols, more left than c. acids
ethanol is weakest
e- donating methyl group strengthens O-H bond
methyl donates -ve charge to -COO-, more likely to accept H+
Esters - Production
Esters: organic compounds with -COO- functions group
nice smelling
Naming esters
alcohol
acyl chloride / carboxylic acid
e.g.
Producing esters from alcohols and carboxylic acids
condensation / esterification reaction
Producing esters from alcohols and acyl chlorides
more effective: acyl chlorides more reactive(faster), and reactions go to completion (no eqm, max yield)
Acyl Chlorides
Acyl chlorides: compounds with COCl fn group
Production of acyl chlorides
Carboxylic acid with
solid PCl5
liquid PCl3 + heat
liquid SOCl2
Reactions of acyl chlorides
addition-elimination: addition of small molecule across C=O, followed by elimination of small molecule
acyl chlorides under addition-elimination reactions such as
hydrolysis to form carboxylic acid
Reaction with alcohols and phenols to form esters
Reaction with ammonia and amines to form amides
General mechanism for addition-elimination reactions
addition of nucleophile across C=O bond
elimination of a small molecule such as HCl or H20
lone pair on nucleophile carries out initial attack on carbonyl carbon
this is followed by the elimination of an HCl molecule
Hydrolysis of acyl chlorides forms carboxylic acid and HCl
water (nucleophile) adds across C=O bond
HCl eliminated
Formation of esters from acyl chlorides
esterification reaction
reactants: alcohol/phenol, acyl chloride
conditions: rt
reaction with phenol is slower than alcohol, base such as NaOH used to speed up reaction (deprotonates phenol to form phenoxide, better nucleophile to carry out attack)
alcohol /phenol (nucleophile) adds across C=O bond, HCl / NaCl eliminated
Formation of amides from acyl chlorides
condensation reaction: nucleophile adds to C=O, Cl- eliminated, HCl formed
reactants: ammonia or 1°/2 ° amines, acyl chloride
nucleophile: Nitrogen (has lone pair of e- )
HCl formed is immediately neutralised by a 2nd molecule of ammonia / amine in excess, forming ammonium salt (e.g. NH4Cl, CH3NH3Cl)
Relative ease of hydrolysis of aryl chlorides
from most readily broken / easily hydrolysed
acyl chloride > alkyl chloride > aryl chloride
trend explained by strength of C-Cl bond
acyl chlorides - hydrolysed at rt R-COCl
C bonded to Cl is also bonded to O
both Cl and O are very electronegative, pulling e- away from C, leaving it very δ+
C-Cl bond is weakened
nucleophilic attack is more rapid
alkyl chloride - hydrolysed by refluxing with strong alkali nucleophile R-C-Cl
C bonded to Cl attached to 1 electronegative atom (Cl), pulls e- away from C
C is not very δ+ (e- deficient)
C-Cl bond is stronger than in acyl chlorides
aryl chlorides - hydrolysis will not occur
C bonded to Cl is part of delocalised π bonding system of benzene ring
one lone pair of e- on Cl overlaps with delocalised system
C-Cl bond has some double bond character, becoming stronger
34. Nitrogen compounds
Primary and secondary amines
1 ° amines: organic compounds with NH2 fn group attached to an alkyl / aryl group
2 ° amines: organic compounds with group attached to a alkyl /aryl groups
Naming
-amine: after alkyl prefix (e.g. methyl amine)
amino- : if another suffix present or branched/complex amines (e.g. amino methane)
Production of Amines
1 ° and 2 ° amines prepared form diff reactions
1 ° amines:
reaction of haloalkanes with ammonia
reduction of nitriles
reduction of amides
2 ° amines:
reaction of haloalkanes with 1 ° amine
Forming 1 ° amines from haloalkanes with ammonia
nucleophilic substitution reaction
reactants: excess, hot ethanolic ammonia
nucleophile: N in ammonia (replaces X)
conditions: heat under pressure
Forming 1 ° amine from reduction of nitrile
reactants: nitrile vapour, H2 gas
conditions: LiAlH4 in dry ether (reducing agent), or Ni catalyst
Forming 1 ° / 2 ° amines from reduction of amides
reactants: LiAlH4 in dry ether (reducing agent)
type of amine depends on nature of amide
Forming 2 ° amines from haloalkanes with 1 ° amine
nucleophilic substitution reaction
reactants: 1 ° amine in ethanol
nucleophile: N in 1 ° amine (replaces X)
conditions: heat under pressure, sealed tube
Reactions of Amines
Condensation reaction between Acyl chloride and ammonia or amine produces Amides
Cl in acyl chlorides is electronegative, draws e- density from carbonyl C
C is e- deficient, attacked by nucleophile like N in ammonia and amines
C-Cl bond broken, amide formed
reaction with amines ⇒ substituted amide
reaction with ammonia ⇒ nonsubstituted amide
Why is acyl chloride a more effective reactant than carboxylic acids in formation of amides?
c acids less reactive than acyl chloride
reaction with c acids dosent fo to completion
Basicity of aq soln of Amines and ammonia
N in ammonia and amines can accept proton H+, can ∴ act as bases in aq soln (when donating lone pair to form dative bonds)
Strength of bases
strength of amine depends on availability of lone pair of e- on N to form a dative covalent bond with H+
lone pair more readily available to form dative bond = stronger base
positive inductive effect ↑ basicity: alkyl groups donate e- density to N, causing lone pair to become more available
delocalisation ↓ basicity: aromatic rings cause lone pair to be delocalised into benzene ring, causing lone pair to be less available
Phenylamine and Azo compounds
Phenylamine: organic compound with benzene ring and amine fn group
Production of phenylamine
3 step synthesis, followed by separation / purification
Nitration (forming nitrobenzene)
reactants: benzene, conc HNO3
catalyst: conc H2SO4
conditions: 25-60 ° C
Reduction (forming phenylamine ion)
conditions: hot tin (Sn) and conc HCl under reflux
Deprotonation (forming phenylamine)
reactants: NaOH, phenylamine ion
separation/purification
phenylamine separated from reaction mixture by steam distillation
Reactions of Phenylamine
Both benzene ring and NH2 can take part in chemical reactions
bromination of phenylamine
formation of diazonium salt
Bromination of phenylamine to form 2, 4, 6- tribromophenylamine
electrophilic substitution reaction
electrophile: N, donate e- density into benzene ring, ↑ e- density in benzene ring activates it, more readily attacked by electrophiles
reactants: aqueous bromine, phenylamine
conditions: rt
Formation of diazonium salt (salt containing N2+ group)
reactants: nitrous acid/HNO2, HCl, phenylamine
HNO2 formed in test tube with NaNO2 and dilute HCl
conditions: below 10 ° C
diazonium salt forms phenol upon warming with water
Azo compounds
Azo compounds: organic compounds with R-N=N=R / azo group
often used as dyes
Diazonium compounds: contain N2+ group
Formation of Azo compounds from coupling reaction of benzene diazonium chloride salt with phenol
multi-step process
forming nitrous acid
reactants: NaNO2, HCl
conditions: test tube, under 10 °C / ice
NaNO2 + HCl ⇒(ice) HNO2 + NaCl
Diazotisation
reactants: phenylamine, HNO2, dilute HCl
conditions: below 10 ° C / ice (prevent thermal decomposition of diazonium ion to phenol, N and HCl)
coupling reaction
reactants: phenol, diazonium ion (acts as electrophile, sub into benzene ring of phenol)
conditions: alkaline conditions (NaOH deprotonates phenol to phenoxide, more reactive to e+)
Why are azo compounds very stable?
The delocalised e- in π bonding systems extend across both benzene rings and the –N=N– bridge
Other days formed from similar route
e.g. formation of yellow dye from coupling reaction between benzene diazonium chloride and C6H5N(CH3)2
Amides
Amide: organic compounds with CONR2 amide group
non-substituted / 1 ° amide: N bonded to 2H atoms
substituted / 2 ° amide: 1 or both H replaced by a hydrocarbon group
Production of Amides
Formation of Amides from acyl chlorides / c. acid with ammonia / amine
condensation reaction: nucleophile adds to C=O, Cl- eliminated, HCl formed
nucleophile: Nitrogen (has lone pair of e- )
conditions: rt
HCl formed is immediately neutralised by a 2nd molecule of ammonia / amine in excess, forming ammonium salt (e.g. NH4Cl, CH3NH3Cl)
Reactions of Amides
Amide group can undergo reactions
Hydrolysis with aqueous alkali or aqueous acid
Reduction with LiAlH4
Hydrolysis of amides to form c. acid and ammonia/ 1 ° amine
conditions: reflux with aq acid / alkali
products differ depending with amide is substituted
nonsubstituted ⇒ c. acid + ammonia
substituted ⇒ c. acid + 1 ° amine
when refluxed in excess
acid⇒ ammonium salt formed
base⇒ carboxylate ion formed (deprotonation)
Reduction of amides to form amine
conditions: LiAlH4 reducing agent
nonsubstituted amide ⇒ 1 ° amine + H2O
substituted amide ⇒ 2 ° amine + H2O
Relative basicity of Amides and Amines
lone pair of e- more readily available for dative bonding = stronger base
more e- =stronger base
amides - weaker base
e- withdrawing O in C=O causes lone pair on N to delocalise into C=O group, removed e- density from N
lone pair on N less readily available for bonding
amine - stronger base
e- donating alkyl groups ↑ e- density on N
lone pair on N more readily available for bonding
Amino Acids
Amino acids: organic compounds with 2 fn groups
basic amino -NH2 group
acidic carboxylic acid -COOH group
due to presence of basic and acidic group, they are amphoteric (act as acid and base)
R group can be acidic, basic, or neutral
Acid-base properties of amino acids + formation of Zwitterion
amino acids can undergo most reactions of amines and c. acids, including
acid-base reactions (amine + acid, C. acid + base)
Amino acids can interact intramolecularly to form zwitterion
Zwitterion: ion with both +ve (NH3+) and -ve (COO-) charge
formed from amino acids in water
due to charges, there are strong intermolecular forces of attraction between amino acids
this makes amino acids soluble crystalline solids
Zwitterions act as buffer soln
acid added: pH ↓ COO- accepts H+ to reform COOH, zwitterion becomes +vely charged
base added: pH ↑ NH3+ donates H+ to reform NH2, zwitterion becomes -vely charged
Isoelectric point: point at which charge = 0 / zwitterion is neutral
a pH level unique to each amino acid due to R group
Formation of Peptide bonds
NH2 group of one amino acid can react with COOH group of another amino acid to form dipeptide
condensation reaction: H2O eliminated
peptide bond / link formed
dipeptides can undergo condensation reaction again to form tripeptide ( 3 amino acids)
polypeptide: many amino acids join to form long chain of molecules
proteins are polypeptides
Electrophoresis
Analytical separation technique, separates ions by placing them in an electric field
Sample of amino acids placed between 2 oppositely charged electrodes
+ve ions move to -ve electrode
-ve ions move to +ve electrode
Rate of ion movement depends on
size of ions (larger = slower)
charge of ions (highly charged = quicker)
Electropherogram: bands on gel after electrophoresis
each band = particular species
Amino acids can be separated according to charge and mass
they are colourless, sprayed with ninhydrin (brown/purple stain) to show where they have gone to
Changes in charge of amino acid due to pH of solution
isoelectric point: no overall charge, no movement of zwitterions
pH > isoelectric point: -ve charge, move to +ve electrode
pH < isoelectric point: +ve charge, move to -ve electrode
-ve charged amino acid: NH3+ donated H+, NH2 and COO- left
+ve charged amino acid: COO- accepted H+, NH3+ and COOH left
35. Polymerisation
Condensation Polymerisation - Forming polymers
Forming polymers such as polyesters, polyamides, proteins
Formation of polyesters
addition polymerisation: monomers with C=C bonds join to form polymers
condensation polymerisation: Small molecule is lost when monomers join together to form polyester
polyesters have ester links
Monomers for polyesters
OH on dicarboxylic acid and H of diol expelled as H2O to form polyester
diol (2 -OH)
+
dicarboxylic acid (2 -COOH)
2 hydroxy carboxylic acids (OH and COOH fn group)
Formation of polyamides
condensation polymerisation
polyamides have peptide links
Monomers for polyamides
H on diamide and OH on dicarboxylic acid / Cl on dichloride expelled as H2O / HCl as polyamide forms
diamide (2 -NH2)
+
dicarboxylic acid
diamide
+
dichloride (2 -COCl)
2 aminocarboxylic acids (OH and COOH fn group)
Making proteins/polypeptides from amino acid monomers (aminocarboxylic acid)
amine group (-NH2) and carboxyl group (-COOH) of an amino acid polymerise with another amino acid
forms peptide links
proteins can be broken into its constituent amino acids through hydrolysis
You can deduce the repeat units of a condensation polymer by identifying the linking between monomers
chain of C: addition polymer
ester link: polyester
peptide link: polyamide
You can identify monomers from small molecule expelled
H2O: c acid
H: amine
Cl: dioyl chloride
Predicting type of polymerisation
Identifying type of polymerisation by looking at functional groups
Addition polymerisation
monomer contains C=C bond
double bond opens up to add monomers to either side of starting polymer
forms polyalkenes
co-polymer: 2+ monomers used to make 1 repeating unit
Condensation polymerisation
makes polyamides and polyesters
summary:
Identifying type of polymerisation by considering structure of polymer backbone
Addition polymerisation
no fn groups on polymer backbone, usually chain of C atoms
sidechains might branch off from backbone (e.g. benzene rings, nitrile groups (CN), halogens
Condensation polymerisation
backbone of polyesters contain ester links (-COO-)
backbone of polyamides contain peptide links (-CONH-)
Degradability of Polymers
Polyalkenes and biodegradability
polyalkenes are nonpolar and saturated, making them nonbiodegradable and chemically inert
instead, they can be melted and recycled for new uses
The energy released from burning can be used to generate electricity
Burning plastics in oxygen releases carbon dioxide and water (complete combustion) which can contribute to global warming
Photodegradation of polymers
some polymers are biodegradable due to their ability to breakdown using light
C=O groups along polymer chains absorb UV light
this weakens C=O areas of polymers and breaks them down into smaller molecules
e.g for polyesters and polyamides
Disadvantages of photodegradability
weaker polymers
UV light not available (e.g. in landfills)
Biodegrading polyesters and polyamides via hydrolysis
polyesters and polyamides can be broken down by hydrolysis (advantage)
hydrolysis: breakdown of molecules using water
acid hydrolysis: acid catalyst used
alkaline hydrolysis: alkaline catalyst used
heating with catalyst breaks polymer
Hydrolysis of polyamides
acid hydrolysis: breaks polymer into di carboxylic acid and ammonium ions
alkali hydrolysis breaks polymer into its sodium salts (e.g. dicarboxylic acid salt and diamines)
Hydrolysis of polyesters
acid hydrolysis: diol + dicarboxylic acid
alkali hydro lists: diol + dicarboxylic acid salt
36. Organic synthesis
Elucidating organic molecules - fn groups, properties, reactions
Fn groups in AS required as well as:
Production of fn groups
benzoic acids
acyl chlorides
amides
Producing benzoic acid
reactants: methylbenzene
oxidation: reflux with hot alkaline KMnO4 to form potassium benzoate
protonation with dilute HCl to form benzoic acid
Producing acyl chlorides
reactants: c. acid
substitution reaction: -OH replaced by Cl
Producing amides
condensation reaction
reactants: acyl chloride, NH3 (nonsubstituted amide) / methyl amine (substituted amide)
conditions: rt
Reactions of functional groups
Electrophilic substitution reactions of benzene
reaction
electrophile
halogenation
X+
nitration
NO2+
alkylation
R+
acylation
RCO+
Hydrogenation of benzene
addition reaction
Esterification of phenol
more efficient with acyl chloride than carboxylic acid
Acid-base reactions of phenol
phenol + base ⇒ soluble salt + water
Nitration of phenol
Bromination of phenol
Multi-step synthetic routes
Aliphatic reactions
Aromatic reactions
Analysis
37. Analytical techniques
Thin- Layer Chromatography (TLC)
Separate small samples
2 phases: stationary and mobile phase
Stationary phase
thin metal sheet coated in Al2O3, or SiO2
solutes adsorb onto surface and travel thru plate
Mobile phase
flows over stationary phase
polar (water or alcohol) or nonpolar (alkanes), any liquid solvent or gas solvent
carries components investigated
in coloured components: investigated under UV light using ninhydrin
Conducting a TLC analysis
prep beaker with small amount of solvent
on TLC plate, draw baseline in pencil
place spot of reference compound on the left, then a spot of sample on right of baseline and air dry
reserve allows identification of compounds in sample
put TLC plate inside beaker with solvent - make sure pencil line above solvent, place lid to cover beaker
as solvent reaches top, remove plate and draw another pencil line where solvent reached (solvent front) to use to calculate Rf
Rf values
The extent to which components are separated depends on solubility of each component in the mobile phase and interaction between each component and the stationary phase
less polar components travel further up ⇒ larger Rf ⇒ more soluble in mobile phase
more polar components do not travel far up ⇒ more attracted to polar stationary phase
Gas / Liquid chromatography
Analysing bases, volatile liquids, solids in vapour form
Stationary phase
nonpolar, non-volatile liquid applied to the inner walls of a capillary column
mobile phase
unreactive carrier gas
noble, or inert diatomic
e.g. He, N2, Ar
Retention time: time taken for sample molecule to travel through the column
from time inserted to time detected
often recorded on chromatogram
each peak = a volatile compound in the analysis sample. each peak has its own rt
rt compared with data book values to identify unknown molecules
longer rt: nonpolar components, attractiveness to nonpolar liquid in stationary phase
shorter rt: polar components, interact with carrier gas
Relative concentration of each component = area under each peak
Area under peak = ½ base x height
peaks roughly take shape of Δ
% composition of a particular molecule in a mixture
Carbon-13 NMR spectroscopy
carbon-13 NMR : analytical tool used to analyse samples and determine the structure of substances
Each Carbon environment gives a peak
solvent used to dissolve samples for 13C-NMR : CDCl3 (solvent peak around 70 ppm)
all 13C-NMR spectra measured relative to TMS / tetramethylsilane ( 0 ppm)
predict the number of carbon environments
refers to a specific c atom and the combination of atoms, bonds, fn groups attached
each unique c environment produces a peak in 13C - NMR
if there is a line of symmetry through the molecule, they belong to same environment and product a single peak
e.g. predict no. peaks in 13C-NMR spectrum of 1,3-dihydroxybenzene
4 environments = 4 peaks
predict each carbon’s chemical shift
this is the frequency of an atomic nucleus relative to TMS reference compound
measured in ppm
example- 13C-NMR spectrum for propanone
both CH3 groups are in same environment
2 environments = 2 peaks
C=O peak over 200
C-H peaks 0-50
Proton (1H) NMR spectroscopy
Technique used to determine structure of molecules
paired with other techniques like Mass spec
Magnetic field strengths of protons in organic compounds measured and recorded on H NMR spectrum
all samples measured against reference compound TMS / Tetramethylsilane
single sharp peak at 0 ppm
Sample peaks plotted as a ‘shift’ away from this reference peak
gives rise to ‘chemical shift’ values for protons on the sample compound (measured in ppm)
Features of NMR spectrum
intensity of peak against chemical shift
area under peak: no. protons in particular environment
height: intensity / absorption from protons
Molecular / chemical environments
diff. arrangements of H atoms have diff environments (identical / symmetry = same environment)
no. environments = no. diff chemical shifts / peaks
e.g. How many different 1H chemical environments occur in 2-methylpropane?
2 1H chemical environments
2 peaks on 1H NMR spectrum
Chemical shift values for H molecular environments
symmetrical H = same environment
1 ppm → Alkane
2 ppm → Next to C=O
7 ppm → Benzene ring
9–10 ppm → Aldehyde
10–12 ppm → Carboxylic acid
Low resolution 1H NMR
peaks on a low resolution NMR spectrum refer to the molecular environments of an organic compound
High resolution 1H NMR
number of peaks a signal splits into = n + 1
n = number of H on adjacent C atom
n+1 splitting patterns
Common pairs of splitting patterns
TMS and deuterated solvents
TMS gives sharp peak at 0 on NMR spectrum
why is TMS an ideal reference chemical?
it is inert and volatile
it reduces undesirable chemical reactions with compound analyses
it mixes well with most organic compounds
Deuterated solvents in 1H NMR
CDCl3 used as a solvent, no peak
why is a solvent needed for samples in 1H NMR?
dissolving samples in solvent help separate molecules and prevent them from interacting
what must the solvent do?
be a good solvent for organic molecules
not contain any H atoms, does not interfere with NMR signals
examples of non-deuterated solvents
Carbon tetrachloride, CCl4:
This solvent does not contain hydrogen, so it does not produce 1H NMR signals.
It is suitable for 1H NMR but does not dissolve all molecules well.
examples of deuterated solvents
Deuterated solvents are often used in 1H NMR because deuterium (²H) is an isotope of H whose nuclei absorb in a different region from protons, so it gives no signal in a ¹H NMR spectrum
Deuterochloroform / Chloroform-d, CDCl3:
preferred, contains deuterium (2H) instead of H, does not interfere with the H NMR
Deuterium oxide / Heavy water, D2O:
deuterium atoms exchange reversibly with the H in the -OH and -NH, signals identified in NMR
Identifying -OH and -NH signals
Protons in -OH (hydroxyl) and -NH (amine) groups give singlet peaks in ¹H NMR
but these signals can be tricky:
peaks are broad or sometimes fall outside normal chemical shift ranges
H in these groups exchanges quickly with protons from water or acids, so only one peak appears
Their chemical shift ranges may overlap with other types of protons
To identify these groups more clearly, proton exchange with deuterium oxide (D2O) is used:
The deuterium atoms in D2O exchange reversibly with the protons in the -OH or -NH groups
-OH + D2O ⇌ -OD + HOD
-NH-CO- + D2O ⇌ -ND-CO- + HOD
Since deuterium does not absorb in the same region as protons in the NMR spectrum, the signal for -OH or -NH disappears after D2O is added
This confirms the presence of -OH or -NH groups in the molecule
If peak disappears after D2O, it is due to the exchange of a proton from an -OH or -NH group
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