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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