organic chem 2.0

Nucleophile

A 'nucleus-loving' species, a substance that is an electron pair donor.  Are often anions or the negative side of a dipole. E.g. NH3, OH-,CN-

Electrophile

Electron pair acceptor. E.g. NO2+, H+, 𝛿+, H2SO4

Organic Reactions

Substitution

One atom or group of atoms takes place of another atom or group of atoms, forming a new substance.

Elimination 

Atoms are removed forming multiple bonds, releases small molecules as products like water.

Condensation

When two molecules come together, with the elimination of water.

 

Addition

Atoms are added to the hydrocarbon across a multiple bond, no condensation of small molecules.

Hydrolysis 

When a molecule is split in two by the addition of water.

Oxidation and reduction

An oxidation reaction in which the sum of the oxidation number of an atom increases .

Reduction reaction number decreases.

TOPIC 4.2 HYDROCARBONS 

ALKANES

Unreactivity of Alkanes

  • strong carbon-carbon single bonds and strong carbon-hydrogen bonds

  • C-H bonds are slightly polar can't attract other molecules or ions

  • can be burned completely

  • reaction with halogens breaks C-H bonds

  • cracking breaks C-C bonds

Combustion of Alkanes

COMPLETE COMBUSTION

alkane + oxygen ⟶ carbon dioxide + water

INCOMPLETE COMBUSTION

alkane + oxygen ⟶ carbon monoxide + water

Soot (Cs) can also be a product of incomplete combustion.

Free Radical Substitution of Alkanes

  • H atom substituted by halogen e.g. Cl/Br

  • Alkanes are unreactive; UV light needed for reaction

  • Steps: initiation, propagation, termination

Substitution Mechanism 

INITIATION 

PROPAGATION  

Sources of Alkanes

  • ⟶Primary source is petroleum 

  • ⟶Petroleum is an complex mixture of organic compounds, mostly alkanes

  • ⟶Aliphatic and aromatic hydrocarbons also sourced form alkanes

Aliphatic: straight chain hydrocarbons

Aromatic: containing benzine rings

Benzine rings: organic molecule molecular formula C6H6, C atoms joined in form of a hexagon.

Cracking 

Catalytic Cracking

  • 500˚C

  • low pressure

  • zeolite catalyst

  • removes H atoms and electrons ⟶ leaving carbocations

  • final product: branched alkanes, cycloalkanes, aromatic compounds

Thermal Cracking

  • 800˚C

  • 70 atm

  • radicals produced

 

ALKENES

Electrophilic Addition of Alkenes

Hydrogenation of alkenes

  • reagent: hydrogen

  • Pt or Ni catalyst

  • 250˚C  

Hydro-halogenation of alkenes 

    

Hydration of alkenes

Reaction with water break double bond produces alcohol. 

Oxidation addition reactions with potassium 

cold potassium manganate VII 

hot concentrated manganate  

Addition polymerisation

Polythene 

  • Cling wrap

  • Water pipes

Polychloroethene PVC

  • Water pipes

  • Insulation wires

 General conditions: high pressure, high temperature and catalyst

Disposal of polyalkenes

Depolymerisation 

  • removing the monomer units, strongly burning the plastics in oxygen

Polymers as fuel 

  • burned to produce heat, petroleum based

Halogen derivatives 

Classifying halogenoalkanes

  • Primary halogenoalkane: one carbon attached to the carbon atom adjoining the halogen.

  • Secondary halogenoalkane: two carbons attached to the carbon atom adjoining the halogen.

  • Tertiary halogenoalkane: three carbons attached to the cation atom adjoining the halogen. 

REACTION OF HALOGENOALKANES

Halogenoalkanes undergo either:

  • Substitution 

  • Elimination 

NUCLEOPHILIC SUBSTITUTION REACTIONS 

Hydrolysis 

Reagent: strong alkali, NaOH or KOH

Condition: heat/reflux

  • Fluoroalkanes not hydrolysed, C-F bond to strong

  • Ease of hydrolysis increase: primary < secondary < tertiary

  • Tertiary halogenoalkanes can be hydrolysed without alkali 

Nitrile (cyanide) 

Reagent: KCN or NaCN in ethanol 

Conditions: Solvent ⟶ ethanol

Heat/reflux 

Primary amines

Reagent: ammonia 

Conditions: ammonia in ethanol under pressure in sealed container 

Nucleophilic substitution mechanism

  • ⟶C-X bond is a polar bond, partial charges due to high electronegativity of halogen    

ELIMINATION REACTIONS

Reagent: ethanolic NaOH or KOH

Conditions: 60ºC, reflux

  • OH– acts as a proton acceptor, accepts H+ loss from the halogenoalkanes during elimination

  • Becomes progressively easier, primary < secondary < tertiary  

Alcohols 

Naming alcohols

  • ⟶OH group in addition to groups that has a higher priority, OH named with prefix hydroxy-

  • ⟶Two or more -OH groups, tri, di used 

  • ⟶Add  e to stem name

Source of alcohols: 

  • ⟶hydration of alkenes and fermentation 

Types of alcohols

 

Properties of alcohols 

  • Colourless liquids at r.t.p 

  • BP and densities increases with increasing C atoms and with increasing OH groups

  • Low volatility & high BP due to ability to form hydrogen bonds between alcohols

 

Solubility of alcohols in water

  • Smaller alcohols mix completely with water, hydrogen bonds

  • Chain length increases, solubility decreases

  • Small alcohols good solvents 

REACTIONS OF ALCOHOLS 

SUBSTITUTION REACTIONS WITH ALCOHOLS GIVE HALOGENOALKANES

Reaction with HCl

  • ⟶Tertiary alcohols react with concentrated HCl 

  • ⟶@rtp

  • ⟶Tertiary alcohol is made 

Mechanism of the reaction 

Reaction with Hydrogen Bromide (hydrobromic acid)

  • ⟶Alcohols react with hydrogen halide produces halogenoalkane 

Reaction with sodium

  • ⟶H2 given off

  • ⟶CH3CH2O–Na+ = sodium ethoxide; gives alkaline solution in water

Oxidation of alcohols

PRIMARY ALCOHOLS

  • Oxidised to aldehydes & carboxylic acids

  • Aldehyde: alcohol heated with K2Cr2O7

  • Carboxylic acids: alcohol heated strongly under reflux, excess K2Cr2O7, so aldehydes also converted

SECONDARY ALCOHOLS

  • Ketones

  • K2Cr2O7, heated under reflux

TERTIARY ALCOHOLS

  • Doesn't undergo oxidation

Dehydration of alcohols

  • ⟶Elimination reaction, water molecule removed and alkene is produced

  • ⟶Excess hot concentrated sulfuric acid H2SO4 / phosphoric acid H3PO4

  • ⟶170ºC

Esterification 

  • ⟶Condensation reaction between an alcohol and carboxylic acid 

  • ⟶Heated in concentrated H2SO4, acts as catalyst

  • ⟶Water molecule is eliminated

Formation of tri-iodomethane (CHI3)

  • ⟶Iodine solution added to alcohol then NaOH

  • ⟶Colour of iodine fades to very pale yellow precipitate CHI3

  • ⟶All secondary alcohols and ethanol is the only primary alcohol that reacts

ISOMERISM 

  • ⟶same molecular formula but different arrangements of atoms

Structural Isomers

  • chain

  • positional

  • functional

Stereoisomers

  • geometrical

  • optical

Chain

  • ⟶differ by length of C backbone 

Functional

  • ⟶belong to different functional groups 

Positional

  • ⟶different positions of functional group 

Geometrical Isomers

Geometric isomers occur in molecules that have restricted rotation around a carbon-carbon double bond (C=C) or a similar structure. 

  • ⟶Cis isomers:In cis isomers, the identical groups are on the same side of the double bond.

  • ⟶Trans isomers:In trans isomers, the identical groups are on opposite sides of the double bond.

Optical Isomerism

Optical isomers arise in molecules that have a chiral centre. A chiral centre is a carbon atom that is bonded to four different groups.

 

Enantiomers:

  • ⟶non-superimposable mirror images of each other

  • ⟶same physical properties and most chemical properties

  • ⟶interact differently with biological molecules

Carbonyl compounds

ALDEHYDES AND KETONES

  • ⟶carbonyl compounds; carbon atom and a oxygen atom bonded with a double bond, not associated with other atoms  

Naming aldehydes and ketones

  • Aldehydes: suffix -al e.g. propanal

  • ROCH used for aldehydes

  • Ketones end in -one e.g. propan-2-one

Formation of aldehydes and ketones 

  • ⟶produced by oxidation of primary and secondary alcohols

  • Alcohols heated with K2Cr2O7 gives aldehydes, distilled prevent further oxidation (ketones)

Carboxylic acids

  • ⟶weak organic acids, COOH

  • ⟶CnH2n+1 COOH

  • ⟶Fictional group only found at the end

FORMATION OF SALTS

Reacting with active metals

  • Produces hydrogen gas and a salt

  • Sodium + ethanoic acid ⟶ sodium ethanoate + hydrogen gas

Reacting with alkalis

  • Neutralisation reaction

  • Salt and water

Reacting with a carbonate or a hydrogen carbonate

  • Salt, carbon dioxide and water

Esterification 

ESTERS AND AMINES

ESTERS

  • ⟶Volatile compound, less dense than water

  • ⟶Fruity smell 

  • ⟶Do not have strong hydrogen bonds

Naming esters 

  • Alcohol name becomes the alkyl part

  • Carboxylic acid becomes the stem name

  • End in -oate 

Hydrolysis of esters

  • Nucleophilic substitution reaction (opposite of esterification)

  • Carbonyl carbon has 𝜹+ charge, attacked by water acid catalyst

  • Ester breaks up producing carboxylic acid and alcohol

  • Slow reaction, water not strong nucleophile

  • Reversible reaction

Uses of esters

  • ⟶Solvents for paints and varnishes

  • ⟶Perfumes, flowers, fruits

  • ⟶Artificial flavours

  • ⟶Soaps

Amines

  • ⟶Related to ammonia, one hydrogen atom replaced by an alkyl chain

  • ⟶NH2 

NAMING AMINES

Primary amines 

Secondary amines 

Tertiary amines