M6S1: Aromatic Compounds and Carbonyls

Benzene

• C₆H₆

• Cyclic structure

Delocalised Model

P-orbitals of all six carbons overlap to crate a π-system made of a clouds of electrons above and below

Evidence of delocalisation

• Enthalpy of hydration of benzene is less exothermic than expected

• Indicating it is more stable due to delocalised electrons being spread over more atoms

Naming aromatic compunds

• If benzene ring is main functional group,

  • prefix = other group

  • suffix = benzene

• If benzene is not main functional group

  • prefix = phenyl

  • suffix = from functional group of other group

Why doesn’t benzene react with bromine water

Due to benzene’s π-system, it is very stable and negative charge is spread out which is unfavourable for this reaction

Prefers reacting with electrophiles

Benzene electrophilic substituion

1. Electron dense region attracts electrophile

2. Electrophile take pair of electrons from centre and form bond

3. Partially breaking delocalised ring = positive charge

4. Lose hydrogen to regain stability

Halogenation of benzene

• Uses halogen carrier to allow electrophilic substitution

• Halogen carrier polarises the halogen allowing it to react

• Eg. AlCl₃, FeCl₃

Nitration of benzene

• Warm benzene to 55°C

• Concentrated nitric acid

• Concentrated sulfuric acid (catalyst), makes NO₂⁺

• HNO₃ + H₂SO₄ → HSO₄^- + NO₂⁺ = H₂O

• Increase temp = more substitutions

Acyl group

-C(=O)-R

Acylation

• Used to add acyl group to benzene

• Need to use halogen carrier to make acyl group positive

Alkylkation

• Used to add alkyl group to benzene ring

• Haloalkane and halogen carrier used

Phenol

• C₆H₅OH

• Suffix: -phenol

Phenol neutralisation

• Phenol weakly acidic

• Reacts with base to make salt and water

Phenol electrophilic substitution

• Undergoes electrophilic substitution in bromine water

• OH group has electron donating effect = more reactive

Electron donating groups

• -OH, -NH₂

• Have electrons in orbitals that overlap with π-system

• Increasing electron density at carbons 2,4, 6

Electron withdrawing groups

• -NO₂

• Doesn’t have orbitals that overlap with delocalised ring

• So withdraws electron density from ring at carbons 2, 4, 6

• Directs electrophilic substitution to 3 & 5 position

Reduction of carbonyls

• Can reduce carbonyl groups to alcohol using reducing agent

• Usually NaBH₄

Carbonyls and Hydrogen Cyanide

• React to produce hydroxy nitrile

• Nucleophilic addition

Brady’s Reagent - test for carbonyl

• Formed from 2,4-DNPH dissolved in methanol & conc H₂SO₄

• Forms bright orange precipitate if carbonyl group present

• Each carbonyl group forms different crystalline derivative with different melting points - can be used to identify

Tollens’ Reagent

• Used to distinguish aldehyde & ketone

• AgNO₃ dissolved in (aq) ammonia

• Heated in test tube with aldehyde = silver mirror

• Ag⁺ + e^- → Ag_(s)

Carboxylic acid

• -COOH

• -oic acid

• Always on 1st carbon when naming

• Polar = small carboxylic group very soluble

• Weak acid

Carboxylic acid reactions

Undergoes same reaction with metals, carbonates and bases as a weak acid

Acyl chlorides

• COCl-

• C_nH_2n-1OCl

• Suffix: -oyl chloride

Formation of acyl chlorides

React carboxylic acid and SOCl₂

-OH group replaced with -Cl

Nucleophilic substitution

Acyl chloride with water

• -Cl group replaced by -OH

• Produces carboxylic acid & HCl

Acyl chloride with alcohols

Produces ester

Acyl chloride with ammonia

Produces primary amide

Acyl chloride with amines

Produces secondary amide

Acyl chloride with phenol

Produces ester

Esters

Formed form carboxylic acid and alcohols

Naming ester

• First part form alcohol

• Second part from carboxylic acid & replace ‘-oic acid’ with ‘oate’

• Eg. Ethyl methanoate

Ways of producing ester

• Alcohol & carboxylic acid

• Alcohol & acid anhydride (dehydration)

Alcohol & Carboxylic acid

• Heat carboxylic acid with alcohol in presence of strong catalyst

• Usually conc H₂SO₄ used

• Esterification

Dehydration

• Alcohol and acid anhydride

• Acid anhydride made from two identical carboxylic acid molecules, joined via oxygen 

• Acid anhydride also called dimers

Hydrolysis of esters

• Splits ester

• Acid/Alkali used to speed up 

• Both produce alcohol but second product is different

Acid hydrolysis

• Ester split into alcohol and carboxylic acid

• As it is reversable, needs water to push reaction forwards

Base hydrolysis

• Reflux ester with dilute alkali, such as NaOH

• Forms alcohol and carboxylate salt