M6 Aromatic + Carbonyl compounds, carboxylic acids and esters

Evidence to disprove Kekulés Model

Benzene does not react with bromine water at room temperature & pressure

The carbon-carbon bond lengths in benzene are all the same

The enthalpy of hydrogenation of benzene is less exothermic than expected

Orbital overlap in benzene

Kekulé model

Delocalised Model

Similarities between Kekulé & delocalised model

Both models show sideways overlap of p orbitals

Both models have a pi bond which lies above and below the plane of the carbon atoms

Differences between Kekulé & delocalised model

Delocalised - p orbitals overlap forming a delocalised pi system

Kekulé - there are 3 pi bonds.Each is localised over one carbon to carbon bond

Benzene

Delocalised electrons in a pi system

Lower electron density

Less able to polarise an electrophile (requires halogen carrier

Cyclohexene

Localised electrons in a pi bond between two carbon atoms

Higher electron density

More able to polarise an electrophile

Electron withdrawing groups

NO2

Electron Directing effects → 3,5

Electron donating groups

Electron donating effect

OH → 2,4,6

NH2 → 2,4,6

Cl/Br/I → ?

Nitration of benzene

Nitration of benzene (mechanism)

Reduction of nitrobenzene (+ conditions)

Halogenation of benzene

Halogenation of Benzene (mechanism)

Alkylation of benzene

Alkylation of benzene (mechanism)

Acylation of benzene

Acylation of benzene (mechanism)

NOTE

PHENOLS DONT NEED HALOGEN CARRIERS

Why does phenol undergo electrophilic substitution more readily than benzene

The lone pair of electrons from the p orbital on the oxygen of the OH can be donated into the delocalised pi system of the benzene ring

The electron density increases inside the ring

More able to polarise the electrophile

Preparation of aliphatic amines

Excess ethanolic ammonia

why excess ammonia

Used to prevent further substitution of the amine group that forms

Ethyl amine could further react with chloroethane to make diethylamine/triethylamine

Nucleophilic addition to carbonyls

Every carbonyl if a ketone/aldehyde needs 2[H]

Nucleophilic addition of propanal + hydride DRAW

Nucleophilic addition to propanone using hydrogen cyanide

Chemical test for a carbonyl

Bradys reagents - 2,4dinitrophenylhydrazine

Forms an orange precipitate

Chemical test for carbonyl (3)

Use bradys reagent to form an orange precipitate

Purify that solid by recrystillisation

Measure the melting point of the solid and compare to database

Chemical test for an Aldehyde

Tollens reagent

Aldehyde - silver mirror forms

Ketone - Bradys reagent orange ppt THEN no reaction using tollents

Ethanolic acids + NaOH/OH-

Esterification

Warmed in the presence of H2SO4

Making esters from acid anhydrides

anhyrdride + alcohol → ester + carboxylic acid

Acid hydrolysis

Hot aqueous acid eg. HCl

Ester + H2O → carboxylic acid + alcohol

Base Hydrolysis

Hot aqueous alkali eg. NaOH KOH LiOH

Ester + NAOH → carboxylic acid + alcohol

Making acyl chloride

Acyl chloride + water

Acyl chloride + water → carboxylic acid + HCl

Acyl chloride + alcohol

Acyl chloride + alcohol → ester + HCl

acyl chloride + ammonia

Acyl chloride + 2NH2 → primary amide + HCl

forming secondary amides from acyl chloride