6.1. Aromatic Compounds, Carbonyls and Acids: Module 6: Organic Chemistry & Analysis Chemistry OCR A A Level

Arenes

Arenes are aromatic compounds that contain a benzene ring as part of their structure.

Benzene

Benzene is an arene consisting of a ring of six carbon atoms each bonded to one hydrogen atom, giving it the molecular formula C6H6.

X-ray Diffraction and Infrared Data

X-ray diffraction experiments have shown that all the bond lengths between carbon atoms in benzene are the same. If the cyclohexatriene structure was correct, three of the bond lengths would be the length of a single carbon bond and three would be the length of a double carbon bond. In reality, each bond in the benzene ring has an intermediate length in between that of a double and single bond.

Electrophilic Substitution

Benzene is resistant to electrophilic addition reactions, such as bromination, which other compounds with carbon-carbon double bonds, such as alkenes, readily undergo. Benzene does not undergo electrophilic addition since this would involve breaking up the stable delocalised ring of electrons. Benzene instead undergoes electrophilic substitution reactions.

Halogenation

Halogenation is a type of electrophilic substitution reaction in which benzene reacts with halogens in the presence of a catalyst, such as iron(III) bromide (FeBr3).

Nitration

Nitration is a form of electrophilic substitution, where the electrophile is an NO2 + ion. This is a reactive intermediate, produced in the reaction of concentrated sulfuric acid (H2SO4) with concentrated nitric acid (HNO3). Sulfuric acid behaves as a catalyst since it is not used up in the reaction.

Friedel-Crafts Acylation

The delocalised electron ring in benzene can also act as a nucleophile, leading to their nucleophilic attack on acyl chlorides. This reaction is known as Friedel-Crafts acylation.

Bromine Water

The test for unsaturation would be expected to work for benzene like it does for alkenes, however, benzene is resistant to bromination. This is due to the delocalised electron density of the π-system in benzene compared with the localised electron density of the π-bond in alkenes. The delocalised model makes benzene relatively stable and hence not reactive enough to decolourise bromine water.

Phenols

Phenols are organic compounds containing a benzene ring with an OH alcohol group. This makes them aromatic alcohols. Phenols are weak acids. They can be neutralised in a reaction with NaOH but will not react with carbonates.

Electrophilic Substitution Reactions

Phenol, an aromatic compound with the formula C6H5OH, is produced in electrophilic substitution reactions with benzene. Phenol can react with bromine water via multiple substitutions to produce 2,4,6-tribromophenol which forms as a white precipitate with a distinct smell of antiseptic. This reaction decolourises bromine water.

Directing Effects

Electron donating groups such as OH and NH₂ direct electrophiles to substitute at the 2- and 4- positions. Electron-withdrawing groups such as NO₂ are 3-directing in electrophilic substitution of aromatic compounds. These effects occur to favour the most stable charged intermediate.

Directing effects

Directing effects can be used to predict substitution products. This is very important in organic synthesis as it allows you to control the structure of the products.

Carbonyl Compounds

Carbonyl compounds are organic compounds containing a carbonyl group, C=O. This gives them the functional group -CO. The most common carbonyl compounds are aldehydes and ketones.

Aldehydes

Aldehydes are produced from the initial oxidation and distillation of primary alcohols. Aldehydes have a carbonyl group on a carbon atom at the end of the carbon chain (only attached to one other carbon atom).

Ketones

Ketones are recognised by the functional group -C=O, a carbonyl group. They are produced from the oxidation of secondary alcohols with acidified potassium dichromate(VI). Ketones have a carbonyl group on a carbon atom that is attached to two other carbon atoms.

Oxidation

Primary and secondary alcohols can be oxidised to produce various products, but tertiary alcohols are not easily oxidised.

Potassium Dichromate(VI) (K2Cr2O7)

Potassium dichromate(VI) is used as an oxidising agent in the oxidation of alcohols. As the alcohol is oxidised, potassium dichromate(VI) is reduced. This reduction is observed as a colour change from orange to green, which indicates the alcohol has undergone oxidation. The colour change occurs due to the change in oxidation state of the chromium ion.

Nucleophilic Addition

All of the oxidation reactions involved in the production of carbonyl compounds from alcohols can be reversed via reduction reactions.

NaCN (sodium cyanide)

NaCN (sodium cyanide) is often used as the reagent along with a H⁺ source to provide the HCN (hydrogen cyanide). This is because HCN is hard to store and is a toxic gas which reacts to produce dangerous byproducts.

Hydroxynitriles

Hydroxynitriles commonly contain a chiral carbon centre meaning optical isomers of the product are produced. The :CN- nucleophile can attack from either above or below the planar double bond, causing different enantiomers to be produced.

Properties of Carboxylic Acids

Carboxylic acids are organic compounds identified by the functional group -COOH, which contains a carbonyl group (C=O) and an -OH acid group. When naming carboxylic acids, the suffix -anoic acid is used. For example, a carboxylic acid containing a chain of four carbon atoms would be called butanoic acid.

Reactions of Carboxylic Acids

Carboxylic acids are weak acids and therefore react with bases in a neutralisation reaction to produce a salt.

Esters

Esters have the functional group -COO-. They are named after the alcohol and carboxylic acid from which they are formed.

For example, the ester formed from methanol and propanoic acid is methyl propanoate and the ester formed from butanol and ethanoic acid is butyl ethanoate.

Esterification

Carboxylic acids can react with alcohols in the presence of a strong acid catalyst to form esters. Concentrated sulfuric acid is often used as the acid catalyst. This reaction is called esterification and is carried out under reflux.

Hydrolysis of Esters

Ester hydrolysis is the reverse reaction to esterification, converting esters back into alcohols and carboxylic acids. This process is done by adding water, but can be carried out under different conditions to produce different products.

Acyl Chlorides

Acyl chlorides have the functional group -COCl and have the suffix -oyl chloride, with the stem of their name representing the longest chain of carbon atoms.

Acylation

Carboxylic acids have derivative molecules where the -OH group is replaced by another group. Acyl Chlorides are one such derivative that reacts violently due to the very polar COCl group.

Reactions of Acyl Chlorides

Acyl chlorides react via nucleophilic addition-elimination reactions. In these reactions, the addition of a nucleophile leads to the elimination of a product under aqueous conditions.