Preparation Of Aromatic Aldehydes And Ketones

1. By oxidation of alkyl benzene

Aromatic aldehydes are obtained by oxidation of side chain in the aromatic ring.

2. By Friedel crafts reaction (Refer to hydrocarbons)

3. By Gattermann Koch Reaction

In this method aromatic aldehydes are prepared by formylation of aromatic ring with carbon monoxide.

4. From Grignard’s reaction

Both aliphatic & aromatic aldehyde can be obtained by this method.

HCN on treatment with Grignard’s reagent & subsequently followed by hydrolysis yield an aldehyde.

Similarly

PHYSICAL PROPERTIES OF ALDEHYDES & KETONES

(i) Physical state

Most of aldehydes (except formaldehyde which is a gas) are liquids at room temperature. The lower ketones are colourless liquids and have pleasant smell.

(ii) Boiling points

Aldehydes & ketones have relatively high boiling points as compared to hydrocarbons of comparable molecular masses due to polar carbonyl group, which bring stronger intermolecular dipole – dipole interactions between the opposite ends of C = O dipoles.

Ketones are relatively more polar than their corresponding isomeric aldehydes due to the presence of two electron repelling alkyl group around the carbonyl carbon.

(iii) Solubility

The lower members of aldehydes & ketones (upto four carbon atoms) are soluble in water. It is due to their capability of forming hydrogen bonds with water molecules. The solubility of these compounds in water decreases with the increase in the size of alkyl group because of the increase in magnitude of non polar part in the molecule.

CHEMICAL PROPERTIES

Aldehydes & ketones are highly reactive compounds, they undergo nucleophilic addition reactions. Their reactivity is due to presence of a polar carbonyl group. The positively charged carbon atom of carbonyl group is readily attacked by nucleophilic species for initiation of the reaction. This leads to formation of intermediate anion which further undergoes the attack of H+ ion or other positively charged species to form the final product. The reaction in general may be represented as:

Relative reactivity of aldehydes & ketones

In general ketones are less reactive than aldehydes on a account of following facts:

1. Electron releasing effect of two alkyl groups, decreases the magnitude of positive charge on ketones.
2. Steric effect caused by two alkyl groups also hinders the approach of the nucleophile to the carbonyl carbon.