Aldehydes and Ketones

Introduction

Aldehydes and ketones are two similar homologous groups both having the carbonyl group:

aldehydes 1

The Carbon on the carbonyl group is slightly positive wince the Oxygen is pulling the electrons towards it, leaving the possibility of nucleophilic addition.

Terminal carbonyl group: Aldehydes

Non-terminal carbonyl group: Ketones

Solubility

The solubility of ketones in water is minimal, due to the fact that the carbonyl group offers no Hydrogen bonding, but this is one of the most important solvents in any lab since it can dissolve practically all organic compounds.

Reactivity

Carbonyl groups react via nucleophilic addition, with the mechanism being represented as follows:

aldehydes 2

Where the nucleophile attacks the positive carbon on the carbonyl, with the electrons being shifted to the Oxygen to produce a negative Oxygen. This Oxygen then collects a positive charge, normally an H+ to produce an alcohol.

Preparation

Halogenoalkanes

Gem-dihalogenoalkanes, which are halogenoalkanes with 2 halogens on the same Carbon can be used to prepare carbonyl groups by substituting the halogens with the hydroxyl group. The product would be unstable and it would decompose to form the carbonyl group

Oxidation

Alcohols can be oxidised to aldehydes and ketones. A specialised set up has to be used in order to oxidise primary alcohols to aldehydes since these can then be further oxidised to carboxylic acids.

This reaction is performed:

aldehydes 3

Using dichromate as the oxidising agent since it is weaker then permanganate.

Dropping the dichromate dropwise in order for the alcohol to always be in excess

The temperature of the system is set up so that the aldehyde boils off as soon as it is prepared

Dehydrogenation

The removal of hydrogen from alcohols will result in an enol (alkene with a vicinal alcohol). Keto-enol tautomerisation would then give the ketone/aldehyde, depending on the starting material. Primary alcohols give rise to aldehydes, while secondary alcohols give rise to ketones.

Ozonolysis

Ozone can be used to cleave an alkene from the double bond into two aldehydes.

aldehydes 4

Reactions

Addition of HCN

aldehydes 5

HCN is a very toxic gas, and therefore this is prepared in-situ via the reaction of sulfuric acid with potassium cyanide.

Since HCN is a weak acid, and therefore the concentration of the CN in solution would be low, reducing the rate of the reaction. In order to avoid this a base is added which would then shift the equilibrium to the cyanide ion, increasing the rate of the reaction. Acid hydrolysis would then give the alcohol.

Addition of Na+ HSO3

aldehydes 6

The reaction with hydrogen sulfate is a reversible reaction and this is used to purify aldehydes. When the salt is added to the aldehyde (reaction works best with aldehydes due to the bulkiness of the alkyl groups) a solid is precipitated out (the charges on the salt would make the whole molecule insoluble) which can then be filtered and washed. This reaction is reversible in both acidic and basic conditions.

Reduction of aldehydes and ketones

aldehydes 7

Aldehydes and ketones can be reduced to give alcohols, with the mechanism being the same as a nucleophilic addition, with an H being the nucleophile.

The reducing agents can be either LiAlH4 in ether or NaBH4 which can be used in water.

Addition with Grignard

One of the best ways to add an alkyl chain is the addition of a Grignard’s to a carbonyl group.

aldehydes 8

Nucleophilic Addition followed by elimination

Some reactions can lose water after the nucleophilic addition. This can happen when a Nitrogen which has a Hydrogen attached to it joins to the positively charged Carbon. Condensation will result with a double bond between the Nitrogen and the Carbon.

aldehydes 9

Some reactions that follow this pattern are:

Hydrazine

aldehydes 10

Hydroxylamine

aldehydes 11

Phenylhydrazine

aldehydes 12

2,4-DNPH

aldehydes 13

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