Aldehyde

Ketone

So far we have a reactive group attached to the carbon of the hydrocarbon via a σ−bond. In aldehydes and ketones an oxygen is attached to the carbon via a π−bond. So the chemistry of aldehydes and ketones is the chemistry of the carbonyl group, C=O.

For aldehydes the carbon of the carbonyl group has at least one proton bonded to it. Both groups to the carbon of the carbonyl in a ketone are alkyls. Since the chemistry of the carbonyl group is very much the consequence of the polarity of this group, the presence of a proton in place of an alkyl is very significant.

In the early years aldehydes were seen as derivatives of the corresponding acids, since the acids are better known.

Formic acid		Formaldehyde		Acetic acid		Acetaldehyde

As for the ketones, the system used for ethers was followed.

Ethyl methyl ketone

4-chlorohexanal

Of course the academia would prefer the IUPAC Nomenclature. It is good to note that the numbering of the carbon position starts at the carbon of the carbonyl group of the aldehyde.

(A)

For some aldehydes it might be easier to use "carbaldehyde" for the carbonyl group. So we can name compound (A) as 2-chlorocyclohexanecarbaldehyde.

CARBONYL

The oxygen atom is much more electronegative compared to the carbon atom. So we will get a very significant dipole moment for the carbonyl group. This is especially so since one of the bond is a π−bond. The π−bond is more polarisable; meaning the electrons in the bond can be shifted easier.

The dipole moment for formaldehyde, H−C(O)−H is 2.27 D, while that for acetone,
CH₃−C(O)−CH₃ is 2.85 D.

PHYSICAL PROPERTIES

The difference between the boiling point of the lower aldehydes and ketones (up till 4 carbons) reflects the difference in the dipole moment of the carbonyl groups. For molecules larger than 4 carbons the difference is only down to 2 degrees centigrade. The effect of the carbonyl is greatly reduced since it is not so exposed for intermolecular interaction. This also explains why the boiling point is also quite similar to an alkyl chloride of comparative molecular size.

CHEMICAL PROPERTIES

Aldehydes and ketones will react with protonated solvents in the presence of acid via a series of equilibrium. With water a gem-diol is obtained.

Gem-diol

When two moles of alcohol (for each mole of carbonyl group) are used the acetal and ketal will be formed. In general acetalisation is easier than ketalisation, and smaller carbonyl molecules are favoured over larger molecules. So for linear aldehydes and ketones with more than 10 carbons the reaction is insignificant.

An important observation is that in acetalisation (and ketalisation) three molecules are bonded to form one molecule. Thermodynamically the entropy change for the reaction is negative, and this does not favour the process. The reaction will only take place when the reaction is highly exothermic (∆H is highly negative) (reference). This explains why diols are better reactants than alcohols. The reaction of cyclohexanone and ethylene glycol gave about 80% of the cyclic ketal.

Also the reaction consists of a series of equilibrium, so the yield of the products is low (especially with larger aldehydes and ketones) unless you keep removing the products as they are formed.

HYDROGEN CYANIDE

The second last step is a migration of a proton to the oxygen of the peroxide. For ketone, where there is no proton available, an alkyl will migrate instead. So for aldehyde we will get the corresponding acid; where the ketone will give an ester (R-COOR'). The overall result is the oxidation of the carbonyl to a carboxylate. This reaction is known as the Baeyer-Villiger oxidation.

Ketones can have two different alkyl substituents to the carbonyl group, this means two types of esters can be obtained. In general the migration of the alkyl is in the order:

H > tertiary > secondary > phenyl> primary> methyl

With cyclic ketones we will get cyclic esters, otherwise known as lactones.

HYDROXIDES and ALKOXIDES

The carbonyl group can react with water and alcohols in the presence of a base.

With a water / hydroxide mixture the gem-diol is produced.

⇆

H2O ⇆

With an alcohol / alkoxide mixture the reaction will give the hemiacetal or hemiketal.

⇆

ROH ⇆

Similar when two moles of alcohol (for each mole of carbonyl group) are used the acetal and ketal will be formed. Acetals and ketals are generally stable in basic conditions and are hydrolised to the carbonyl compounds only in acidic solutions. This can be exploited to protect the carbonyl group when carrying out a reaction on another reactive group in the molecule.

R−CH(X)−(CH2)i−CO−R'   +   HOCH2CH2OH   �

Ethers (ketals are ethers) are rather inert, the reaction can now can carried out upon X. The product obtained is than hydrolysed in the presence of acid to regenerate the ketone, provided the other group will not be hydrolysed.

Tutorial 11.1

A project requires the preparation of 4-heptynal from 2-bromopropanal and 1-butyne, show the reaction paths you would used.     Answer

It is important to remember that acetals and ketals are ethers. As such they can form peroxides when left standing in air. So when distilling these compounds that have been exposed to air for a "long" period of time it is best to wash off the peoxides first, and never distill till the "last drop".

Tutorial 11.2

Propose the mechanism for the reaction between a ketone and a primary amine to produce the imine.     Answer