Carbonyl Compounds (2) Aldehydes and Ketones

Carbonyl compounds covers a great many different types of organic compounds.

We’ll just discuss in this blog two of the simpler types: aldehydes and ketones.

Let’s begin by looking at the structural formulae of aldehydes.

  

These are three aldehydes.

Note the nomenclature:     eth—an—al

The suffix –al denotes the aldehyde homologous series, the prefix eth– denotes the number of carbon atoms in the molecule and includes the carbon of the carbonyl group and the –an– denotes the compound is saturated. 

Can you draw the structural formula of methanal the simplest aldehyde?

Space filling structures of some of these aldehydes look like this:

Ethanal  CH₃CHO

Ketones on the other hand have alkyl groups on both sides of the carbonyl group

These are three of the simplest ketone structures.

Here is a typical space–filling picture of the simple ketone: propanone

Solubility in water:

The simpler aldehydes and ketones are completely miscible with water.

The solubility in water decreases as the carbon chain length increases.

Van der Waals forces (temporary dipole forces) between carbon chains begin to dominate over the hydrogen bonds formed between the aldehyde and water. 

The hydrogen bond is shown as a dotted line.

Note that the hydrogen bond is linear with a bond angle of 180^o

Distinguishing an aldehyde from a ketone

There are a couple of chemical tests you can carry out to see if an aqueous solution contains a simple aldehyde or a simple ketone.

Both aldehydes and ketones will form a yellow crystalline precipitate with Bayer’s Reagent. 

But only aldehydes will form a red precipitate on heating gently with either blue Fehling’s solution or alkaline Benedict’s solution.

Aldehydes can be oxidised to carboxylic acids in this redox reaction.

CH₃CHO   +   H₂O      →    CH₃COOH   +   2H⁺     +     2e–

The blue copper(II) solution is reduced to a red precipitate  of copper(I) oxide

2Cu ²⁺       +     2e–     →      2Cu⁺

Typical reaction of an aldehyde or ketone

Aldehydes and ketones undergo nucleophilic addition reactions with appropriate nucleophiles such as HCN.

The reaction with the weak acid HCN occurs in the presence of the soluble salt KCN.

The KCN is necessary since it provides sufficient CN– ions cyanide/nitrile ions.

KCN is fully ionised in aqueous solution whereas HCN is partially ionised since HCN is a weak acid:

HCN      +     H₂O       ⇋    H₃O⁺   +    CN^—

The mechanism of nucleophilic addition looks like this:

Step one:

Step Two:

The negatively charged oxygen abstracts a proton from the HCN molecule

 

Step Three:

The catalyst :CN– is regenerated

 

If the ketone is asymmetrical as in the example below the product of the reaction with HCN is optically active but both optically active isomers form giving rise to a racemic mixture:

Problem:

What happens if an aldehyde, say CH₃CH₂CHO, is used instead of the ketone?

Can you work out the structures of the reaction products and see if they are optically active or not.

Is in fact a racemic mixture produced?