KETONES

William, Nanda, Hafiz & Mio

Introduction

Alcohols are found in two forms:

Primary: -OH group attached at the end of

the chain.

Secondary: -OH group attached to a

carbon atom not located at the end of the

chain.

Formation of ketones

When a secondary alcohol is heated with the oxidising agent potassium dichromate (VI), it oxidises into a ketone.

CH3CH(OH)CH3 + [O] CH3COCH3 +

H2Opropan-2-ol potassium dichromate propanone

water

Ketones cannot be oxidised any further.

So what is a ketone?

A ketone is a carbonyl compound where one of the carbon atoms along the chain (not at the ends) is double-bonded to an oxygen atom.

Naming convention

Carbon atoms

Name

3 propanone

4 butanone

5 pentanone

6 hexanone

7 heptanone

8 octanone

9 nonanone

10 decanone

Applications

Polymer precursor Pharmaceuticals Solvents e.g.:

acetone methylethyl ketone cyclohexanone

Reduction of ketones

When a ketone is warmed with the reducing agent NaBH4 (sodium tetrahydridoborate), it reverts into a secondary alcohol.

CH3COCH3 + 2[H] CH3CH(OH)CH3

propanone reducing agent propan-2-ol

We can think of it as a nucleophilic addition, where H- ions from the reducing agent act as the nucleophile.

Testing for ketones

Testing for ketones and aldehydes in a solution may be done with 2,4-dinitrophenylhydrazine (2,4-DNPH).

If the solution contains either, an orange precipitate will be formed.

This is an example of condensation reaction between 2,4-DNPH and the ketone.

Testing with Tollens’ reagent

Both aldehydes and ketones form an orange precipitate with 2,4-DNPH.

To differentiate, we may react the solution with Tollens’ reagent – a solution of silver nitrate in excess ammonia.

When it is warmed with an aldehyde, a silver precipitate builds up.

Ketones, however, will remain colourless.

Fehling’s solution

is an alkaline solution containing copper (II) ions.

It oxidises and turns from clear blue to an opaque red/orange when heated with an aldehyde.

Ketones cannot be oxidised any further, so the solution remains blue.

Nucleophilic addition with HCN The carbonyl group C=O is highly

polarised, as the oxygen atom has more electron pairs (and therefore more negative).

The carbon atom, being more positive, is open to addition reactions* from nucleophilic ions (i.e. negative ones).

*”nucleophilic attack” is a proper term and sounds cooler.

O C----

-

---

--

Mechanism

Addition, as you’ll recall, involves the breakdown of a double bond into a single bond.

An example of a nucleophile is the CN- ion in HCN, hydrogen cyanide.

O C

--

--

CN

O- C

--

--

CN

-

HH+

O C

--

--

CN

H

CN- -

Thank You