Alcohols

Chemistry in Action

Chemistry in Action

Within organic chemistry, oxidation is considered as either:

  • the addition of oxygen
  • the removal of hydrogen

The oxidising agent in equations is represented simply as [O]

Mild oxidation of primary and secondary alcohols

When a primary alcohol is mixed with an oxidising agent and two hydrogen atoms are subsequently removed an aldehyde forms. For example:

CH3CH2OH + [O] ? CH3CHO + H20

Remember for you A level Chemistry revision: an aldehyde molecule contains the group:

When a secondary alcohol is mixed with an oxidising agent and two hydrogen atoms are subsequently removed a ketone forms. For example:

CH3CH(OH)CH3 + [O] ? CH3COCH3 + H2O

A ketone molecule contains the group.

Collectively, aldehydes and ketones are called carbonyls. Together they use the general formula CnH2nO. The R groups are different for each:

  • in aldehydes one is a H atom
  • in ketones neither is a H atom

As there are no H atoms available to give up, tertiary alcohols do not readily oxidise.

  • Further oxidation of aldehydes

When an aldehyde mixes with an oxidising agent it is possible for an oxygen atom to be added to the group forming a carboxylic acid. For example:

CH3CHO + [O] ? CH3COOH

A carboxylic molecule contain the group:

As ketones do not contain a C-H bond into which an oxygen atom can insert itself, thery cannot be oxidised.

——————————————————

Reagents and conditions for oxidation

In organic chemistry potassium dichromate (K2Cr2O7) in dilute sulphuric acid (H2SO4) is the most commonly used oxidising agent.

Cr2O72-(aq) + 14H+(aq) + 6e ? 2Cr3+(aq) + 7H2O(l)

In the reduction process there is a colour change from orange to green because the Cr2O72- ion is green and the Cr3+ ion green. Aldehydes and carboxylic acids can be produced by oxidising primary alcohols. The conditions used dictate the major product.

As there is no hydrogen bonding between aldehyde molecules, carbonyls are more volatile than either carboxylic acids or alcohols. Therefore, it is possible to distil off volatile aldehyde has it is formed using distillation apparatus.

If reflux apparatus is used instead then the aldehyde remaining in the reaction vessel gets converted into carboxylic acid. The yield of carboxylic acid can be improved by using heat and an excess of the oxidising agent.

Secondary alcohols form ketones when oxidised. Using distillation apparatus the volatile ketone can be distilled off as it forms.

——————————————————

Tests to distinguish between aldehydes and ketones

Tollen’s reagent

Tollen’s reagent, otherwise known as ammoniacal silver nitrate, is an oxidising agent and reacts with aldehydes when boiled. The colourless Ag+ ions are reduced into a metallic silver that forms on the test tube’s surface to create a ‘silver mirror’.

Ketones, on the other hand, are not reducing agents.

Fehling’s solution

Fehling’s solution is a complex solution which contains Cu2+ ions. Again, it uses the principle that aldehydes are reducing agents while ketones are not. On heating, the Cu2+ ions react with the aldehydes and the blue Cu2+ is reduced to Cu2O, a brick red precipitate.