Alcohols
 

Home | Chemistry | Chemistry GCSE


Alcohols RCH2OH, R2CHOH, R3COH.

Links

Primary alcohol → aldehyde
Reagents: potassium dichromate(VI) solution and dilute sulphuric acid.
Conditions: the dichromate/alcohol mixture is added to hot acid so that the aldehyde distils out of the
                  reaction mixture and further oxidation is minimised:

CH3CH2OH + [O] → CH3CHO + H2O

The aldehyde may oxidise further to a carboxylic acid

CH3CHO + [O] → CH3COOH


Alkanes & alkenes
Arenes
Halogenoalkanes
Alcohols
Aldehydes and ketones

Carboxylic acids
Acyl chlorides
Amines
Nitro compounds
Nitriles
Amides

Secondary alcohol → ketone
Reagents: potassium dichromate(VI) solution and dilute sulphuric acid.
Conditions: heat under reflux

CH3CH(OH)CH3 + [O] → CH3COCH3 + H2O

There is no further oxidation of the ketone under these conditions.


 

Tertiary alcohols are unaffected by the reactions above.


 

Alcohol → alkene
Reagents: concentrated sulphuric acid
Conditions: heat to 170o with excess acid.

CH3CH(OH)CH2CH3 → CH3CH=CHCH3 + H2O

The ease of elimination of water is primary < secondary < tertiary. Tertiary alcohols will sometimes lose water just on heating. If there are two alkenes which can be produced, the major product is that bearing most alkyl groups on the C=C group.

Other reagents can be used which do not cause the charring and oxidation that conc sulphuric acid does. Either (i) syrupy phosphoric(V) acid H3PO4 + heat, or (ii) alcohol vapour passed over hot aluminium oxide, are effective.


 

Alcohol → halogenoalkane. All types of alcohol react similarly.
Reagents: phosphorus(V) chloride
Conditions: at room temperature

CH3CH2OH + PCl5 → CH3CH2Cl + POCl3 + HCl


 

Alcohol → halogenoalkane. All types of alcohol react similarly.
Reagents: thionyl chloride SOCl2
Conditions: at room temperature:

CH3CH2OH + SOCl2 → CH3CH2Cl + HCl + SO2

This gives a very clean reaction since the products apart from the halogenoalkane are gaseous.


 

Alcohol → halogenoalkane.
Reagents: sodium bromide and 50% sulphuric acid. HBr is generated in situ.
Conditions: heat under reflux.

NaBr + H2SO4 → HBr + NaHSO4

CH3CH2CH2CH2OH + HBr → CH3CH2CH2CH2Br + H2O

Primary alcohols react best; others tend to eliminate to give alkenes. This reaction cannot be used for iodides, since KI is oxidised by sulphuric acid. The reaction below is used instead.


 

Alcohol → bromo- or iodoalkane.
Reagents: moist red phosphorus and bromine or iodine
Conditions: warm.

The phosphorus trihalide is prepared in situ by this reaction.

3CH3CH2OH + PBr3 → 3CH3CH2Br + P(OH)3


 

JRG Beavon 2002