2.10.1c: describe the following chemistry of alcohols: (i) combustion (ii) reaction with sodium (iii) substitution reactions to form halogenoalkanes, including reaction with PCl5 and its use as a qualitative test for the presence of the –OH group (iv) oxidation using potassium dichromate(VI) in dilute sulfuric acid on primary alcohols to produce aldehydes and carboxylic acids and on secondary alcohols to produce ketones

COMBUSTION

Some alcohols, such as ethanol and methanol, can be used as fuels

• ethanol is used as a fuel substitute in countries with limited oil reserves
• methanol is used as a petrol additive to improve combustion and prevent the engine from knocking

REACTION WITH SODIUM

Alcohols react with sodium to release hydrogen and sodium ethoxide

The gas generated can be collected in an upturned test tube and tested with a lighted splint

Observations:

• effervescence
• the white solid (sodium) disappears

2CH3CH2OH + 2Na → 2C2H5ONa+ + H2

REACTION WITH PCL5

PCl5 is used to test for the presence of the OH group in alcohols

Postive test = misty white HCl fumes given off

SUBSTITUTION REACTIONS

Nucleophilic substitution, where a nucleophile (electron-rich) is attracted to the electron-deficient C+-O- bond

• the nucleophile replaces the -OH group
• eg. substitution reaction to form a chloroalkane from ethanol and PCl5

To prepare:

• a chloroalkane = alcohol + concentrated hydrochloric acid
• a bromoalkane = alcohol + sodium/potassium bromide and concentrated sulphuric acid
• a chloroalkane = alcohol + sodium/potassium iodide and concentrated phosphoric (V) acid/red phosphorus