Question

How is ethyl bromide prepared from :
(i)Ethyl alcohol (ii) Ethane (iii) Ethene

Answer 1

Hint: For the preparation of ethyl bromide from ethyl alcohol, you will need a reagent that can replace OH from the alcohol by bromide ion. For the preparation of ethyl bromide from ethane, refer to the free radical halogenation of alkanes and for the preparation of ethyl bromide from ethene, recall the addition of halogen acids to alkenes.

Complete step by step answer:
(i) Preparation of ethyl bromide from ethyl alcohol:
By treating the ethyl alcohol (${C_2}{H_5}OH$) with phosphorus tribromide $\left( {PB{r_3}} \right)$, we get ethyl bromide (${C_2}{H_5}Br$). It is a substitution reaction because the hydroxyl group (OH) of the alcohol is substituted by the bromide ion (Br).
$3{C_2}{H_5}OH + PB{r_3} \to 3{C_2}{H_5}Br + {H_3}P{O_3}$

(ii) Preparation of ethyl bromide from ethane:
When ethane (${C_2}{H_6}$) reacts with bromine in the presence of $AlB{r_3}$, we get ethyl bromide as a product.
${C_2}{H_6} + B{r_2}\xrightarrow{{AlB{r_3}}}C{H_3}C{H_2}Br + HBr$

(iii) Preparation of ethyl bromide from ethene:
Ethene (${C_2}{H_6}$) can be converted to ethyl bromide by reaction with hydrogen bromide. This reaction is called addition of hydrogen halide to alkene (or addition hydrohalogenation reaction) where hydrogen halide acts as electrophile and attacks the ${\text{C = C}}$ bond of alkene.
${C_2}{H_6} + HBr \to {C_2}{H_5}Br$

Additional Information:
Alkyl halides are best prepared from alcohols, which are easily accessible. The order of reactivity of alcohols with any given haloacid is ${3^0} > {2^0} > {1^0}$ . By free radical halogenations of alkanes, we get a complex mixture of isomeric mono and polyhaloalkanes, which are difficult to separate as pure compounds.

Note: Constant boiling of ethyl alcohol with HBr (48%) is also used for preparing ethyl bromide. In the laboratory, addition of bromine in $CC{l_4}$ to an alkene, results in discharge of a reddish brown colour of bromine. This is an important method for the detection of double bonds in a molecule.