Question

For the preparation of n-propyl bromide from n-propyl alcohol, which of the following reagents is most preferred?
A.${{\text{P}}_4}/{\text{B}}{{\text{r}}_2}$
B.${\text{HBr}}$
C.${\text{B}}{{\text{r}}_2}$
D.${\text{NaBr}}$

Answer 1

Bromine alone is used with alkenes to form the Addition product. Tertiary alcohol prefers hydrogen bromide for the formation of alkyl halide. Activated bromine is best for primary alcohols.

Complete step by step answer:
The best reagent for the preparation of alkyl halide from the alcohol especially primary alcohol is ${{\text{P}}_4}/{\text{B}}{{\text{r}}_2}$ that is phosphorus and bromine. Red phosphorus and bromine reacts to form the reagent as follow:
${{\text{P}}_4} + 6{\text{B}}{{\text{r}}_2} \to 4{\text{PB}}{{\text{r}}_3}$
Hydrogen bromide that is ${\text{HBr}}$ , is best to use when the alcohol is tertiary alcohol. Tertiary alcohols are those alcohols in which hydroxyl group is attached to that carbon which is directly attached with 3 more carbon atoms. This is because the tertiary carbocation formed is most stable and the reaction will be highly favourable.
The bromine alone is not used for the conversion of alcohol to alkyl halide. Hence this option stands eliminated.
${\text{NaBr}}$ that is sodium bromide is a weak nucleophile and is not able to substitute hydroxide which is a stronger nucleophile from the alcohol. There is no reaction between alcohol and sodium bromide.
The given alcohol is n-propyl alcohol which is ${\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{OH}}$ which is a primary alcohol and hence the best reagent for the conversion to alkyl halide will be ${{\text{P}}_4}/{\text{B}}{{\text{r}}_2}$ .

Thus, the correct option is A.

Note:
Phosphorus has many allotropes, one of them is red phosphorus. It is an amorphous solid. It is non poisonous in nature unlike the while phosphorous.