The reagent used to convert propanol to 1-bromopropane is:

$PBr_3$

The correct answer is Option (1) → $PBr_3$

The reagent commonly used to convert propan-1-ol to 1-bromopropane is phosphorus tribromide ($PBr_3$). Alternatively, a mixture of hydrobromic acid ($HBr$) or red phosphorus and bromine ($P + Br_2$) can also be used to facilitate this substitution reaction.

Reaction:

$3 C_3H_7OH + PBr_3 \longrightarrow 3 C_3H_7Br + H_3PO_3$

Method: Phosphorus tribromide reacts with the hydroxyl group, replacing it with a bromine atom.

Conditions: The reaction typically proceeds via a nucleophilic substitution ($S_N2$) mechanism, ensuring the 1-bromopropane product.

Detailed Explanation

Option 1: $PBr_3$

$PBr_3$ is the standard reagent used to convert alcohols into alkyl bromides.

Reaction:

$3 C_3H_7OH + PBr_3 \longrightarrow 3 C_3H_7Br + H_3PO_3$

Here:

Propanol $\longrightarrow$ 1-bromopropane

Mechanism: The $-OH$ group is replaced by $Br$ via $S_N2$ mechanism, making it very effective for primary alcohols.

Hence correct.

Option 2: $\text{Br}_2$

Bromine alone does not replace $-\text{OH}$ group in alcohols. It typically reacts with alkenes or in free radical substitution, not suitable for this conversion.

Option 3: $\text{CH}_3\text{Br}$

This is methyl bromide, not a reagent for substitution of $-\text{OH}$ group.

Option 4: $\text{CBr}_4$

Not a valid reagent for converting alcohol to alkyl bromide.