Question

How many of the given reagents can be used to convert alcohol into alkyl halide? I. NaBr II. NaCl III. HBr IV. PCl₃ V. SOCl₂ VI. HCl (conc.)/ZnCl₂ (anhyd.) VII. NaI VIII. PCl₅

Answer 1

5

Answer 2

To convert an alcohol into an alkyl halide, the hydroxyl (-OH) group needs to be replaced by a halogen atom. This typically requires converting the poor leaving group (-OH) into a good leaving group (like $\text{H}_2\text{O}$ or a halide-phosphorus complex) and then allowing a halide ion to attack the carbon.

Let's analyze each given reagent:

1. I. NaBr: Sodium bromide is an ionic salt. The bromide ion ($\text{Br}^-$) is a nucleophile, but it is not strong enough to displace the -OH group from an alcohol directly, as -OH is a poor leaving group. To use $\text{Br}^-$ effectively, the alcohol must first be protonated (e.g., by $\text{H}_2\text{SO}_4$) to convert -OH into a good leaving group ($\text{H}_2\text{O}^+$). Therefore, NaBr alone cannot convert alcohol to alkyl halide. It is typically used in conjunction with a strong acid to generate HBr in situ.

2. II. NaCl: Similar to NaBr, sodium chloride is an ionic salt. The chloride ion ($\text{Cl}^-$) cannot displace -OH directly from an alcohol.

3. III. HBr: Hydrogen bromide is a strong acid and a source of $\text{Br}^-$. It protonates the alcohol to form $\text{R-OH}_2^+$, which then loses water to form a carbocation (for 2° and 3° alcohols) or undergoes $\text{S}_{\text{N}}2$ displacement (for 1° alcohols) by $\text{Br}^-$. Thus, HBr can be used.

   $\text{R-OH} + \text{HBr} \rightarrow \text{R-Br} + \text{H}_2\text{O}$

4. IV. PCl₃: Phosphorus trichloride is a commonly used reagent to convert alcohols to alkyl chlorides. It reacts to form phosphite acid as a byproduct.

   $3\text{R-OH} + \text{PCl}_3 \rightarrow 3\text{R-Cl} + \text{H}_3\text{PO}_3$

5. V. SOCl₂: Thionyl chloride (often in the presence of pyridine) is an excellent reagent for converting alcohols to alkyl chlorides (Darzens reaction). The gaseous byproducts ($\text{SO}_2$ and HCl) make purification of the alkyl halide easy.

   $\text{R-OH} + \text{SOCl}_2 \xrightarrow{\text{pyridine}} \text{R-Cl} + \text{SO}_2 \uparrow + \text{HCl} \uparrow$

6. VI. HCl (conc.)/ZnCl₂ (anhyd.): This combination is known as the Lucas reagent. It is effectively used to convert alcohols to alkyl chlorides. $\text{ZnCl}_2$ acts as a Lewis acid, coordinating with the oxygen of the alcohol, making -OH a better leaving group.

   $\text{R-OH} + \text{HCl} \xrightarrow{\text{ZnCl}_2} \text{R-Cl} + \text{H}_2\text{O}$

7. VII. NaI: Similar to NaBr and NaCl, sodium iodide is an ionic salt and cannot convert alcohol to alkyl halide directly. It requires a strong acid (like $\text{H}_3\text{PO}_4$ or $\text{H}_2\text{SO}_4$) to generate HI in situ.

8. VIII. PCl₅: Phosphorus pentachloride is another effective reagent for converting alcohols to alkyl chlorides.

   $\text{R-OH} + \text{PCl}_5 \rightarrow \text{R-Cl} + \text{POCl}_3 + \text{HCl}$

Based on the analysis, the reagents that can be used to convert alcohol into alkyl halide are: III. HBr IV. PCl₃ V. SOCl₂ VI. HCl (conc.)/ZnCl₂ (anhyd.) VIII. PCl₅

There are 5 such reagents.