Name: Preparation of a Haloalkanes
Uploaded: 2019-07-19
Duration: 5 min 52 s
Description: The haloalkanes are a group of chemical compounds derived from alkanes containing one or more halogens, and in this video, we will learn about their preparat...

Question

Why is sulphuric acid not used during the reaction of alcohols with

KI

?

Accepted Answer

In the presence of sulphuric acid $(H_{2} {SO}_{4})$ , $KI$ produces $HI$ .
$\underset{Potassium iodide}{2 KI (s)} + \underset{Sulphuric acid}{H_{2} {SO}_{4} (l)} \to \underset{Potassium hydrogen sulphate}{2 {KHSO}_{4} (s)} + \underset{Hydrogen iodide}{2 HI (g)}$

Since $H_{2} {SO}_{4}$ is an oxidising agent, it oxidises $HI$ (produced in the reaction to $I_{2}$ ).
$\underset{Hydrogen iodide}{2 HI (g)} + \underset{Sulphuric acid}{H_{2} {SO}_{4} (l)} \to \underset{Sulphur dioxide}{{SO}_{2} (g)} + \underset{Iodine}{I_{2} (s)} + \underset{Water}{H_{2} O (l)}$

Thus, the reaction between alcohol and $HI$ to form an alkyl iodide cannot occur. Hence, sulphuric acid is not used during the reaction of alcohols with $KI$ . Instead, a non-oxidising acid such as $H_{3} {PO}_{4}$ is used.

$\underset{Methanol}{{CH}_{3} OH (l)} + \underset{Potassium iodide}{KI (s)} + \underset{Phosphoric acid}{H_{3} {PO}_{4} (l)} \overset{∆}{\to} \underset{Iodomethane}{{CH}_{3} - I (l)} + \underset{Potassium dihydrogen phosphate}{{KH}_{2} {PO}_{4} (s)} + \underset{Water}{H_{2} O (l)}$

Andhra Pradesh Board Solutions for Chapter: Haloalkanes and Haloarenes, Exercise 2: Intext Questions

Andhra Pradesh Board Chemistry Solutions for Exercise - Andhra Pradesh Board Solutions for Chapter: Haloalkanes and Haloarenes, Exercise 2: Intext Questions

Questions from Andhra Pradesh Board Solutions for Chapter: Haloalkanes and Haloarenes, Exercise 2: Intext Questions with Hints & Solutions

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