Question

Methanol can be distinguished from ethanol by:
(A) Heating with ${{I}_{2}}$ and alkali
(B) Treating with Schiff’s reagent
(C) Treating $Cr{{O}_{3}}$solution in dil. ${{H}_{2}}S{{O}_{4}}$
(D) Treating with Lucas reagent

Answer 1

Hint: The methanol and ethanol can be distinguished from each other by one of the tests given in above options. Physically, we cannot differentiate both the alcohols as they appear the same. Just, ethanol can be consumed but methanol is toxic.

Complete answer:
To distinguish methanol and ethanol we have a test known as Iodoform test. Let us know about the iodoform test in detail.
Iodoform test-
This test can be conducted more efficiently in the laboratory, but care must be taken while experimenting as both the alcohols are highly flammable. This test calls for iodine solution (dissolved in potassium iodide solution) and sodium hydroxide (it is corrosive).
Procedure (rough description)-
1. Add a few (but equal) drops of ethanol and methanol each in separate test tubes.
2. Add a few (but more than alcohol droplets) drops of iodine solution to the test tubes containing alcohols each.
3. Now add a few drops of sodium hydroxide (base) to each alcohol.
4. Swirl the test tubes gently for a few minutes. The dark colour of iodine starts to fade.
5. After two mins observe the test tubes.
You will see that the solution of ethanol in the test tube will be cloudy and will form yellow precipitate. The yellow precipitate will be of tri-iodo-methane (iodoform) which would have an antiseptic smell. Whereas, the methanol solution containing the test tube will remain clear.
Reaction-
\[C{{H}_{3}}C{{H}_{2}}OH+{{I}_{2}}\xrightarrow[\Delta ]{NaOH}CH{{I}_{3}}+HCO{{O}^{-}}N{{a}^{+}}\]
                                (yellow ppt.)

Therefore, option (A) Heating with ${{I}_{2}}$ and alkali is the correct answer.

Note:
The iodoform test is positive for acetaldehyde, methyl ketones, secondary alcohols that contain methyl groups in alpha position; along with ethanol.
Ethanol is the only primary alcohol which will give reaction to the mixture of iodine and base.