Which of the following will not form a yellow precipitate on heating with an alkaline solution of iodine? [2004]a)CH3CH(OH)CH3b)CH3CH2CH(OH)CH3c)CH3OHd)CH3CH2OHCorrect answer is option 'C'. Can you explain this answer?

Question

Answer

Formation of a yellow precipitate on heating a compound with an alkaline solution of iodine is known as iodoform reaction.

• Methyl alcohol does not respond to this test. iodoform test is exhibited by ethyl alcohol, acetaldehyde, acetone, methyl ketones and those alcohols which possess CH3CH(OH) - group.

Answer

Yeah ....Option C is the correct answer..
Why...See...》》CH3OH cannot participate in halofrom reaction or iodoform reaction because methanol doesn't have any keto methyl producing grp or keto methyl grp...so..it is the correct answer.
Mind it....only those compounds participate in halofrom reaction which has keto methyl producing grp or keto methyl grp...

Answer

Explanation:

To understand why option C will not form a yellow precipitate on heating with an alkaline solution of iodine, we need to consider the reaction that occurs when iodine reacts with alcohols.

When an alcohol reacts with iodine in an alkaline solution, the following reaction takes place:

R-OH + I2 + 2OH- → R-OI + H2O + 2I-

In this reaction, the iodine (I2) is reduced to iodide ions (I-) while the alcohol is oxidized to an alkyl iodide (R-OI). The alkyl iodide formed is insoluble in water and precipitates out, resulting in a yellow precipitate.

Now let's analyze each option:

a) CH3CH(OH)CH3 (2-propanol):
This alcohol has a secondary carbon atom, and when it reacts with iodine, it will form a yellow precipitate of 2-iodopropane.

b) CH3CH2CH(OH)CH3 (2-butanol):
This alcohol also has a secondary carbon atom, and when it reacts with iodine, it will form a yellow precipitate of 2-iodobutane.

c) CH3OH (methanol):
This option is methanol, which is a primary alcohol. Primary alcohols do not undergo oxidation with iodine in the presence of an alkaline solution. Therefore, it will not form a yellow precipitate.

d) CH3CH2OH (ethanol):
This alcohol has a primary carbon atom, and when it reacts with iodine, it will form a yellow precipitate of ethyl iodide.

Therefore, the correct answer is option C (CH3OH) because it is a primary alcohol and will not form a yellow precipitate on heating with an alkaline solution of iodine.

Key Points:
- Alcohols react with iodine in an alkaline solution to form alkyl iodides.
- Secondary and tertiary alcohols form yellow precipitates when reacting with iodine.
- Primary alcohols do not undergo oxidation with iodine in the presence of an alkaline solution and, therefore, do not form a yellow precipitate.

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