Chemistry Class 12 NCERT Solutions: Chapter 11 Alcohols Phenols and Ethers Part 7

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Q: 19. Write the mechanism of acid-catalysed dehydration of ethanol to yield ethene.


The mechanism of acid dehydration of ethanol to yield ethene involves the following three steps:

Step 1:

Protonation of ethanol to form ethyl oxonium ion:

Q 19 1 Protonation of Ethanol to form Ethyl Oxonium Ion

Q 19 1 Protonation of Ethanol to Form Ethyl Oxonium Ion

Step 2:

Formation of carbocation (rate determining step):

Q 19 2 Formation of carbocation

Q 19 2 Formation of Carbocation

Step 3:

Elimination of a proton to form ethene:

Q 19 3 Elimination of a Proton to form ethene

Q 19 3 Elimination of a Proton to Form Ethene

The acid consumed in step 1 is released in Step 3. After the formation of ethene, it is removed to shift the equilibrium in a forward direction

Q: 20. How are the following conversions carried out?

(i) Propene → Propan-2-ol

(ii) Benzyl chloride → Benzyl alcohol

(iii) Ethyl magnesium chloride → Propan-1-ol

(iv) Methyl magnesium bromide → 2-Methylpropan-2-ol


(i) If propene is allowed to react with water in the presence of an acid as a catalyst then propan-2-ol is obtained.

Q 20 i Propene → Propan-2-ol

Q 20 I Propene → Propan-2-Ol

(ii) If benzyl chloride is treated with (followed by acidification) then benzyl alcohol is produced

Q 20 ii Benzyl chloride → Benzyl alcohol

Q 20 II Benzyl Chloride → Benzyl Alcohol

(iii) When ethyl magnesium chloride is treated with methanal, an adduct is the produced which gives propan-1-ol on hydrolysis.

Q 20 iii Ethyl magnesium chloride → Propan-1-ol

Q 20 III Ethyl Magnesium Chloride → Propan-1-Ol

(iv) When methyl magnesium bromide is treated with propane, an adduct is the product which gives 2-methylpropane-2-ol on hydrolysis.

Q 20 iv Methyl magnesium bromide → 2-Methylpropan-2-ol

Q 20 IV Methyl Magnesium Bromide → 2-Methylpropan-2-Ol

Q: 21. Name the reagents used in the following reactions:

(i) Oxidation of a primary alcohol to carboxylic acid

(ii) Oxidation of a primary alcohol to aldehyde

(iii) Bromination of phenol to 2, 4, 6-tribromophenol.

(iv) Benzyl alcohol to benzoic acid.

(v) Dehydration of propan-2-ol to propene

(vi) Butan-2-one to butan-2-ol.


(i) Acidified potassium permanganate

(ii) Pyridinium chlorochromate (PCC)

(iii) Bromine water

(iv) Acidified potassium permanganate

(v) phosphoric acid


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