Class 12 Chemistry – Preparation of Alcohols

Class 12 Chemistry – Preparation of Alcohols (10 Questions)

1. Hydrolysis of Haloalkanes (2 Marks)

Write the chemical equation for the preparation of ethanol from ethyl bromide using aqueous KOH.

2. Hydration of Alkenes (2 Marks)

How is propan-2-ol prepared from propene? Write the balanced chemical equation and mention the catalyst used.

3. Reduction of Aldehydes (2 Marks)

Name the reagent used to convert ethanal into ethanol and write the reaction.

4. Reduction of Ketones (2 Marks)

Write the chemical equation for the conversion of propanone into propan-2-ol using a suitable reducing agent.

5. Grignard Reagent (3 Marks)

What happens when methylmagnesium bromide (CH₃MgBr) reacts with methanal, followed by hydrolysis? Write the reaction and name the alcohol formed.

6. Grignard Reagent (3 Marks)

Predict the product when ethylmagnesium bromide reacts with ethanal, followed by acidic hydrolysis. Write the complete reaction.

7. Fermentation Method (3 Marks)

Describe the preparation of ethanol by fermentation of glucose. Mention:

• the microorganism used,

• the optimum temperature,

• and write the balanced chemical equation.

8. Higher Order Thinking (3 Marks)

A compound A (C₂H₄) reacts with steam in the presence of an acid catalyst to form compound B.

1. Identify A and B.

2. Write the balanced chemical equation.

3. Name the type of reaction involved.

9. Conversion-Based Question (3 Marks)

Complete the following conversion with suitable reagents:

CH3CH=CH2→CH3CH(OH)CH3CH

Mention the reaction conditions.

10. Assertion and Reason (1 Mark)

Assertion (A): Primary alcohols can be prepared by reducing aldehydes.

Reason (R): Sodium borohydride (NaBH₄) is a reducing agent that converts aldehydes into primary alcohols.

Choose the correct option:

• (A) Both A and R are true, and R is the correct explanation of A.

• (B) Both A and R are true, but R is not the correct explanation of A.

• (C) A is true, but R is false.

• (D) A is false, but R is true.

ANSWERS:-

Class 12 Chemistry – Preparation of Alcohols

10 Questions with Answers

1. How is ethanol prepared from ethyl bromide using aqueous KOH?

Answer:
Ethyl bromide reacts with aqueous KOH to form ethanol.

$$CH_3CH_2Br + KOH_{(aq)} \rightarrow CH_3CH_2OH + KBr$$

This is a nucleophilic substitution reaction.

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2. How is propan-2-ol prepared from propene?

Answer:

$$CH_3CH=CH_2 + H_2O \xrightarrow{H^+} CH_3CH(OH)CH_3$$

Propene undergoes acid-catalysed hydration to give propan-2-ol.

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3. How is ethanol prepared by reduction of ethanal?

Answer:

$$CH_3CHO \xrightarrow{NaBH_4/H^+} CH_3CH_2OH$$

Reagent: $NaBH_4$ or $LiAlH_4$

Aldehydes are reduced to primary alcohols.

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4. How is propan-2-ol prepared by reduction of propanone?

Answer:

$$CH_3COCH_3 \xrightarrow{NaBH_4} CH_3CHOHCH_3$$

Propanone is reduced to propan-2-ol, a secondary alcohol.

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5. What happens when methylmagnesium bromide reacts with methanal followed by hydrolysis?

Answer:

Step 1:

$$CH_3MgBr + HCHO \rightarrow CH_3CH_2OMgBr$$

Step 2:

$$CH_3CH_2OMgBr + H_3O^+ \rightarrow CH_3CH_2OH + Mg(OH)Br$$

Alcohol formed: Ethanol (CH₃CH₂OH)

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6. What happens when ethylmagnesium bromide reacts with ethanal followed by hydrolysis?

Answer:

$$C_2H_5MgBr + CH_3CHO \rightarrow CH_3CH(OMgBr)C_2H_5$$

On hydrolysis:

$$CH_3CH(OMgBr)C_2H_5 + H_3O^+ \rightarrow CH_3CH(OH)C_2H_5$$

Alcohol formed: Butan-2-ol

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7. How is ethanol prepared by fermentation of glucose?

Answer:

Glucose is fermented in the presence of yeast at about 303–313 K (30–40°C).

$$C_6H_{12}O_6 \xrightarrow[\text{303–313 K}]{\text{Yeast}} 2C_2H_5OH + 2CO_2$$

Products: Ethanol and carbon dioxide.

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8. A compound A (C₂H₄) reacts with steam in the presence of an acid catalyst to form B. Identify A and B.

Answer:

A = Ethene $(CH_2=CH_2)$

B = Ethanol $(CH_3CH_2OH)$

$$CH_2=CH_2 + H_2O \xrightarrow{H^+} CH_3CH_2OH$$

Reaction: Acid-catalysed hydration of alkene.

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9. Complete the conversion:

$$CH_3CH=CH_2 \rightarrow CH_3CH(OH)CH_3$$

Answer:

Use water in the presence of an acid catalyst:

$$CH_3CH=CH_2 + H_2O \xrightarrow{H^+} CH_3CH(OH)CH_3$$

Product: Propan-2-ol.

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10. Assertion–Reason

Assertion: Primary alcohols can be prepared by reducing aldehydes.

Reason: $NaBH_4$ is a reducing agent that converts aldehydes into primary alcohols.

Answer: A

Both Assertion and Reason are true, and the Reason correctly explains the Assertion.