Problem 120 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is correct? (a) Reduction of any aldehyde gives secondary alcohols (b) Reduction of vegetable oil with \(\mathrm{H}_{2} \mathrm{SO}_{4}\) gives glycerine (c) Reaction of ethanolic iodine with \(\mathrm{NaOH}\) gives iodoform (d) Sucrose on reaction with \(\mathrm{NaOH}\) gives invert sugar

Step-by-Step Solution

Answer

The correct answer is (c).

Step 1: Analyze Option (a)

Option (a) states that the reduction of any aldehyde gives secondary alcohols. Let's consider a general aldehyde with the structure R-CHO. When this aldehyde is reduced, the carbonyl group (C=O) is converted into an alcohol group (-OH), resulting in a primary alcohol (R-CH2OH). Therefore, option (a) is incorrect as aldehydes reduce to primary alcohols.

Step 2: Analyze Option (b)

Option (b) states that the reduction of vegetable oil with \(\mathrm{H}_{2} \mathrm{SO}_{4}\) gives glycerine. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) is sulfuric acid, not a reducing agent. Glycerine is typically produced from the hydrolysis of triglycerides, not their reduction. Therefore, option (b) is incorrect.

Step 3: Analyze Option (c)

Option (c) involves the reaction of ethanolic iodine with \(\mathrm{NaOH}\), which is known as the iodoform reaction. This reaction characteristically produces iodoform (\(\mathrm{CHI}_3\)) when methyl ketones or alcohols with a methyl group adjacent to a hydroxyl group are present. Hence, option (c) is correct.

Step 4: Analyze Option (d)

Option (d) suggests that sucrose reacts with \(\mathrm{NaOH}\) to give invert sugar. Sucrose does not react with \(\mathrm{NaOH}\) to give invert sugar. Invert sugar refers to the equimolar mixture of glucose and fructose, typically obtained by the hydrolysis of sucrose, often using acidic conditions rather than basic. Thus, option (d) is incorrect.

Key Concepts

Primary Alcohols

When discussing organic chemistry, primary alcohols hold significant importance. They are the result when a general aldehyde undergoes reduction. Aldehydes are organic compounds characterized by the carbonyl group, represented as R-CHO, where R signifies a hydrocarbon group. The carbonyl carbon atom possesses a double bond with oxygen.

During the reduction process, the carbon-oxygen double bond in the aldehyde is converted to a single bond, forming an alcohol group (-OH). The product of this reaction is a primary alcohol with the structure R-CH2OH. The term "primary" indicates that the -OH group is attached to a carbon atom that is directly bonded to only one other carbon atom.

• The reaction is typically performed using reducing agents such as NaBH4 (sodium borohydride) or LiAlH4 (lithium aluminum hydride).
• This simple change transforms the aldehyde into a more reactive alcohol, which can then participate in further chemical reactions.

Iodoform Reaction

The iodoform reaction is a notable chemical test for the presence of methyl ketones or specific alcohols. When you mix ethanolic iodine (\( I_2 \)) with sodium hydroxide (\( NaOH \)), it has the peculiar ability to produce a yellow precipitate of iodoform (\( CHI_3 \)).

This reaction occurs because methyl ketones, or alcohols with a methyl group next to a hydroxyl group, can be oxidized to form a compound that reacts with iodine.

• Methyl ketones have the structure \( R-CO-CH_3 \), and the reaction can be used as a test because it's visually identifiable due to the formation of the characteristic yellow iodoform.
• Alcohols like ethanol will also participate in this reaction after they are oxidized to form acetaldehyde, a kind of methyl ketone.

Aside from being a laboratory test, this reaction serves in organic synthesis as a method to check for certain functional groups in a compound.

Sucrose Hydrolysis

Sucrose hydrolysis is a fascinating chemical reaction where sucrose, a disaccharide sugar, is broken down into its simpler components: glucose and fructose. This process is commonly referred to as the formation of "invert sugar".

Unlike the reaction with sodium hydroxide, which does not yield invert sugar, hydrolysis typically occurs under acidic conditions.

• Enzymatic methods can also catalyze this transformation; the enzyme sucrase is a natural catalyst found in the digestive system.
• The process changes the sweetness and chemical properties of the original sugar, significantly affecting its use in culinary and industrial applications.

The equimolar mixture of glucose and fructose formed has differing chemical properties from sucrose, which is why this process is crucial in many aspects of food science and nutrition.