Question

A compound (A) (which gives haloform reaction), on reduction with zinc amalgam and conc.HCl gives (B). (B) is also obtained by treating \(1,1,2,2\) - tetrachlorobutane with zinc dust. The compound (A) is (a) C#CC(C)O (b) C=C(C)C=O (c) C=CC(C)O (d) C#CC=O

Short answer

Question: Identify the compound (A) that gives a haloform reaction and on reduction with zinc amalgam and conc. HCl gives (B), which is also obtained by treating 1,1,2,2-tetrachlorobutane with zinc dust. Choose the correct compound (A) out of the given options: (a) \(\mathrm{CH_3C\equivCCH_2OH}\), (b) \(\mathrm{CH_3CH=C(CH_3)CHO}\), (c) \(\mathrm{CH_3CH=C(CH_3)CH_2OH}\), (d) \(\mathrm{CH_3C\equiv CCHO}\). Answer: Compound (A) is (b) \(\mathrm{CH_3CH=C(CH_3)CHO}\).

Step-by-step solution

Identify the functional group in each given compound and determine if they can give haloform reaction or not.

(a) \(\mathrm{CH_3C\equivCCH_2OH}\): This compound has a terminal alkyne functional group and a primary alcohol group. Alcohols (except for methyl ketones) do not give haloform reactions. So, option (a) can be ruled out. (b) \(\mathrm{CH_3CH=C(CH_3)CHO}\): This compound has an aldehyde functional group and an alkene functional group. Aldehydes with at least one alpha hydrogen can give haloform reactions. In this case, there's one alpha hydrogen present. So, option (b) is a candidate for compound (A). (c) \(\mathrm{CH_3CH=C(CH_3)CH_2OH}\): This compound has an alkene functional group and a primary alcohol functional group, which does not give haloform reactions. So, option (c) can be ruled out. (d) \(\mathrm{CH_3C\equivCCHO}\): This compound has a terminal alkyne functional group and an aldehyde functional group. Like we discussed earlier, aldehydes with one alpha hydrogen can give haloform reactions, which is true for this compound as well. Therefore, option (d) is also a candidate for compound (A). Now, we have two candidates for compound (A): option (b) and option (d).

Find the product (B) obtained by reducing each compound with zinc amalgam and conc. HCl.

(b) Reducing \(\mathrm{CH_3CH=C(CH_3)CHO}\) with zinc amalgam and conc. HCl will reduce the aldehyde group to a primary alcohol: \(\mathrm{CH_3CH=C(CH_3)CH_2OH}\). (d) Reducing \(\mathrm{CH_3C\equiv CCHO}\) with zinc amalgam and conc. HCl will reduce the aldehyde group to a primary alcohol: \(\mathrm{CH_3C\equiv CCH_2OH}\).

Compare the obtained products with the product formed by treating 1,1,2,2-tetrachlorobutane with zinc dust.

Treating 1,1,2,2-tetrachlorobutane with zinc dust will reduce two carbon-chlorine bonds to form a double bond: \(\mathrm{CCl_2CHCHCl_2}\) \(\xrightarrow{\mathrm{Zn\ \mathrm{dust}}}\) \(\mathrm{CH_2=CH-CHCl_2}\). Comparing this product with the products obtained by reducing compounds (b) and (d), we see a match between this product and the one obtained by reducing compound (b). Therefore, compound (A) is: (b) \(\mathrm{CH_3CH=C(CH_3)CHO}\).

Key concepts

Haloform Reaction

The haloform reaction is a chemical process where a molecule containing a methyl ketone group reacts with halogens in the presence of a base to form a haloform (such as chloroform, bromoform, or iodoform) and a carboxylate ion. This reaction is particularly useful in identifying methyl ketones and

• is effective with compounds that have an R-CO-CH₃ structure.
• The process involves the halogenation of the methyl group, followed by cleavage of the carbon-carbon bond.

Notably, this reaction can also occur with aldehydes that have at least one alpha hydrogen. For example, in our exercise, compound (b) could undergo a haloform reaction due to the presence of an aldehyde group with an alpha hydrogen.

Zinc Amalgam Reduction

Zinc amalgam reduction is a method used to transform organic compounds by reducing specific functional groups. This reduction technique employs a mixture of zinc metal and mercury, known as zinc amalgam, along with concentrated hydrochloric acid (HCl). The process is widely used for eliminating - Nitro groups are converted into amines. - Aldehydes are reduced to primary alcohols. This is clearly observed in the exercise where compounds containing aldehyde groups are reduced to their corresponding alcohols when treated with zinc amalgam and HCl.

Functional Groups Identification

Functional groups are vital to the structure and reactivity of organic molecules. Identifying these groups requires analyzing the groups attached to the carbon backbone. Each functional group exhibits distinct properties and reactions, such as:

• Alcohols contain the -OH group.
• Aldehydes possess the -CHO group.
• Terminal alkynes have a -C≡C-H structure.

For instance, in the exercise, identifying the aldehyde group with one alpha hydrogen is crucial for determining if the compound can undergo a haloform reaction. Recognizing - The presence of these groups is essential for predicting the chemical behavior and transformations under specific reactions.

Compound Reduction

Compound reduction involves decreasing the oxidation state of the molecules' atoms, often transforming a molecule with a double or triple bond into one with a single bond. Techniques like zinc amalgam reduction are commonly used in chemistry to achieve this transformation, especially for reducing: - Carbonyl groups to alcohols. - Nitro groups to amines. In the exercise scenario, compound reduction is needed to understand how compound (A) transforms into compound (B) through a zinc amalgam reduction. Such a process highlights reduction methodologies' critical role in synthetic chemistry, facilitating modifications in complex organic structures.