Question

The number of structural and configurational isomers of a bromo compound, \(\mathrm{C}_{5} \mathrm{H}_{9} \mathrm{Br}\), formed by the addition of \(\mathrm{HBr}\) to 2 -pentyne respectively are: (a) 1 and 2 (b) 2 and 4 (c) 4 and 2 (d) 2 and 1

Short answer

The answer is (b) 2 and 4.

Step-by-step solution

Understanding the Problem

We need to determine the possible isomers formed by the addition of HBr to 2-pentyne (a compound with triple carbon-carbon bonds) and identify the structural and configurational isomers that can result from this reaction.

Identify Initial Reaction

The initial molecule, 2-pentyne, is: CH₃≡C-CH₂-CH₂-CH₃. When HBr adds to this alkyne, it converts it to an alkene, and further addition results in an alkyl halide. We need to analyze possible alkyl halides that can be formed after full hydrogenation.

Determine Structural Isomers

Adding HBr leads to two possible positions for the bromine to attach to: either next to the original triple bond or at the end of the chain, creating 1-bromo-pentane and 2-bromo-pentane. Specify isomers: 1. 1-bromo-pentane (Br on C1) 2. 2-bromo-pentane (Br on C2).

Identify Configurational Isomers

After Br addition, the newly formed chiral centers in the formed alkenes allow for different spatial arrangements, leading us to identify stereoisomers: For each structural isomer, if applicable, determine the chiral carbon: 1. 2-bromo-pentane possesses a chiral center, allowing for two enantiomers (R and S configurations).

Count Isomers

Add the structural isomers determined in Step 3 and configurational isomers from Step 4: - Structural Isomers: 2 - Configurational Isomers: 2 (from 2-bromo-pentane).

Select Correct Option

Based on the number of isomers: - We have 2 structural isomers (1-bromo-pentane and 2-bromo-pentane). - We have 2 configurational isomers (R and S for 2-bromo-pentane). Thus, the correct choice is (b) 2 and 4.

Key concepts

Structural Isomers

Structural isomers are compounds with the same molecular formula but different structural arrangements of atoms. In the case of our compound, \( \mathrm{C}_5\mathrm{H}_9\mathrm{Br} \), formed by the addition of \( \mathrm{HBr} \) to 2-pentyne, we identify structural isomers by considering different positions for the bromine attachment.
2-pentyne initially has a triple bond between the carbon atoms. When \( \mathrm{HBr} \) is added, it breaks the triple bond, converting it into a single bond and allowing for a bromo compound to form. The primary structural isomers in this reaction are:

• 1-bromo-pentane: Where bromine attaches to the first carbon of the chain.
• 2-bromo-pentane: Where bromine attaches to the second carbon of the chain.

Even though these isomers share the same formula, their unique structures result in different chemical and physical properties, like boiling points and reactivity.

Configurational Isomers

Configurational isomers are a type of stereoisomer with molecules that have the same connectivity of atoms but differ in the spatial arrangement. This occurs due to the presence of chiral centers within the molecules.
In the context of the exercise, once the \( \mathrm{HBr} \) is added to 2-pentyne, we focus on 2-bromo-pentane, which features a chiral carbon. This chiral center is the point where the molecule can have different configurations.
For 2-bromo-pentane, the chiral center allows for two configurations:

• The R-enantiomer, where the arrangement of groups around the chiral center follows a certain order (right-handed).
• The S-enantiomer, which arranges the groups in the opposite order (left-handed).

These configurations lead to configurational isomers, which may have different interactions with polarized light and other chiral substances.

Addition Reactions in Alkynes

Addition reactions involve adding atoms or groups to a molecule, typically seen when reacting alkenes or alkynes with halogens or hydrogen halides. In the exercise, we focus on how \( \mathrm{HBr} \) adds to 2-pentyne, an alkyne with a triple bond.
During the addition reaction:

• The first step involves the breaking of the triple bond to form a double bond, making an alkene intermediate.
• Further addition of \( \mathrm{HBr} \) results in the formation of a bromoalkane, where bromine atoms attach to one of the carbon atoms that were originally part of the alkyne.

Importantly, this process can result in different structural isomers, depending on the position where the bromine atom adds to the carbon chain.

Chirality in Organic Compounds

Chirality is an important concept in organic chemistry, referring to a molecule's lack of symmetry or non-superimposability on its mirror image. This property is primarily due to the presence of a chiral center, which is a carbon atom bonded to four different groups.
In the case of 2-bromo-pentane, chirality arises from its central carbon being attached to four distinct substituents, leading to two mirror-image forms that cannot be superimposed - these are known as enantiomers.
Chiral molecules have significant implications in synthesis and pharmacology because their enantiomers can interact very differently with biological systems, despite having identical molecular formulas. Each enantiomer can vary greatly in terms of activity, taste, smell, and how they metabolize in the body. Understanding chirality is crucial for the development of many chemical products and medications.