Problem 129 - A Complete Resource Book in Chemistry for JEE Main

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

Step-by-Step Solution

Answer

The correct option is (d) 2 and 1.

Step 1: Identify the Reaction Product

The addition of HBr to 2-pentyne, which contains a triple bond, typically follows Markovnikov's rule. This rule suggests that the hydrogen atom bonds to the carbon with more hydrogen atoms, while the bromide (Br) attaches to the carbon with fewer hydrogen atoms, resulting in a Markovnikov addition product. In the case of 2-pentyne, this leads to choosing the two carbons in the alkyne bond and forming an initial product: 2-bromopent-2-ene.

Step 2: Determine Structural Isomers

After forming 2-bromopent-2-ene, we must consider possible isomers. However, since this is based on the alkyne addition followed by immediate conversion to a single compound, no structural isomers occur because the positions of the carbon backbone and bromine do not allow for different arrangements that satisfy all structures. Thus, only clean addition products result without alternative structures.

Step 3: Determine Configurational Isomers

Configurational isomers result from geometric shapes around the double-bond formed after the first addition. Given 2-bromopent-2-ene, two geometric isomers form: the cis-2-bromopent-2-ene, where the CH₃ groups on the double-bonded carbons are on the same side, and the trans-2-bromopent-2-ene, where the CH₃ groups are on opposite sides. Therefore, there are two configurational isomers.

Step 4: Analyze Options Against Findings

Given that Step 2 presents 1 structural isomer and Step 3 presents 2 configurational isomers, we can now compare our calculations against the multiple-choice options: (1 structural isomer and 2 configurational isomers). Checking the options: (a) doesn't match (1, 2); (b) doesn't match (2, 4); (c) doesn't match (4, 2); (d) matches precisely with (2, 1).

Key Concepts

Structural Isomers

Structural isomers are molecules with the same molecular formula but different structural arrangements. This means they have the same number and type of atoms but differ in how these atoms are connected. While the atoms might be the same, the connectivity leads to different compounds.For example, if you have a compound described by the formula \(\mathrm{C}_5\mathrm{H}_9\mathrm{Br}\), the atoms within could be arranged in various ways. However, in the context of the reaction between HBr and 2-pentyne, the only possibility for a structural isomer is the original product, 2-bromopent-2-ene. This limits the number of structural isomers here because the addition occurs only in one specific configuration due to Markovnikov's rule, as further explained below.

Configurational Isomers

Configurational isomers arise from different orientations of atoms or groups across a double or triple bond without breaking these bonds. This often results in geometric (cis-trans) isomers, where different spatial arrangements are possible. In this case, after HBr adds to 2-pentyne, we form 2-bromopent-2-ene with a double bond. The double bond restricts the rotation, allowing for geometric isomers:

• Cis-isomer: The two CH₃ groups on the double-bonded carbons are on the same side.
• Trans-isomer: The two CH₃ groups are on opposite sides.

This results in two configurational (geometric) isomers. Even though they share the same connectivity, their spatial arrangements are distinct.

Markovnikov's Rule

Markovnikov's rule is a guiding principle in organic chemistry that predicts the outcome of certain addition reactions. This rule states that in the addition of a protic acid (like HBr) to an alkene or alkyne, the acid's hydrogen (H) attaches to the carbon with the most hydrogen atoms already attached, and the halide (like bromine) prefers the adjacent carbon with fewer hydrogen atoms. In the reaction involving HBr and 2-pentyne, the alkyne undergoes this type of addition, resulting in the formation of 2-bromopent-2-ene. The hydrogen atom from HBr attaches to the less substituted carbon atom, which is more hydrogen-rich. Bromine, meanwhile, latches onto the other carbon, leading to a favorable Markovnikov addition product.

Geometric Isomers

Geometric isomers are a type of configurational isomer that emerge when the structure of a compound restricts the rotation around a double bond or ring, resulting in different spatial configurations. In 2-bromopent-2-ene, the presence of a double bond creates potential for these isomers. Only two unique spatial arrangements exist:

• Cis-isomer: The significant substituents or groups (like the CH₃) are on the same side of the double bond.
• Trans-isomer: The substituents are on opposing sides.

These isomers can possess different physical and chemical properties, making their identification and differentiation essential in chemical synthesis and applications.