Question

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

Short answer

Answer: 2 structural isomers and 4 stereoisomers.

Step-by-step solution

Identify the starting compound

Draw the structure of starting alkene - pent-2-yne. It has the molecular formula C5H8 and the structure is CH3-C≡C-CH2-CH3. A triple bond is present between the second and third carbon atoms.

Add HBr to form a bromo compound

The addition of HBr to an alkyne results in an alkene, this is called a hydrohalogenation reaction. In this case, the alkyne is pent-2-yne and HBr is added, which will result in the formation of a bromoalkene.

Determine structural isomers

When HBr is added to pent-2-yne, the carbocation intermediate can be formed at two different positions, and then the Br- ion can attach to either of the carbocation sites. This will give two different structural isomers, which are: 1. 2-Bromo-2-pentene (Br is added to 2nd carbon and a double bond exists between carbon 2 and 3) 2. 3-Bromo-2-pentene (Br is added to 3rd carbon and a double bond exists between carbon 2 and 3) There are 2 structural isomers.

Determine stereoisomers

Each of the above structural isomers in step 3 can have two different spatial arrangements (stereoisomers) due to the possibility of cis (Z) and trans (E) configuration around the double bond. For trans: Terminal groups (Br and H) are on opposite sides and for cis: Terminal groups (Br and H) are on the same side 1. 2-Bromo-2-pentene : - E-2-Bromo-2-pentene - Z-2-Bromo-2-pentene 2. 3-Bromo-2-pentene : - E-3-Bromo-2-pentene - Z-3-Bromo-2-pentene There are 4 stereoisomers. Hence, the correct answer is (c) 2 structural isomers and 4 stereoisomers.

Key concepts

Structural Isomerism

Structural isomerism occurs when compounds with the same molecular formula have different arrangements of atoms. This kind of isomerism is one of the foundational concepts of organic chemistry and is essential for understanding how molecules can change in structure without altering their overall composition.
In the case of adding HBr to pent-2-yne, structural isomers are formed based on the position where the bromine (Br) attaches to the carbon chain.

• When the Br atom attaches to the second carbon, it forms 2-bromo-2-pentene with a double bond between carbon 2 and 3.
• When the Br atom attaches to the third carbon, it forms 3-bromo-2-pentene with the double bond still between carbon 2 and 3.

This demonstrates the concept of structural isomerism because both structures share the \( \text{C}_5\text{H}_9\text{Br} \) formula, but differ in their structural arrangements.

Stereoisomerism

Stereoisomerism refers to the spatial arrangement of atoms in molecules that share the same structural formula. It's crucial in many chemical reactions because these variations can influence the physical and chemical properties of molecules.
After the hydrohalogenation of pent-2-yne, the structural isomers can further display stereoisomerism. This is due to the presence of a double bond which allows for a variation in the arrangement of substituent groups around it, leading to cis and trans configurations.

• Cis (Z) configuration: This occurs when similar and major groups, such as bromine and hydrogen, are on the same side of the double bond.
• Trans (E) configuration: This occurs when these groups are on opposite sides of the double bond.

This spatial differentiation is what leads to stereoisomerism in the formed bromoalkenes from the reaction.

Hydrohalogenation Reaction

Hydrohalogenation is a type of chemical reaction where a hydrogen halide (such as HBr) is added to an unsaturated compound like an alkene or alkyne, resulting in the formation of a haloalkane or haloalkene.
In our specific case, HBr is added to pent-2-yne, a type of alkyne. When the hydrogen bromide reacts with pent-2-yne, it first forms an intermediate carbocation. The positively charged intermediate then captures the bromine ion, converting the alkyne into a bromoalkene.
This process is guided by Markovnikov's rule, which indicates that hydrogen is added to the carbon with more hydrogen atoms already bound to it. Hence, the reaction can yield two structural isomers, due to different possibilities of the carbocation intermediate.

Cis-Trans Isomerism

Cis-trans isomerism is a kind of stereoisomerism that occurs particularly around double bonds or in cyclic structures, where the relative positioning of atoms or groups can differ.
In organic compounds like bromoalkenes, the cis or "Z" isomer has substituents on the same side of the double bond, while the trans or "E" isomer has them on opposite sides.

• Within the compound 2-bromo-2-pentene, you can form cis and trans isomers based on how Br and other groups position themselves in relation to the C=C double bond.
• Similarly, in 3-bromo-2-pentene, despite having a carbon-bromine bond in a different position, it still allows for this type of isomerism due to the presence of a double bond.

This type of isomerism is significant as it can result in differing physical properties like boiling points and solubility, due to the different spatial arrangements of the atoms.