Question

When 2-pentene is treated with \(\mathrm{Cl}_{2}\) in methanol, three products are formed. Account for the formation of each product (you need not explain their relative percentages).

Short answer

Answer: The three products formed are 2,3-dichloropentane, 2-chloropentane, and 3-chloropentane.

Step-by-step solution

Identify the structure of 2-pentene

2-pentene is an alkene with five carbon atoms and a double bond between the second and third carbon. Its structure can be represented as CH₃CH=CHCH₂CH₃.

Identify the possible sites of reaction

Since alkenes are capable of undergoing addition reactions, the double bond in the 2-pentene molecule will be the major site of reaction. Two chlorine atoms can react at the double bond in the following ways: 1. Both chlorine atoms can add to the double bond, resulting in a di-chloro product. 2. One chlorine atom can add to the double bond, creating a mono-chloro product. This can happen in two possible configurations.

Formation of the di-chloro product

When both chlorine atoms add to the double bond, the reaction follows an electrophilic addition mechanism. The double bond breaks, and each chlorine atom bonds to a carbon atom at the site of the double bond. In this case, the resulting product is 2,3-dichloropentane. Its structure can be represented as CH₃CHClCHClCH₂CH₃.

Formation of the mono-chloro products

When only one chlorine atom adds to the double bond, the product can be either 2-chloropentane (CH₃CHClCHCH₂CH₃) or 3-chloropentane (CH₃CH₂CHClCH₂CH₃). These products are formed via different pathways: 1. In the case of 2-chloropentane, the chlorine atom attacks the less-substituted carbon atom of the double bond (the one with fewer alkyl groups attached), following Markovnikov's rule. 2. In the case of 3-chloropentane, the chlorine atom attacks the more substituted carbon atom of the double bond (the one with more alkyl groups attached), following an anti-Markovnikov addition. This alternative pathway may be facilitated by the solvent (methanol) or any other external factors. Thus, the three products formed when 2-pentene reacts with Cl₂ in methanol are 2,3-dichloropentane, 2-chloropentane, and 3-chloropentane.