Question

There are three isomers of dichlorobenzene, one of which has now replaced naphthalene as the main constituent of mothballs. a. Identify the ortho, the meta, and the para isomers of dichlorobenzene. b. Predict the number of isomers for trichlorobenzene. c. It turns out that the presence of one chlorine atom on a benzene ring will cause the next substituent to add ortho or para to the first chlorine atom on the benzene ring. What does this tell you about the synthesis of $m$ -dichlorobenzene? d. Which of the isomers of trichlorobenzene will be the hardest to prepare?

Short answer

The ortho, meta, and para isomers of dichlorobenzene have chlorine atoms at positions 1,2; 1,3 and 1,4 respectively. There are three isomers of trichlorobenzene: 1,2,3; 1,2,4 and 1,3,5. Since the presence of one chlorine atom directs the next substituent to add ortho or para, synthesizing m-dichlorobenzene would be challenging. The hardest isomer of trichlorobenzene to prepare is the one that does not follow the ortho/para rule, which is the 1,3,5-trichlorobenzene.

Step-by-step solution

Identify the ortho, meta, and para isomers of dichlorobenzene.

When two substituents are attached to a benzene ring, the position of the substituents can determine the type of isomer. - Ortho (o-) isomer: The two substituents are adjacent (1,2 positions). - Meta (m-) isomer: The two substituents are separated by one carbon (1,3 positions). - Para (p-) isomer: The two substituents are across from each other (1,4 positions). In the case of dichlorobenzene, the two substituents are chlorine atoms. Thus, we have the following isomers: - Ortho-dichlorobenzene (o-dichlorobenzene): Chlorine atoms at 1,2 positions. - Meta-dichlorobenzene (m-dichlorobenzene): Chlorine atoms at 1,3 positions. - Para-dichlorobenzene (p-dichlorobenzene): Chlorine atoms at 1,4 positions.

Predict the number of isomers for trichlorobenzene.

For trichlorobenzene, there are three chlorine atoms attached to the benzene ring. We will consider all possible positions: 1. All three chlorine atoms adjacent: 1,2,3-trichlorobenzene. 2. Two adjacent chlorine atoms and one separated by one carbon: 1,2,4-trichlorobenzene. 3. All three chlorine atoms separated by one carbon: 1,3,5-trichlorobenzene. There are three possible isomers of trichlorobenzene.

Analyzing the synthesis of m-dichlorobenzene given the substituent rule.

The presence of one chlorine atom on a benzene ring causes the next substituent to add ortho (1,2 position) or para (1,4 position) to the first chlorine atom. This preference is due to the electrophilic properties of chlorine that direct the incoming substituents to ortho and para positions. Given this rule, the synthesis of m-dichlorobenzene will be challenging since the electrophilic behavior of the chlorine atom will inherently push for adding the second chlorine atom to ortho or para positions, not to the meta (1,3) position.

Determine the hardest to prepare isomer of trichlorobenzene.

Given the directive influence of chlorine atoms on the substitution pattern, the hardest trichlorobenzene isomer to prepare will be the one that doesn't follow the ortho/para rule. The three isomers we found in Step 2 are: 1. 1,2,3-trichlorobenzene: Two chlorine atoms are ortho to each other, and the third is meta. 2. 1,2,4-trichlorobenzene: Two chlorine atoms are ortho to each other, and the third is para. 3. 1,3,5-trichlorobenzene: Chlorine atoms are separated by one carbon, which does not follow the ortho/para rule. Therefore, the hardest isomer of trichlorobenzene to prepare is 1,3,5-trichlorobenzene.