Question

(a) What is the difference between a substitution reaction and an addition reaction? Which one is commonly observed with alkenes, and which one with aromatic hydrocarbons? (b) Using condensed structural formulas, write the balanced equation for the addition reaction of 2 -pentene with \(\mathrm{Br}_{2}\) and name the resulting compound. (c) Write a balanced chemical equation for the substitution reaction of \(\mathrm{Cl}_{2}\) with benzene to make paradichlorobenzene in the presence of \(\mathrm{FeCl}_{3}\) as a catalyst.

Short answer

(a) Substitution reactions involve the replacement of one atom or group of atoms by another, while addition reactions involve the combination of two molecules to form a larger molecule. Alkenes typically undergo addition reactions, while aromatic hydrocarbons like benzene undergo substitution reactions. (b) The balanced equation for the addition reaction of 2-pentene with \(\mathrm{Br_2}\) is \(\mathrm{C_5H_{10} + Br_{2} \rightarrow C_{5}H_{10}Br_{2}}\), and the resulting compound is 2,3-dibromopentane. (c) The balanced equation for the substitution reaction of \(\mathrm{Cl}_2\) with benzene to make paradichlorobenzene in the presence of \(\mathrm{FeCl}_3\) as a catalyst is \(\mathrm{C_6H_6 + 2Cl_2 \rightarrow C_6H_4Cl_2 + 2HCl}\).

Step-by-step solution

Understanding Substitution and Addition Reactions

A substitution reaction is a chemical reaction in which one atom or group of atoms is replaced by another atom or group of atoms. On the other hand, an addition reaction is a chemical reaction in which two molecules combine to form a larger molecule. In general, alkenes undergo addition reactions while aromatic hydrocarbons, like benzene, undergo substitution reactions.

(a) Reaction Types Commonly Observed with Alkenes and Aromatic Hydrocarbons

Alkenes, which are unsaturated hydrocarbons containing a carbon-carbon double bond, commonly undergo addition reactions. Aromatic hydrocarbons, such as benzene, which contain a ring of alternating single and double bonds, typically undergo substitution reactions.

(b) Addition Reaction of 2-pentene with \(\mathrm{Br}_{2}\)

The addition reaction of 2-pentene with \(\mathrm{Br}_2\) is as follows: 2-pentene + \(\mathrm{Br_2} \rightarrow\) 2,3-dibromopentane In the balanced equation form: \(\mathrm{C_5H_{10} + Br_{2} \rightarrow C_{5}H_{10}Br_{2}}\) The resulting compound is 2,3-dibromopentane.

(c) Substitution Reaction of \(\mathrm{Cl}_{2}\) with Benzene

In the presence of \(\mathrm{FeCl}_3\) as a catalyst, the substitution reaction between benzene and \(\mathrm{Cl}_2\) to produce paradichlorobenzene is as follows: Benzene + 2 \(\mathrm{Cl}_2 \rightarrow\) paradichlorobenzene + 2 HCl In the balanced equation form, using the formula for benzene (\(\mathrm{C_6H_6}\)) and paradichlorobenzene (\(\mathrm{C_6H_4Cl_2}\)): \(\mathrm{C_6H_6 + 2Cl_2 \rightarrow C_6H_4Cl_2 + 2HCl}\)