Problem 31 - Chemistry: The Central Science

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.

Step-by-Step Solution

Answer

(a) In a substitution reaction, a functional group or atom in a molecule is replaced with another functional group or atom, commonly observed with aromatic hydrocarbons like benzene. In an addition reaction, the unsaturated bonds of a molecule break, and new atoms or groups of atoms are added to the resulting broken bonds, commonly observed with alkenes like ethene, propene, etc. (b) The balanced equation for the addition reaction of 2-pentene with \(\mathrm{Br}_{2}\) is: CH3-CH=CH-CH2-CH3 + Br2 -> CH3-CH(Br)-CH(Br)-CH2-CH3 The resulting compound is named 2,3-dibromopentane. (c) The balanced equation for the substitution reaction of \(\mathrm{Cl}_{2}\) with benzene is: Benzene + 2 \(\mathrm{Cl}_{2}\) + \(\mathrm{FeCl}_{3}\) -> Paradichlorobenzene + \(\mathrm{FeCl}_{4}^{-}\) + 2 \(\mathrm{HCl}\).

Step 1: Substitution reaction

In a substitution reaction, a functional group or atom in a molecule is replaced with another functional group or atom. This type of reaction is commonly observed with aromatic hydrocarbons like benzene.

Step 2: Addition reaction

In an addition reaction, the unsaturated bonds of a molecule break, and new atoms or groups of atoms are added to the resulting broken bonds. This type of reaction is commonly observed with alkenes like ethene, propene, etc. b) Addition reaction of 2-pentene with \(\mathrm{Br}_{2}\)

Step 3: Write the equation

To write the equation, first, determine the initial and final structures of the reactants and product: 2-pentene: CH3-CH=CH-CH2-CH3 \(\mathrm{Br}_{2}\): Br-Br The Br-Br bond breaks, and one Br atom attaches to each of the carbon atoms with the double bond. Product: CH3-CH(Br)-CH(Br)-CH2-CH3 Now, write the balanced equation using the structures: CH3-CH=CH-CH2-CH3 + Br2 -> CH3-CH(Br)-CH(Br)-CH2-CH3

Step 4: Name the product

The product is named by counting the positions of the functional groups (in this case, Bromine) and the starting material (pentene). So, the resulting compound's name is 2,3-dibromopentane. c) Substitution reaction of \(\mathrm{Cl}_{2}\) with benzene

Step 5: Write the equation

In this case, we are given that the catalyst is \(\mathrm{FeCl}_{3}\), which helps in generating the electrophile by combining with \(\mathrm{Cl}_{2}\): \(\mathrm{Cl}_{2}\) + \(\mathrm{FeCl}_{3}\) -> \(\mathrm{FeCl}_{4}^{-}\) + \(\mathrm{Cl}^{+}\) The electrophilic substitution takes place in the benzene ring, replacing two hydrogen atoms with chlorine atoms to form paradichlorobenzene: Benzene + 2 \(\mathrm{Cl}^{+}\) -> Paradichlorobenzene By balancing, the overall chemical equation, we get: Benzene + 2 \(\mathrm{Cl}_{2}\) + \(\mathrm{FeCl}_{3}\) -> Paradichlorobenzene + \(\mathrm{FeCl}_{4}^{-}\) + 2 \(\mathrm{HCl}\)

Key Concepts

Substitution Reaction

In chemistry, a substitution reaction is an essential process where one atom or a group of atoms in a molecule is swapped with another atom or group. This type of reaction is quite prevalent among aromatic hydrocarbons like benzene. Here’s why:

• Benzene is very stable, mainly due to its aromatic nature. When substitution occurs, the aromatic nature remains intact.
• In substitution reactions, a catalyst like \( \mathrm{FeCl}_{3} \) is often used to facilitate the reaction.

For instance, using \( \mathrm{Cl}_{2} \) in the presence of \( \mathrm{FeCl}_{3} \) creates an electrophile that reacts with benzene to form products like paradichlorobenzene. This transformation is essential in making various chemical compounds used in everyday products.
Understanding substitution reactions are crucial for students, as they form the basis for creating many complex organic molecules.

Addition Reaction

An addition reaction is another fundamental concept in organic chemistry, especially with alkenes. Alkenes are hydrocarbons that contain carbon-carbon double bonds. In these reactions, the double bond opens up to allow atoms or groups to add to the molecule. Here's how it works:

• Addition reactions are limited to unsaturated molecules like alkenes.
• The double bond breaks, allowing the new atoms to attach to the formerly double-bonded carbon atoms.

An example is the addition of \( \mathrm{Br}_{2} \) to 2-pentene:
\[ \text{CH}_{3}-\text{CH}=\text{CH}-\text{CH}_{2}-\text{CH}_{3} + \text{Br}_{2} \rightarrow \text{CH}_{3}-\text{CH(Br)}-\text{CH(Br)}-\text{CH}_2-\text{CH}_3 \] The product formed is 2,3-dibromopentane. Understanding addition reactions is vital as they show how unsaturated molecules can interact with other substances to form more saturated structures, contributing to forming various useful chemicals and materials.

Aromatic Hydrocarbons

Aromatic hydrocarbons, known as arenes, constitute a class of hydrocarbons characterized by their unique stability and structure. Benzene is the most archetypal aromatic hydrocarbon. These compounds are crucial in chemistry for several reasons:

• They possess a ring-like structure, held together by delocalized electrons, making them unusually stable.
• Their stability means they resist reactions that typically break bonds, such as addition reactions.

Hence, arenes typically undergo substitution reactions instead. The stability afforded by the aromatic ring facilitates reactions that allow benzene to retain its aromatic nature while acquiring new substituents, crucial in producing dyes, drugs, and perfumes. Understanding aromatic hydrocarbons helps students grasp the concept of electron delocalization and its influence on chemical reactivity.

Alkenes

Alkenes are fundamental organic compounds notable for their carbon-carbon double bonds. They fall into the category of unsaturated hydrocarbons and exhibit reactivity in a unique manner:

• The double bonds allow alkenes to participate in addition reactions.
• They serve as precursors for more complex chemical compounds.

These reactions make them essential in industrial chemistry applications, such as plastic production, where small alkene molecules are polymerized into larger structures. A practical example includes the addition of bromine to alkenes like in the case of 2-pentene:
\( \text{CH}_3-\text{CH}=\text{CH}-\text{CH}_2-\text{CH}_3 \) reacting with \(\text{Br}_2\) to form \( \text{CH}_3-\text{CH(Br)}-\text{CH(Br)}-\text{CH}_2-\text{CH}_3 \). Recognizing the role of alkenes in forming valuable industrial materials is vital for students, aiding in understanding reactivity and molecular structures.

Electrophilic Substitution

Electrophilic substitution is a common reaction mechanism in aromatic chemistry. This process involves an electrophile replacing a hydrogen atom in the aromatic ring—a fundamental type of substitution reaction in aromatic hydrocarbons. Here's why it's crucial:

• Involves a two-step mechanism where the aromatic ring first attacks an electrophile.
• The positive nature of the electrophile makes it susceptible to attack by the electron-rich aromatic ring.

For example, in the reaction of benzene with \( \mathrm{Cl}_{2} \) in the presence of \( \mathrm{FeCl}_{3} \), the electrophilic substitution yields compounds such as paradichlorobenzene, maintaining the aromaticity of benzene.Mastering the concept of electrophilic substitution enables students to predict and understand the outcomes of reactions involving aromatic systems, hence its significance in organic chemistry.