Question

(a) True or false: Alkenes undergo addition reactions and aromatic hydrocarbons undergo substitution reactions. (b) Using condensed structural formulas, write the balanced equation for the reaction of 2 -pentene with \(\mathrm{Br}_{2}\) and name the resulting compound. Is this an addition or a substitution reaction? (c) Write a balanced chemical equation for the reaction of \(\mathrm{Cl}_{2}\) with benzene to make para-dichlorobenzene in the presence of \(\mathrm{FeCl}_{3}\) as a catalyst. Is this an addition or a substitution reaction?

Short answer

The given statement is true. The reaction between 2-pentene and Br2 is an addition reaction, forming 2,3-dibromopentane: \(CH_{3}CH=CHCH_{2}CH_{3} + Br_{2} \rightarrow CH_{3}CHBrCHBrCH_{2}CH_{3}\). The reaction between benzene and Cl2 in the presence of FeCl3 is a substitution reaction, producing para-dichlorobenzene: \(C_{6}H_{6} + 2Cl_{2} \xrightarrow[]{FeCl_{3}} C_{6}H_{4}Cl_{2} + 2HCl\).

Step-by-step solution

Question (a): True or false statement

The given statement is True. Alkenes generally undergo addition reactions due to the presence of a double bond, and aromatic hydrocarbons, such as benzene, typically undergo substitution reactions because of their stable ring structure formed by the resonance of delocalized electrons.

Question (b): 2-pentene with Br2

In the reaction between 2-pentene and Br2, we have the following reactants: - 2-pentene: \(CH_{3}CH=CHCH_{2}CH_{3}\) - Bromine: \(Br_{2}\) This is an addition reaction, as the double bond of 2-pentene reacts with Br2, and both bromine atoms get added to the molecule. Balanced reaction: \(CH_{3}CH=CHCH_{2}CH_{3} + Br_{2} \rightarrow CH_{3}CHBrCHBrCH_{2}CH_{3}\) The resulting compound is called 2,3-dibromopentane.

Question (c): Benzene with Cl2

In the reaction between benzene and Cl2, we have the following reactants: - Benzene: \(C_{6}H_{6}\) - Chlorine: \(Cl_{2}\) This is a substitution reaction, as one or more hydrogen atoms of benzene are replaced by chlorine atoms. To make para-dichlorobenzene, two hydrogen atoms at para position will be replaced by chlorine atoms using FeCl3 as a catalyst. Balanced reaction: \(C_{6}H_{6} + 2Cl_{2} \xrightarrow[]{FeCl_{3}} C_{6}H_{4}Cl_{2} + 2HCl\) In this reaction, para-dichlorobenzene is formed, and it is a substitution reaction.

Key concepts

Alkenes

Alkenes are a fascinating class of hydrocarbons characterized by at least one carbon-carbon double bond. This double bond is crucial because it makes alkenes more reactive than their single-bonded counterparts, known as alkanes. Alkanes only have single bonds, while alkenes have at least one double bond, which is shown as \( \text{C=C} \). The double bond in alkenes provides a site for chemical reactions. This is why alkenes commonly undergo addition reactions.

The double bond consists of a sigma bond and a pi bond. The pi bond is weaker and more reactive than the sigma bond, making it the primary site for chemical reactions. When an alkene reacts, the pi bond breaks, and new atoms add on, converting the double bond into two single bonds.

• For example, reactions such as hydrogenation, halogenation, and hydrohalogenation are typical addition reactions experienced by alkenes.
• In chemistry, understanding the reactivity of alkenes can help predict outcomes and transformations in organic synthesis.

Aromatic Hydrocarbons

Aromatic hydrocarbons are a unique subset of hydrocarbons that consist of ring structures and delocalized electrons, resulting in enhanced stability. The most well-known aromatic hydrocarbon is benzene, represented by the chemical formula \( \text{C}_6\text{H}_6 \). This molecule consists of six carbon atoms forming a ring with alternating single and double bonds. However, instead of distinct single and double bonds, the electrons are shared equally as a cloud over all the carbon atoms.

This unique structure is called resonance, and it contributes to the stability of aromatic compounds. This stability generally prevents aromatic hydrocarbons like benzene from undergoing addition reactions. Instead, they typically undergo substitution reactions.

• Substitution reactions in aromatic hydrocarbons often involve replacing a hydrogen atom in the ring with another atom or group of atoms.
• This allows the aromatic ring to maintain its stable structure while still reacting to form new compounds.

Understanding the behavior of aromatic hydrocarbons is essential for creating many important compounds, including dyes, plastics, and pharmaceuticals.

Addition Reactions

Addition reactions are a type of chemical reaction prominent with unsaturated hydrocarbons, like alkenes. In an addition reaction, atoms or groups of atoms are added to the carbon atoms involved in a double or triple bond. This type of reaction changes the degree of saturation of the organic molecule.

During the addition reaction, the reactive double bond is converted into single bonds. For instance, when alkenes undergo halogenation, halogen atoms like bromine and chlorine add across the double bond.

• A classic example of an addition reaction is the reaction between 2-pentene and bromine \( \text{Br}_2 \) in which the bromine atoms add across the double bond to form a dibromoalkane.
• This results in the transformation of a double bond into two single bonds, which significantly alters the molecule's properties.

Addition reactions are very useful in organic synthesis, enabling chemists to create more complex molecules from simpler ones.

Substitution Reactions

Substitution reactions involve the replacement of an atom or a group of atoms in a molecule with a different atom or group. This type of reaction is commonly seen in aromatic hydrocarbons due to their stable electronic structure. Instead of adding new atoms, as in addition reactions, substitution reactions preserve the stable aromatic ring by replacing existing atoms.

In benzene, for example, halogenation occurs through substitution reactions. In the presence of a catalyst like iron(III) chloride (\( \text{FeCl}_3 \)), chlorine (\( \text{Cl}_2 \)) can replace a hydrogen atom in benzene.

• An example is the formation of para-dichlorobenzene when two hydrogen atoms in benzene are substituted with chlorine atoms on opposite sides of the benzene ring, maintaining the aromatic structure.
• These reactions are fundamental in creating various derivatives of benzene, which are integral in producing numerous industrial products.

Substitution reactions highlight the chemical nature of aromatic compounds and their ability to form stable but reactive compounds.