Problem 35 - Chemistry: The Central Science in SI Units, Global Edition

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 paradichlorobenzene in the presence of \(\mathrm{FeCl}_{3}\) as a catalyst. Is this an addition or a substitution reaction?

Step-by-Step Solution

Answer

(a) True, alkenes undergo addition reactions and aromatic hydrocarbons undergo substitution reactions. (b) The balanced equation for the reaction of 2-pentene with bromine is: \(CH_3-CH=CH-CH_2-CH_3 + Br_2 \rightarrow CH_3-CHBr-CHBr-CH_2-CH_3\). The resulting compound is 2,3-dibromopentane, and it is an addition reaction. (c) The balanced chemical equation for the reaction of chlorine with benzene to make paradichlorobenzene is: \(C_6H_6 + 2Cl_2 \xrightarrow{FeCl_3} C_6H_4Cl_2 + 2HCl\). This is a substitution reaction.

Step 1: Statement validity

Alkenes contain a carbon-carbon double bond which makes them undergo addition reactions. Aromatic hydrocarbons, such as benzene, possess an unusually stable ring electron system, and typically undergo substitution reactions, where one of the atoms in the ring is exchanged for another atom or group of atoms. Therefore, the given statement is true. #b) Reaction of 2-pentene with bromine#

Step 2: Write the balanced equation

The balanced equation for the reaction of 2-pentene (\(C_5H_{10}\)) with bromine (\(Br_2\)) can be written as follows: \[CH_3-CH=CH-CH_2-CH_3 + Br_2 \rightarrow CH_3-CHBr-CHBr-CH_2-CH_3\]

Step 3: Name the resulting compound

The resulting compound is named as: 2,3-dibromopentane

Step 4: Identify the reaction type

Since the carbon-carbon double bond in 2-pentene is broken and two bromine atoms are added to the molecule, the reaction is an addition reaction. #c) Reaction of chlorine with benzene to produce paradichlorobenzene#

Step 5: Write the balanced chemical equation

The balanced chemical equation for the reaction of chlorine (\(Cl_2\)) with benzene (\(C_6H_6\)) to produce paradichlorobenzene (\(C_6H_4Cl_2\)) using iron(III) chloride (\(FeCl_3\)) as a catalyst is as follows: \[C_6H_6 + 2Cl_2 \xrightarrow{FeCl_3} C_6H_4Cl_2 + 2HCl\]

Step 6: Identify the reaction type

Two hydrogen atoms in the benzene are replaced by chlorine atoms, so this is a substitution reaction.

Key Concepts

Addition Reactions

In chemistry, addition reactions are a fundamental type of chemical reaction where atoms or groups of atoms are added to a molecule. This is especially common in organic chemistry with molecules that have carbon-carbon double bonds, such as alkenes. During an addition reaction, a double bond in the molecule is broken, and new atoms are added to the resulting open sites.

To visualize this, imagine breaking a carrot into two pieces and then sticking other objects on the exposed broken ends. In molecular terms, double bonds serve as these broken ends, becoming a site for new attachments. Alkenes, due to their unsaturated nature, are particularly susceptible to addition reactions, helping convert them into more saturated molecules.

Addition reactions often involve symmetrical addition like halogenation, or asymmetrical addition, where molecules like hydrogen chloride add across the double bond. Each leads to different possible reaction products.

Substitution Reactions

A substitution reaction is another fundamental type of chemical reaction where an atom or a group within a molecule is replaced by another atom or group. This can be thought of as finding a new replacement for a part in a device.

In organic chemistry, substitution reactions often occur in aromatic compounds, where a stable ring structure can maintain its configuration despite the scrutiny of external factors. An aromatic ring like benzene features unique electron stabilization, making outright additions across its bonds less likely. Instead, a substitution at one of its many hydrogen sites is more feasible, resulting in the introduction of a new substituent group without altering the underlying ring configuration.

Examples include halogenation using a catalyst, where iodine, bromine, or chlorine can replace a hydrogen atom on an aromatic compound, often mediated by metal catalysts to maintain stability during the exchange.

Alkenes

Alkenes are a type of hydrocarbon characterized by at least one carbon-carbon double bond in their molecular structure. This double bond is a region of high electron density, which makes alkenes more reactive than their single-bonded counterparts, alkanes.

The double bond in alkenes provides a site for chemical reactions, primarily addition reactions, where atoms or groups can be added to the carbon atoms involved in the double bond. The straightforward nature of alkenes’ addition reactions turns them into crucial starting materials in the synthesis of a wide array of chemical products, including polymers and other complex molecules.

Moreover, each alkene is built with unique IUPAC naming conventions based on its structure, where the longest chain of carbon atoms serves as a backbone, and the location of the double bond is denoted by numerical indices. The reactivity and versatility of alkenes are central themes in organic chemistry, highlighting their role in both laboratory and industrial processes.

Aromatic Hydrocarbons

Aromatic hydrocarbons, often referred to as arenes, are organic compounds that contain a planar unsaturated ring of atoms that are usually stabilized by resonance. Benzene is the archetype of aromatic hydrocarbons, exhibiting exceptional stability despite seemingly unsaturated pi bonds. This stabilization is due to the delocalization of electrons within what is known as an aromatic ring.

This unique electron configuration gives rise to distinctive characteristics in aromatic hydrocarbons. Unlike other unsaturated hydrocarbons, they generally resist addition reactions because such reactions require the disruption of the stable electron cloud. Instead, aromatic hydrocarbons more commonly undergo substitution reactions, where one hydrogen atom in the ring is replaced with another atom or group without disrupting the configuration of the ring itself.

Aromatic compounds are prevalent in a variety of real-world applications, from pharmaceuticals and dyes to polymers and agrochemicals, owing their widespread use to their chemical stability and unique properties.