Question

Using condensed structural formulas, write a balanced chemical equation for each of the following reactions: (a) hydrogenation of cyclohexene; (b) addition of \(\mathrm{H}_{2} \mathrm{O}\) to trans- 2 pentene using \(\mathrm{H}_{2} \mathrm{SO}_{4}\) as a catalyst (two products); (c) reaction of 2 -chloropropane with benzene in the presence of \(\mathrm{AlCl}_{3}\).

Short answer

(a) \( \mathrm{C}_{6} \mathrm{H}_{10} + \mathrm{H}_{2} \to \mathrm{C}_{6} \mathrm{H}_{12} \) (b) \( \mathrm{C}_{5} \mathrm{H}_{10} + \mathrm{H}_{2} \mathrm{O} \to \mathrm{C}_{5} \mathrm{H}_{12} \mathrm{O} + \mathrm{C}_{5} \mathrm{H}_{12} \mathrm{O} \) (c) \( \mathrm{C}_{3} \mathrm{H}_{7} \mathrm{Cl} + \mathrm{C}_{6} \mathrm{H}_{6} \to \mathrm{C}_{9} \mathrm{H}_{10} + \mathrm{H} \mathrm{Cl} \)

Step-by-step solution

(a) Hydrogenation of cyclohexene

Hydrogenation is a reaction in which hydrogen is added to a double bond in an unsaturated organic compound. In this case, cyclohexene is an unsaturated hydrocarbon with a carbon-carbon double bond. During hydrogenation, the double bond will be broken, and hydrogen atoms will be added to the carbons. To balance the equation, we need one molecule of hydrogen gas (H2). Cyclohexene + H2 -> Cyclohexane \( \mathrm{C}_{6} \mathrm{H}_{10} + \mathrm{H}_{2} \to \mathrm{C}_{6} \mathrm{H}_{12} \)

(b) Addition of water to trans-2-pentene using sulfuric acid as a catalyst

In this reaction, a molecule of water (H2O) is added to the carbon-carbon double bond of trans-2-pentene with the aid of a catalyst, sulfuric acid (H2SO4). This reaction results in two products: 2-pentanol and 3-pentanol. Trans-2-pentene + H2O (with H2SO4 as a catalyst) -> 2-pentanol + 3-pentanol \( \mathrm{C}_{5} \mathrm{H}_{10} + \mathrm{H}_{2} \mathrm{O} \to \mathrm{C}_{5} \mathrm{H}_{12} \mathrm{O} + \mathrm{C}_{5} \mathrm{H}_{12} \mathrm{O} \)

(c) Reaction of 2-chloropropane with benzene in the presence of aluminum chloride.

This reaction is an electrophilic aromatic substitution, specifically a Friedel-Crafts alkylation, in which 2-chloropropane reacts with benzene in the presence of a catalyst, aluminum chloride (AlCl3). The chlorine atom from 2-chloropropane is substituted by the benzene ring, forming isopropylbenzene and releasing hydrogen chloride (HCl). 2-chloropropane + Benzene (with AlCl3 as a catalyst) -> Isopropylbenzene + HCl \( \mathrm{C}_{3} \mathrm{H}_{7} \mathrm{Cl} + \mathrm{C}_{6} \mathrm{H}_{6} \to \mathrm{C}_{9} \mathrm{H}_{10} + \mathrm{H} \mathrm{Cl} \)

Key concepts

Hydrogenation

Hydrogenation is a chemical reaction where hydrogen molecules add to unsaturated compounds, converting them into saturated ones. In organic chemistry, this typically involves the addition of hydrogen across a carbon-carbon double bond in alkenes, resulting in alkanes. For example, when cyclohexene undergoes hydrogenation, it becomes cyclohexane.

Key points to remember about hydrogenation:

• The reaction requires molecular hydrogen (\(\mathrm{H}_{2}\)) and often a catalyst such as palladium or nickel to facilitate the reaction.
• The addition of hydrogen breaks the double bond, adding one hydrogen atom to each carbon formerly involved in the bond.
• This process saturates the molecule, as it converts an unsaturated compound (one with a double bond) into a saturated compound (single bonds only).

In our example, the balanced chemical equation is: \[\mathrm{C}_{6}\mathrm{H}_{10} + \mathrm{H}_{2} \to \mathrm{C}_{6}\mathrm{H}_{12}\] This shows the cyclohexene (\(\mathrm{C}_{6}\mathrm{H}_{10}\)) reacting with hydrogen to form cyclohexane (\(\mathrm{C}_{6}\mathrm{H}_{12}\)). By breaking the double bond and adding hydrogen, the ring structure remains intact but is now fully saturated.

Addition Reactions

Addition reactions are fundamental in organic chemistry and involve the joining of two molecules to form a single product. These reactions commonly occur with unsaturated hydrocarbons that contain double or triple bonds, such as alkenes or alkynes.

A classic example in hydrocarbons is the hydration of alkenes, where water is added to a double bond with an acid catalyst, such as sulfuric acid (\(\mathrm{H}_{2}\mathrm{SO}_{4}\)). The reaction typically results in the formation of alcohols.

Example: Addition of Water to Trans-2-pentene

In this scenario, trans-2-pentene reacts with water in the presence of sulfuric acid, leading to two possible alcohol products: 2-pentanol and 3-pentanol. The mechanism proceeds through:

• The acid catalyst protonates the alkene, creating a carbocation intermediate.
• Water then attacks this carbocation, resulting in the formation of the alcohol.
• The outcome is steered by Markovnikov's rule, where the hydrogen atom attaches to the less substituted carbon of the double bond.

The balanced equation for the reaction is: \[\mathrm{C}_{5}\mathrm{H}_{10} + \mathrm{H}_{2}\mathrm{O} \to \mathrm{C}_{5}\mathrm{H}_{12}\mathrm{O} + \mathrm{C}_{5}\mathrm{H}_{12}\mathrm{O}\]},

Friedel-Crafts Alkylation

Friedel-Crafts alkylation is a technique employed in organic chemistry to alkylate (add an alkyl group to) an aromatic compound, such as benzene. It is an example of an electrophilic aromatic substitution reaction.

In this reaction, the aromatic ring reacts with an alkyl halide in the presence of a strong Lewis acid catalyst, like aluminum chloride (\(\mathrm{AlCl}_{3}\)). The catalyst facilitates the formation of a carbocation from the alkyl halide, enabling the substitution.

Example: Reaction of 2-chloropropane with Benzene

When 2-chloropropane reacts with benzene under the influence of \(\mathrm{AlCl}_{3}\), it forms isopropylbenzene, also known as cumene. The process involves:

• The \(\mathrm{AlCl}_{3}\) catalyst polarizing the chlorine-carbon bond in 2-chloropropane, thus forming a carbocation.
• This carbocation then attacks the electron-rich benzene ring, replacing a hydrogen atom with the isopropyl group.
• Hydrogen chloride (\(\mathrm{HCl}\)) is released as a byproduct of the reaction.

The reaction can be represented by the equation: \[\mathrm{C}_{3}\mathrm{H}_{7}\mathrm{Cl} + \mathrm{C}_{6}\mathrm{H}_{6} \to \mathrm{C}_{9}\mathrm{H}_{10} + \mathrm{HCl}\] This elegant method allows chemists to introduce complex alkyl chains into hydrocarbons, creating derivatives with diverse chemical properties.