Problem 36 - Chemistry: The Central Science in SI Units, Global Edition

Question

Using condensed structural formulas, write a balanced chemical equation for each of the following reactions: (a) hydrogenation of cyclohexene, \((\mathbf{b})\) 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 \(\mathrm{AlCl}_{3}\)

Step-by-Step Solution

Answer

The balanced chemical equations for the given reactions are: a) Hydrogenation of cyclohexene: \[ C_6H_{10} + H_2 \rightarrow C_6H_{12} \] b) Addition of H2O to trans-2-pentene using H2SO4 as a catalyst (two products): \[ CH_3CH=CHCH_2CH_3 + H_2O \rightarrow CH_3CH(OH)CH=CHCH_3 + CH_3CH=CHCH(OH)CH_3 \] c) Reaction of 2-chloropropane with benzene in the presence of AlCl3: \[ C_6H_6 + CH_3CHClCH_3 \rightarrow C_6H_5CH(CH_3)_2 + HCl \]

Step 1: Write the condensed structural formula for cyclohexene

Cyclohexene is a six-membered ring with a carbon-carbon double bond. The condensed structural formula is written as \(C_6H_{10}\).

Step 2: Add hydrogen gas in the presence of a catalyst

The hydrogenation reaction requires hydrogen gas, which has the formula \(H_2\). A catalyst, usually a metal like palladium (Pd) or platinum (Pt), is added to the reaction, but it does not appear in the balanced reaction.

Step 3: Write the condensed structural formula for cyclohexane

The product of hydrogenation of cyclohexene is cyclohexane, which has the condensed structural formula \(C_6H_{12}\).

Step 4: Write the balanced chemical equation

Putting it all together, the balanced chemical equation is: \[ C_6H_{10} + H_2 \rightarrow C_6H_{12} \] #b) Addition of H2O to trans-2-pentene using H2SO4 as a catalyst (two products)#

Step 1: Write the condensed structural formula for trans-2-pentene

Trans-2-pentene is a five-carbon chain with a double bond between the second and third carbons. The condensed structural formula is written as \(CH_3CH=CHCH_2CH_3\).

Step 2: Add water and the H2SO4 catalyst

Water (\(H_2O\)) is added to the reaction and H2SO4 acts as a catalyst, but again, it is not included in the balanced chemical equation.

Step 3: Write the condensed structural formula for the two products

The products of the reaction are two alcohols: 2-Pentanol with an OH group at the second carbon, written as \(CH_3CH(OH)CH=CHCH_3\), and 3-Pentanol with an OH group at the third carbon, written as \(CH_3CH=CHCH(OH)CH_3\).

Step 4: Write the balanced chemical equation

Putting it all together, the balanced chemical equation is: \[ CH_3CH=CHCH_2CH_3 + H_2O \rightarrow CH_3CH(OH)CH=CHCH_3 + CH_3CH=CHCH(OH)CH_3 \] #c) Reaction of 2-chloropropane with benzene in the presence of AlCl3#

Step 1: Write the condensed structural formula for 2-chloropropane and benzene

2-Chloropropane is a three-carbon chain with a chlorine atom at the second carbon. The condensed structural formula is written as \(CH_3CHClCH_3\). Benzene is a six-membered ring with alternating single and double carbon-carbon bonds, written as \(C_6H_6\).

Step 2: Add the AlCl3 catalyst

The Friedel-Crafts alkylation reaction requires a catalyst, which in this case is AlCl3, but it is not included in the balanced chemical equation.

Step 3: Write the condensed structural formula for the product

The product of the reaction is isopropylbenzene, also known as cumene, which has the condensed structural formula \(C_6H_5CH(CH_3)_2\).

Step 4: Write the balanced chemical equation

Putting it all together, the balanced chemical equation is: \[ C_6H_6 + CH_3CHClCH_3 \rightarrow C_6H_5CH(CH_3)_2 + HCl \]

Key Concepts

Hydrogenation

Hydrogenation is a fundamental reaction in organic chemistry where an unsaturated compound, like an alkene, reacts with hydrogen gas (\(H_2\)) to form a saturated compound, such as an alkane.
This process requires a catalyst, commonly palladium (Pd) or platinum (Pt), to proceed efficiently. In our exercise, cyclohexene, a six-membered carbon ring with one double bond, undergoes hydrogenation to form cyclohexane.

• The double bond in cyclohexene breaks, allowing the addition of two hydrogen atoms.
• The resulting product, cyclohexane, has all single bonds, making it more stable.

This reaction exemplifies how catalysts facilitate the addition of hydrogen without appearing in the balanced equation, making the transformation seamless while maintaining chemical simplicity. Through hydrogenation, unsaturated compounds can become more stable by reducing the number of double or triple bonds.

Addition Reaction

An addition reaction involves the combination of molecules by adding atoms or groups across a double or triple bond, typically found in alkenes and alkynes. In the example given, trans-2-pentene undergoes an addition reaction with water (\(H_2O\)) in the presence of sulfuric acid (\(H_2SO_4\)) as a catalyst to form two different alcohols: 2-pentanol and 3-pentanol.

• The double bond in the trans-2-pentene opens up, allowing an -OH group to attach to one of the carbons originally in the double bond.
• The position of the -OH attachment can vary, resulting in different structural isomers, as evidenced by 2-pentanol and 3-pentanol.

This reaction highlights the versatility of alkene structures to form a variety of products depending on how components add to them. Catalysts like \(H_2SO_4\) speed up the reaction without being consumed by providing a reaction-friendly environment.

Friedel-Crafts Alkylation

Friedel-Crafts alkylation is a significant organic reaction used to introduce alkyl groups into an aromatic compound, such as benzene. This reaction type involves the substitution of a hydrogen atom on the aromatic ring with an alkyl group, in this case facilitated by a catalyst.
In the provided scenario, benzene reacts with 2-chloropropane in the presence of aluminum chloride (\(AlCl_3\)), a Lewis acid catalyst, to yield isopropylbenzene.

• 2-Chloropropane provides the isopropyl cation \((CH_3CH^+CH_3)\), facilitated by the catalyst, which creates a more reactive species.
• The aromatic ring of benzene undergoes an electrophilic substitution, where the alkyl group attaches and displaces a hydrogen atom.
• As a result, the benzene becomes isopropylbenzene, also known as cumene.

Friedel-Crafts alkylation is a powerful tool for chemists to create complex molecules by adding carbon chains to aromatic structures. The catalyst \(AlCl_3\) is crucial as it stabilizes the carbocation needed for successful alkylation.