Which compound of each pair will react faster with the...

Which compound of each pair will react faster with the specified reagent? Explain your answer. a. 1-hexene or $E-3$-hexene with bromine in acetic acid. b. cis- or trans-4-( $t$-butyl)cyclohexylmethyl bromide with $\mathrm{KOC}\left(\mathrm{CH}_{3}\right)_{3}$ in $t$-butyl alcohol. c. 2-phenylpropene or 4-(1-methylethenyl)benzoic acid with sulfuric acid in water.

Which compound of each pair will react faster with the specified reagent? Explain your answer.
a. 1-hexene or $E-3$-hexene with bromine in acetic acid.
b. cis- or trans-4-( $t$-butyl)cyclohexylmethyl bromide with $\mathrm{KOC}\left(\mathrm{CH}_{3}\right)_{3}$ in $t$-butyl alcohol.
c. 2-phenylpropene or 4-(1-methylethenyl)benzoic acid with sulfuric acid in water.

Key Concepts

Acid-Catalyzed Hydration of Alkenes

This mechanism involves the protonation of the alkene to form a carbocation, followed by nucleophilic attack by water and subsequent deprotonation to yield an alcohol. The electronic properties of substituents attached to the alkene, such as resonance donation or electron withdrawal, greatly affect the reactivity and overall reaction rate by stabilizing or destabilizing the intermediates.

Carbocation Stability in Acid-Catalyzed Reactions

Carbocation intermediates are often formed in acid-catalyzed reactions such as hydration of alkenes. The stability of these intermediates, which can be enhanced by resonance effects from adjacent aromatic rings or destabilized by electron-withdrawing groups, is a major factor influencing the reaction rate. More stable carbocations are formed more readily and lead to faster reactions.

Stereoelectronic Factors in Elimination Reactions

In elimination reactions like E2, the spatial arrangement of substituents (for example, axial versus equatorial positions in cyclic systems) plays a decisive role. The isomer that offers a more favorable antiperiplanar orientation between the leaving group and the proton is more reactive, making stereochemistry an important factor in predicting which compound will react faster.

Steric and Electronic Effects on Alkene Reactivity

The rate at which an alkene reacts in electrophilic addition is governed by both steric hindrance and electronic factors. A less substituted, less-hindered double bond tends to react faster with electrophiles because there is greater accessibility, despite the fact that more substituted alkenes may form more stable intermediates. These effects are crucial in comparing different alkene isomers in a given reaction.

Electrophilic Addition to Alkenes

This concept deals with how alkenes react with electrophiles such as bromine. The reactivity is influenced by the electron density of the double bond and the degree of substitution; terminal alkenes generally have more accessible electron density compared to more substituted internal alkenes, which can be sterically hindered even though substitution can sometimes stabilize intermediates.

E2 Elimination Mechanism

E2 elimination is a bimolecular process in which a strong base removes a proton while the leaving group departs, leading to the formation of a double bond. The geometry of the substrate, especially the need for an antiperiplanar arrangement between the hydrogen to be removed and the leaving group, is critical in determining the reaction rate and outcome.

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