Question

Which of the following alkenes will undergo acid catalysed hydration most readily? (a) (b) Cis-2-butene (c) trans-2-butene (d) 1 -butene

Short answer

(a) unspecified, (b) cis-2-butene, (c) trans-2-butene, or (d) 1-butene? Answer: Both (b) cis-2-butene and (c) trans-2-butene will undergo acid-catalyzed hydration more readily than (d) 1-butene. However, as option (a) is unspecified, it cannot be compared to the other options.

Step-by-step solution

Draw the alkenes

Draw the structures of the four alkenes listed in the exercise. (a) is unspecified, so we can't analyze it. (b) Cis-2-butene: H2C=C(H)-CH2-CH3 (c) trans-2-butene: H2C=C(-H)-CH2-CH3 (d) 1-butene: H2C=CH-CH2-CH3

Determine the carbocations formed from each alkene

Determine the carbocations formed from each of these alkenes upon the addition of water across the double bond. (b) Cis-2-butene: H3C-C^(+)(-H)-CH2-CH3 (c) trans-2-butene: H3C-C^(+)(-H)-CH2-CH3 (d) 1-butene: H3C-CH2-C^(+)-CH3

Determine the intermediate carbocation stability

Now, compare the stability of the intermediate carbocations. Carbocation stability is generally in the order: 3°>2°>1° (b) Cis-2-butene: A 2° carbocation (c) trans-2-butene: A 2° carbocation (d) 1-butene: A 1° carbocation

Compare the reactivity of the alkenes

Since both cis-2-butene and trans-2-butene form a 2° carbocation during acid-catalyzed hydration, they will undergo the reaction more readily than 1-butene, which forms a less stable 1° carbocation. While cis and trans isomers can have different properties, the reaction rate does not significantly vary between them for this specific case. So the final answer is: Both (b) Cis-2-butene and (c) trans-2-butene will undergo acid-catalyzed hydration more readily than (d) 1-butene. However, as option (a) is unspecified, it cannot be compared to the other options.

Key concepts

Acid-catalyzed Hydration

Acid-catalyzed hydration is a critical concept in organic chemistry. This reaction involves the addition of water to an alkene in the presence of an acid catalyst, typically a strong acid like sulfuric acid or phosphoric acid.

Here's how it works: the double bond of the alkene is protonated by the acid, forming a carbocation intermediate. Then, a water molecule attacks this positively charged carbocation, leading to the formation of an alcohol.

Acid-catalyzed hydration is a vital transformation in synthesizing alcohols from alkenes, commonly seen in industrial applications. The reaction efficiency relies heavily on the stability of the carbocation formed during the process, an essential point we will explore next.

Carbocation Stability

Carbocation stability is paramount when discussing acid-catalyzed hydration, as it significantly influences the reaction's progress.

Carbocations are positively charged carbon atoms, and their stability increases with the number of alkyl groups attached. This is understood through the stability order: tertiary (3°) > secondary (2°) > primary (1°).

Alkyl groups stabilize carbocations through hyperconjugation and inductive effect. Hyperconjugation involves the spatial overlap of orbital electrons, while the inductive effect is the electron-donating nature of alkyl groups.

For the alkenes in the exercise, cis-2-butene and trans-2-butene both form a secondary carbocation, which is more stable than the primary carbocation formed by 1-butene. Therefore, these alkenes are more likely to undergo hydration efficiently.

Cis-trans Isomerism

Cis-trans isomerism, a form of stereoisomerism, affects the physical and chemical properties of compounds, though it has a minor influence on acid-catalyzed hydration reactivity.

Cis and trans isomers arise due to restricted rotation around a double bond. In cis isomers, substituents are on the same side, while in trans isomers, they are on opposite sides.

This difference can lead to variations in boiling point, melting point, and solubility. However, in the specific case of acid-catalyzed hydration in our exercise, both cis-2-butene and trans-2-butene form the same type of carbocation (secondary). Thus, the reactivity in hydration does not differ significantly between these isomers, making both quite reactive compared to 1-butene.