Question

Four structures (1) - (4) of different alcohols are given below: (1) CCC(O)c1ccccc1 (2) CC(O)Cc1ccccc1 (3) CCC(O)CC (4) CC(C)CCO The order of facility, from fastest to slowest, of acid catalysed dehydration will be (a) \(2>1>3>4\) (b) \(1>2>3>4\) (c) \(4>3>2>1\) (d) \(2>3>4>1\)

Short answer

The correct order of dehydration facility is (a) \(2>1>3>4\).

Step-by-step solution

Understanding the Concept

Acid catalyzed dehydration involves the loss of water from an alcohol to form an alkene. The ease of dehydration depends on the stability of the carbocation intermediate formed during the reaction. More stable carbocations dehydrate more easily.

Analyze Alcohol (1)

Alcohol (1) is benzyl alcohol with a propyl group, structured as: phenyl group - CH(OH) - CH2 - CH3. The carbocation formed here when the alcohol is protonated and water is lost is a benzyl carbocation, which is resonance stabilized.

Analyze Alcohol (2)

Alcohol (2) also forms a benzyl carbocation upon dehydration, similar to alcohol (1). The structure of this alcohol is: phenyl group - CH2 - CH(OH) - CH3. This also results in a resonance-stabilized carbocation.

Analyze Alcohol (3)

Alcohol (3), propyl alcohol, structured as: CH3 - CH2 - CH(OH) - CH3, forms a secondary carbocation after water loss. This is less stable compared to the benzyl carbocations due to lack of resonance stabilization.

Analyze Alcohol (4)

Alcohol (4) is tert-butanol, structured as: CH3 - C(CH3)2 - CH(OH) - CH3. This forms a tertiary carbocation, which is generally more stable than a secondary carbocation but lacks resonance stabilization features available to benzyl carbocations.

Arrange According to Dehydration Facility

Now, comparing the stabilities: Resonate-stabilized benzyl carbocations (from alcohols 1 and 2) have higher stability compared to simple alkyl carbocations. Despite tertiary carbocations usually being more stable, the unique stabilization of benzyl carbocations makes alcohols 1 and 2 dehydrate more readily than 3 and 4. Among them, (2) provides a slightly better stabilization due to less steric hindrance compared with (1), leading to: Order = 2 > 1 > 3 > 4.

Key concepts

Carbocation Stability

Understanding the stability of a carbocation is crucial when considering acid-catalyzed dehydration of alcohols. When an alcohol undergoes dehydration, it generally forms a carbocation intermediate. The rule of thumb is: the more stable the carbocation, the easier it is for the dehydration to proceed. But what makes a carbocation stable?

• Primary, secondary, and tertiary carbocations: In simple terms, tertiary carbocations (attached to three other carbon atoms) are more stable than secondary ones (attached to two other carbon atoms), which are more stable than primary carbocations (attached to one other carbon atom). This is due to the electron-donating effect of alkyl groups, which stabilize the positive charge.
• Hyperconjugation: This is a stabilizing interaction that involves the overlap of \(\sigma\) (bonding) orbitals with an adjacent empty \(\pi\) or \(p\) orbital, leading to a spread of the positive charge over a larger region.
With these principles in mind, we can see how different alcohols will form carbocations of varying stabilities, significantly affecting the ease with which they undergo dehydration.

Resonance Stabilization

Resonance stabilization is a key player in enhancing the stability of certain carbocations. It occurs when the positive charge on a carbocation is delocalized over a structure through resonance—a situation where electrons can "move" across different atoms, creating multiple resonance structures.

• Benzyl carbocations: These carbocations occur when a positive charge is adjacent to a benzene ring. The charge can be spread over the aromatic system, significantly enhancing stability. This results in several resonance structures that "spread out" the positive charge, reducing its intensity at any single point.
• Importance in alcohol dehydration: When acid-catalyzed dehydration forms a benzyl carbocation, its stability is greatly increased, compared to non-resonance-stabilized carbocations. This means dehydration reactions can proceed more readily.

In summary, when an alcohol can form a resonance-stabilized carbocation upon losing water, it tends to dehydrate more easily than those forming only simple alkyl carbocations.

Alcohol Dehydration Process

The alcohol dehydration process in an acid-catalyzed environment is an important reaction in organic chemistry. This process converts alcohols into alkenes and involves a few critical steps.

• Protonation: The first step is the protonation of the alcohol's hydroxyl group by an acid, often using sulfuric acid (H₂SO₄), forming an oxonium ion.
• Formation of carbocation: After protonation, the oxonium ion loses water, resulting in a carbocation intermediate.
• Deprotonation to form alkene: Finally, the carbocation loses a hydrogen ion, leading to the formation of a double bond and, subsequently, an alkene.

The ease and rate of these steps largely depend on the stability of the formed carbocation. Therefore, alcohols leading to resonance-stabilized carbocations, or particularly stable tertiary carbocations, will dehydrate more easily and rapidly.