Problem 50 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Arrange the following alcohols in order of increasing ease of dehydration (i) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (ii) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{OH}\) (iii) \(\mathrm{Cl}_{3} \mathrm{CCH}_{2} \mathrm{OH}\) (iv) \(\mathrm{F}_{3} \mathrm{CCH}_{2} \mathrm{OH}\) (a) \(\mathrm{ii}<\mathrm{i}<\mathrm{iv}<\mathrm{iii}\) (b) \(\mathrm{iv}<\mathrm{iii}<\mathrm{ii}<\mathrm{i}\) (c) \(\quad\) iv \(<\) iii \(

Step-by-Step Solution

Answer

The correct order is (iv) < (iii) < (i) < (ii), matching option (c).

Step 1: Understand Dehydration of Alcohols

Dehydration of alcohols involves the removal of water (0") from the alcohol molecule. It typically follows the E1 mechanism, favored by acid catalysis and formation of a stable carbocation intermediate.

Step 2: Analyze Each Alcohol

In dehydration, primary carbocations form less readily than secondary and tertiary carbocations. Stabilizing groups like phenyl or electron-withdrawing groups (e.g., 02 03) on the alpha carbon can also affect carbocation stability.

Step 3: Assess Trichloromethyl Alcohol 03020203

Trichloromethyl alcohol 03020203) has strong electron-withdrawing chlorine atoms, which destabilize the carbocation, hence it is difficult to dehydrate.

Step 4: Evaluate Trifluoromethyl Alcohol 060603

Trifluoromethyl alcohol 060603) also has electron-withdrawing groups but is less effective compared to chlorine, making it still difficult but slightly easier to dehydrate than 03020203.

Step 5: Consider Benzyl Alcohol 020303

Benzyl alcohol (020303) forms a relatively stable benzyl carbocation due to resonance stabilization, making it easier to dehydrate than 03020203 and 060603.

Step 6: Examine Ethanol 030303

Ethanol (030303) is a simple primary alcohol without additional stabilizing groups, making it easier to dehydrate than those with electron-withdrawing groups but more difficult than benzyl alcohol.

Step 7: Arrange in Order of Increasing Ease of Dehydration

Based on analysis, the order of increasing ease of dehydration is 03020203 < 060603 < 030303 < 020303.

Key Concepts

E1 mechanism

In organic chemistry, the E1 mechanism refers to a two-step elimination reaction that plays a key role in the dehydration of alcohols. To break it down simply, in this process, alcohols lose a water molecule. This typically occurs in the presence of an acid catalyst. Here's how it goes:

• **Step One:** The alcohol is protonated by the acid. This means the hydroxyl group (\(-OH\)) of the alcohol forms a bond with a hydrogen ion (\(H^+\)) from the acid, creating a good leaving group, water (\(H_2O\)).
• **Step Two:** Upon losing water, a carbocation intermediate is formed. This carbocation is crucial for the E1 mechanism.
• **Step Three:** Finally, a base in the mixture extracts a proton from a neighboring carbon, leading to the formation of an alkene.

This sequence highlights why E1 is termed an elimination ('E') and 'unimolecular' ('1') process. The unimolecular step refers to the rate-limiting step where only one molecular entity, the carbocation, is formed.

carbocation stability

Central to the E1 mechanism is the stability of the carbocation intermediate. Some carbocations are more stable than others, and this stability determines how easily an alcohol can undergo dehydration.

• **Tertiary carbocations** are typically the most stable. They are surrounded by more alkyl groups, which donate electron density through hyperconjugation and inductively stabilize the positive charge.
• **Secondary carbocations** are moderately stable for similar reasons but have fewer alkyl groups compared to tertiary ones.
• **Primary carbocations** are the least stable as they lack the electron-donating support of multiple alkyl groups.

Understanding carbocation stability is essential when arranging alcohols in order of their dehydration ease. For example, when phenyl (\(C_6H_5\)) or benzyl groups are present, they provide additional stabilization via resonance. Another factor to consider is whether any substituents, like electronegative atoms, are close to the site of the positive charge. Such electron-withdrawing groups can affect stability, shifting the dehydration trend.

electron-withdrawing groups

Electron-withdrawing groups (EWGs) play a significant role in determining the ease of dehydration of alcohols through the E1 mechanism. These groups pull electron density away from surrounding atoms, impacting carbocation stability.

• When an alcohol contains EWGs like fluorine or chlorine as substituents on the alpha carbon, these effect carbocation stability differently.
• **Chlorine** is a stronger electron-withdrawing group than fluorine, making alcohols with chlorine substituents less reactive due to decreased carbocation stability.
• The presence of EWGs tends to destabilize carbocations because they increase the positive charge density, making it difficult for stable carbocations to form.

This is why trichloromethyl alcohol and trifluoromethyl alcohol are more challenging to dehydrate compared to ones without such electron-withdrawing groups, like benzyl alcohol. Understanding how these groups affect electron distribution helps predict and explain reaction trends.