Question

Consider the following compounds: 1\. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) 2\. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{3}\) 3\. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COH}\) These compounds are dehydrated by treatment with sulphuric acid. The correct sequence of increasing order of the reactivity of these three compounds towards dehydration is (a) \(1,3,2\) (b) \(1,2,3\) (c) \(2,1,3\) (d) \(3,1,2\)

Short answer

Option (b): 1, 2, 3.

Step-by-step solution

Identify the Alcohol Type

First, identify the type of alcohol for each compound. Compound 1 is butanol, a primary alcohol. Compound 2 is 2-butanol, a secondary alcohol. Compound 3 is tert-butanol, a tertiary alcohol.

Recall Dehydration Reactivity Order

Know that for alcohol dehydration, tertiary alcohols are more reactive than secondary alcohols, which are more reactive than primary alcohols when treated with sulfuric acid. This is because the stability of the carbocation formed during the dehydration increases in the order: primary < secondary < tertiary.

Rank the Compounds Based on Reactivity

Using the knowledge from Step 2, rank the compounds from least to most reactive: 1. 2. 3. Therefore, tert-butanol (Compound 3) will be the most reactive, followed by 2-butanol (Compound 2), and then butanol (Compound 1).

Match the Ranking with Given Options

Compare the determined order (1, 2, 3) for increasing reactivity to the options provided: - Option (a): 1, 3, 2 - Option (b): 1, 2, 3 - Option (c): 2, 1, 3 - Option (d): 3, 1, 2 The correct order, based on reactivity, is option (b): 1, 2, 3.

Key concepts

Reactivity of Alcohols

In the world of organic chemistry, the reactivity of alcohols towards dehydration is fundamental. Dehydration involves removing a water molecule from an alcohol to form an alkene. The process relies heavily on the type of alcohol involved: primary, secondary, or tertiary. Each type has distinctive properties affecting its reactivity:

• Primary Alcohols: These have only one alkyl group attached to the carbon bearing the -OH group. They are usually less reactive towards dehydration due to the formation of less stable primary carbocations.
• Secondary Alcohols: These have two alkyl groups, which offer some stabilization to the carbocation formed during dehydration.
• Tertiary Alcohols: These types are the most reactive in dehydration. The three alkyl groups provide significant stability to the intermediate carbocation, increasing reaction rates.

Understanding these properties helps predict the sequence of reactivity during dehydration.

Sulfuric Acid as Dehydrating Agent

Sulfuric acid ( H_2SO_4 ) is a key player in the dehydration of alcohols due to its properties as a strong acid and effective dehydrating agent. During dehydration, sulfuric acid protonates the hydroxyl group (-OH) of the alcohol. This transformation enhances the leaving group's ability to depart, forming a carbocation.
Beyond the formation of the intermediate carbocation, sulfuric acid also facilitates the removal of a water molecule, assisting in the formation of the double bond characteristic of alkenes.

• This approach effectively speeds up the elimination reaction.
• Sulfuric acid's dual role as both an acid and an absorbent of water ensures the reaction proceeds smoothly.

By understanding sulfuric acid's function, we appreciate how it aids in overcoming the energy barriers typically associated with slow reactions.

Carbocation Stability

One of the most critical factors in alcohol dehydration is carbocation stability. This intermediate is formed when the -OH group leaves, taking its electron pair, leaving behind a positively charged carbon ion called a carbocation. The stability of this ion is crucial because it directs the rate of reaction.
The stability of carbocations increases with the number of alkyl groups attached to the positively charged carbon:

• Primary Carbocations: Least stable, making primary alcohol dehydration slower.
• Secondary Carbocations: Have moderate stability due to the ability of surrounding groups to donate electron density.
• Tertiary Carbocations: Most stable due to maximum hyper-conjugation and electron donation from the surrounding groups.

The more stable the carbocation, the quicker it is for the dehydration to proceed, making tertiary alcohols the fastest to react in such conditions.

Primary, Secondary, Tertiary Alcohols

The classification of alcohols into primary, secondary, and tertiary forms depends on their structural configurations. It is essential to understand these definitions to predict reactivity in dehydration reactions.

• Primary Alcohols: The carbon attached to the hydroxyl group is linked to only one other carbon atom. Example: 1-butanol is a primary alcohol.
• Secondary Alcohols: Here, the carbon connected to the -OH group is bound to two other carbon atoms. Example: 2-butanol fits this category.
• Tertiary Alcohols: In this case, the carbon bonded with the hydroxyl group is connected to three carbon atoms. Example: tert-butanol is classified as tertiary.

Identifying these types allows chemists to anticipate the behavior during dehydration. Tertiary ones dehydrate more easily, taking less energy and time to turn into alkenes, making such classifications invaluable for conducting and understanding reactions efficiently.