Question

Consider the following species: 1\. \(\mathrm{RCHCH}_{3}\) 2\. \(\mathrm{RCH}_{2} \mathrm{CH}_{2}^{+} \quad\) 3. \(\mathrm{RCH}_{2} \mathrm{CH}_{2} \mathrm{OH}_{2}^{+}\) In the dehydration of primary alcohols, the correct sequence of formation of the species involved is (a) \(1,2,3\) (b) \(1,3,2\) (c) \(3,2,1\) (d) \(2,1,3\)

Short answer

(c) 3,2,1

Step-by-step solution

Understand Primary Alcohol Dehydration

The dehydration of primary alcohols involves the removal of water (H₂O) to form alkenes. This involves creating a carbocation intermediate.

Identify the Species Involved

The species given are 1. \(\mathrm{RCHCH}_{3}\) (alkene),2. \(\mathrm{RCH}_{2} \mathrm{CH}_{2}^{+}\) (carbocation - positively charged intermediate), and3. \(\mathrm{RCH}_{2} \mathrm{CH}_{2} \mathrm{OH}_{2}^{+}\) (oxonium ion - protonated alcohol).

Analyze the Formation Sequence

The dehydration process starts with the alcohol being protonated to form an oxonium ion, \(\mathrm{RCH}_{2}\mathrm{CH}_{2}\mathrm{OH}_{2}^{+}\) (species 3). Next, the water leaves, creating a carbocation \(\mathrm{RCH}_{2}\mathrm{CH}_{2}^{+}\) (species 2). Finally, a hydrogen is removed to form an alkene \(\mathrm{RCHCH}_{3}\) (species 1).

Determine Correct Sequence

Based on the understanding of primary alcohol dehydration, the species form in the sequence: start with species 3 (oxonium ion), then species 2 (carbocation), and finally species 1 (alkene). Thus the correct answer is (c) 3,2,1.

Key concepts

Carbocation Formation

A key step in the dehydration of primary alcohols is the formation of a carbocation. This happens when water is removed from the alcohol molecule. The process involves a few critical transformations. Initially, the alcohol is protonated to make it a better leaving group. As the alcohol loses water, a positive charge is left on the carbon, creating a carbocation. Carbocations are highly unstable and reactive. They serve as intermediates in many organic reactions, including dehydration and some types of rearrangement reactions. Their instability arises because carbon atoms do not favorably host a positive charge. Understanding carbocations is important as they represent a transition phase that leads to the formation of other molecules such as alkenes. In this stepwise conversion, recognizing how the carbocation forms and stabilizes (through bond rearrangements or interactions with other molecules) is crucial for predicting the outcome of a reaction.

Alkene Formation

The ultimate goal of dehydrating a primary alcohol is to produce an alkene. Following the formation of the carbocation, the next step involves creating the double bond characteristic of alkenes. The carbocation can lose a hydrogen atom, usually through the abstraction of a proton by a base, leading to the development of a double bond between the two adjacent carbons. Alkenes are unsaturated hydrocarbons, which means they have fewer hydrogen atoms than their saturated counterparts like alkanes. This lack of saturation implies alkenes are more reactive and can participate in a variety of chemical reactions such as addition and polymerization reactions. In practical terms, controlling this step by influencing conditions like temperature, or using specific catalytic agents, is important to ensure high yield and desired selectivity of the alkene product.

Oxonium Ion

The beginning of the dehydration process of a primary alcohol involves the formation of an oxonium ion. This compound arises when an alcohol is protonated, resulting in the temporary formation of a positively charged ion. The addition of a proton (from a strong acid) to the alcohol increases the susceptibility of the molecule to lose water and form an intermediate carbocation.The oxonium ion is essential in these reactions because it acts as a stepping stone that bridges the stable alcohol and the less stable carbocation. The stability of an oxonium ion is what allows it to initiate the dehydration process effectively.This ion typically has the general structure \

• \( \text{+} \underline{O}\text{---}R \),\where the oxygen atom bears the positive charge due to its additional proton.

Understanding the role of the oxonium ion helps in comprehending how molecules like primary alcohols can transform under acidic conditions to create more complex structures like alkenes.