Question

Alcohol can be coverted into alkene by reaction with (A) conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (B) \(\mathrm{KHSO}_{4}\) (C) conc. \(\mathrm{H}_{3} \mathrm{PO}_{4}\) (D) alc. KOH

Short answer

The correct reagent for converting alcohol into alkene is (D) alc. KOH, as it proceeds via the E2 mechanism, eliminating the possibility of rearrangements and giving a cleaner product with fewer rearrangement possibilities.

Step-by-step solution

Rule out options with unsuitable reagents for the conversion of alcohol to alkene

First, we will consider which of the given reagents are not suitable for converting alcohols to alkenes. Since alcohols don't undergo dehydration reactions easily, we can rule out any reagents that might not provide the necessary conditions for the dehydration to occur. Option (B) \(\mathrm{KHSO}_{4}\) is not suitable for the conversion of alcohol to alkene, as it is not a strong enough dehydrating agent to facilitate the reaction. Option (C) conc. \(\mathrm{H}_{3} \mathrm{PO}_{4}\) is not suitable for the conversion of alcohol to alkene. Although phosphoric acid can act as a dehydrating agent, it is milder as compared to sulfuric acid and not efficient for dehydration of alcohols. Now, we are left with options (A) and (D).

Compare the remaining two reagents and their mechanisms for the conversion of alcohol to alkene

Now we will compare the mechanisms of the remaining two reagents, conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (option A) and alc. KOH (option D), and their suitability in the conversion of alcohol to alkene. Option (A) conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\): Concentrated sulfuric acid is a strong acid and an effective dehydrating agent. It can undergo the following mechanism to convert alcohol to alkene via the E1 mechanism: 1. Protonation of the alcohol oxygen 2. Loss of water to form a carbocation 3. Loss of proton from a neighboring carbon atom to form a double bond (alkene formation) Option (D) alc. KOH: Alcoholic potassium hydroxide is a strong base. It can undergo the following mechanism to convert alcohol to alkene via the E2 mechanism: 1. De-protonation of a neighboring carbon atom to the alcohol oxygen 2. Simultaneous breaking of the C-O bond and formation of a C=C double bond (alkene formation)

Determine the correct option for the conversion of alcohol to alkene

Both conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) and alc. KOH can convert alcohol to alkene, but the question asks for the most suitable reagent. Option (A) conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) might lead to rearrangements if the reaction proceeds through carbocation formation which may give rise to multiple products. However, rearrangements can be controlled and minimized under appropriate reaction conditions. Option (D) alc. KOH eliminates the possibility of rearrangements since it proceeds via an E2 mechanism. It is generally preferred for primary and secondary alcohols and gives a cleaner product with fewer rearrangement possibilities. Therefore, the correct option for the conversion of alcohol to alkene is: (D) alc. KOH

Key concepts

Dehydration Reaction

Dehydration reactions are a class of chemical processes that involve the removal of water (H2O) from a molecule. They are a vital part of organic chemistry, particularly in the transformation of alcohols into alkenes.
In the context of converting alcohol to alkene, dehydration reacts to eliminate a water molecule from the alcohol compound. This typically requires an acid catalyst to proceed. Common catalysts for this kind of reaction are sulfuric acid (H2SO4) and phosphoric acid (H3PO4). The process not only eliminates water but also forms a double bond in the resulting alkene, significantly altering both the structure and properties of the original alcohol.
Understanding dehydration is crucial for comprehending the mechanisms behind the conversion of alcohol to alkene. The choice of reagent can have profound effects on the reaction's path, efficiency, and the purity of the final product, as well as the potential for additional chemical rearrangements or side reactions.

E1 Mechanism

The E1 mechanism stands for 'Unimolecular Elimination' and is one of the two primary mechanisms in which dehydration reactions can occur. This process is characterized by its two-step reaction mechanism:

Step 1: Formation of a Carbocation

The alcohol is first protonated, thanks to the acidic conditions provided by the catalyst. This protonation makes the departure of water (as a leaving group) from the molecule more favorable, leading to the formation of a carbocation - a highly reactive intermediate with a positively charged carbon atom.

Step 2: Alkene Formation

In the second step, a base - which can be part of the solvent or the acid itself - removes a hydrogen ion (proton) from a carbon atom adjacent to the carbocation. The electrons left behind from this deprotonation move towards the carbocation, resulting in the formation of a double bond and the creation of the alkene.
The E1 mechanism tends to be more common with secondary and tertiary alcohols, as these can more easily stabilize the carbocation intermediate. One potential downside of the E1 mechanism is the possibility of carbocation rearrangement, which can lead to multiple products and reduce the purity of the desired alkene.

E2 Mechanism

Unlike the E1 mechanism, the E2 mechanism - or 'Bimolecular Elimination' - involves a single concerted step. This mechanism is typically favored in primary and secondary alcoholes, where carbocation formation is less favorable.

Concerted Elimination

In the E2 mechanism, the base (such as alc. KOH, where 'alc.' stands for 'alcoholic' or 'in an alcohol solution') abstracts a proton from the carbon adjacent to the carbon bearing the hydroxyl group of the alcohol. Simultaneously, the electrons from the C-H bond being broken are used to form a double bond between the two carbon atoms as the alcohol oxygen leaves as a water molecule. This process does not involve the formation of an intermediate, reducing the chances of side reactions or rearrangements.
Opting for an E2 mechanism can provide a cleaner reaction yielding the desired alkene with fewer by-products. The choice of a strong base like alcoholic KOH facilitates this mechanism, making it a robust method for synthesizing alkenes from alcohols.