Question

Which acid is used during formation of Alkyl Iodide by reaction between alcohol and \(\mathrm{Kl}\) (A) Conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (B) Conc. \(\mathrm{H}_{3} \mathrm{PO}_{4}\) (C) \(\mathrm{CH}_{3} \mathrm{COOH}\) (D) \(\mathrm{HCN}\)

Short answer

In conclusion, the acid used during the formation of alkyl iodide by the reaction between alcohol and Kl is \( \text{Conc. } H_2SO_4 \).

Step-by-step solution

Identify the reaction type

First, we need to identify that the reaction involved in the formation of alkyl iodide includes an acid-catalyzed substitution reaction between an alcohol and Kl.

Determine the role of the acid

We will now analyze the role of an acid in the reaction. The acid acts as a catalyst, which means it is necessary for the reaction to occur, but it will not be consumed during the reaction. The acid protonates the alcohol, increasing the electrophilic nature of the alcohol's carbon atom (making it more susceptible to nucleophilic attack by iodide ions).

Analyze the acidity of provided options

We will now analyze the acidity of the given options and decide which one would be suitable for the reaction: (A) Conc. H2SO4: Concentrated sulfuric acid is a strong mineral acid that is a powerful dehydrating agent and a strong electrophile. It is known to protonate alcohols and assist in nucleophilic substitution reactions. This is a potential candidate. (B) Conc. H3PO4: Concentrated phosphoric acid is also a mineral acid, albeit less strong than sulfuric acid. It can act as an acid catalyst for substitution reactions involving alcohols, but it is typically less effective than sulfuric acid. This is a less likely candidate. (C) CH3COOH: Acetic acid is a weak carboxylic acid that is not a suitable candidate for the protonation of alcohols on its own. In addition, it cannot produce a strong nucleophile capable of displacing an alcohol group. (D) HCN: Hydrogen cyanide is a weak acid that is not capable of protonating alcohols and facilitating the formation of alkyl iodides.

Choose the right acid

After analyzing the acidity and reactivity of the provided options, we can conclude that option (A) Conc. H2SO4 is the correct answer as it is the most suitable acid for protonating the alcohol and facilitating the formation of alkyl iodide. In conclusion, the acid used during the formation of alkyl iodide by the reaction between alcohol and Kl is Conc. H2SO4.

Key concepts

Acid-Catalyzed Reaction

To understand the formation of alkyl iodides, we must first grasp the concept of an acid-catalyzed reaction. This type of reaction involves the use of an acid to speed up or facilitate the process. In our scenario, concentrated sulfuric acid (\( \mathrm{H}_2\mathrm{SO}_4 \)) acts as the acid catalyst.
The acid does not directly partake in the transformation but plays a crucial role by making the reaction environment more reactive.
This occurs because the acid increases the ability of a molecule to participate in the reaction.

In the synthesis of alkyl iodide, our primary goal is to replace the alcohol group with an iodide ion. Here, the acid helps by creating the right conditions for substitution to occur. It modifies the alcohol structure, making it more susceptible to attack by iodide ions. Thus, the right choice of acid is vital for ensuring an efficient reaction.

Protonation of Alcohols

Protonation is a key step when synthesizing alkyl iodides from alcohols. It is where an acid donates a proton (\(H^+\)) to the alcohol molecule. This donation is significant because it turns the alcohol's hydroxyl group (\(-OH\)) into a better leaving group, a requirement for substitution reactions.
The acid's proton attaches to the oxygen in the alcohol molecule, transforming it into a water molecule (\( \text{H}_2\text{O} \)), which exits easily from the molecule.
As a result of protonation, the carbon atom bonded to the original hydroxyl group becomes highly electrophilic.

Electrophilicity describes an atom's ability to attract nucleophiles (electron pair donors). Increasing this property enhances the molecule's readiness to undergo nucleophilic attack. Concentrated sulfuric acid is particularly effective at this because it is a strong acid, able to donate protons readily. This makes it a perfect candidate for facilitating the alcohol's conversion into a more reactive compound.

Nucleophilic Substitution Reaction

Nucleophilic substitution is the action where a nucleophile replaces a leaving group on a molecule. In our synthesis of alkyl iodides, the iodide ion acts as the nucleophile. It approaches the positively charged carbon atom, formed after protonation of the alcohol.
At this point, the iodide ion successfully displaces the water molecule.This results in the formation of the desired alkyl iodide.

Two types of nucleophilic substitution reactions exist, known as \( \text{S}_N1 \) and \( \text{S}_N2 \). The alkyl iodide formation typically follows the \( \text{S}_N2 \) mechanism. This mechanism entails a one-step process where the nucleophile attacks from the backside. This allows the simultaneous formation of a new bond as the old bond breaks.

Thanks to the protonation and increased carbon electrophilicity, the iodide ion has an easier path to replace the leaving group, resulting in efficient synthesis of alkyl iodide. This highlights why choosing the right acid is crucial for this type of chemical reaction.