Problem 89 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

If only one equivalent of \(\mathrm{NH}_{3}\) (one mole of \(\mathrm{NH}_{3}\) reacts with one mole of cyclohexyl chloride) was used, the reaction may not go to completion. Why? (a) \(\mathrm{NH}_{3}\) is not a strong nucleophile. Therefore, more \(\mathrm{NH}_{3}\) is needed. (b) \(\mathrm{Cl}\) is not a good leaving. Therefore, more \(\mathrm{NH}_{3}\) is needed. (c) Cyclohexyl group presents large steric hindrance. Therefore, more \(\mathrm{NH}_{3}\) is needed. (d) \(\mathrm{NH}_{3}\) will be protonated by \(\mathrm{HCl}\) as reaction proceeds. Therefore, the reaction will be incomplete.

Step-by-Step Solution

Answer

(d) \(\mathrm{NH}_3\) will be protonated by \(\mathrm{HCl}\) as the reaction proceeds, leading to incomplete reaction.

Step 1: Understand the Reaction Components

The reaction involves ammonia (\(\mathrm{NH}_3\)), which is a nucleophile, reacting with cyclohexyl chloride. During the reaction, the chlorine atom will leave as a chloride ion (\(\mathrm{Cl}^-\)), and \(\mathrm{NH}_3\) will potentially be converted to an alkylammonium chloride.

Step 2: Identify the Role of Ammonia

In this reaction, \(\mathrm{NH}_3\) functions as a nucleophile. A nucleophile is a species that donates an electron pair to form a chemical bond in relation to a positively charged or electron-deficient site, typically carbon in this case.

Step 3: Analyze Reaction Details

When \(\mathrm{NH}_3\) reacts with cyclohexyl chloride, the chlorine leaves and \(\mathrm{NH}_3\) forms a bond with the cyclohexyl group. The by-product of this reaction is hydrochloric acid (\(\mathrm{HCl}\)). However, \(\mathrm{HCl}\) can further react with \(\mathrm{NH}_3\) to form ammonium chloride (\(\mathrm{NH}_4^+\mathrm{Cl}^-\)).

Step 4: Consider Protonation of Ammonia

As the reaction progresses, the \(\mathrm{NH}_3\) present will start reacting with the formed \(\mathrm{HCl}\), leading to a conversion of \(\mathrm{NH}_3\) into \(\mathrm{NH}_4^+\). This reduces the amount of \(\mathrm{NH}_3\) available to proceed with the initial reaction with cyclohexyl chloride.

Step 5: Evaluate Why the Reaction May Not Complete

The protonation of \(\mathrm{NH}_3\) to form \(\mathrm{NH}_4^+\) in the presence of \(\mathrm{HCl}\) depletes the effective concentration of \(\mathrm{NH}_3\) available to react with any unreacted cyclohexyl chloride. This reduction in nucleophilic \(\mathrm{NH}_3\) leads to the incomplete reaction.

Key Concepts

Ammonia Reactivity

Ammonia (\(\mathrm{NH}_3\)) is known for being a readily available nucleophile, which means it is fond of donating a pair of electrons to form a bond. However, its effectiveness as a nucleophile isn't as high as other stronger nucleophiles. This is due to its relatively smaller size, which offers less repulsion of its lone pair from the molecule. Because it is a weaker nucleophile, \(\mathrm{NH}_3\)

• May not always effectively attack more sterically hindered centers.
• Needs to be used in larger quantities to drive a reaction to completion, particularly when competition is present, such as side reactions that remove it from the reaction system. Therefore, simply using one equivalent of ammonia might not suffice.

Understanding its limited reactivity helps in adjusting the reaction conditions to favor desired outcomes.

Protonation Effects

During the reaction between \(\mathrm{NH}_3\) and cyclohexyl chloride, a significant by-product is hydrochloric acid (\(\mathrm{HCl}\)). \(\mathrm{HCl}\) is a strong acid and has a tendency to protonate bases present in the reaction mixture, such as ammonia.When \(\mathrm{NH}_3\)gains a hydrogen ion (\(\mathrm{H}^+\)), it becomes ammonium ion (\(\mathrm{NH}_4^+\)). This

• Changes the initially nucleophilic structure of the molecule, diminishing its capability to further participate as a nucleophile.
• Decreases the concentration of the active \(\mathrm{NH}_3\), impacting the reaction efficiency and causing incomplete conversion if only one equivalent is used initially.

Due to protonation, an increase in the initial amount of ammonia can be beneficial to ensure enough of it remains to complete the desired reaction.

Steric Hindrance

Steric hindrance is an essential aspect in organic chemistry that affects how easily molecules can approach and react with each other. In the case of cyclohexyl chloride, the cyclohexyl group creates a bulky environment around the chloride leaving group.This steric bulk

• Makes it more challenging for the incoming \(\mathrm{NH}_3\) to access the carbon atom bound to the chlorine, since the surrounding atoms get in the way.
• Means that reactions involving bulky groups often require stronger or more abundant nucleophiles to overcome the physical barriers.

To successfully proceed with a nucleophilic substitution on a sterically hindered center, adjusting the reaction conditions, such as increasing nucleophile concentration, might be crucial for better results.

Leaving Groups in Organic Reactions

In any nucleophilic substitution reaction, the quality of the leaving group significantly impacts the reaction's success. Chloride (\(\mathrm{Cl}^-\)) is the leaving group involved when ammonia reacts with cyclohexyl chloride. Good leaving groups

• Are typically weak bases, as they stabilize the negative charge they assume upon bond cleavage.
• Facilitate bond breaking, enabling the nucleophile to form a new bond with the substrate.

Although chloride is considered an acceptable leaving group, sterically hindered sites can still challenge its departure. If it's not efficiently displaced, the reaction might stall or become incomplete. Therefore, it's crucial to consider the nature of the leaving group when designing the reaction setup, ensuring sufficient kinetic favorability to complete the substitution process.