Problem 138 - A Complete Resource Book in Chemistry for JEE Main

Question

How many of the following amines can be used for the symmetrical cleavage of diborane? \(\mathrm{CH}_{3} \mathrm{NH}_{2},\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH},\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{3} \mathrm{~N},\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N},\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{NH}\), \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NH}_{2}, \mathrm{C}_{5} \mathrm{H}_{5} \mathrm{~N}\), (i \(\left.-\mathrm{Pr}\right)_{3} \mathrm{~N}\), quinuclidine

Step-by-Step Solution

Answer

Six amines can be used for the symmetrical cleavage of diborane.

Step 1: Understand Diborane Cleavage

Diborane (_2H_6) is known for its symmetrical cleavage with amines. In this reaction, diborane forms adducts with amines via the lone pair present on the nitrogen atom. This is possible when the nitrogen atom in the amine molecule is easily accessible and not completely hindered by bulky groups.

Step 2: Identify Suitable Amines

To participate in the symmetrical cleavage of diborane, the amine must have a free and accessible lone pair on nitrogen. Amines with bulky substituents or those that are sterically hindered (like tertiary amines) typically do not react efficiently because the lone pair on nitrogen is not readily available.

Step 3: Analyze Each Amine

Consider each amine from the list: - CH_3NH_2 (Methylamine): Primary amine, free lone pair - (CH_3)_2NH (Dimethylamine): Secondary amine, free lone pair - (C_2H_5)_3N (Triethylamine): Tertiary amine, hindered lone pair - (CH_3)_3N (Trimethylamine): Tertiary amine, hindered lone pair - (C_2H_5)_2NH (Diethylamine): Secondary amine, free lone pair - AC_2H_5NH_2 (Ethylamine): Primary amine, free lone pair - C_5H_5N (Pyridine): Aromatic amine, available lone pair - i-Pr_3N (Triisopropylamine): Tertiary amine, highly hindered - Quinuclidine: Has accessible lone pair and can react

Step 4: Count Suitable Amines

From the analysis, suitable amines for symmetrical cleavage of diborane include primary and secondary amines where the lone pair on nitrogen is easily accessible and includes quinuclidine with its unique structure allowing reactivity. Pyridine, although not common for aliphatic cleavage, also participates in such cleavages due to its nitrogenous structure. Suitable Amines: 1. CH_3NH_2 2. (CH_3)_2NH 3. (C_2H_5)_2NH 4. C_2H_5NH_2 5. C_5H_5N 6. Quinuclidine

Key Concepts

Diborane Cleavage

Diborane ( B_2H_6) is a chemical compound known for its unique reaction capabilities, especially its cleavage in the presence of amines. The process of diborane cleavage involves the breaking apart of its bonds, allowing it to form adducts with other molecules. This occurs specifically with amines because they have a lone pair of electrons on their nitrogen atoms that can interact with the electron-poor boron atoms.

However, not all amines are capable of interacting with diborane effectively. The structure of the amine plays a critical role, as it must allow the lone pair on nitrogen to be accessible. Amines without this readily available lone pair cannot engage in successful diborane cleavage, which makes understanding the structural requirements of the amine crucial for predicting their reactivity.

Symmetrical Cleavage

Symmetrical cleavage of diborane happens when both boron atoms in diborane interact equally with an amine's lone pair electrons. This type of reaction is called 'symmetrical' because the cleavage leads to two equally divided parts of the borane structure, often forming stable complexes.

The symmetrical cleavage is most efficient when the incoming amine does not have bulky groups that hinder access to the nitrogen atom's lone pair. Bulky substituents, commonly found in tertiary amines, can block the lone pair from effectively interacting with diborane, thus preventing symmetrical cleavage from occurring.

When selecting an amine to participate in this reaction, one must consider how freely the nitrogen's lone pair is available for interaction to achieve symmetrical cleavage.

Accessible Lone Pair on Nitrogen

For diborane cleavage, the accessibility of the lone pair on nitrogen is essential. This lone pair is the site of action where the borane moiety is attracted, resulting in the cleavage of diborane and the formation of an adduct.

Amines with easily accessible lone pairs, typically found in primary or secondary amines, are better candidates for the reaction. Primary amines, with only one alkyl group, offer little hindrance around the nitrogen. Secondary amines, having two alkyl groups, can still maintain an available lone pair if these groups are not excessively large. On the other hand, tertiary amines are usually hindered by three alkyl substituents, making the lone pair difficult to access.

The structure of amines plays a pivotal role in determining the availability of the lone pair, thus influencing the amine's ability to participate in diborane cleavage.

Primary Amines

Primary amines are organic compounds where the nitrogen atom is attached to only one carbon-containing group (alkyl or aryl) and two hydrogen atoms. This simple structure leaves the lone pair on nitrogen quite exposed and free to serve as a site for chemical interactions, such as forming adducts with boron atoms in diborane.

Compounds like methylamine ( CH_3NH_2) and ethylamine ( C_2H_5NH_2) are examples of primary amines that can effectively engage with diborane due to their accessible nitrogen lone pair. These amines are less hindered by steric factors, allowing the lone pair to be available for bonding. As such, primary amines are often highly reactive towards diborane, enabling efficient symmetrical cleavage.

The key takeaway for primary amines is their straightforward structure that favors accessibility, leading to their effectiveness in diborane reactions.

Secondary Amines

Secondary amines have a nitrogen atom bonded to two carbon-containing groups, such as two alkyl groups, and one hydrogen atom. This structural characteristic can still offer some free access to the lone pair on nitrogen, allowing these amines to react with diborane, though sometimes with slightly lowered efficiency compared to primary amines.

Examples like dimethylamine ( (CH_3)_2NH) and diethylamine ( (C_2H_5)_2NH) are typical secondary amines that participate in the symmetrical cleavage of diborane due to their manageable steric effects from the two attached groups. These groups need to be not too large or bulky, which would otherwise hinder the nitrogen's lone pair

The effectiveness of secondary amines in diborane cleavage largely revolves around the balance of steric hindrance and the availability of the lone pair, making them versatile yet selective participants in chemical reactions like diborane cleavage.