Question

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

Short answer

5 amines can cleave diborane.

Step-by-step solution

Understanding Symmetrical Cleavage of Diborane

Diborane (\(B_2H_6\)) can undergo symmetrical cleavage with amines, resulting in the formation of two \(BH_3\) molecules bonded to two amine molecules. This reaction requires amines which have a lone pair of electrons to donate to the empty orbitals of boron atoms.

Identify Amines with Available Lone Pairs

Analyze each given amine:- \(\text{CH}_3\text{NH}_2\): Contains a nitrogen with a lone pair and can participate in cleavage.- \((\text{CH}_3)_2\text{NH}\): Contains a nitrogen with a lone pair and can participate in cleavage.- \((\text{C}_2\text{H}_3)_3\text{N}\) and \((\text{CH}_3)_3\text{N}\): Tertiary amines with no hydrogen bonded to N, harder to cleave without steric bulk.- \((\text{C}_2\text{H}_3)_2\text{NH}\): Contains a nitrogen with a lone pair and can participate in cleavage.- \(\text{C}_2\text{H}_5\text{NH}_2\): Contains a nitrogen with a lone pair and can participate in cleavage.- \(\text{C}_5\text{H}_5\text{N}\): Pyridine, nitrogen lone pair is available, but not as reactive for cleavage.- \((\text{i-Pr})_3\text{N}\): Often too sterically hindered.- Quinuclidine: Tertiary amine, lone pair on nitrogen but with unique steric properties that may hinder cleavage. However, generally reactive with symmetrical cleavage.

Determine Suitable Candidates for Cleavage

Among the amines, primary and secondary amines like \(\text{CH}_3\text{NH}_2\), \((\text{CH}_3)_2\text{NH}\), \((\text{C}_2\text{H}_3)_2\text{NH}\), and \(\text{C}_2\text{H}_5\text{NH}_2\) are typically suitable due to optimal lone pair availability without excessive steric hindrance. Quinuclidine, despite being tertiary, can still typically react due to its unique structure.

Count the Reactive Amines

From the analysis, the suitable amines are \(\text{CH}_3\text{NH}_2\), \((\text{CH}_3)_2\text{NH}\), \((\text{C}_2\text{H}_3)_2\text{NH}\), \(\text{C}_2\text{H}_5\text{NH}_2\), and quinuclidine. Therefore, 5 out of the given amines can participate effectively in the symmetrical cleavage of diborane.

Key concepts

Symmetrical cleavage of diborane

The symmetrical cleavage of diborane, represented by the chemical formula \(B_2H_6\), involves the breaking of its central B-B bond. This process results in the formation of two \(BH_3\) molecules. These molecules, in turn, bond with amines, forming stable complexes. This reaction is an excellent example of electron pair donation where the electron-deficient boron atoms react with amines that have an available lone pair of electrons.For this reaction to be successful, the diborane needs an amine with a lone pair ready to interact with its empty boron orbitals. With this setup, diborane can undergo cleavage seamlessly, resulting in two boron-nitrogen bonds. This is the core of borane chemistry, highlighting the electron-deficient nature of boron and the electron-donating ability of nitrogen in amines.

Lone pair availability

Amines are nitrogen-containing compounds capable of donating a lone pair of electrons. This lone pair is crucial for interacting with molecules like diborane. Without this available lone pair, the crucial bond formation between boron and nitrogen cannot take place. - **Amines with lone pair:** These are typically primary and secondary amines, where the nitrogen atom has an available lone pair due to less steric hindrance from surrounding groups. - **Tertiary amines:** While they also have lone pairs, the presence of bulky groups can reduce their effectiveness in reactions like the symmetrical cleavage of diborane. The electronic environment around nitrogen in a tertiary amine is more crowded, making electron pair donation less efficient compared to less hindered amines.

Primary and secondary amines

Primary and secondary amines play distinct roles in chemical reactions due to their structure. In primary amines, \(\text{NH}_2\) is bonded to one alkyl or aryl group. They are generally more reactive than secondary and tertiary amines because the nitrogen atom is less surrounded by other groups.- **Primary amines:** They have an open and accessible lone pair of electrons. For example, \(\text{CH}_3\text{NH}_2\) and \(\text{C}_2\text{H}_5\text{NH}_2\) have lone pairs that are readily available for reactions.- **Secondary amines:** These have two groups attached to the nitrogen. For instance, \((\text{CH}_3)_2\text{NH}\) and \((\text{C}_2\text{H}_3)_2\text{NH}\) still maintain a lone pair but with slightly more steric effects. However, they are often competent for reactions like the cleavage of diborane due to their moderate steric hindrance.

Steric hindrance

Steric hindrance refers to the slowing or blocking of chemical reactions due to the size of groups within a molecule. In the context of amines reacting with diborane, steric hindrance can significantly affect the reaction's outcome. Large groups attached to the nitrogen atom can impede the approach of diborane, inhibiting efficient bond formation.- **Impact on reaction:** For example, tertiary amines like \((\text{CH}_3)_3\text{N}\) face greater steric hindrance as the nitrogen atom is surrounded by bulky groups, which can prevent the lone pair from reaching out to the boron atom in diborane.- **Balancing reactivity:** Amines like quinuclidine, despite being tertiary, have a unique structure that may accommodate boron atoms differently, sometimes mitigating the usual steric hindrance expected in other tertiary amines. Ensuring the right configuration and size of substituents is key to optimizing reactivity in such scenarios.

Reactive amines

Reactive amines are those which efficiently donate their lone pair for bond formation in chemical reactions. In the context of amines and diborane, reactivity depends on both the availability of the lone pair and the structural environment surrounding the nitrogen.- **Identifying reactive amines:** In our set of amines, \(\text{CH}_3\text{NH}_2\), \((\text{CH}_3)_2\text{NH}\), \((\text{C}_2\text{H}_3)_2\text{NH}\), \(\text{C}_2\text{H}_5\text{NH}_2\), and quinuclidine are reactive. They possess the necessary lone pair, and their structural configurations allow effective interaction with diborane.- **Structural favorability:** The primary reason these amines react well is due to their relatively less crowded environment around the nitrogen atom, ensuring that electron-rich areas are accessible for reaction. Understanding these dynamics helps in predicting and utilizing reactive amines effectively in chemical synthesis.