Problem 40 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

The incorrect statement amongest the following is (a) The order of reduction reaction by LiAlH \(_{4}\) and that of the reduction by \(\mathrm{NaBH}_{4}\) each is two (b) Al-H bond is more ionic than B-H bond and hence LiAlH \(_{4}\) can produce larger concentration of hydride ion than that of \(\mathrm{NaBH}_{4}\) (c) LiAlH \(_{4}\) cannot reduce \(-\mathrm{NO}_{2}\) group (d) Inspite of very small rate constant of reduction of any carbonyl function other than aldehydes and ketones, the rate of reduction with LiAlH \(_{4}\) becomes appreciable due to large concentration of hydride ion

Step-by-Step Solution

Answer

The incorrect statement is (d).

Step 1: Analyze Statement (a)

Statement (a) claims that the order of reduction by LiAlH \(_{4}\) and NaBH \(_{4}\) each is two. This statement is generally accurate because both reagents act as sources of hydride ions, which reduce carbonyl compounds in a step-wise manner. The kinetics of these reduction reactions commonly suggest a second-order reaction, involving both the hydride donor and the carbonyl compound.

Step 2: Analyze Statement (b)

Statement (b) discusses the ionic character of Al-H and B-H bonds. Al-H bonds in LiAlH \(_{4}\) are indeed more ionic than B-H bonds in NaBH \(_{4}\), due to aluminum's lower electronegativity compared to boron. This ionic character contributes to a greater release of hydride ions by LiAlH \(_{4}\), which can influence reduction strength and efficiency.

Step 3: Analyze Statement (c)

Statement (c) claims that LiAlH \(_{4}\) cannot reduce nitro groups (-NO \(_{2}\) ). This is correct as LiAlH \(_{4}\) is a strong reducing agent but is not typically used to reduce strongly electron-withdrawing groups like nitro groups, which require different reduction conditions, such as catalytic hydrogenation.

Step 4: Analyze Statement (d)

Statement (d) states that LiAlH \(_{4}\) can speed up reductions due to its large hydride ion concentration. While LiAlH \(_{4}\) provides a high concentration of hydride ions, this statement implies a substantial rate increase that may not account for the practical limitations and selectivity issues of LiAlH \(_{4}\). The rate of reduction won’t necessarily become appreciable just due to increased hydride concentration if the rate constant is inherently small, particularly for esters or acids compared to aldehydes and ketones.

Key Concepts

Ionic Character of Al-H Bond

In LiAlH\(_{4}\), the bond between aluminum (Al) and hydrogen (H) is highly ionic. This occurs because aluminum has lower electronegativity compared to boron in NaBH\(_{4}\). With a higher difference in electronegativity, the Al-H bond has a greater ability to donate electrons to the hydrogen, enhancing its ionic character.

• The ionic nature helps LiAlH\(_{4}\) to release hydride ions more readily than NaBH\(_{4}\).
• This increased release of hydrides makes LiAlH\(_{4}\) a more powerful reducing agent.
• In reduction reactions, the degree of ionic character affects the efficiency and speed of the reaction.

Thus, understanding this ionic characteristic can help predict how efficiently a hydride ion can be transferred in a reduction process.

Kinetics of Reduction Reactions

Reduction reactions involving LiAlH\(_{4}\) follow a second-order kinetic model. This reaction involves two entities: the hydride donor and the carbonyl compound. Despite the second-order nature, various factors can influence the overall rate of reaction, such as the concentration of the reactants and the inherent nature of the reactive sites.

• LiAlH\(_{4}\) reacts efficiently with carbonyl groups like aldehydes and ketones.
• The substantial concentration of hydride ions available from LiAlH\(_{4}\) facilitates swift reactions.
• External conditions such as temperature and solvent can further influence the kinetics.

Understanding these kinetics helps in designing controlled reduction processes and predicting the reaction courses with precision.

Reduction of Nitro Groups

LiAlH\(_{4}\) is a strong reducing agent, but it has limitations when it comes to reducing nitro groups (-NO\(_{2}\)). Nitro groups are particularly stabilized through resonance and require conditions that LiAlH\(_{4}\) cannot provide effectively.

• LiAlH\(_{4}\) is more suited for reducing carbonyl groups rather than strong electron-withdrawing groups like nitro groups.
• Effective reduction of nitro groups often requires catalytic hydrogenation, a different technique from those employed with LiAlH\(_{4\).
• Alternative reducing agents are preferred for reducing nitro groups due to selectivity issues.

Recognizing these limitations is crucial for chemists when choosing suitable reagents for various reduction reactions.