Question

Choose the incorrect option about \(\mathrm{LiAlH}_{4}\). (A) It is a very strong nucleophilic reducing agent (B) It can reduce acid to corresponding alcohol (C) It reduces to \(\mathrm{R}-\mathrm{CH}_{2}-\mathrm{NH}_{2}\) (D) One mole of \(\mathrm{LiAlH}_{4}\) can reduce only one mole of ester

Short answer

Option (D) is incorrect because one mole of LiAlH4 can actually reduce four moles of ester.

Step-by-step solution

Option (A) - Nucleophilic reducing agent

Lithium aluminum hydride (LiAlH4) is a strong nucleophilic reducing agent. It can donate hydrides (H-) to electrophilic substrates, resulting in the reduction of the substrate molecule. Therefore, this statement is correct.

Option (B) - Reduction of acids to alcohols

LiAlH4 is capable of reducing carboxylic acids to their corresponding primary alcohols. This occurs through a two-step process involving a nucleophilic attack by hydride on the carbonyl carbon and protonation of the alkoxide intermediate. Thus, this statement is also correct.

Option (C) - Reduction to R-CH2-NH2

LiAlH4 has the ability to reduce certain functional groups, such as amides, to an amine with additional hydrogens added onto the carbonyl carbon, resulting in the formation of R-CH2-NH2. This statement is correct.

Option (D) - Reduction of one mole of ester

Contrary to this statement, one mole of LiAlH4 can reduce four moles of ester. The reducing agent has four hydride (H-) ions that can be donated to electrophilic carbonyl groups of the ester, converting them into their corresponding alcohols. Therefore, this statement is incorrect. In conclusion, option (D) is the incorrect statement about LiAlH4.

Key concepts

Nucleophilic Reducing Agent

Lithium aluminum hydride (LiAlH4) is a highly efficient nucleophilic reducing agent. It is known for its aggressive ability to donate hydride ions \(\text{(} \text{H}^− \text{)}\), which act as nucleophiles. Hydrides are negatively charged hydrogen ions with extra electrons, enabling them to attack positively charged or electron-deficient centers (electrophiles) in other molecules.
This property makes LiAlH4 an excellent reagent for reducing a wide variety of functional groups in organic chemistry. Its strength and reactivity stem from the fact that each LiAlH4 molecule holds four hydride ions, capable of reducing multiple molecules or functional groups at once under appropriate conditions.
When working with LiAlH4, it is crucial to handle it with care, given its highly reactive nature, especially with moisture.

Reduction of Carboxylic Acids

LiAlH4 is prominently used for the reduction of carboxylic acids to primary alcohols. This transformation is vital in synthetic organic chemistry, where altering the functional group can be crucial for further reactions or the synthesis of different compounds. The mechanism through which LiAlH4 reduces carboxylic acids involves a couple of key steps:

• First, the hydride ion from LiAlH4 attacks the carbonyl carbon present in the carboxylic acid. This step results in the formation of an alkoxide intermediate by breaking the C=O bond
• Following this, the reaction typically involves protonation, usually through a subsequent acid workup, leading to the production of the alcohol

This conversion is highly efficient, but care must be taken to ensure that complete reduction occurs, as partial reductions can sometimes lead to undesired products.

Reduction Mechanism

Understanding the reduction mechanism of LiAlH4 helps to comprehend why it is so effective in transforming many functional groups. The core of LiAlH4's reduction power lies in its capability to donate hydride ions effectively to various electrophiles.
The general simplification of the mechanism involves these pivotal steps:

• Hydride ion donation: The negatively charged hydride ion approaches the electrophilic center, often a carbon in carbonyl groups
• Attack and bond formation: The hydride ion attacks the electrophile, causing the double bond to break and form a new carbon-hydrogen bond
• Intermediate formation: An alkoxide or similar intermediate forms, stabilizing the reactive molecule

The reaction often concludes with protonation, allowing the completion of the reduction and converting the intermediate into the final reduced product, such as a primary alcohol or an amine, depending on the initial functional group.

Ester Reduction

Ester reduction by LiAlH4 is a well-known reaction in organic chemistry, turning esters into their respective alcohols. It underscores LiAlH4's strength as a reducing agent, being able to handle even more complex substrates than simple carbonyl groups.
The process operates similarly to the reduction of carboxylic acids, with the following typical steps:

• The hydride ion attacks the carbonyl carbon of the ester, disrupts the C=O bond, and forms an alkoxide ion
• This intermediate can then rearrange, often culminating in two alcohol products; one derived from the original ester portion, and another from the alkoxy part of the ester

LiAlH4 is thus highly efficient, as one molecule can theoretically convert up to four moles of ester due to its four hydride ions, making it a cost-effective reagent for large-scale organic synthesis. Nonetheless, this reaction demands meticulous control to prevent over-reduction and generation of unwanted by-products.