Question

For the complete reduction of \(5.8 \mathrm{~g}\) of acetone to isopropyl alcohol, the quantity of \(\mathrm{LiAlH}_{4}\) required (assuming chemical yield to be \(100 \%\) ) is approximately [Mass: \(\mathrm{Li}=6.9, \mathrm{Al}=27\) ] (A) \(3.8 \mathrm{~g}\) (B) \(0.95 \mathrm{~g}\) (C) \(1.9 \mathrm{~g}\) (D) \(15.2 \mathrm{~g}\)

Short answer

The mass of LiAlH4 required for the complete reduction of 5.8 g of acetone to isopropyl alcohol, assuming 100% chemical yield, is approximately \(3.8 \, g\).

Step-by-step solution

Write down the balanced chemical equation

First, we need to write the balanced equation for the conversion of acetone to isopropyl alcohol using LiAlH4 as a reducing agent. \(C_3H_6O (acetone) + LiAlH_4 \rightarrow C_3H_8O (isopropyl\, alcohol) + \, ...\) (Other products). Since LiAlH4 is the limiting reagent, we need to determine how much is required for the complete reduction of given quantity of acetone.

Calculate the moles of acetone

We are given the mass of acetone as 5.8 g. We can use this to calculate the moles of acetone using its molar mass. The molar mass of acetone (C3H6O) is: Molar mass of acetone = \(3(12.01 \,g/mol) + 6(1.01 \,g/mol) + 1(16.00 \,g/mol)= 58.08 \, g/mol\) So, moles of acetone = \(\frac{5.8 \, g}{58.08 \, g/mol} = 0.1 \, moles\)

Calculate the moles of LiAlH4 required

From the balanced chemical equation, the stoichiometric ratio of acetone to LiAlH4 is 1:1. Therefore, for complete reduction of 0.1 moles of acetone, 0.1 moles of LiAlH4 are needed.

Calculate the mass of LiAlH4 required

Now we calculate the mass of the 0.1 moles of LiAlH4 required, using the given atomic masses of Li and Al: Molar mass of LiAlH4 = \((6.9 \,g/mol + 27 \,g/mol + 4(1.01 \,g/mol)) = 37.94 \, g/mol\) Mass of LiAlH4 required = \(0.1 \, moles \times 37.94 \, g/mol = 3.794 \, g\) The mass of LiAlH4 required is approximately 3.8 g, which is option (A).

Key concepts

Acetone to Isopropyl Alcohol Conversion

The conversion of acetone to isopropyl alcohol is an example of a reduction reaction in organic chemistry. In this process, acetone (C\(_3\)H\(_6\)O) undergoes a transformation to become isopropyl alcohol (C\(_3\)H\(_8\)O). This change involves the addition of hydrogen, a hallmark of reduction reactions, specifically converting the carbonyl group of acetone into a secondary alcohol.

Here’s how it happens in simple terms:

• Acetone has a carbon-oxygen double bond, known as a carbonyl group.
• In reduction, this double bond is broken, and additional hydrogen atoms are introduced.
• This results in the formation of a hydroxyl group (OH) in place of the double-bonded oxygen, converting acetone to isopropyl alcohol.

During the reaction, we often require a specific reducing agent and conditions to facilitate the conversion safely and efficiently, which brings us to the role of LiAlH\(_4\).

Stoichiometry in Organic Chemistry

Stoichiometry is a major concept in chemistry that involves calculating the quantitative relationships of the reactants and products in a chemical reaction. In organic chemistry, stoichiometry helps determine the correct proportions for substances to ensure complete reaction.

For reducing acetone to isopropyl alcohol, understanding stoichiometry is key for calculating the needed amount of reducing agent, LiAlH\(_4\). Here’s what you need to know:

• The reaction is balanced, meaning each type of atom is present in equal numbers on both sides of the equation.
• In most cases, including this one, the stoichiometry ratio is 1:1 for acetone to LiAlH\(_4\). This means one mole of acetone requires one mole of LiAlH\(_4\) to be completely reduced.
• By knowing the molar mass of the compounds involved, chemistry students can convert mass to moles, allowing them to use stoichiometry to determine the precise quantities required.

Using stoichiometry ensures not only efficient use of reactants but also helps avoid waste and prevent unnecessary chemical consumption, promoting both cost-effectiveness and safety in chemical processes.

Use of LiAlH4 as a Reducing Agent

Lithium aluminium hydride (LiAlH\(_4\)) is a powerful reducing agent widely used in organic chemistry. It’s particularly effective for reducing carbonyl compounds, such as ketones and aldehydes, to alcohols.

LiAlH\(_4\) is unique due to several reasons:

• High reactivity, which makes it effective in reducing even less reactive carbonyl groups.
• It functions by providing hydride ions (H\(^-\)), which are important for reducing the carbon-oxygen double bonds in carbonyl compounds to single hydrogen-oxygen bonds, forming alcohols.
• Despite its effectiveness, LiAlH\(_4\) is sensitive to moisture and air, so it must be handled under dry conditions and often requires careful storage and handling.

In the conversion of acetone to isopropyl alcohol, as discussed, LiAlH\(_4\) is crucial as it facilitates the rapid addition of hydrogen to the acetone, thereby achieving the desired product with high efficiency when used in the correct stoichiometric amounts.