Question

On catalytic hydrogenation, a compound \(\mathrm{X}\left(\mathrm{C}_{7} \mathrm{H}_{12}\right)\) absorbs 2 mol of hydrogen and yields 2 -methylhexane. On treatment with \(\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{OH}, \mathrm{X}\) gives a precipitate which contains silver and which regenerates \(X\) on treatment with dilute \(\mathrm{HNO}_{3}\). The structure of \(\mathrm{X}\) is

Short answer

X is 2-methyl-1-hexyne (\text{CH}_3\text{-CH}_2\text{-CH}_2\text{-C}\text{-}\text{C}\text{-CH}_3 ).

Step-by-step solution

Understand the molecular formula of the compound

The compound X has the molecular formula \(\text{C}_7\text{H}_{12}\).

Analyze Hydrogenation Reaction

Since the compound absorbs 2 moles of hydrogen (\(\text{H}_2 \)), it indicates that there are two degrees of unsaturation corresponding to two double bonds or one triple bond.

Consider the product 2-methylhexane

The product indicates that X must be a compound that results in the formation of 2-methylhexane upon hydrogenation. 2-methylhexane has no double or triple bonds.

Reaction with Ammoniacal Silver Nitrate

X reacts with \(\text{Ag}\text{(NH}_3\text{)}_2\text{OH}\) to form a precipitate containing silver. This implies that X contains one or more triple bonds.

Regeneration of compound X with dilute \(\text{HNO}_3\)

The intermediate compound formed with \(\text{Ag}\text{(NH}_3\text{)}_2\text{OH}\) regenerates X upon treatment with dilute \(\text{HNO}_3\). This indicates that the triple bond might be terminal as it forms a silver acetylide.

Derive the Structure of X

Considering all the clues, X has a molecular formula \(\text{C}_7\text{H}_{12}\), absorbs 2 mol of \(\text{H}_2\) and forms 2-methylhexane upon hydrogenation. The structure of X is 2-methyl-1-hexyne.

Key concepts

Degrees of Unsaturation

When analyzing a compound's structure, identifying the degrees of unsaturation (DU) is crucial. DU helps determine the number of rings, double bonds, or triple bonds present in a molecule.

For compound \(\text{X (C}_7\text{H}_{12})\), the DU can be calculated using the formula: \(\text{DU} = \frac{(2C + 2 - H)}{2}\). Here, \(\text{C} = 7\) and \(\text{H} = 12\):

\[ \text{DU} = \frac{(2(7) + 2 - 12)}{2} = 2 \]

This result indicates that the compound has 2 degrees of unsaturation, which can correspond to two double bonds, one triple bond, or a ring structure combined with a double bond.

Molecular Formula Analysis

Analyzing the molecular formula of a compound is the starting point in understanding its structure. For compound \(\text{X (C}_7\text{H}_{12})\), the molecular formula suggests it contains 7 carbon atoms and 12 hydrogen atoms.

In the context of degrees of unsaturation, this formula implies that there are fewer hydrogen atoms than in a fully saturated counterpart (which would be \(\text{C}_7\text{H}_{16}\)). The difference of 4 hydrogens hints at the presence of 2 degrees of unsaturation. This reinforces the earlier calculation, suggesting either multiple bonds or ring structures.

Understanding this step is essential before moving on to analyze reactions involving the compound.

Reaction with Ammoniacal Silver Nitrate

The reaction of compound X with \(\text{Ag}\text{(NH}_3\text{)}_2\text{OH}\) provides further insight into its structure. When X reacts with this reagent, a silver-containing precipitate forms. This is a key indicator that the compound has a triple bond, which can form a silver acetylide.

Terminal alkynes (compounds with a triple bond at the terminal end) are especially reactive with ammoniacal silver nitrate, yielding a precipitate. This behavior is a strong clue toward the nature of the compound X, suggesting it likely possesses a terminal triple bond.

Regeneration with Dilute Nitric Acid

Finally, the regeneration of compound X from the silver-containing precipitate upon treatment with dilute nitric acid confirms its structure involving a triple bond. The process involves breaking down the silver acetylide to regenerate the original alkyne.

This step verifies the presence of a terminal triple bond in X, as terminal alkynes are known to regenerate under these conditions.

The compound's properties can further help us piece together its full structure. Considering it absorbs 2 mol of \(\text{H}_2\) during hydrogenation to yield 2-methylhexane, we deduce that X is 2-methyl-1-hexyne. This is a compound with both the molecular formula \(\text{C}_7\text{H}_{12}\) and a terminal triple bond capable of forming a silver acetylide.