Problem 113 - A Complete Resource Book in Chemistry for JEE Main

Question

vA hydrocarbon of molecular formula \(\mathrm{C}_{7} \mathrm{H}_{12}\) on catalytic hydrogenation over platinum gives \(\mathrm{C}_{7} \mathrm{H}_{16} .\) The parent hydrocarbon adds bromine and also reacts with \(\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\right] \mathrm{OH}\) to give a precipitate. The parent hydrocarbon is (a) \(\left(\mathrm{CH}_{2}\right)_{3} \mathrm{CCH}_{2} \mathrm{C} \equiv \mathrm{CH}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{2} \mathrm{CH}=\mathrm{CH}_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CHCH}\left(\mathrm{CH}_{3}\right)_{2}\) (d) \(\mathrm{CH}_{2} \mathrm{CH}^{2}=\mathrm{CHCH}=\mathrm{CHCH}_{2}\)

Step-by-Step Solution

Answer

The parent hydrocarbon is option (a): \(\left(\mathrm{CH}_{2}\right)_{3} \mathrm{CCH}_{2} \mathrm{C} \equiv \mathrm{CH}.\)

Step 1: Analyze Hydrogenation Reaction

The given hydrocarbon \((C_7H_{12})\) on catalytic hydrogenation converts to \((C_7H_{16}).\) Hydrogenation of unsaturated compounds (those with double or triple bonds) generally saturates them, adding more hydrogen atoms. Here, \((C_7H_{12})\) becomes \((C_7H_{16}),\) indicating it originally had 2 degrees of unsaturation (like 2 double bonds or 1 triple bond).

Step 2: Reaction with Bromine

The compound adds bromine, indicating the presence of unsaturation i.e., it contains either double or triple bonds. Only compounds with \(C=C\) or \(C \equiv C\) will decolorize bromine water, confirming the presence of pi-bonds.

Step 3: Tollens' Reaction Confirmation

The compound reacts with Tollens' reagent \((\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\right] \mathrm{OH})\) to form a precipitate, indicating the presence of a terminal alkyne functional group \( - C \equiv C - H. \) This test identifies terminal alkynes, which form silver acetylides as a precipitate.

Step 4: Verify with Options

Evaluate the given options:(a) \(\left(\mathrm{CH}_{2}\right)_{3} \mathrm{CCH}_{2} \mathrm{C} \equiv \mathrm{CH} \) is a terminal alkyne and meets all conditions (hydrogenation, bromine addition, Tollens' test).(b) and (d) are likely to contain only double bonds and cannot form a precipitate with Tollens' reagent.(c) This has a triple bond but not at the end (internal alkyne), so it doesn't react with Tollens' reagent.

Step 5: Conclusion

The only option that fits all criteria (conversion to \(C_7H_{16}\) upon hydrogenation, reacts with bromine, and reacts with Tollens' reagent) is option (a), a terminal alkyne \(\left(\mathrm{CH}_{2}\right)_{3} \mathrm{CCH}_{2} \mathrm{C} \equiv \mathrm{CH}.\)

Key Concepts

Hydrocarbons

Hydrocarbons are organic compounds composed exclusively of carbon (C) and hydrogen (H) atoms. They are the simplest type of organic compounds and form the basis for more complex molecules. Hydrocarbons can be categorized into different types based on the types of bonds between carbon atoms:

• Alkanes: These are saturated hydrocarbons with single covalent bonds (C-C). They have the general formula \(C_nH_{2n+2}\).
• Alkenes: These unsaturated hydrocarbons contain at least one double bond (C=C). Their general formula is \(C_nH_{2n}\).
• Alkynes: These are unsaturated hydrocarbons with at least one triple bond (C≡C). Their general formula is \(C_nH_{2n-2}\).

Understanding the structure of hydrocarbons is crucial in organic chemistry, as they serve as building blocks for more complex chemical reactions and compounds.

Catalytic Hydrogenation

Catalytic hydrogenation involves the addition of hydrogen to an unsaturated organic compound in the presence of a catalyst, typically a metal such as platinum, palladium, or nickel. This reaction is essential for converting unsaturated compounds like alkenes and alkynes into saturated hydrocarbons (alkanes).
In the current context, the conversion of \(C_7H_{12}\) to \(C_7H_{16}\) demonstrates hydrogenation of the compound's unsaturated bonds. The process breaks the double and/or triple bonds, allowing additional hydrogen atoms to bond with the carbon atoms. This reaction is commonly used in the food industry to hydrogenate oils, turning them into semi-solid forms like margarine.
Key aspects to remember about catalytic hydrogenation include:

• Catalyst Role: The catalyst speeds up the reaction without being consumed.
• Saturation: The process saturates the compound, removing any carbon-carbon multiple bonds.
• Applications: Widely employed in organic synthesis and industrial processes.

Tollens' Test

Tollens' test is a qualitative chemical test used to distinguish aldehydes from ketones and to identify certain functional groups like terminal alkynes. It involves using Tollens' reagent, which is amoniacal silver nitrate. When a compound with specific reactive groups is added, it reduces silver ions to metallic silver, forming a distinct precipitate.
The test is particularly significant for identifying terminal alkynes, such as in this exercise. A terminal alkyne reacts with Tollens' reagent to produce silver acetylide, a gray precipitate, confirming the presence of the terminal \(-C≡C-H\).
Key points about the Tollens' test include:

• Reagent Composition: Consists of \(Ag(NH_3)_2^+\) in water.
• Selective Reactivity: Targets functional groups like terminal alkynes and aldehydes.
• Observation: Positive test indicated by a silver mirror or gray precipitate formation.

Unsaturation in Organic Compounds

Unsaturation in organic compounds refers to the presence of double or triple bonds between carbon atoms. These types of bonds affect the physical and chemical properties of the molecules.
The degree of unsaturation is an important concept that helps in determining the structure and reactivity of hydrocarbons. Each double bond or ring is equivalent to one degree of unsaturation, while a triple bond accounts for two degrees.
Why unsaturation matters:

• Reactivity: Unsaturated compounds are generally more reactive than saturated ones due to the presence of pi bonds which are less stable.
• Stability: Adding hydrogens via reactions such as hydrogenation makes them more stable.
• Tests: Chemical tests like the addition of bromine or the Tollens' test exploit unsaturation to identify compounds.

Understanding unsaturation helps predict how a compound might behave in reactions such as polymerization, addition, or oxidation, crucial in both academic and industrial chemistry.