An example of highly unsaturated hydrocarbon is

Analysis

The question asks us to identify the "highly unsaturated" hydrocarbon from the given list. In organic chemistry, the degree of unsaturation refers to the presence of multiple bonds (double or triple bonds) between carbon atoms.

Concept Explanation

To determine the degree of unsaturation, we look at the general formulas for hydrocarbons:

1. Alkanes (Saturated): Follow the formula $C_nH_{2n+2}$. These contain only single bonds.
2. Alkenes (Unsaturated): Follow the formula $C_nH_{2n}$. These contain at least one double bond.
3. Alkynes (Highly Unsaturated): Follow the formula $C_nH_{2n-2}$. These contain at least one triple bond.

A hydrocarbon is considered "highly unsaturated" if it has a low hydrogen-to-carbon ratio, which is characteristic of compounds with triple bonds.

Step-by-Step Solution

Let's analyze each option based on the general formulas:

• Option A: $C_2H_2$ (Ethyne) • Here, $n=2$. Using the alkyne formula ($2n-2$): $2(2) - 2 = 2$. • This matches $C_2H_2$. It contains a triple bond ($H-C \equiv C-H$). This is an alkyne.
• Option B: $CH_4$ (Methane) • This is an alkane. It is fully saturated with single bonds.
• Option C: $C_2H_4$ (Ethene) • Here, $n=2$. Using the alkene formula ($2n$): $2(2) = 4$. • This contains a double bond ($CH_2=CH_2$). It is unsaturated, but less so than an alkyne.
• Option D: $C_4H_{10}$ (Butane) • Here, $n=4$. Using the alkane formula ($2n+2$): $2(4) + 2 = 10$. • This is a saturated alkane.

Comparison: $C_2H_2$ (Ethyne) has the highest degree of unsaturation because it contains a triple bond, making it the most "highly unsaturated" among the choices provided.

Conclusion

$C_2H_2$ is the correct answer.

Exam Tip

When you see the term "highly unsaturated," immediately look for the compound with the fewest number of hydrogen atoms relative to the number of carbon atoms. Alkynes ($C_nH_{2n-2}$) will almost always be more unsaturated than alkenes ($C_nH_{2n}$) or alkanes ($C_nH_{2n+2}$).