Chapter 21, Problem 5STP

Interpretation Introduction

Interpretation:

The general formula of alkyne having one triple bond should be determined if n is the number of carbon atoms in the hydrocarbon.

1. ${\text{C}}_{\text{n}}{\text{H}}_{\text{n+2}}$
2. ${\text{C}}_{\text{n}}{\text{H}}_{\text{2n+2}}$
3. ${\text{C}}_{\text{n}}{\text{H}}_{\text{2n}}$
4. ${\text{C}}_{\text{n}}{\text{H}}_{\text{2n}-\text{2}}$

Concept introduction:

Compounds consist of carbon and hydrogen is known as hydrocarbons. Hydrocarbons are classified as saturated hydrocarbon and unsaturated hydrocarbon.

The hydrocarbons in which carbon-carbon multiple bonds are present is said to be unsaturated hydrocarbons.

The unsaturated hydrocarbon which contains one or more double bonds in the structure is known as alkene.

The unsaturated hydrocarbon which contains one or more triple bonds in the structure is known as alkyne.

Answer to Problem 5STP

The general formula of alkyne having one triple bond is ${\text{C}}_{\text{n}}{\text{H}}_{\text{2n}-\text{2}}$.

Explanation of Solution

An alkyne is an unsaturated hydrocarbon which contains one or more triple bond. In alkyne, carbon atoms linked with triple bond are bound with each other by one sigma and two pi bonds.

The general formula given in A option is ${\text{C}}_{\text{n}}{\text{H}}_{\text{n+2}}$. This is not possible for any hydrocarbon.

The general formula given in B option is ${\text{C}}_{\text{n}}{\text{H}}_{\text{2n+2}}$. In this general formula, number of hydrogen atoms is more in comparison to number of carbon atoms. Thus, this formula belongs to alkane.

The general formula given in C option is ${\text{C}}_{\text{n}}{\text{H}}_{\text{2n}}$. In this general formula, number of hydrogen atoms is double in comparison to number of carbon atoms. As per the formula, hydrocarbon consists of double bond(s) between carbon atoms. Thus, this formula belongs to alkene.

The general formula given in D option is ${\text{C}}_{\text{n}}{\text{H}}_{\text{2n}-\text{2}}$. In this general formula, number of hydrogen atoms is less than the number of carbon atoms. As per the formula, hydrocarbon consists of triple bond between carbon atoms. Thus, this formula belongs to alkyne as it has less number of hydrogen atoms in comparison to other given general formulas as well hydrocarbon consists of triple bond.

Hence, correct option is (D).