Chapter 2, Problem 22P

The general molecular formula for alkanes is ${\text{C}}_{\text{n}}{\text{H}}_{{\text{2n+2}}^{\text{.}}}$. What is the general molecular formula for:

Cycloalkanes

Alkenes

Alkynes

Cyclic hydrocarbons that contain one double bond

Interpretation Introduction

Interpretation:

The general molecular formula for the given hydrocarbons is to be written.

Concept introduction:

The general molecular formula of a class of compounds represents the ratio of the number of atoms of different types of elements present.

When the functional group changes, the general molecular formula changes.

Answer to Problem 22P

Solution:

${\text{C}}_{\text{n}}{\text{H}}_{2\text{n}}$

${\text{C}}_{\text{n}}{\text{H}}_{2\text{n}}$

${\text{C}}_{\text{n}}{\text{H}}_{2\text{n}-2}$

${\text{C}}_{\text{n}}{\text{H}}_{2\text{n}-2}$

Explanation of Solution

(a) Cycloalkanes.

These contain a ring of three or more carbon atoms. Because of this, the number of hydrogen atoms reduces by two as compared to corresponding alkanes having similar configuration.

The general molecular formula of cycloalkanes is ${\text{C}}_{\text{n}}{\text{H}}_{2\text{n}}$.

(b) Alkenes.

These contain at least one carbon-carbon double bond. Alkenes have two hydrogen atoms less as compared to alkanes having the same number of carbon atoms.

The general molecular formula of alkenes is ${\text{C}}_{\text{n}}{\text{H}}_{2\text{n}}$

(c) Alkynes.

Alkynes are hydrocarbons that contain at least one carbon-carbon triple bond. Alkynes have four hydrogen atoms less as compared to alkanes that have the equal number of carbon atoms.

The general molecular formula of alkynes is ${\text{C}}_{\text{n}}{\text{H}}_{2\text{n}-2}$

(d) Cyclic hydrocarbons that contain one double bond.

They have one carbon-carbon double bond that are cycloalkenes. Cycloalkenes have four hydrogen atoms less as compared to alkanes having the same number of carbon atoms.

The general molecular formula of cyclic hydrocarbons that contain one carbon-carbon double bond is ${\text{C}}_{\text{n}}{\text{H}}_{2\text{n}-2}$