You Asked for It

Introduction

Have you figured out yet why the formula for a saturated, acyclic hydrocarbon is C_nH_(2n+2)? Look at Figure 4 and the reason should become apparent.

Figure 4

Origin of the C_nH_(2n+2) Rule

As Figure 4 indicates, linear hydrocarbons may be viewed as a chain of CH₂ groups in which the first and last carbons are each bonded to an extra hydrogen atom. The formula for n CH₂ groups may be written as C_nH_2n. Addition of the terminating hydrogen atoms produces the formula C_nH_(2n+2). This formula applies to non-linear saturated acyclic hydrocarbons as well.

Because it is divalent, oxygen may be incorporated into the chain building scheme outlined in Figure 4 without altering the C/H ratio of a particular carbon chain. Look at Figure 5.

Figure 5

The C_nH_(2n+2) Rule Also Works with Oxygen

Taken together, Figures 4 and 5 demonstrate that the inclusion of oxygen atoms in a structure does not alter the C/H ratio in comparison to a hydrocarbon containing the same number of carbon atoms. The same conclusion applies to any other divalent atom as well, e.g. sulfur. In fact, it applies to all atoms that have an even valence.

Such is not the case when a molecular formula includes an atom with an odd valence. Consider Figure 6, which shows what happens when you try to incorporate a nitrogen atom into a chain of CH₂ groups.

Figure 6

The C_nH_(2n+2) Rule Doesn't Work for Nitrogen

Since nitrogen is trivalent, you have to use an extra hydrogen to create a satisfactory structure. Hence the formula for a saturated acyclic compound that contains C, H and 1 nitrogen atom is C_nH_(2n+3)N.

Exercise 1 What is the general formula of a saturated acyclic compound that contains C, H, and 1 fluorine atom?

Exercise 2 If X represents any of the halogen atoms, what is the general formula of a saturated acyclic compound that contains C, H, and 1 X atom?

Exercise 3 What is the general formula of a saturated acyclic compound that contains C, H, and 2 nitrogen atoms?

Exercise 4 What is the general formula of a saturated acyclic compound that contains C, H, and 2 halogen atoms?

Exercise 5 If a compound contains an odd number of nitrogen atoms will the number of hydrogen atoms in it be odd or even?

Back