Successive members differ by CH₂

The idea

The formulae of successive members of a homologous series differ by a –CH2– unit.

One step along the series

Methane is CH₄ and ethane is C₂H₆. These are neighbours in the list of saturated chains.

What is the difference between the formulae of the two?

• One carbon atom and two hydrogen atoms
• Two carbon atoms and one hydrogen atom
• Four hydrogen atoms

Add a unit

Use the + button to add one more carbon atom. The dashed box shows the new CH₂ unit.

The same step each time

Going from one member to the next always adds one carbon atom and two hydrogen atoms: that is a -CH₂- unit.

The formulae of successive members of a homologous series differ by a -CH₂- unit.

• Neighbours | Difference
• CH₄ and C₂H₆ | -CH₂-
• C₂H₆ and C₃H₈ | -CH₂-
• C₃H₈ and C₄H₁₀ | -CH₂-

Notes

The formulae of successive members of a homologous series differ by a -CH₂- unit.

Check yourself

What is the difference between the formulae of successive members of a homologous series?

Pentane is C₅H₁₂. The next member of the series is hexane. How many hydrogen atoms does hexane have?

Answer: 14

Adding a CH₂ unit adds 2 hydrogen atoms: 12 + 2 = 14, which is C₆H₁₄.

Which two compounds are successive members of the same series?

How many carbon atoms are added in going from C₃H₈ to C₄H₁₀?

Answer: 1

4 − 3 = 1 carbon atom, together with 2 hydrogen atoms: a CH₂ unit.

• A -CH₃ unit. A -CH₃ unit has three hydrogen atoms. Going from C₃H₈ to C₄H₁₀ adds one carbon and two hydrogen atoms.
• A -CH- unit. A -CH- unit has one hydrogen atom. Going from C₃H₈ to C₄H₁₀ adds one carbon and two hydrogen atoms.
• A -CH₂- unit — correct. Yes!
• C₃H₈ and C₄H₁₀ — correct. Yes!
• C₂H₆ and C₄H₁₀. These differ by two CH₂ units, so they are not successive members.
• C₂H₄ and C₂H₆. These differ by two hydrogen atoms, not by a CH₂ unit.