# Question Video: Determining the Formula for the Next Compound in a Homologous Series Chemistry

What is the formula for the next alkene in this homologous series? C₂H₄, C₃H₆, C₄H₈

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### Video Transcript

What is the formula for the next alkene in this homologous series? C2H4, C3H6, C4H8.

A homologous series is a family of compounds that have the same functional group, similar chemical properties, and the same general formula with each compound in the series differing by a structural unit.

In this question, we need to determine a formula for the next alkene in the homologous series. In the alkene homologous series, the functional group that each of the compounds contains is a carbon-carbon double bond. The first alkene in the series is ethene with a molecular formula of C2H4. The next alkene in the series is propene with a molecular formula of C3H6. We can see that both of these molecules contain a carbon-carbon double bond.

We can also see that propene is one CH2 unit larger than ethene. The next alkene in the series is but-1-ene. But-1-ene contains a carbon-carbon double bond and is one CH2 unit larger than propene. From this, we can see that each alkene in the homologous series differs from the next by one CH2 structural unit. So we know that the next alkene in the series should be one CH2 group larger than but-1-ene.

If we redraw but-1-ene, then add an additional CH2 structural unit to the carbon chain, we get the displayed formula for the next alkene in this series. This is the structure of pent-1-ene, which has the molecular formula C5H10. We could have also arrived at this answer if we knew the general formula of an alkene.

Looking at the molecular formula of ethene, propene, and but-1-ene, we can see that there are twice as many hydrogen atoms as carbon atoms in an alkene. So for 𝑛 number of carbon atoms, there will be two 𝑛 hydrogen atoms. We could tell from the formulas that the next alkene in the series would contain five carbon atoms. This means that this alkene would have two times five hydrogen atoms, and the molecular formula would be C5H10. So the formula for the fourth alkene in this homologous series is C5H10.