What is the general formula for alkanes?

What is the general formula for alkanes?

• A. CnH2n+2
• B. CnH2n
• C. CnH2n+1
• D. CnH2n-2

Answer: (A)

The main idea is that alkanes are saturated hydrocarbons with only single bonds, so each carbon forms four bonds in total. For an acyclic alkane with n carbon atoms, the carbon skeleton has n−1 carbon–carbon single bonds, using 2(n−1) of the valence bonds between carbons. The total valence from all carbons is 4n, so the remaining bonds must be to hydrogen: 4n − 2(n−1) = 2n + 2 hydrogens. This gives the general formula CnH2n+2, which matches known examples like methane (CH4), ethane (C2H6), and propane (C3H8). The other formulas don’t fit alkanes: CnH2n would describe unsaturated hydrocarbons or cycloalkanes with rings, and CnH2n−2 applies to alkynes (and some cycloalkynes), not to alkanes.

The main idea is that alkanes are saturated hydrocarbons with only single bonds, so each carbon forms four bonds in total. For an acyclic alkane with n carbon atoms, the carbon skeleton has n−1 carbon–carbon single bonds, using 2(n−1) of the valence bonds between carbons. The total valence from all carbons is 4n, so the remaining bonds must be to hydrogen: 4n − 2(n−1) = 2n + 2 hydrogens. This gives the general formula CnH2n+2, which matches known examples like methane (CH4), ethane (C2H6), and propane (C3H8). The other formulas don’t fit alkanes: CnH2n would describe unsaturated hydrocarbons or cycloalkanes with rings, and CnH2n−2 applies to alkynes (and some cycloalkynes), not to alkanes.