Naming Alkanes

There is only one structural formula for methane, CH4: HI IH

There is only one structural formula for ethane, C2H6: – C–C– H HI IH

HI HI HI H – C–C–C– H IH IH IH There is only one structural formula for propane, C3H8: HI HI HI H – C–C–C– H IH IH IH

HI HI HI HI H – C–C–C–C– H IH IH IH IH However, when it comes to butane, C4H10, there are two possible structural formulas: HI HI HI HI H – C–C–C–C– H IH IH IH IH

This is also butane, C4H10 – C–C–C– H- H HI IH HIC| -H

Isomers are compounds that have the same molecular formula, but different structural formulas. Isomers usually have different chemical and physical properties as well. We must have some way of specifying which form of butane we are talking about.

Normal Alkanes Alkanes that have a continuous chain of carbons are called normal alkanes. The first isomer of butane is a normal alkane and is sometimes referred to as n-butane.

Branched Alkanes The second isomer has a branch off the second carbon. These branches are referred to as alkyl groups. The name of the alkyl group is determined by the prefix according to the number of carbons and the ending “-yl” The branch in this case would be called methyl

Rules for Naming Alkanes Count the carbons of the longest continuous chain. This will be the prefix. Add –ane to it. Name the alkyl groups attached to the main chain. If there are two or more of the same name, use the counting prefixes (di, tri, etc.)

Rules for Naming Alkanes, cont’d Number the carbons, starting with the carbon closest to the branch. List the alkyl groups first, in alphabetical order, using a number to indicate which carbon it branches from.

This would be called 2-methyl propane – C–C–C– H- H HI IH HIC| -H

This is a short-hand way of writing the same formula: CH3–CH–CH3 CH3|