Alkanes, Alkenes & Alkynes
Alkanes, alkenes and alkynes are simple hydrocarbon chains with no functional groups. Alkanes are identified because the carbon chain has only single bonds. Common alkanes include methane (natural gas), propane (heating and cooking fuel), butane (lighter fluid) and octane (automobile fuel). Alkenes have at least one double bond and alkynes have at least one triple bond. The most common alkyne is ethyne, better known as acetylene. The generic formula for alkanes is CnH2n+2, where n is the number identified by the prefix. Alkenes have the formula CnH2n and alkynes use the formula CnH2n-2.
Writing the formulas for simple alkanes, alkenes and alkynes is as simple as determining how many carbons are in the formula and then putting that number into the generic formula for that hydrocarbon. Since alkanes do not have any real parts to identify, unlike all other organic molecules, there is no need to number the carbons.
prop = 3 oct = 8 pent = 5 dec = 10
C3H2(3)+2 C8H2(8)+2 C5H2(5) C10H2(10)-2
C3H8 C8H18 C5H10 C10H18
Naming the alkanes, alkenes and alkynes is the opposite of the formula writing process. To determine which type of molecule you are working with you should compare the carbons to the hydrogens.
CH4 C6H10 C2H6 C7H14
C:H = 1:2(1)+2 C:H = 6:2(6)-2 C:H = 2:2(2)+2 C:H = 7:2(7)
alkane alkyne alkane alkene
1C = meth 6C = hex 2C = eth 7C = hept
methane hexyne ethane heptene
To determine the position of the double or triple bond would require seeing a Lewis Dot Diagram.