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.

 

propane                                    octane                                      2-pentene                                  4-decyne

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.