Alkenes and Alkynes

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Hydrocarbons with double bonds make up the alkene family. Hydrocarbons with triple bonds make up the alkyne family. Open chain alkenes with one double bond have the general formula, CnH2n, where n equals the number of carbon atoms. Open chain alkynes with one triple bond have the general formula, CnH2n - 2.
Like the alkanes and other hydrocarbons, they are insoluble in water and are flammable. The most familiar alkenes are ethene and propene. Ethyne (acetylene) is an important alkyne.
CH2=CH2
ethene
CH3-CH=CH2
propene
HC-(triple bond)-CH
ethyne
The geometry of ethyne is linear, as shown above, but the structure of ethene and propene are different than what you see in the structural formula. In fact, the two double bonded carbons are sp2 hybridized (see Chemical Bonding: Geometry), and therefore are trigonal planar, giving ethene this shape: 
H       H
 \     /
  C = C
 /     \
H       H
And in propene, this shape: 
    H       H
     \     /
      C = C
     /     \
H - C       H
   / \
  H   H
because of the sp3 in the single bonded carbon.

IUPAC Naming of Alkenes

The rules for naming alkenes are basically the same as those of alkanes (see previous section), but with two differences. The parent chain must include the double bond even if it makes it shorter than the others. And the parent alkene chain must be numbered from whichever end gives the first carbon of the double bond the lower of two possible numbers. Also, the location number should be given as to where the double bond is (except ethene or propene, where the location will always be 1). For example: 
CH3CH2CH=CH2
4  3  2  1
1-butene
 
CH3CH=CHCH3
1  2  3 4
2-butene
 
      CH3     CH3
      |       |
CH3CH2CHCH2CH=CCH3
7  6  5 4  3  21
2,5-dimethyl-2-heptene
Alkanes which have two double bonds are dienes, those with three are trienes, and so forth. Each double bond has to be located by a number. 
CH2=CHCH=CHCH3
1   2 3  4 5
1,3-pentadiene
 
CH2=CHCH2CH=CH2
1   2 3  4  5
1,4-pentadiene
 
CH2=CHCH=CHCH=CH2
1   2 3  4 5  6
1,3,5-hexatriene
* alkynes also follow similar naming conventions *

Geometric Isomerism among the Alkenes

There is no free rotation of a double bond. Doing so would break it. Therefore, many alkenes exhibit geometric isomerism For example, cis-2-butene and trans-2-butene are geometric isomers. Cis means "on the same side," while trans means "on opposite sides." They refer to the orientation of the structure presented. To illustrate: 
CH3     CH3
 \     /
  C = C
 /     \
H       H
cis-2-butene
 
CH3     H
 \     /
  C = C
 /     \
H       CH3
trans-2-butene
Generally, the differences in physical properties are measurable, but their chemical properties are very similar.

Addition Reactions of Alkenes

Alkenes are Lewis bases (electron pair donors) because the pi bond of the carbon-carbon double bond is projected outward where electron-seeking reactants are able to get it. They will react very readily with Lewis acids (electron pair acceptors) and strong Brønsted acids (proton donors). The addition reactions of alkenes make pieces of a reactant become separately attached to the carbons at the ends of a double bond. Ethene readily reacts with hydrogen chloride to make 1-chloroethane: 
H       H                H H  +                H H
 \     /                 | |                   | |
  C = C   + H-Cl  ==>  H-C-C-H  +  Cl-  ==>  H-C-C-H
 /     \                 | |                   | |
H       H                H                     H Cl

 CH2=CH2  +  H-Cl  ==>  CH3-CH2+  +  Cl-  ==>  CH3-CH2-Cl
                     ethyl carbocation
                  (exists for short duration)
Another reaction involves ethene and water, giving ethanol (ethyl alcohol), while sulfuric acid acts like a catalyst. 
H       H                          H H  +                          H H
 \     /                           | |                             | |
  C = C   +  H2SO4  +  H2O  ==>  H-C-C-H  +  HSO4-  +  H2O  ==>  H-C-C-H  +  HSO4-  +  H+  ==>  C2H5OH  +  H2SO4 
 /     \                           | |                             | |
H       H                          H                               H OH

Hydrogenation--Addition of Hydrogen

When an alkene is hydrogenated, it becomes and alkane. It requires a catalyst--powedered platinum, for example-- and often high heat and pressure though. The following is the hydrogenation of 2-butene: 
CH3CH=CHCH3 + H-H ==> CH3CH-CHCH3 or CH3CH2CH2CH3
                         |  |
                         H  H
 2-butene                             butane
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