Naming+Alkenes

H3C - CH2 - CH2 - CH =CH - CH3
 * 1) Select the longest carbon chain.
 * 2) Number each carbon, starting from the end nearest the double bond
 * 3) Give the alkene the appropriate name according to the number of carbons.
 * = Number of carbons ||= 2 ||= 3 ||= 4 ||= 5 ||= 6 ||= 7 ||= 8 ||
 * = Name of alkene ||= ethene ||= propene ||= butene ||= pentene ||= hexene ||= heptene ||= octene ||

6 5 4 3 2 1 So this alkene would be hexene.

__Position Isomers__
However, there are a number of different isomers of hexene, as the double bond can be in a number of different positions:

H3C - CH2 - CH2 - CH2 - CH = CH2

6 5 4 3 2 1 This would be hex-1-ene, because the double bond is on the 1st carbon.

H3C - CH2 - CH2 - CH = CH - CH3

6 5 4 3 2 1 this would be hex-2-ene, as the double bond is on the 2nd carbon

H3C - CH2 - CH= CH - CH2 - CH3

6 5 4 3 2 1 This would be hex-3-ene, as the double bond is on the 3rd carbon

H3C - CH= CH- CH2 - CH2 - CH3

6 5 4 3 2 1 However, when we move the bond to the carbon atom which we labelled '4' (as above), we must review our second rule which says: **number each carbon, starting from the end nearest the double bond**, so we much change our numbering system:

H3C - CH2 = CH2 - CH - CH - CH3

1 2 3 4 5 6 so this is hex-2-ene again.

__Geometrical Isomers__
Geometrical isomerism is a form of stereoisomerism. The two stereoisomers have exactly the same structural formula, however, **the bonds are arranged differently in space.** It occurs only around C=C double bonds. For example, but-2-ene can exist as shown below

The isomer in which both -CH3 groups are on the same side of the souble bond is called //Z//-but-2-ene and the one in which they are on opposite sides is called //E//-but-2-ene. This type of isomerism is often called //E-Z// isomerism. //E-Z// isomerism, unitl fairly recently used to be knows as //cis-trans// isomerism, and the prefixes //cis-// and //trans-// were used instead of Z- and E- (//Z-// was //cis-, E-// was //trans-)// Simply the //E-Z// notation is based on the atomic numbers. We look at the atoms attatched to each of the carbon atoms in the double bond. When the two atoms (of each pair) of higher atomic number are on the same side of the C=C, the isomer is described as //Z//, from the German word for together, //zusammen//.

So this is //Z//-1-bromo-2-chloro-1-fluroethene.

The other isomer has the positions of the H and Cl atoms reversed. So this is //E//-1-bromo-2-chloro-1-fluroethene.

The simplest interpretations of this naming system is that if the two atoms with the greatest atomic number are on the //same// side of the double bond it is //Z. If not, it is E.//