Hydrocarbons: Alkenes
The Alkenes have the general formula of CnH(2n)
and are referred to as a homologous series: they’re a group of compounds
with the same functional group and similar chemical properties that differ
from each other by CH2.
The Functional Group of Alkenes (the most reactive part of the molecule that largely determines the chemical properties of the molecule) is the carbon-carbon double bond.
Alkenes are referred to as unsaturated molecules: they contain carbon-carbon double bonds and therefore they can combine with other molecules in addition reactions.
Naming Alkenes
These are named similar to alkanes, except you give preference to the
double bond and specify its position (you start counting from whichever
end gives the lowest number to the double bond).
The longest chain is 4 carbons (butene) with double bond position on the first carbon and there is one side-chain located on the second carbon atom.
Difference between alkanes and alkenes
Alkane
Shape: Tetrahedral
Bond Angle: 109.5°
Alkene
Shape: Triginal Planar
Bond Angle: 120°
Alkene Bonding
As you know alkenes have a double bond, but it isn’t quite as simple
as you may think. One of the double bonds is referred to as a (s) sigma
bond and the second as a (p) pi bond. You aren’t required to understand
the complex properties of these bonds, just the basics.
· Sigma Bond (s)
A single covalent bond formed by the direct overlap of s orbitals
· Pi Bond (p)
The indirect overlap of adjacent p orbitals
You may be required to represent these bonds in a diagram as follows:
The pi bond is the weaker of the two bonds and it therefore breaks when alkenes react. The pi bond also creates a high electron density above and below the plane of the molecule, which restricts motion about the double bond.
Isomerism
Like alkanes, alkenes also have structural isomers – molecules with
the same molecular formula but a different structural formula. However,
some alkenes may also have geometrical isomers (sometimes referred to
as cis-trans isomers).
Definition
Geometrical Isomers
Molecules with the same molecular and structural formula but a different
arrangement of atoms in space.
For Geometrical isomers you need a carbon-carbon double bond (because
this restricts movement about the double bond) and two different groups
attached to the double bond
This molecule has a carbon-carbon double bond and has two different groups attached to the double bond. The Br atoms (or H atoms) form a ‘cross’ and therefore the molecule is called a trans isomer (trans as in cross e.g. transatlantic). It’s called trans-1,2-dibromoethene
This molecule has a carbon-carbon double bond and has two different groups attached to the double bond. The Br atoms (or H atoms) do not form a ‘cross’ and therefore the molecule is called a cis isomer. It’s called cis-1,2-dibromoethene
Note: These are not geometrical isomers because they contain the same
group attached to one of the carbon atoms in the double bond.
