Geometrical Isomerism
In alkenes a new type of isomerism is encountered, this is cis-trans isomerism, and arises due to the fact that a double bond is rigid, and cannot rotate. this would thus create different possibilities for same structural compounds. An example would be 1,2-dibromoethene, where the two possibilities are the following:
Due to the fact that the double bond cannot rotate these two compounds are not equivalent, and thus a different type of naming would have to be introduced in alkenes to make up for this geometrical isomerism.
This naming including the prefixes cis and trans, where cis is used for compounds which would have similar groups on the same side of the Carbon atoms, while trans is used to indicating two similar groups which are on opposite sides of the Carbon atoms.
The example used would thus give:
Addition reactions
In addition reaction through a double bond, it is of utter importance to have an electrophile present. An electrophile is any compound, element or ion which is electron loving, and this would not normally either possess a positive charge or/and not possess a freely available lone pair of electrons.
The mechanism of addition is as follows:
where the first step is the use of the electrophile which would open up the double bond. Once the double bond is open a cation would be produced (a cation is a positive charge Carbon atom) and this would be open for a nucleophilic attack. A nucleophile is any compound or ion which has got a freely available lone pair of electrons, which can normally be seen as a negative sign on the ion.
Markovnikov’s Rule
When alkenes undergo electrophilic addition the mechanism establishes that not all products are formed in equal amounts and this is explained through Markovnikov’s Rule;
In an addition reaction of a protic acid HX to an alkene or alkyne, the hydrogen atom of HX becomes bonded to the carbon atom that had the greatest number of hydrogen atoms in the starting alkene or alkyne.
What this rule says is that when a positive atom interacts with the electron cloud of the alkene the new bond will form on the Carbon that has the highest number of hydrogens (this can also be explained as the Carbon that has the least number of alkyl groups attached to it).
This is due to the fact that alkyl chains stabilise positive charges through a process called hyperconjugation. This ends up having a similar effect to the positive inductive effect, in which electrons are pushed towards the positive carbon, stabilising the charge and thus making the intermediate more stable and more likely to react.
What this means is that a tertiary carbon would be more stable than a secondary carbon which is more stable than a primary carbon. It must also be noted that there is no difference in stabilisation when the alkyl group is short or long. A methyl would offer the same stabilisation as an ethyl or a propyl.
Preparation
Alkenes can be produced from alcohols by dehydrating the alcohols producing a double bond. A dehydrating reaction is any reaction which loses water as a by-product. The dehydrating agents used are normally acids, such as sulfuric acid and phosphoric acid.
CH3CH2OH CH2=CH2
Hydrogenation
There are two types of hydrocarbons, those that are saturated and those that are unsaturated. Saturated hydrocarbons are those hydrocarbons that have got no C to C double bonds, while unsaturated hydrocarbons would be those were C to C double bonds are present. Hydrogenation would thus be a process of transforming an unsaturated hydrocarbon to a saturated hydrocarbon.
The reaction would need a catalyst and normally a moderate temperature, where the catalyst can be Ni or Pt while the temperature would be around 140oC
CH3CH=CH2 CH3CH2CH3