In alkenes there are two adjacent sp�-hybridised carbon atoms. A pair of sp�-valence orbitals interact with each other to form a σ−bonds, while the two adjacent p-orbitals interact to form the π−bond.
Tutorial 3.1
What is the difference between the two terms: CC double bond and CC π−bond? Answer
The chemistry of alkenes is the chemistry of the CC σ-bond, the CH σ-bond and the CC π-bond. The first two would be similar to that discussed for alkanes. The CC σ-bond would break into free radicals when heated to above 400�C, and the proton is easily attacked by free radicals.
The π-bond will have its own chemistry but it can also influence the chemistry of the CH σ-bond.
CC π−BOND CHEMISTRY: ELECTROPHILIC ADDITION
The CC π−bond is electron rich and so it can wheel-and-deal to form new relationships. Remember that bonds are formed by valence electrons and any group that has valence electrons to trade can make deals. Also the bond energy for a CC π-bond is ca 270 kJ mole‾� as compared with ca 350 kJ mole‾� for a CC σ-bond. So the CC π-bond would like very much to react to form σ-bonds to lower its energy for greater stability.
Any species that is looking for an electron source will try to strike a deal with the CC π−bond. Proton being the smallest atom is the most aggressive trader. So protons from HCl acid, HBr acid or dilute sulphuric acid will form carbonium ion with the CC π−bond.
Reaction with Hydrogen Chloride
If we bubbled hydrogen chloride gas into alkenes at room temperature we would obtain an alkyl chloride. The mechanism for the reaction is;
| I I −C=C− + HCl |
� |
I I H−C−C+ + Cl‾ I I carbonium ion |
� |
I I H−C−C−Cl I I |
Experiment |
Since groups are added to the molecule this class of reaction is known as an addition reaction.
The addition is started by an electrophile, an electron hungry species. (philos is Greek for "loving"). So this is classified as an Electrophilic Addition reaction. The transition carbon with a positive charge is known as the carbonium ion.
Markonikov's Rule
Now if the alkene is 2−methyl−2−butene the addition of HCl should give two possible products; 1,1−dimethyl−propyl chloride or 1−(isopropyl)−ethyl chloride. Because of the electron-donating-inductive effect of alkyl groups, the more stable carbonium ion would be the one with more alkyl groups attached to it. Thus the more favoured product would be the 1,1−dimethyl−propyl chloride.
Long before this mechanism was known, Markonikov made the observation that the halide was always attached to the more substituted carbon of the π−bond. For a long time this was known as Markovnikov's Rule for the addition reaction of alkenes. Now we known why it is so.
For alkenes with equal number of alkyl groups attached to the π−bond then we would end up with a mixture of isomers and faced a separation problem to isolate the products. An example would be the reaction of 2-pentene with HCl. The products are a mixture of 1-methyl-butyl chloride and 1-ethyl-propyl chloride. In synthesis chemistry this aspect must always be one of the factors to consider before setting up a manufacturing plant. Choose a reaction that gives a high yield of the intended product which can be easily isolated.
Tutorial 3.2
Alkyl chloride can be prepared by reacting an alkane with chlorine with the help of UV light, or by reacting an alkene with hydrogen chloride. Which method would you choose if you were to set up a plant to produce alkyl chloride? Answer
Reaction with Sulphuric acid
Tutorial 3.3
Commercial alcohol is prepared by the hydration of alkenes. For example isobutylene is absorbed in 65% aqueous sulphuric acid. The mixture is then hydrolysed to give tert-butyl alcohol, (CH3)3C-OH. Present the mechanism for the process. What is another name for isobutylene? Answer
Reaction with Mercuric Acetate
Another way to prepare alcohol from alkene is to react it with mercuric acetate, followed by sodium boronhydration with NaBH4 / NaOH.
| RCH=CH2 + Hg(OAc)2 | � | RC(+)H−CH2HgOAc + AcO‾ | |
| RC(+)H−CH2HgOAc + H2O | � | RCH(OH)−CH2HgOAc + H+ | |
| RCH(OH)−CH2HgOAc + NaBH4 | � | RCH(OH)−CH3 + NaBH3OAc + Hg |
The sodium hydroxide is needed to neutralise the acetic acid generated during the first stage of the reaction.
| AcO‾ + H+ | � | CH3COOH | Experiment |
Reaction with Chlorine and Bromine
Proton is not the only species that can react with the π-bond. Chlorine and bromine gases can also do the same. However these molecules are not as aggressive as the proton so the mechanism is slightly different.
         |
� |
  |
� |     |
 |
  |
� |
|
Why do we propose that the bromine starts its attack on one side of the plane and the last step was from the other side of the plane? Should not it be on the same side of the plane?
Well, experiments showed that the bromination of cyclopentene or cyclohexene produces trans−1,2−dibromocyclopentane and trans−1,2−dibromocyclohexane. So in science the obvious must always be confirmed by experiments.
These reactions must be conducted in an inert solvent like methylene chloride, chloroform, carbon tetrachloride, etc. If a protic solvent is used, like water or alcohol, the second step of the reaction would be the addition of OH‾ or OR‾, to give a "bromo hydroxyl" or a "bromo ether" product, either as a major of a minor product.
Tutorial 3.4
Hypochlorous acid, Cl2/NaOH, is prepared by passing chlorine gas into aqueous sodium hydroxide. (Household bleaching solution like chlorox is a 5% solution of hypochlorous acid). If a solution of hypochlorous acid is stirred with cyclohexene at room temperature what would you expect to get? If cyclohexene in methanol is stirred with bromine water at room temperature what would you expect to get? Answer
FREE RADICAL REACTION
We know that the only notable reaction for alkanes is free radical chain reaction, so the alkyl part of alkene should also react in a similar manner. But before the free radical can attack the proton it is attracted by the π−bond who gives him a better offer that he cannot refuse. So free radical attacks π-bond first before it goes for the proton in the alkyl portion. This is the chemistry for Free Radical Polymerisation.
| AROMATIC | ALKYNES | CONTENT |