Electrophilic addition is a two−stage process. First an electrophile attaches to the carbon of the π−bond giving a carbonium ion. Then the carbonium ion seeks out an "anion" to complete the transaction.
Not all deals can be made by credit card. Some business is by cash and carry, no cash, no purchase, a single transaction. The reagent would attach to both carbons of the π−bond simultaneously. (Technospeak: syn addition. Syn in Greek means together). Syn additions is also known as concerted reactions.
In syn addition the position of attack is dependent on steric factor. The bulkier end will bond with the carbon with less bodyguards; that is carbon with more proton groups.
HYDROBORATION
The boron in borane, BH3, is electron−deficient and cannot exist by itself, so it forms a bridged diborane, B2H6. It has a planer structure. (Diborane is a colourless toxic gas). Commercially borane is sold as an ether−complex, R2O:BH3, like THF:borane.
In borane we have a electron hungry compound that can do business with the π-bond.
| RCH=CH2 + THF:BH3 | THF � |
(R-CH2−CH2)3B |
Unlike the reaction with chlorine or bromine gases, when the boron approaches one carbon of the π−bond the other carbon simultaneously grab hold of the proton. The boron is now stabilised by the alkyls with the help of their electron−donating−inductive effect. As expected the boron will attach to the carbon which is not overcrowded by alkyl groups, although it can be argued that this will provide greater stabilisation of the carbon. So the steric factor is more critical than the electronic factor. The exchange is executed simultaneously resulting in a trans−addition.
The trialkylboron complex is seldom isolated but reacted to give the desired product. For the synthesis of alcohol it is mixed with a hydroperoxide/NaOH mixture;
| (RCH2CH2)3B + 3 ‾OOH | � | (RCH2−CH2O)3B + 3 ‾OH | |
| (RCH2−CH2O)3B + 3 H2O + ‾OH | � | 3 RCH2−CH2OH + 3 B(OH)4‾ |
Tutorial 3.5
When 1−methyl−cyclopentene is first reacted with THF:borane, followed by hydrolysis with an alkaline hydroperoxide solution, the product obtained was trans-2-methylcyclopentanol. Propose a mechanism for the reaction. Explain why a cis−product was not obtained and why the reaction did not follow the Markonikov rule. Answer
OZONATION
| \ / C=C / \ |
+ O3 | � |           |
� |   |
Experiment |
| Ozonide | ||||||
The ozonide can be cleaved with zinc / acetic acid to give ketones.
|
+ H2O | Zn / acetic acid � |
\ C=O + / |
/ O=C \ |
+ ZnO |
It can also be cleaved by sodium boronhydride to give alcohols.
|
+ H2O | NaBH4 � |
\ HC−OH + / |
/ HO−CH \ |
+ NaB(O)H2 |
Beside ozone, peroxides can also attack the π-bond in a similar manner. Since hydrogen peroxide solution do not mix well with alkenes (oil and water), organic peracids, RC(O)OOH, like peracetic acid or perbenzoic acids, are used. These are two of the more stable peracids.
| CH2=CH2 + |
O II CH3C−OOH |
CH2Cl2 → |
  |
+ | O II CH3C−OH |
This reaction is also known as Prileschajew epoxidation. The epoxide is in most cases not very stable (as expected since the ring has a bond angle very much less than 109�). It is easily hydrolysed in the acidic environment to a hydroxyl−acetate. And if water is added it would proceed to give a glycol.
|
|
+ CH3COOH | � | CH3COO OH I I −C−−C− I I |
H2O � |
HO OH I I −C−−C− I I |
Experiment |
METAL COMPLEXES
  |
  |
One of the chemistry of transition metals is the ability to form complexes with groups that are rich in electrons. The permanganates and osmium tetroxide are able to form complexes with CC π−bond. This can then be hydrolysed to give glycols. These are syn additions and the hydroxyl groups are added onto the same side of the π−bond.
Alkene reacts easily with acidified potassium permanganate solution at room temperature. The reaction is shown by the disappearance of the violet colour of the permanganate.
Osmium tetroxide is a toxic crystalline compound soluble in both water and organic solvents. When used in conjunction with hydrogen peroxide, the black alkene-osmium tetroxide precipitate that is formed, is easily converted to the glycol and the osmium tetroxide regenerated. (Experiment)
In both cases the glycol formed is cis-stereospecific.
HYDROGENATION
This is a special type of complexation where the double bond is complexed to the metal surface directly. Metals like platinum, palladium and nickel, amongst others, have such an affinity for the π-bond and also for the hydrogen molecule. It became a facilitator for a syn addition between the alkene and the hydrogen molecule, to give an alkane.
These reactions are highly sensitive to protic solvents like acids, water, alcohol, etc. so it is not easy to carry out by students in the laboratory. A little moisture present in the reagents or solvents will destroy the chemical reactivity of the metal surface.
However this is a very important commercial reaction. For example it is used to convert vegetable oils into margarine.
It is also good to note that while the syn additions to form alcohol and glycol are oxidation processes, hydrogenation is a reduction process.
