≡
Empty p-orbital

The carbon atom can also form a positively charge transition species known as a carbonium ion (or a carbocation). The structure can be representated as shown.

The alkyl group can have an electron-donating-inductive effect on the carbon centre. So the stability is in the order:

Carbonium ion:    tertiary > secondary > primary > methyl

In instances where the initial carbonium ion generated is a secondary carbonium ion and there is an adjacent tertiary carbon, the carbonium ion could even undertake a 1,2-proton shift to form the tertiary carbonium.

GENERATION of CARBONIUM ION

There are many reactions that can produce carbonium ion. Two features must be noted for all these reactions:

• All these reactions are reversible; and
• The carbonium formed is according to the Markonikov's Rule.

ATTACK OF π−BOND BY PROTON

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The most effective reagents are HCl acid or sulphuric acid. Other reagents are; the halogens and mercuric acetate.

DISSOCIATION OF ALKYL HALIDES

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The dissociation is dependent on three factors:

• The strength of the C-halide bond. Since the bond strength is in the order F > Cl > Br, it would be easier to generate the carbonium ion with the bromide. Generally we cannot get carbonium ion from the alkyl flourides.
• The stability of the carbonium ion. Primary carbonium ion is not stable enough for the reaction to take place. Only the tertiary alkyl chloride; and the secondary and tertiary alkyl bromides will form carbonium ion.
• The solvent. A polar solvent will help "pull" the halide away from the carbon and also stablise the carbonium ion. The reaction is faster in aqueous and hydrolytic solvents.

ATTACK OF OXYGEN ATTACHED TO CARBON

Alcohol, ROH

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The oxygen of the alcohol can be protonated with a strong acid (HCl acid or sulphuric acid) leading to the dissociation of the hydroxyl group. With this reaction it is possible to generate primary carbonium ion since the bond strength between the carbon and the protonated hydroxyl group is very weak.

With concentrated sulphuric acid the carbonium ion actually form an akyl hydrogen sulphate; R−O−SO₂−OH.

Preparation of iso-Propyl bromide from iso-Propyl Alcohol
Preparation of tert-Butyl chloride from tert-Butyl Alcohol

However primary carbonium ion is so reactive that in general it will not be around long enough to experience any 1,2-rearrangement to form a more stable secondary or tertiary carbonium ion.

(A)

(B)

(C)

There is an exception though when the primary carbonium ion is adjacent to a quaternary carbon. Here we do not have a 1,2-proton shift, but a 1,2-methyl shift. Note the last reaction is not reversible, so a product is formed.

(A)		(B)		(C)

If we have a strained cyclic ring adjacent to the carbonium ion we can even witness a 1,2-shift to form a more stable ring. Again the last step is irreversible.

Tutorial 6.4

When 30 g of pinacol, (CH₃)₂C(OH)−C(OH)(CH₃)₂, was refluxed with 130 ml of 6N H₂SO₄ at 120�C, about 17.8 of pinacolone, (CH₃)₃C−C(=0)CH₃, was produced.

• Present the mechanism of the reaction;
• Calculate the yield of pinacolone; and
• Design the set up to remove the pinacolone during the reaction to drive the reaction towards the product; and
• Propose a procedure for carrying out the reaction.

Pinacol: M.Wt. = 118.2; B.Pt. = 170�C; ρ = 1.3 g ml‾�.
Pinacolone: M.Wt. = 100.2; B.Pt. = 103 - 107�C; ρ = 0.805 g ml‾�.      Answer

Ether, ROR'

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The ether also has an oxygen but it is attached to two carbons. The oxygen can be protonated but it will not proceed to form a carbonium ion.

However for cyclic ethers (also known as epoxides or oxiranes) the oxygen is under heavy (bond angle) strain and would be too happy to break free. Needless to say the more stable carbonium ion will be formed. This is an irreversible reaction; the product would not want to return to its stressed out stage.

The replacement of one substituent by another va such a carbonium is known as a S_N1 Reaction.

If the result is the eliminaton of the substituent to form an alkene then it is known as E1 Reaction.

So it is not surprising to have both reactions operating at the same time to give a mixture of products as shown below.

LIMITATIONS OF CARBONIUM CHEMISTRY

It is basically a proton driven chemistry. The common sources of proton are strong acids which unfortunately are inorganic compounds. So mixing them with organic compounds can be a problem. If you cannot find a common solvent to mix the two there will not be any meaningful reaction. Remember no collision, no reaction.

Those organic compounds with polar groups (oxygen and nitrogen groups) might not have this problem for a polar solvent can be found to dissolve both.

The second problem is the high reactivity of the carbonium ion. We have seen that the reaction

can give an ether, R−O−R', and an alkene. When there is water present we can also get R-OH. Then there is a possibility of rearrangement. So carbonium reaction is likely to generate quite a few by-products. This has a negative impact for industry chemistry. We always aim for reaction that gives only the product we want. It saves on chemical and also on the purification of the product; increasing productivity of the process.

VINYLIC CARBONIUM ION

These are carbonium ion with the positive charge on a carbon with a π−bond and so the chemistry is different from that of the alkyl carbonium ion discussed above.

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A vinyl carbonium ion is formed by the attack of a proton on a δ−carbon. It is less stable than an alkyl carbonium ion since the empty p-orbital belongs to an sp-hybrid carbon rather than a sp�-hybrid carbon.

Carbon with more s-character has a greater hold on the valence electrons and is less likely to share the electrons with the p-orbital. The vinyl carbonium ion will react to give a vinyl compound.

Tutorial 6.5

One method to prepare the commercially important monomer, vinyl chloride, is the reaction between HCl acid and acetylene. Outline the difficulties presented by this process.    Answer

One interesting reaction with a δ−bond is with nitrile (C≡N). Nitrogen is electronegative and can attract a proton.

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Of course we would need a strong acid like HCl or sulphuric acid and a higher temperature, like 50�C or more, for this reaction to occur since the nirogen is not as electronegative as oxygen.