Alkenes - Stability of intermediates.

In the electrophilic addition mechanism, you have observed that a carbo-cation intermediate is produced. In addition, in an earlier article, you would have read about another intermediate, the radical.

Reaction intermediates are formed when a chemical reaction occurs in at least two steps. They are usually not produced in the last step of the reaction, and have to generally reactive so as the reaction can go to completion.

The carbo-cation intermediate is similar to the radical intermediate. In the latter's case, the carbon is electron deficient because it is one less from fulfilling the octet. While in the former's case, the carbon is also electron deficient. But in this case, the carbon actually is two short of fulfilling the octet.

Let's examine the geometry for the carbo-cation. With 6 valence electrons; there are 3 bonded electron pairs. Hence, by VSEPR theory, the geometry of the carbo-cation will be planar. Therefore, there will be an empty p-orbital remaining.

Due the effect of hyperconjugation (Note: This term is not part of the GCE A syllabus), alkyl substituents are electron donating. Hence, increasing the number of alkyl substituent around the carbon which carries the positive charge will make the cation more stable.

Another effect which helps to stabilise the carbo-cation would be to delocalise the positive charge. This effect was seen in the stabilising of the carbo-cation intermediate formed in electrophilic substitution reaction(it is actually known as the Wheland intermediate).

In conclusion, a simple chemistry rule - localising of electrons, or charges will make the substance more reactive. Generally, a chemical reaction occurs when there is a flow of electrons, which results in bonds to be broken/formed and/or instances where the oxidation number to change.

Therefore, when you intensify charges or localise electrons, these will promote the flow of electrons, because it makes the electrons more attracted to a positive charge or more exposed to attacks, which promotes a chemical reaction - i.e. substance is more reactive.

Hence, despite a more reactive intermediate favours a faster reaction, it is not produced. This is because you actually need to first produce the reactive intermediate, which is dependent on the reaction energetics. Therefore, this actually results in a multi-step reaction to produce the more stable intermediate as oppose to a less stable intermediate.

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^{Article written by Kwok YL 2009.}

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