2.1.3 Intermediate Stability

Intermediate is a molecular entity that is formed from the reactants (or preceding

intermediates) and reacts further to give the directly observed products of a chemical reaction. In

other words, it is a reacting species which is no longer a starting material or reactant, and has not

yet become product. When the necessary conditions of the reaction no longer prevail, these

intermediates react further and no longer remain in the reaction mixture. Hence, they do not

appear in the final products of the reaction.

Intermediates are similar to transition states in that they allow the mechanism to be

defined or theorized, but they differ from each other, although the differences are subtle.

Intermediates have discrete lifetimes be it a few nanoseconds or many days, whereas transition

states last for just one bond vibration cycle. Besides that, intermediates are part of a stepwise

change and can occasionally be directly observed to exist while transition states are usually part

of a coordinated change and not directly observable.

Intermediate may be unstable molecules, in which case they are called reaction

intermediate, or highly stable molecules. Many of them are short-lived and highly reactive, thus

having a low concentration in the reaction mixture. Reaction intermediates are often free radicals

or unstable ions such as carbocations.

The most common intermediate in organic reactions is carbocations. Since carbocation is

electron-deficient and therefore unstable, it will be stabilized by nearby electron-donating group.

The stability of carbocations increase from primary to tertiary due to three factors, which are

neighbouring carbon atoms, neighbouring carbon-carbon multiple bonds and neighbouring atoms

with lone pairs.

In the first factor which is neighbouring carbon atoms, carbons or alkyl groups in

particular are considered as electron-releasing groups through inductive effect. The inductive

effect means that the carbon that is connected to the hydrogen will be electron-rich since it is

more electronegativity than hydrogen, and can donate some of those electrons to the

neighbouring carbocation. This method can also be explained through hyperconjugation, a

process which invokes stabilization through donation of the electrons in C-H sigma bonds to the

empty p-orbital of the carbocation. Therefore, the more alkyl groups attached to carbocation, the

more stable the carbocation is.

The second factor is neighbouring carbon-carbon multiple bond. Carbocations adjacent to

another carbon-carbon double or triple bond have special stability because overlap between the

empty p orbital of the carbocation with the p orbitals of the π bond allows for charge to be shared

between multiple atoms. This effect is called delocalization and is a major stabilizing influence.

The last factor is neighbouring atoms with lone pairs. The neighbouring atom will

donates a pair of electrons to the electron-poor carbocation which results in formation of double

bond and the charge will move to the atom donating the electron pair. This process often goes by

the name of “π donation”. The strength of this effect varies with basicity, so nitrogen and oxygen

are the most powerful π donors. Even halogens can help to stabilize carbocations through

donation of a lone pair.

In the reaction of hydrohalogenation of alkenes and dehydrohalogenation of haloalkanes,

the reactants will become intermediates before turning into the final products, which are

haloalkane and alkene respectively. Therefore, the stability of the intermediates, which are

carbocations in these reactions, plays an important part as they affect the production of the final

product.

The stability of carbocation increase from primary to secondary, and finally, to tertiary.

In general, the pathway leading to more stable intermediate will be of lower energy; hence it will

be the preferred pathway. The more stable the carbocation, the lower the activation energy of the

reaction which will lead to a more stable transition state. The increased stability of the rate-

limiting transition state in turn will increases the rate of reaction and substantially increase the

number of products form which is haloalkane for the reaction of hydrohalogenation of alkenes

and dehydrohalogenation of haloalkanes. Therefore, the effect of intermediate stability on the

final product of hydrohalogenation of alkenes and dehydrohalogenation of haloalkanes is

increasing the amount of products formed.

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