Question

How does ring size, hybridization and degree of carbon (1st degree, 2nd degree, 3rd degree) affect SN1 and SN2 reactions?

Answer 1

Nucleophilic Substitution

S_N1 = Substitution Nuclophillic Unimolecular

1. The effect of ring size on reactivity.

S_N1 will occur if a stable carbocation is formed that is if the ring structure can form stable carbocation it will favour S_N1 irrespective of ring size.

For example a ring with pi bonds like the cyclopropenium carbocation (three membered ring with one double bond) obeys huckel rule, is aromatic and will favour SN1 mechanism. Similar result can be obtained when the seven membered ring, three pi bonds forms a carbocation as in cyclohexatrienyl carbocation. However, with antiaromatic carbocation (cyclopentane with two alkenes) this substitution is not favourable.

N.B., The more resonance structures available to the carbocation, the greater the stability

2. Hybridization.

In SN1 reactions there is considerable build-up of positive charge on the carbon bearing the leaving group. The stability of sp and sp² carbocations is much lesser. Hence, an sp³ hybridized carbocations is more favoured. However, S_N1 at Sp² carbon are well documented.

3. Effect of alkyl group

The alkyl group must be able to form a relatively stable carbocation in order to react via an SN1 mechanism. Hence Tertiary is preferred and fastest. The order is as follows:

Tertiary 3° > secondary 2° >>> primary 1° > methyl

S_N2 = Substitution Nucleophillic Bimolecular

1. The effect of ring size on reactivity.

S_N2 also occurs with rings. However, it is very difficult with C3 and C4 rings because the reaction has an unstable transition state, in which the nucleophile and the leaving group are only partially bounded but the three substituents are trigonal planar with 120 °C bond angles. The ring strain in cyclopropane and cyclobutane with 60 and 90 degree bonds respectively will significantly hinder the reaction. There is no problem with C5 and C6 and larger rings if there is no steric hindrance. It is noteworthy, that the ring flips during S_N2.

2. Hybridization.

S_N2 reactions ONLY occur at sp³ hybridized carbons.

3. Effect of alkyl group

Tertiary 3° < secondary 2° << primary 1° < methyl