Difference between SN1 and SN2 reaction mechanism.
SN2
Unhindered alkyl halides, those in which the back side of the a-carbon is not blocked, will react fastest in SN2 reactions, that is: Me° >> 1° >> 2° >> 3°
While a methyl halides reacts quickly in SN2 reactions, a 3° does not react. The a-carbon in vinyl and aryl halides, as in 3° carbocations, is completely hindered and these alkyl halides do not undergo SN2 reactions.
SN1
The order of reactivity of substrates for SN1 reactions is the reverse of SN2
3° > 2° > 1° > vinyl > phenyl > Me°
R3C-Br R2HC-Br RH2C-Br CH2=CH-Br f-Br H3C-Br
SN2 is Bimolecular
Bimolecular (or 2nd order) means that the rate of an SN2 reaction is directly proportional to the molar concentration of two reacting molecules, the alkyl halide ‘substrate’ and the nucleophile:
Rate = k [RX] [Nu:-]
SN1 is unimolecular
SN1 reactions obey 1st order kinetics, i.e., Rate = k×[RX].
Protic solvents, which contain –OH or –NH2 groups. Protic solvents slow down SN2 reactions.
Polar aprotic solvents like acetone, which contain strong dipoles but no –OH or –NH2 groups. Polar aprotic solvents speed up SN2 reactions.
Non polar solvents, e.g., hydrocarbons. SN2 reactions are relatively slow in non polar solvents.
SN1
Polar solvents, both protic and aprotic, will solvate and stabilize the charged transition state (C+ intermediate), lowering the activation energy and accelerating SN1 reactions.
Nonpolar solvents do not lower the activation energy and thus make SN1 reactions relatively slower
SN2 No carbocation formation takesplace
SN1 carbocation is formed
Unhindered alkyl halides, those in which the back side of the a-carbon is not blocked, will react fastest in SN2 reactions, that is:
Me° >> 1° >> 2° >> 3°
While a methyl halides reacts quickly in SN2 reactions, a 3° does not react.The a-carbon in vinyl and aryl halides, as in 3° carbocations, is completely hindered and these alkyl halides do not undergo SN2 reactions.
SN1
The order of reactivity of substrates for SN1 reactions is the reverse of SN2
3° > 2° > 1° > vinyl > phenyl > Me°
R3C-Br R2HC-Br RH2C-Br CH2=CH-Br f-Br H3C-Br
SN2 is Bimolecular
Bimolecular (or 2nd order) means that the rate of an SN2 reaction is directly proportional to the molar concentration of two reacting molecules, the alkyl halide ‘substrate’ and the nucleophile:
Rate = k [RX] [Nu:-]
SN1 is unimolecular
SN1 reactions obey 1st order kinetics, i.e., Rate = k×[RX].
Protic solvents, which contain –OH or –NH2 groups. Protic solvents slow down SN2 reactions.
Polar aprotic solvents like acetone, which contain strong dipoles but no –OH or –NH2 groups. Polar aprotic solvents speed up SN2 reactions.
Non polar solvents, e.g., hydrocarbons. SN2 reactions are relatively slow in non polar solvents.
SN1
Polar solvents, both protic and aprotic, will solvate and stabilize the charged transition state (C+ intermediate), lowering the activation energy and accelerating SN1 reactions.
Nonpolar solvents do not lower the activation energy and thus make SN1 reactions relatively slower
SN2 No carbocation formation takesplace
SN1 carbocation is formed
