A guide to the two pathways for nucleophilic substitution.
SN1 Mechanism
The mechanism involves two-steps.
With a tertiary halide:
![](https://static.wixstatic.com/media/df0959_cf392e486b8d4ea99bb6edfc2cee371b~mv2.png/v1/fill/w_88,h_30,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/df0959_cf392e486b8d4ea99bb6edfc2cee371b~mv2.png)
In the first step, which is the slow step, the C-Cl bond will break and both electrons will go to Cl. A carbocation intermediate and Cl- form.
![](https://static.wixstatic.com/media/df0959_dbc2c4fa975e4144ba1f8350efdbd4ef~mv2.png/v1/fill/w_49,h_16,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/df0959_dbc2c4fa975e4144ba1f8350efdbd4ef~mv2.png)
The second (fast) step sees the hydroxide, OH-, attacking the carbocation and forming the alcohol.
![](https://static.wixstatic.com/media/df0959_5b5b8d54b1d548ddbae6fd0ece5ad3b6~mv2.png/v1/fill/w_84,h_23,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/df0959_5b5b8d54b1d548ddbae6fd0ece5ad3b6~mv2.png)
In the slow step, there is only one halogenoalkane, hence the rate is first order with respect to halogenoalkane and zero order with respect to hydroxide.
The rate equation:
rate = k[RX]
The overall order is one, hence SN1. Tertiary halides favour the SN1 mechanism - there are more electron-donating alkyl groups to stabilise the carbocation intermediate (carbocation is more likely to form).
SN2 Mechanism
The SN2 mechanism is a one-step reaction.
With a primary halide:
![](https://static.wixstatic.com/media/df0959_ec0e687e35e04e1b961eeb305e13914b~mv2.png/v1/fill/w_72,h_23,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/df0959_ec0e687e35e04e1b961eeb305e13914b~mv2.png)
The hydroxide attacks the carbon from behind the C-Cl bond. The C-O bond is formed while the C-Cl bond breaks with, in the transition state, the OH, C and Cl groups along the same axis.
![](https://static.wixstatic.com/media/df0959_cda4d03c037c402a9168ea1f3b4a94aa~mv2.png/v1/fill/w_49,h_10,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/df0959_cda4d03c037c402a9168ea1f3b4a94aa~mv2.png)
This is the only step, so it is the rate determining step. One halogenoalkane and one OH- are involved - rate is first-order with respect to both the halogenoalkane and the hydroxide.
The rate equation:
rate = k[RX][OH-]
Overall, a second-order mechanism (SN2) favoured by primary halides (less bulky alkyl groups = less steric hindrance).
![](https://static.wixstatic.com/media/df0959_4a1b1566344b4cb6b4d673dbc2c7dac4~mv2.png/v1/fill/w_59,h_18,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/df0959_4a1b1566344b4cb6b4d673dbc2c7dac4~mv2.png)