Substitution reactions of haloalkanes+
Figure 125
Bromoethane
Any halogen could be used instead of Br
45.1.1 Reaction mechanisms
Nucleophilic substitution can occur in two different ways. SN2 involves abackside attack
. SN1 involves acarbocation intermediate.
SN2 mechanism
Figure 126 Illustration of the Sn2 mechanism. First, the electrons in the nucleophile
attack the central carbon atom from the side opposite the leaving group (in this case, a
halogen). The electrons forming the bond between the central carbon atom and the
halogen move to the halogen, causing the halogen to leave the molecule.
SN1 mechanism
Figure 127 Illustration of the Sn1 mechanism. First, in the presence of a polar solvent,
the C-X bond breaks, forming the carbocation. This carbocation intermediate is highly
reactive. In this case, it reacts with water. Note that the water may attack from either
side.
45.1.2 Comparison of SN1 and SN2 mechanism
Stereochemistry:
SN2 - Configuration is inverted (i.e. R to S and vice-versa).
SN1 - Product is a mixture of inversion and retention of orientation because the carbocation
can be attacked from either side. In theory the products formed are usually racemic due to
the 50% change of attack from the planar conformation. Interestingly, the amount of the
inverted product is often up to 20% greater than the amount of product with the original
orientation. Saul Winstein has proposed that this discrepancy occurs through the leaving
group forming an ion pair with the substrate, which temporarily shields the carbocation
from attack on the side with the leaving group.
Rate of reaction:
SN2 - Rate depends on concentrations of both the haloalkane and the nucleophile. SN 2