Experimental data from nucleophilic substitution reactions on substrates that have optical activity (the ability to rotate plane-polarized light) shows that two general mechanisms exist for these types of reactions. The first type is called an SN2 mechanism. This mechanism follows second-order kinetics (the reaction rate depends on the concentrations of two reactants), and its intermediate contains both the substrate and the nucleophile and is therefore bimolecular. The terminology SN2 stands for “substitutiThe second type of mechanism is an SN1 mechanism. This mechanism follows first-order kinetics (the reaction rate depends on the concentration of one reactant), and its intermediate contains only the substrate molecule and is therefore unimolecular. The terminology SN1 stands for “substitution nucleophilic unimolecular.”
The alkyl halide substrate contains a polarized carbon halogen bond. The SN2 mechanism begins when an electron pair of the nucleophile attacks the back lobe of the leaving group. Carbon in the resulting complex is trigonal bipyramidal in shape. With the loss of the leaving group, the carbon atom again assumes a pyramidal shape; however, its configuration is inverted. on nucleophilic bimolecular.”SN2 reactions require a rearward attack on the carbon bonded to the leaving group. If a large number of groups are bonded to the same carbon that bears the leaving group, the nucleophile's attack should be hindered and the rate of the reaction slowed. This phenomenon is called steric hindrance. The larger and bulkier the group(s), the greater the steric hindrance and the slower the rate of reaction. Table 1 shows the effect of steric hindrance on the rate of reaction for a specific, unspecified nucleophile and leaving group. Different nucleophiles and leaving groups would result in different numbers but similar patterns of resul
Alkyl Group (ALK)
Relative Rate of Substitution
−CH2CH3 (larger group)...
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