NUCLEOPHILIC SUBSTITUTION REACTIONS:
. All the nucleophilic substitution reactions begins with a Heterolytic cleavage of
the bond between the sp 3 carbon and the leaving group.
. This bond cleavage makes the sp3 carbon electron-deficient.
. With the leaving group and a pair of electrons gone, the substrate is now a positively charged carbocation .
. The nucleophile then reacts with this carbocation in a heterogenic bond forming the product.
Most of the Aliphatic nucleophilic substitution reactions follow one of two mechanisms , SN1 , SN2
The basic difference between the two mechanisms is
. When the leaving group departs
. Rate-determining step of the reaction ( i.e., whether the rate of the reaction is dependent on the substrate or the nucleophile)
. Whether any intermediate is formed.
| Type of reaction | SN 1, or a Substitution, Nucleophilic, unimolecular (1) reaction | SN 2, or a Substitution, Nucleophilic, bimolecular ( 2) reaction |
| When the leaving group departs | The leaving group departs before the nucleophile arrives. | The leaving group departs while the nuclepohile is reacting i.e., the rate at which the nucleophile attacks the carbon is equal to the rate at which the leaving group departs. |
| Whether any intermediate is formed. | A carbocation intermediate is formed first. | The bond between the carbon and the leaving group breaks, and the bond between the carbon and the nucleophile forms simultaneously, |
| Rate-determining step of the reaction | The rate of the reaction depended only on the concentration of the substrate. i.e, it is a first order reaction(hence the name unimolecular reaction) | The rate for a second order reaction depends on the concentrations of two The rate of this reaction depends on the concentration of both reagents in the reaction mixture. changes in the concentrations of both reactants affect the rate of reaction. Thus it is a second order reaction. (hence the name bimolecular reaction) |
The SN1 reaction is a two-step reaction in which 1.The leaving group leaves, forming a carbocation. This is the slow step, and so the rate is dependent only on the concentration of the substrate.
2.The nucleophile attacks the carbocation. It can do this from either side, typically in a 50/50 ratio.
3.Therefore about half the product has retained the original configuration, and about half is inverted. Protonated nucleophiles, such as methanol, then lose a proton to the solvent.
The SN2 reaction
1.The SN2 reaction is a single-step displacement of a leaving group by a nucleophile.
2.During the transition state, the bond to the nucleophile forms at the same time that the bond to the leaving group breaks .
3.Therefore the nucleophile is required to approach from the back , and configuration at carbon is inverted.
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