Mechanismof Nucleophilic substitution reaction The reactionmechanism of nucleophilic substitution is classified asa) BimolecularNucleophilic substitution reaction (SN2)b) UnimolecularNucleophilic substitution reaction (SN1)Stereochemical aspects of SN reaction Tounderstand the stereo chemical aspects of Nucleophilic substitution reactionswe must understand basic idea of stereo chemistry (Refer unit 11 Class XI forexplanation) Thestereo chemistry plays a significant role in nucleophilic substitution reactionsi) The SN2 reaction proceedswith complete stereo chemical inversion ie the optical activity of the productis opposite to that of reactant.ii) The SN1 reaction proceedswith racemisation ie the product formed is optically inactive but can beresolved into two optically active isomers.Let us discuss thefollowing terms to know the above stereo chemical aspects by followingillustration.1) Chiral carbonJ.Vant Hoff and C.LeBel bothindependently in the year 1874 pointed out that the four valancies of a carbonatom are directed towards the corners of a regular tetra hedron and if all theatoms of groups attached to a carbon atom are different, such a carbon iscalled asymmetric carbon or chiral carbon or stereo centre.2) EnantiomersOptical isomers whose molecularstructures are non super imposable mirror images of each other and which rotatethe plane polarised light usually but in opposite direction are called enantiomers.3) RacemisationA mixture containing equal amountsof enantiomers (dextro and leavo forms) is called racemic mixture and areoptically inactive.
The process of conversion of one enantiomer (+ or -) into aracemic mixture is called racemisation.4) Inversion and retention If the relative configuration ofatoms/groups around the chiral centre in an optically active molecule remainsthe same after and before the reaction, the reaction is said to proceed with retention of configuration. On theother hand, if the relative configuration of the atoms/groups around a stereocentre in the product is opposite to that of reactant, the reaction is said toproceed with inversion of configuration.Example If(Y) is the only product, the process is called the retention of configurationbecause (Y) has the same configuration as reactant. If(Z) is the only product, the process is called inversion of configurationbecause (Z) has the configuration opposite to the reactant (X). Ifan equimolar mixture of (Y) and (Z) is formed, then the process is calledracemisation and the product is optically inactive because one isomer willrotate light in the direction opposite to another. Thusform the above it is clear three different products may be formed when achemical reaction involves bond cleavage or bond formation at a chiral carbon(optically active) Inshort, in case of optically active haloalkane the product formed by SN2reactions will have inversion of configuration and in SN1 reactionracemisation takes place.
SN2MechanismSN2stands for bimolecular Nucleophilic substitution “S”stands for substitution “N”stands for nucleophilic “2”stands for bimolecular (two molecules are involved in the rate determining step) Therate of SN2 reaction depends upon the concentration of both alkylhalide as well as the nucleophile. Rateof reaction = K alkylhalideNucleophile Thissuggest that the reaction is second order and occurs in one step. Thisreaction involves the formation of a transition state in which both thereactant molecules are partially bonded to each other. The attack ofnucleophile occurs from the back side and the halide ion leaves from the frontside. The carbon at which substitution occurs has inverted configuration duringthe course of reaction just as an umbrella has tendency to invert in a windstorm. This inversion of configuration is called Walden inversion; after paul waldsen who first discovered theinversion of configuration of a compound in SN2 reaction. Wewill understand SN2 reaction mechanism by taking an example ofreaction between chloromethane and aqueous KOH.
(14.44) SN2 reaction of opticallyactive haloalkane are always accompanied by inversion of configuration at theasymmetric centre.Example (-) – 2 – Bromo octane is heatedwith sodium hydroxide (+) – 2 – Octanol is formed in which – OH group occupies a position opposite towhat bromine had occupied, (-)2 – Bromo octane (+)2 – Octanol (product) (-) – 2 – bromo octane and (+) – 2 -octanol have opposite sign of optical rotation even though they are notenantiomers.
SN1Mechanism SN1stands for unimolecular Nucleophilic substitution ‘S’ stands for substitution ‘N’ stands for nucleophilic ‘1’ stands for unimolecular (onemolecule is involved in the rate determining step) The rate of SN1 reactiondepends upon the concentration of alkyl halide and is independent of theconcentration of the nucleophile. Rate of reaction = Kalkyl halide This suggest that the reaction isfirst order and occurs in two steps. We will understand SN1reaction mechanism by taking a reaction between tertiary butyl bromide withaqueous KOH. This reaction takes place in twosteps as shown belowStep – 1 Formation ofcarbocation The polar C – Br bond breaks slowlyand carbocation and bromide ion are formed. This step is slow and hence it isthe rate determining step.Step – 2 Nucleophilicattack on carbocation The carbocation formed immediatelyreacts with the nucleophile. This step is fast and hence does not affect therate of the reactions. This step can be illustrated asshown below.
As shown above, the nucleophilicreagent OH- can attack carbocation from both the sides, they will bemirror image of each other. In the above example substratetertiary butyl bromide is not optically active, hence the obtained product isoptically inactive. If halo alkane substrate is optically active then productobtained will be optically inactive racemic mixture. As nucleophilic reagent OH-can attack carbocation from both the sides, equal proportion of dextro and levorotatory optically active isomer which forms optically inactive racemicmixture.Example Hydrolysis of optically active 2 -Bromo butane which give racemic (or) butan-2-olStep – 1 : Formation ofcarbocation Step – 2 : Nucleophilicattack on both faces (front and rear) with almost equal giving 50 : 50 mixture of twoenantiomers (Racemic mixture) The order of reactive reactivity ofhaloalkanes towards SN1 and SN2 reaction is given as.SN2 reactionTertiary halide, Secondary halide, Primary halide,CH3XSN1 reactionComparision of SN2and SN1 mechanism S.No Concept SN2 SN1 1 Number of steps One step process Two step process 2 Kinetics Second order First order 3 Molecularity Bimolecular Unimolecular 4 Transition state One step, one transition state Two steps, two transition 5 Direction of attack of Nucleophile Only back side attack Back side attack and front side attack 6 Type of alkyl halides Primary alkyl halide Tertiary alkyl halide 7 Stereo chemistry if substrate is optically active Inversion of configuration Racemisation 8 Nature of solvent Low polar solvent favours this mechanism High polar solvent favours this mechanism.