nucleophilic substitution reaction (sn2)-1

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Nucleophilic substitution bimolecular (SN2) ReactionProf. R. B. SunojIIT Bombay

IntroductionSN2 correspond to substitution Nucleophilic Bimolecular.

This Reaction is a concerted reaction in which bond forming and bondbreaking occur simultaneously.

Figure SN2 reaction Energy required to break the bond is compensated by bond formation.

IntroductionExample 1:

Example 2:

Example 3:

Kinetics The rate of the reaction is found to be vary linearly with non zero slope with respect to electrophile as well as nucleophile .

Rate=K [elec][Nucl]2 in the SN2 correspond to bimolecular.

K= equilibrium constant

In presence of large excess of nucleophile kinetics is found to be first order experimentally even though mechanism is bimolecular. rate=K[elec] Nucleophile affect the rate even being in large excess but concentration does not vary significantly and rate is found to be determined alone by electrophile. Such kinetics are known as Pseudo first order reaction.

Mechanism During SN2 reaction nucleophile first approaches to antibonding orbital of C-Y bond. Attractive interaction between orbital, develop bond between incoming group and carbon. Simultaneously the leaving group start to depart away from carbon center. Substitution may be possible either via front side or backside attack of nucleophile.

In SN2 substitution backside attack is preferred in which approach of nucleophile is 180 away from the leaving group.

Walden Inversion Complete Inversion in stereochemistry is observed during aliphaticnucleophilic substitution via SN2 pathway, confirming that backside attack is preferred over front side attack. Stereochemical outcome of the SN2 reaction is termed as Walden inversion in honor of his discovery.

Only backside attack of nucleophile provide the most favorable interaction between antibonding orbital of C-Y bond and orbital of incoming nucleophile.

Nucleophile and Nucleofuge Nucleophile is a species looking for a nucleus (positive charge) to which it can donate its electron. Usually nucleophiles are electron rich species.

A Nucleofuge is nucleophile departing from a molecule instead of adding.

The term Nucleophile and Nucleofuge are kinetic term, used for discussing reactivity and kinetics.

Effect of Solvent Solvent may play a vital role in the rate of SN2 reaction as it involves eithercreation or dispersion of charge. Charged species are more stabilized in the polar solvent than non-polar solvent. Difference between the solvation capacity of reactant and transition state in various solvent lead to the solvent effect in a reaction. Reactants Transition State Charge Creation/DispersionCharge dispersion

Effect of Increasing the Polarity of SolventRetard the reaction

Nuc

R-LG

Nuc

R R

LG LG LG LG

NucNuc

R-LGR-LG

Nuc Nuc Nuc

Charge creationCharge dispersion Charge dispersion

speed the reactionRetard the reaction Retard the reaction

RR

Nuc

R-LG

In case of negative nucleophile a remarkable change in rate of SN2 reaction is observed while changing the solvent from polar protic to polar non-protic solvent. In polar aprotic solvent negatively charged nucleophile are less soluble but solvent are polar enough to solubilize the nucleophile making them highly reactive.

SolventMethanol Water Formamide Acetonitrile Acetone

K relative0.9 1.0 14.1 3.58 x 104 1.41 x 106

Solubility of nucleophile is a major problem in substitution reaction, particularly in less polar aprotic solvent. Crown ether is added to solvate the counter-cation which induce the solubility

of corresponding anionic nucleophile.

Structure Function Correlation With Nucleophile In SN2 reaction stronger the nucleophile faster the reaction will occur. Strength of a nucleophile can be determined by given principle1. A nucleophile with negative charge is more powerful than its conjugate acid. Example: NH2 >NH3, OH >H2O, 2. Nucleophilicity is almost in same order as basicity in row. Example: R3C > R2N > RO > F NH2 > RO >R2NH > ArO > NH3 > Pyridine > F > H2O > ClO4

3. Going down in a group nucleophilicity increase while basicity decrease.Example: I >Br > Cl > F (less solvation, Higher polarization) S >O

Structure Function Correlation With Nucleophile4. More free nucleophile, more nucleophilicity. Example 1: Rate of reaction using NaOH can be largely enhanced by specifically solvating cation Na+. (use of crown ether). Example 2: NaOHDMSO >NaOHwater (basicity) In water Na+ and HO both are solvated while in DMSO Na+ is solvated preferably than HO leaving HO as free nucleophile.

Structure function correlation with leaving group Negative charge develops on leaving group during the rate determining step.Thus, reaction proceed faster with a leaving group which stabilize better negative charge.

Better leaving group are mostly weak bases which stabilize the negativecharges. Example: TsO , MsO , TfO

In presence of better leaving group, SN1 reaction do not require strong base, but for SN2 reaction it is required. Example: 1. XH is always a weaker base than X . Thus XH is a better leaving group which facilitate the SN2 reaction. HS > H2S, HO > H2O, I > HI, NH2 >NH3 2. Acidic medium also protonate the base making them weaker. ROH is converted to ROH + in acid medium which is a better leaving group.

Structure Function Correlation With R Group R group plays vital role on the rate of reaction as well as in determination of the reaction. R group may have stearic, electronic and neighboring group effect. Stearic hindrance may slower the rate of reaction, as nucleophile adds to the carbon centre in rate determining step. During transition state incoming nucleophile(Nuc) and leaving group(LG) are

90 to the R group. Thus placing larger group at 90 to LG and Nuc, transitionstate will be more strained which causes lesser rate of reaction.R X0

R Y R 900

90 Bulkier R group increase steric hindrance

Structure Function Correlation With R GroupA bulkier adjacent group may bend the trajectory of nucleophile away from the linear approach which leads to a strained transition state.R R R

Nuc R R

LG

Strained transition state due to adjacent group steric effect

electron withdrawing nature of groups having sp2 carbon(vinyl and phenyl) make the adjacent carbon more electrophilic and hence reactivity towards nucleophile increases. Example:

Higher rate for nucleophilic substitution at allyl and benzyl system.

Neighboring Group Participation In presence of electron donor neighboring group, reaction undergo with higherrate of reaction than expected and neither inversion nor racemisation is observed. It shows the neighboring group participation in the reaction.

Two consecutive SN2 substitution, leads to retention of configuration. Example 1

Neighboring Group ParticipationExample 2

The most likely neighboring group participation in three, five, six membered ring. Four membered ring neighboring group participation is higher in case of alkyl substitution on or carbon. The effect of halogen increase as going down the group ( I> Br> Cl ).

Example 3

Example 4

Some neighboring groups are COO , COOR, COAr, OCOR, OR, OH, O, NH2 , NHR , NHCOR, SH, SR, I, Br, S

SN2 Reaction allylic rearrangement in SN2 condition is known as SN2 reaction.

Simultaneous movement of three electron pair during transition stateSN2' attack Usual attack(SN2) X

Attack at carbon with SN2 condition is SN2 reaction.

SN2 Reaction Increasing the size of nucleophile as well as stearic hindrance at - carbon increases the extent of SN2 reaction.

Leaving group also affect the extent of reaction in certain cases.Example: PhCH=CHCH2X treated with LiAlH4 when X=Br,Cl only SN2 reaction =P(Ph)3+Br only SN2 reaction Stereochemistry depends upon the nature of incoming group and leaving group in SN2 reaction, still syn-substitution is preferred over anti.