SN1 and SN2 Flashcards
Direction of nucleophilic attack in SN2?
Approaches carbon from opposite side to
C-X bond (backside attack), maximises overlap of orbital of the nucleophile containing electron pair with empty sigma* orbital of C-X bonds, this orbital has lowest energy of the empty orbitals so best electron accepting orbital - inversion configuration of molecule, called Walden inversion
Why is SN2 stereospecific?
SN2 reactions are stereospecific because different enantiomers of the react produce different enantiomers of the product
Size of substituents effect on rate SN2?
Larger size slower rate, approach of nucleophiles is statically hindered by large substituents, hence why primary halogenoalkanes ready undergo SN2, but secondary halogenoalkanes are slow and tertiary halogenoalkanes do not undergo SN2- larger size of substituents larger steric interactions of transition state and higher energy, hence why rate is slower
Alkyl groups effect on rate SN2?
Greater number of alkyl groups on carbon atom the weaker the partial positive charge on the carbon atom and the slower the rate of attack by the nucleophile
Rate of SN2 depends on?
Halogenoalkane and nucleophile - hence bimolecular
Rate of SN1 depends on?
Just halogenoalkane - hence unimolecular
Why is SN1 racemic?
Carbocation is planar so nucleophile ca equally approach from either side, when a single enantiomer of halogenoalkanes undergoes SN1 the product will be racemic
Tight ion pair SN1?
Many SN1 reactions do not lead to complete racemisation, product derived from inversion of configuration is formed in excess when the C-X bond breaks carbocation and halogen ion are attracted to one another and form a tight ion pair, the halogen ion blocks one face of the carbocation the nucleophile enters most from opposite side to departing halogen ion
Alkyl groups effect on rate SN1?
More alkyl groups faster SN1, +1 groups so stabilise intermediate carbocation, more stable carbocation faster it is formed by cleavage of C-X bond - hence why tertiary halogenoalkanes readily undergo SN1, secondary are slow and primary do not undergo SN1
Exceptions of primary halogenoalkanes undergoing SN1?
A subsequent that can stabilise a carbocation by mesomeric effects will promote SN1 - benzyl halides, allyl halides and alkoxymethyl halides are examples. The intermediate carbocations are stabilised by positive mesomeric effects of the benzene ring, C=C bond and OR group respectively.
Size of substituents on SN1?
Bigger alkyl groups increases rate, steric acceleration release fo steric strain because alkyl groups are further apart in carbocation than halogenoalkanes, faster rate of heterolytic cleavage of C-X bond
Why to vinyl or aryl halides not undergo any nucleophilic substitution?
SN2 not possible because nucleophiles cannot approach sp2 carbon atom at 180 degree angle and carbon atom bonded to halogen is not very reactive. SN1 require formation of unstable aryl and vinyl carbocation where charge cannot be stabilised by delocalisation
Effect of nucleophile?
Increasing concentration of nucleophile or greater nucleophilic strength increases SN2 but not SN1
Polar solvent on SN2?
pNegatively charged nucleophiles, increasing polarity of solvent slightly decreases rate - polar solvents solvates and stabilises small negatively charge better than it solvates the transition state, more polar the solvent the more strongly the negatively charged nucleophile is solvated and slower rate of SN2
Uncharged nucleophile, increasing polarity of solvent slightly increases rate, polar solvents solvate and stabilise transitions state more strongly than uncharged nucleophile
Solvent on SN2?
Changing from polar protic to polar aprotic solvent has a dramatic effect on rate, SN2 fastest in polar aprotic solvents since they cannot solvate negatively charged nucleophiles, unsolvated anions are highly reactive and free to approach and react with halogenoalkane
Polar protic readily solvate negatively charged anions and reduces reactivity of nucleophile since solvent molecules hinder approach to halogenoalkanes, reduce rate.
