Basic Organic Chemistry

Sn2 , Sn1 , E2 , and E1 - Substitution and Elimination Reactions

Nucleophilic Substitution Reactions (SN2 and SN1) replace a leaving group with a nucleophile (Nu: or Nu: - )

Elimination Reactions (E2 and E1) generate a double bond by loss of " A+ " and " B: - "

They may compete with each other

Nucleophilic Substitution Reactions:

SN2 Reaction:


Reaction is:

Stereospecific (Walden Inversion of configuration)

Concerted - all bonds form and break at same time

Bimolecular - rate depends on concentration of both nucleophile and substrate

Substrate: Best if primary (one substituent on carbon bearing leaving group), works if secondary, fails if tertiary

Nucleophile: Best if more reactive (i.e. more anionic or more basic)

Leaving Group: Best if more stable (i.e. can support negative charge well):

TsO- (very good) > I- > Br- > Cl- > F- (poor)

RF , ROH , ROR , RNH2 are NEVER Substrates for SN2 reactions

Leaving Groups on double-bonded carbons are never replaced by SN2 reactions

Solvent: Polar Aprotic (i.e. no OH) is best: for example dimethylsulfoxide ( CH3SOCH3 ), dimethylformamide ( HCON(CH3)2 ), acetonitrile ( CH3CN ). Protic solvents (e.g. H2O or ROH) deactivate nucleophile by hydrogen bonding but can be used in some cases

— —
SN1 Reaction:


Reaction is:

Non-stereospecific (attack by nucleophile occurs from both sides)

Non-concerted - has carbocation intermediate

Unimolecular - rate depends on concentration of only the substrate

Substrate: Best if tertiary or conjugated (benzylic or allylic) carbocation can be formed as leaving group departs, never primary

Nucleophile: Best if more reactive (i.e. more anionic or more basic)

Leaving Group: Same as SN2: best if more stable (i.e. can support negative charge well):

TsO- (very good) > I- > Br- > Cl- > F- (poor)

However, tertiary or allylic ROH or ROR' can be reactive under strongly acidic conditions to replace OH or OR

Solvent: Same as SN2: Polar Aprotic (i.e. no OH) is best: for example dimethylsulfoxide ( CH3SOCH3 ), dimethylformamide ( HCON(CH3)2 ), acetonitrile ( CH3CN ). Protic solvents (e.g. H2O or ROH) deactivate but can be used in some cases

— —


Elimination Reactions:

E2 Reaction:



Reaction is:

Stereospecific (Anti-periplanar geometry preferred, Syn-periplanar geometry possible)

Concerted - all bonds form and break at same time

Bimolecular - rate depends on concentration of both base and substrate

Favoured by strong bases

— —

E1 Reaction:


Reaction is:
Non-stereospecific- follows Zaitsev (Saytseff) Rule

Non-concerted - has carbocation intermediate - favoured for tertiary leaving groups

Unimolecular - rate depends on concentration of only the substrate

Does NOT occur with primary alkyl halides (leaving groups)

Strong acid can promote loss of OH as H2O or OR as HOR if tertiary or conjugated carbocation can be formed

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