Question

Deference between SN¹ and SN² reaction?

Answer 1

Characteristics of SN¹ reactions: -

Two step reaction.
Rate of reaction = K [R — L]
It is a first order reaction.
Carbocation intermediate is formed. rearrangement of carbocation is possible.
Rate is dependent on stability of carbocation. [3°>2°>1°]
Rate of reaction also depends on stability of cation and anion.
Polar Protic Solvent favours SN¹ reaction. (As PPS dissolves both cation and anion in it)
Leaving group should be strong i.e. weak base (same as SN²).
Weak nucleophile performs SN¹ reaction. (Strong nucleophile will directly attack substrate resulting in E¹ reaction)
Rate of reaction is independent of concentration and strength of nucleophile.
Products formed will be both R- and S- form i.e. mirror image.
Low temperature favours SN¹ reactions.
Characteristics of SN² reactions: -

Single step reaction.
Rate of reaction = K [R — L] [Nu(:)]
It is a second order reaction.
Rate is dependent on concentration and strength of Nucleophile.
Transition state is formed which is sp² hybridized, planar structure. (Not sp³d as Carbon doesn't have d orbital.)
No carbocation intermediate formation.
Leaving group should be strong i.e. weak base.
Rate of reaction is inversely proportional to bulkiness of groups attached to C atom. [CH3Cl > CH3CH2Cl > CH3CH(CH3)Cl > CH3C(CH3)²Cl]
Rate of reaction 1° > 2° > 3° substrates.
Strong nucleophile performs SN² reactions. Nucleophile attacks substrate directly from backside.
Polar Aprotic Solvent like DMSO, DMF, DMA favours SN² reactions. (Because PAS doesn't dissolves/solvent cations and it dissolves only anions in solution, so by taking PAS cations are removed and only Nu(:) is only anion present to attack substrate.]
Walden inversion occurs.
Reaction is favored by low temperature. High temperature leads to E² reaction.

Answer 2

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( for mechanism of sn2 see attached file 1)

♦♦♦• 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


♦♦♦• Solvent:

Polar Aprotic (i.e. no OH) is best.

For example dimethylsulfoxide ( CH3 ( HCON(CH3)2 ), acetonitrile ( CH3

Protic solvents (e.g. H2 but can be used in some case are NEVER Substrates for SN2 reactions

Leaving Groups on double-bonded carbons are never replaced by SN2 reactions SOCH3 ), dimethylformamide CN ). O or ROH) deactivate nucleophile by hydrogen bonding


$\mathbb{\red{\huge{\:\:\:\:\:SN\:1}}}$


( for mechanism of sn1 see attached file 2)

♦♦♦• 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)

Examples: 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

Examples: dimethylsulfoxide ( CH3 ( HCON(CH3)2 ), acetonitrile ( CH3

Protic solvents (e.g. H2
SOCH3 ), dimethylformamide CN ).
O or ROH) deactivate but can be used in some cases

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