Question

what is a polar protic solvent?

why are Sn1 reactions carried out in it?

Answer 1

polar protic solvent are those how have carbocation property which is really seen in tertiary but less in secondary an primary
Sn1 reaction are carried out in this becouse in sn1 show inversion property also

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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