Teoría

Reaction mechanisms

1

Ionic Reactions

2

Ionic Reactions

3

Ionic Reactions

4

Ionic Reactions

5

Bond Polarity
Partial charges

6

Nucleophiles and Electrophiles

7

Leaving Groups

8

Radical Reactions

9

Type of Reactions

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Nucleophilic reactions: nucleophilic substitution (SN)
Nucleophilic substitution: -> reagent is nucleophil
->nucleophil replaces leaving group -> competing reaction (elimination + rearrangements)

leaving group

+ Nu N ucleoph ile
•

-

C X

nucleoph ilic sub stitution

C Nu +

X

-

in the following general reaction, substitution takes place on an sp3 hybridized (tetrahedral) carbon
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Nucleophilic Substitution
• Some nucleophilic substitution reactions
Reactio n: Nu
HO RO HSRS I
-

+ CH3 X
CH3 -OH CH3 -OR CH3 -SH CH3 -SR CH3 -I CH3 -NH3 CH3 -O-H H
+

CH3 Nu + XAn alcoh ol An eth er A thiol (a mercap tan) A su lfid e (a th ioeth er) An alk yl iodide An alk ylammoniu m ion An alcoh ol (after p roton tran sfer)
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NH3 HOH

+

Mechanism
• Chemists propose two limiting mechanisms for nucleophilic displacement – a fundamental difference between them isthe timing of bond breaking and bond forming steps At one extreme, the two processes take place simultaneously; designated SN2 – S = substitution – N = nucleophilic – 2 = bimolecular (two species are involved in the rate-determining step) – rate = k[haloalkane][nucleophile]

•

•

In the other limiting mechanism, bond breaking between carbon and the leaving group is entirely completed beforebond forming with the nucleophile begins. This mechanism is designated SN1 where – S = substitution – N = nucleophilic – 1 = unimolecular (only one species is involved in the rate-determining step) – rate = k[haloalkane]
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SN2 reaction: bimolecular nucleophilic substitution
– both reactants are involved in the transition state of the rate-determining step – the nucleophile attacks thereactive center from the side opposite the leaving group

H HO

C Br HO

H C HH

Br

H HO C H H

+
H

+

Br

-

H

Trans ition s tate w ith s imu ltaneou s bond breaking an d bond forming

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SN2
• An energy diagram for an SN 2 reaction – there is one transition state and no reactive intermediate

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SN1 reaction: unimolecular nucleophilic substitution
• SN1 isillustrated by the solvolysis of tert-butyl bromide – Step 1: ionization of the C-X bond gives a carbocation intermediate

H 3C C H3 C H3 C Br

slow , rate d etermining

CH3 C+ H 3 C CH3

+

Br

A carbocation intermediate; carbon is trigonal p lanar

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SN1
– Step 2: reaction of the carbocation (an electrophile) with methanol (a nucleophile) gives an oxonium ion

CH3 CH3 O HH3 C

CH3 O C CH3 CH3 +

H3 C C O H3 C H3 C

CH3 H

+ C+
H3 C CH3

+ OCH3
H

fast H

– Step 3: proton transfer completes the reaction

H3 C H3 C H3 C + C O

CH3 H

H + O CH3

H3 C fas t H3 C H3 C C O

CH3

+ + H O

H

CH3

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SN1
• An energy diagram for an SN1 reaction

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SN1
• • For an SN1 reaction at a stereocenter, the product is a racemic mixturethe nucleophile attacks with equal probability from either face of the planar carbocation intermediate
C6 H5 C H Cl -Cl
-

C6 H5 C+ H CH 3 OH -H
+

C6 H5 CH3 O C +

C6 H5 C OCH 3

H

H

Cl (R)-Enantiomer

Cl Planar carbocation (achiral)

Cl Cl (S)-Enantiomer(R)-Enantiomer

A racemic mixture

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Effect of variables on SN Reactions

– the nature of substituents bonded tothe atom attacked by

nucleophile – the nature of the nucleophile – the nature of the leaving group – the solvent effect

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Effect of substituents on SN2

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Effect of substituents on SN1

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Effect of substituents on SN reactions
• SN1 reactions – governed by electronic factors, namely the relative stabilities of carbocation intermediates – relative rates: 3° > 2° > 1° >...