ALKIL HALIDA

ALKIL HALIDAReaksi Substitusi dan Eliminasi

Reactions of RX• There are two common types of nucleophilic

substitution reactions:– SN1 reactions• substitution, nucleophilic, unimolecular

– 3o, allylic, benzylic halides– weak nucleophiles

– SN2 reactions• substitution, nucleophilic, bimolecular

– methyl and 1o halides– strong nucleophiles

Reactions of RX

Reactions of RX• Common strong nucleophiles:– hydroxide ion– alkoxide ions– many amines– iodide and bromide ions– cyanide ion

• Common weak nucleophiles:– water– alcohols– fluoride ion

SN2 Reactions• The reaction between methyl iodide and hydroxide ion is a

concerted reaction that takes places via an SN2 mechanism

HO + H HOC

H

H

I C

H

H

H + I- -

nucleophile

substrateproduct

Leaving group

Substrate: the compound attacked by a reagent (nucleophile)

SN2 Reactions

• Concerted reaction:– a reaction that takes place in a single step with

bonds breaking and forming simultaneously

• SN2:– substitution, nucleophilic, bimolecular– transition state of rate-determining step involves

collision of 2 molecules• 2nd order overall rate law

– Rate = k[RX][Nuc]

SN2 Reactions

• SN2 Mechanism:– Nucleophile attacks the back side of the electrophilic

carbon, donating an e- pair to form a new bond

– Since carbon can only have 8 valence electrons, the C-X bond begins to break as the C-Nuc bond begins to form

Nuc C Nuc C X

Nuc + X

H

HH X

H

HH

CH

HH

Nuc C Nuc C X

Nuc + X

H

HH X

H

HH

CH

HH

---

SN2 Reactions• SN2 Mechanism for the reaction of methyl iodide and

hydroxide ion:

HO C

HO

HOI

H

HH

C I

HH

H

CH

HH

HO C

HO

HOI

H

HH

C I

HH

H

CH

HH

-

HO C HO

C HO C I

I

H

HH

H

CH

HH

HH

-

HO C

HO

HOI

H

HH

C I

HH

H

CH

HH

+ I -

SN2 Reactions

• Reaction Energy Diagram:– large Ea due to 5-coordinate carbon atom in

transition state• no intermediates

– exothermic

SN2 Reactions

• SN2 reactions occur with inversion of configuration at the electrophilic carbon.– The nucleophile attacks from the back side (the

side opposite the leaving group).– Back-side attack turns the tetrahedron of the

carbon atom inside out.

Inversion of configuration (Walden inversion) in an SN2 reaction is due to back side attack

SN2 Reactions

• Inversion of configuration:– a process in which the groups bonded to a chiral

carbon are changed to the opposite spatial configuration:• R S or S R

SN2 Reactions

Example: Predict the product formed by the SN2 reaction between (S)-2-bromobutane and hydroxide ion. Draw the mechanism for the reaction.

SN2 Reactions

• The SN2 displacement reaction is a stereospecific reaction– a reaction in which a specific stereoisomer reacts

to give a specific diastereomer of the product

H

CH3Br

H

H

CH3

H

Br

H H

BrCH3

H

CH3Br

H

H

CH3

H

Br

H

HCH3

OH+ OH-

Br

OH

H

CH3Br

H

H

CH3

H

Br

HH

CH3

+ Br -

SN2 Reactions• SN2 reactions occur under the following

conditions– Nucleophile:

• strong, unhindered nucleophile– OH- not H2O– CH3O- not CH3OH– CH3CH2O- not (CH3)3CO-

– Substrate:• 1o or methyl alkyl halide (most favored)• 2o alkyl halide (sometimes)• 3o alkyl halides NEVER react via SN2

SN2 Reactions

• The relative rate of reactivity of simple alkyl halides in SN2 reactions is:

methyl > 1o > 2o >>>3o

• 3o alkyl halides do not react at all via an SN2 mechanism due to steric hinderance.– The back side of the electrophilic carbon becomes

increasingly hindered as the number or size of its substituents increases

SN2 Reactions

• Steric hinderance at the electrophilic carbon: