Chapter 3: COMMON REACTION MECHANISMS potx

MECHANISMS Elimination Electrophilic substitution Nucleophilic substitution Nucleophilic addition... Affects of nucleophile concentration & strength 1 Neither the concentration nor the

ORGANIC CHEMISTRY

Dr Nam T S Phan

Faculty of Chemical Engineering

HCMC University of Technology Office: room 211, B2 Building

Phone: 8647256 ext 5681

Email: ptsnam@hcmut.edu.vn

MECHANISMS

Elimination Electrophilic substitution

Nucleophilic substitution Nucleophilic addition

• Substitution reaction: chemical reaction in which

1 atom / group replaces another atom / group in the structure of a molecule

• In a nucleophilic substitution reaction, a

nucleophile attacks / bonds with the positive center

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BIMOLECULAR NUCLEOPHILIC SUBSTITUTION REACTION (SN2)

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Stereochemistry of SN2 reactions

• The nucleophile attacks from the back side / the side

directly opposite the leaving group

• This attacks causes an inversion of configuration

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UNIMOLECULAR NUCLEOPHILIC

SUBSTITUTION REACTION (SN1)

Note: slow step is rate-determining step

Stereochemistry of SN1 reactions

However, few S N 1 reactions occur with

complete racemization

Factors affecting the rates of

1 The structure of the substrate

2 The concentration & reactivity of the nucleophile

3 The reaction solvent

4 The nature of the leaving group

Affects of substrate structure

Steric effect in the S 2 reaction

Steric hindrance

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Affects of nucleophile concentration

& strength

1) Neither the concentration nor the

structure of the nucleophile affects the

rates of SN1 reactions since the nucleophile does not participate in the

rate-determining step

2) The rates of SN2 reactions depend on

both the concentration and the structure

of the nucleophile

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Nucleophiles that have the same attacking atom:

nucleophilicity roughly parallels basicity:

ROH, HOH

Affects of solvents on SN2

• In polar aprotic solvent, nuceophilicity parallels basicity

Polar aprotic solvents solvate cation but not anions

Rates of S N 2 reactions are generally increased in

polar aprotic solvent

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Affects of solvents on SN1

Polar protic solvents solvate cation & anions effectively

Rates of S N 1 reactions are generally increased in

polar protic solvent

Affects of leaving group

The best leaving groups are those that become the most

stable ions after they depart The best leaving groups are weak bases

SN1 vs SN2

BIMOLECULAR ELMINATION (E2)

Strong base

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Regioselectivity of E2 reactions

Zaitsev’s rule for an E2 reaction:

more substituted alkene is

normally obtained

Keep in mind that the major product of an E2 reaction is always the more stable alkene (not

always the more substituted alkene)

In some E2 reactions, the less stable alkene is the

major product due to steric effects

Hofmann’s product Zaitsev’s product

UNIMOLECULAR ELMINATION (E1)

Weak base

Rearrangements in E1 & SN1

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Stereochemistry of E1 reactions

The carbocation formed in the 1 st step is planar,

so both syn & anti-elimination can occur

The major product is the

more stable alkene

SN vs E

+ Are positively charged

+ Have an atom which carries a partial positive charge

+ Have an atom which does not have an octet of electrons

An electrophilic addition reaction is an addition reaction where carbon-carbon double bonds or

triple bonds are attacked by an electrophile

•Not a

carbocation, but a cyclic halonium ion

• More

stable than carbocation

As the cyclic halonium cation is formed in the

1 st step, it can react with whatever nucleophile

presence in the reaction mixture

Stereochemistry of AE reactions

Markovnikov’s rule

Carbocation rearrangement in AE

More stable

More stable

Relative reactivities of alkenes in AE

H H

H H

H H

H H

H H

ELECTROPHILIC SUBSTITUTION

In an electrophilic substitution reaction, an

electrophile substitutes for a hydrogen of an

aromatic compound

Although benzene has 3 double bonds, the overall reaction is electrophilic substitution rather than

electrophilic addition

Reaction mechanism

An electrophile

Rate-determining step

Nonaromatic, not stable, not formed

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