ORGANIC CHEMISTRY (PETRO)

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:
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Chapter 3:

COMMON REACTION
MECHANISMS

Reaction mechanism: the description of the
step-by-step process by which reactants
are changed / converted into products

Nucleophilic substitution

Nucleophilic addition

Electrophilic substitution

Electrophilic addition
Elimination
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NUCLEOPHILIC SUBSTITUTION

REACTIONS (SN)
• A nucleophile: an electron-rich species that can
form a covalent bond by donating 2 electrons to a
positive center
• A nucleophile is any negative / neutral molecule
that has 1 unshared electron pair
• 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)

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Note: slow step is rate-determining step

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Stereochemistry of SN1 reactions

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However, few SN1 reactions occur with
complete racemization

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Factors affecting the rates of
SN1 & SN2
1. The structure of the substrate
2. The concentration & reactivity of
the nucleophile
3. The reaction solvent
4. The nature of the leaving group
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Affects of substrate structure

Steric
hindrance

Steric effect in the SN2 reaction

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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:

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ROH, HOH

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Affects of solvents on SN2
• In polar aprotic solvent, nuceophilicity parallels basicity

Polar aprotic solvents solvate cation but not anions

Rates of SN2 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 SN1 reactions are generally increased in
polar protic solvent

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