BÀI GIẢNG ORGANIC CHEMISTRY chapter3

ORGANIC CHEMISTRY
Dr Nam T. S. Phan
Faculty of Chemical Engineering
HCMC University of Technology
Office: room 211, B2 Building
Phone: 38647256 ext. 5681
Email:
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Chapter 3:

INTRODUCTION TO
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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UNIMOLECULAR NUCLEOPHILIC
SUBSTITUTION REACTION (SN1)

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Note: slow step is rate-determining step

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ELIMINATION REACTIONS

In an elimination reaction:
+ Groups / atoms are eliminated from a reactant
+ A double bond is formed between the 2 carbons
from which atoms are eliminated
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BIMOLECULAR ELMINATION (E2)

Strong base

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UNIMOLECULAR ELMINATION (E1)

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Weak base

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ELETROPHILIC ADDITION
REACTIONS (AE)
• Electrophilic: electron-seeking / loving
• Most electrophiles:
+ 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 13


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•Not a
carbocation,
but a cyclic
halonium
ion
• More
stable than
carbocation

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NUCLEOPHILIC ADDITION
REACTIONS (AN)
The carbonyl group is polar because the oxygen, being
more electronegative, has greater share of double-bond
electrons
The partial positive
carbon can be attacked
by nucleophiles

The addition of nucleophiles to the carbon
atom of the carbonyl group in nucleophilic
addition reactions
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Reaction mechanism
Positive
center

A nucleophile

Rate-determining step

slow

fast

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Examples:

Nucleophiles

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ELECTROPHILIC SUBSTITUTION
REACTIONS (SE)
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
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Reaction mechanism
Rate-determining step

An electrophile

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Examples:

An electrophile

Rate-determining step

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