chapter 3 2011
TRANSCRIPT
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ORGANIC CHEMISTRY
Dr Nam T. S. PhanFaculty of Chemical Engineering
HCMC University of TechnologyOffice: room 211, B2 Building
Phone: 38647256 ext. 5681
Email: [email protected]
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Chapter 3: INTRODUCTION TO
REACTION MECHANISMS
Elimination
Electrophilic substitution
Nucleophilic substitution Nucleophilic addition
Electrophilic addition
Reaction mechanism: the description of thestep-by-step process by which reactants
are changed / converted into products
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NUCLEOPHILIC SUBSTITUTION
REACTIONS (SN) A nucleophile:an electron-rich species that canform 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 thestructure of a molecule
In a nucleophilic substitution reaction, a
nucleophile attacks / bonds with the positive center
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BIMOLECULAR NUCLEOPHILICSUBSTITUTION REACTION (SN2)
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UNIMOLECULAR NUCLEOPHILICSUBSTITUTION REACTION (SN1)
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7Note: 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 positivecharge
+ Have an atom which does not have an octetof electrons
An electrophilic addition reactionis an addition
reaction wherecarbon-carbon double bondsortriple bondsare attacked by an electrophile
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Not acarbocation,but a cyclic
haloniumion
More
stable thancarbocation
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NUCLEOPHILIC ADDITION
REACTIONS (AN)The carbonyl group is polar because the oxygen, beingmore electronegative, has greater share of double-bond
electrons
The partial positivecarbon 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
A nucleophilePositivecenter
slow
Rate-determining step
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 anaromatic compound
Although benzene has 3 double bonds, the overall
reaction is electrophilic substitution rather thanelectrophilic addition
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Reaction mechanism
An electrophile
Rate-determining step
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