Tải bản đầy đủ

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: ptsnam@hcmut.edu.vn
1


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
2


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
3


4


BIMOLECULAR NUCLEOPHILIC
SUBSTITUTION REACTION (SN2)

5


UNIMOLECULAR NUCLEOPHILIC
SUBSTITUTION REACTION (SN1)

6


Note: slow step is rate-determining step


7


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
8


BIMOLECULAR ELMINATION (E2)

Strong base

9


10


UNIMOLECULAR ELMINATION (E1)

11


Weak base

12


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


14


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

15


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
16


Reaction mechanism
Positive
center

A nucleophile

Rate-determining step

slow

fast

17


Examples:

Nucleophiles

18


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
19


Reaction mechanism
Rate-determining step

An electrophile

20


Examples:

An electrophile

Rate-determining step

21



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

Tải bản đầy đủ ngay

×