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Mechanisms of
organic reactions
How Organic Reactions Occur
Homolytic bond breaking (radical):
A-B  A + B
radicals are formed
Heterolytic bond breaking (polar):
A-B  A+ + :B-
ions are formed
Heterolytic Reactions
 Nucleophile
has an electron-rich atom (e.g Cl-, CN-, NH3) and
can form a bond by donating a pair of electrons to electron-poor
atom
 Electrophile
has an electron-poor atom (e.g H+, CH3+ ) and can form a bond by
accepting a pair of electrons from a nucleophile
A+ + :B-  A:B
Kinds of Organic Reaction I.
 Addition reactions
-
two reactants add together to form a single new
product with no atoms „left over“
 Elimination reactions
- single reactant splits into two products
Kinds of Organic Reaction II.
 Substitution reactions
- two reactants exchange parts to give new
products
 Rearrangement reactions
- single reactant undergoes a reorganization of
atoms
bonds and
Nuccleophilic
Substitution reactions
SN1 & SN2
Uni-/Bi- molecular reactions SN1
& SN 2

Unimolecular reaction SN1
only one of the reactant molecules is present in the transition
state
R-X + Y-  R+ + X- + Y-
R+ + Y-  R-Y

Bimolecular reaction SN2
both of reactants must be present together in transition state
Y- + R-X  [Y…R…X]  R-Y + X-
Electrophilic Substitution
- an electrophile (E) reacts with an aromatic ring
substitutes for one of the hydrogens
MECHANISM
Resonance forms of  complex
and
Electrophiles


Substance that is „electron-loving“
Has an electron-poor atom

Lewis acids (AlCl3, FeCl3) catalyze formation of electrophilic molecules

Lewis acid – accepts electron pair

Lewis base – donates electron pair
Aromatic Electrophilic
Substitutions I
 Halogenation
MECHANISM
X = Cl
Aromatic Electrophilic
Substitutions II.
•Friedel-Crafts alkylation reaction
tert- butylbenzene
tert- butylchloride
•Friedel-Crafts acylation reaction
AlCl3
benzene
+ HCl
Aromatic Electrophilic
Substitutions III.
•Nitration
Substituent Effects in Substituted
Aromatic Rings
1.
Ortho- and para-directing substituents
increase the electron
density inside the ring
Y=
2.
- NH2, -OH, -F, -Cl, alkyl
Meta-directing substituents
decrease the electron density
inside the ring
Y = - NO2, -CN, -COOH, -SO3H
2. Nucleophilic Substitution
 Reactions of Alkyl Halides
Nu:- + CH3-Br  CH3 -Nu + Br
-
Nucleophile = HS- > CN- > I- > HO- > Cl-
3. Radical Substitution
Initiation
Cl2  2Cl
UV
Propagation
Cl + H3C-CH3  H3C -CH2 + HCl
CH3 -CH2 + Cl2  H3C -CH2Cl + Cl
Termination
H3C -CH2 + Cl  H3C -CH2Cl
Electrophilic addition
Markovnikov´s Rule: In the addition of HX to
an alkene, the H attaches to the carbon with
more H.
cis addition – both groups attach to the same side of the double bond
trans addition –groups attach to the opposite side of the double bond
Elimination Reactions
 Dehydration
 Dehydrohalogenation
H
Br
C
C
KOH
H3C
H
H
CH3
CH3CH2OH
H3C
CH
CH
CH3 +
KBr + H2O
Organic halide
SN1
Remember the reactivity order
Benzylic > allylic > 3° RX> 2° RX> 1°
RX
SN2
Remember the reactivity order
1° RX > 2° RX > 3° RX > allylic
>Benzylic
SN2 example
E1
Remember the reactivity order
Benzylic > allylic > 3° RX> 2° RX> 1°
RX
E1 & SN1
E2
Remember the reactivity order
1° RX > 2° RX > 3° RX > allylic
>Benzylic
ALCOHOLS & Phenols
Forming the phenoxide ion
Remember that the negative charge here is localized, therefore
this ion more stable than the alkoxide ion (from alcohol)
Mechanism of the Kolbe-Schmitt Reaction
Ethers
Williamson synthesis
Aldehydes & Ketones
IDOFORM REACTION OF THE METHYL
KETONE
Aldol condensation
Amines
HOFMANN rearrangement