organic chemistry iii (chm 320)
TRANSCRIPT
ORGANIC CHEMISTRY III
(CHM 320)
REACTIVE INTERMEDIATES (PART I)
BY :
OKE, DAVID GBENGA
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COURSE CONTENT:
PART I - Reactive Intermediates :
Carbenes, Nitrenes, Carbocations, Carbanions,
and arynes.
PART II - Lipids, Fats and Oils
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Reactive Intermediates Definition:
These are short lived, high energy and highly reactive
molecules, which when generated in chemical reactions
quickly convert into more stable molecules.
Examples:
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H3C.
H2C: H3C+
H3C-
RN
RadicalCarbene Carbocation Carbanion
aryne Nitrene
. .
. .
:
Some common features of reactive
intermediates includes:
Low concentrations with respect to reaction
substrates and final reaction products.
Except for carbanions, they do not obey the Lewis
octet rule hence their high reactivities.
They are often generated on chemical
decomposition of a chemical compounds.
They are often stabilize by conjugation or
resonance.
Their existence can be proven by means of
chemical trapping.3/14/2020BOWEN UNIVERSITY, IWO. NIGERIA 4
CARBENES
These are uncharged electron deficient
molecular species that contain a divalent
carbon atom surrounded by a sextet of
electrons.
Carbenes can be classified into singlet and
triplet carbenes depending on if the two non-
bonded electrons are paired or not.
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Carbenes
The singlet state is more favoured and canbe described in terms of resonancestructures.
Note that carbenes can either act annucleophiles (if substituted with pie donorsligands) or electrophiles (if substituted withpie acceptors ligands)
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Synthesis of Carbenes
We shall consider three of the methods in which
carbenes can be synthesized:
1. From Diazo compounds – Diazo compounds can be
decomposed to carbenes by heat or light. The
formation of stable nitrogen compensates for the
formation of the unstable carbenes.
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N+ -
N:
R2C
R-C-R + N2
. .hv or
Synthesis of carbenes contd.
2. From ketenes: on heating or irradiation, ketenes can
decompose to carbenes and carbon monoxide.
3. Via α-elimination: this is a common method for the
preparation of halo-carbenes. E.g. dichlorocarbene can be
obtained by treating CHCl3 with hydroxide.
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R2C
O
hv or
R-C-R + CO. .
Cl
Cl
HCl
-OH-H
2O -C(Cl)
3-Cl- Cl-C-Cl
. .+
Reactivity of Carbenes
Insertion to a sigma bond
Addition to a double bond
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CH3
CH3 CH3
CH3
+R
:C
R
CH3
CH3CH3
CH3 R
R
CH3
CH3
HCH3
+ :CH2
C(CH3)3- CH
2- H
Reactions contdRearrangement reaction
Dimerization reaction
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O N2
H
1500C
OH
O+
C-
H
O
CH
:CR2
R2C N+
N:. .
+ R2C N+
N+
CR2
. . . .R
2C=CR
2 + N
2
NITRENES
These are uncharged electron deficient molecularspecies that contain a monovalent nitrogen atomsurrounded by a sextet of electrons.
Nitrenes can also exist as singlet or triplet formsdepending on if the two non-bonded electrons arepaired or not.
They can be protonated to give nitrenium ions thatare isoelectronic with carbenes.
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Singlet and triplet nitrenes
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Synthesis of Nitrenes
The methods of generation of nitrenes have
analogues to those used for carbenes:
1. From Azides – Azides can be decomposed to
nitrenes by heat or light. The formation of stable
nitrogen compensates for the formation of the
unstable nitrenes.
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:N-:N
+ N:N:
+ N N
2. From elimination reaction:
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CH3 S
O
NHO R
CH3SO
2H:N: R +:B
3. From Isocyanates
This occurs also with the elimination of a
molecule (CO) as shown below.
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:N-: O
+N O :N:
+ CO
Reactions of Nitrenes
The reactivity of nitrenes is similar to that of
carbenes and most of the synthetically useful
reactions involves actually “nitrenoids”
species generated with transition metals
rather than free nitrenes. In most cases simple
nitrenes tend to rearrange to give imines:
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R
:N-:
R
R
N+
N
hv/
R
:N:
R
R N
R
R
R
Insertion, Addition and Rearrangement
Reactions:
R
:N:
O
insertion
Addition toalkenes
rearrangement R
N
O
R1
R1
HR
1
R1
NH
O
R
R1
R1
N
RO
R1
R1
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Dimerization reaction of Nitrenes
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Nitrene have the ability to react with
another molecule of itself to form a dimer
as shown below.
:N: NN
ARYNES
Arynes are highly reactive species derived
from aromatic rings by removal of two
ortho substituents. Arynes usually are best
described as having a strained triple bond;
however, they possess some biradical
character as well.
Formation:
aryne3/14/2020BOWEN UNIVERSITY, IWO. NIGERIA 20
Cl
NaNH2
NH3(l)
NH21.
NH2
COOH
HNO2
H2O
N2
+
CO2
_
C+
CO2
_
II.
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CH
CH+
CH
CH
[4 + 2]III.
REACTIONSBenzynes react with a wide variety of mono and poly
olefins, heteroatoms containing unsaturated
compounds, as well as with each other to give dimers.
But the most characteristic reaction is the
cycloaddition reaction.
1.
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+CH2
CH2CH3
CH3
CH3CH2
CH3
+
a
b
2. Benzynes react with many nucleophiles to
form different classes of organic compounds as
shown below:
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+ H2O
+ ROH
+ RNH2
+ NH3
+ H2S
RSH
OH
OR
NHR
NH2
SH
SR
3.Dimerization reactions.
Assignment – Give the product of the
trimerization reaction given below:
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+
+
CARBOCATIONS
These are carbon atoms in an organic molecule
bearing a positive formal charge.
Stability
Carbocations are basically stabilized in two ways:
1. Resonance stabilized carbocations and
2. Those that are not stabilized by resonance.
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RC
+
R
R
Stability cont’d
There are five classification of carbocations based on
their stability:
allylic 30 20 10 methyl
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RC
+
R
R+CH2
CH2 RC
+
R
H
HC
+
H
R
HC
+
H
H
Formation of carbocations
They are most often formed in three ways:
1. Ionization of alkyl halide (or protonated of OH)
2. Addition of an electrophile to a pi bond
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R X R++ X:
-
CH2
CH3
CH3
H+
C+
CH3
CH3
CH3
3. From Diazonium ions
R N+
N R N2+R+
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Reactions of carbocations
Generally, carbocations undergo three basic types of
reaction:
1. Nucleophile capture –
2. Elimination to form a pi bond
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HC
+
H
H
+ OH2H H
HOH H
+
+
CH+
CH3
H:B CH2
CH3
+ H+
3. Rearrangement reaction of
carbocation
A less stable carbocation may rearrange to
form a more stable one; e.g a 2o carbocation
may rearrange itself to form a tertiary
carbocation. To achieve this, there may be
either hydride or methyl shift.
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CARBANIONS
This can be defined as any member of a class of
organic compounds in which a negative charge is
predominantly on a carbon atom.
Stability
Carbanions prefer a lesser degree of alkyl substitution.
More so, they prefer to be in the allylic position which
is their most stable form due to resonance.
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HC
-
H
H-CH2
CH2 HC
-
R
H
RC
-
H
R
RC
-
R
R
allylic methyl 1o 2o 3o
Formation of carbanions
1. Loss of proton from a saturated compound.
a.
b.
2. Addition of nucleophile to an unsaturated compound.
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CH3
O2N
:B
CH2
-
O2N
CH2 CH2 + -OH CH2
-
OH
(Ph)3CH + Na+NH
2
- (Ph)3C
-Na+ + NH
3
liq. NH3
REACTIONS
Carbanions are highly basic, so they behave as
nucleophiles thus they can attack carbon to form
carbon-carbon bonds. These can be shown in
different condensation reactions given below:
1. Aldol condensation
2. Cross aldol condensation and
3. Benzoin condensation
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Examples are:
1.
2.
3.
CH3 COH CH3 COH+-OH
CH3
COH
COH
+ CH3 COH-OH
COH
Cinnamaldehyde
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COH
HOC+
KCN
OH O
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