ORGANIC CHEMISTRY III (CHM 320)

ORGANIC CHEMISTRY III

(CHM 320)

REACTIVE INTERMEDIATES (PART I)

BY :

OKE, DAVID GBENGA

3/14/2020BOWEN UNIVERSITY, IWO. NIGERIA 1

COURSE CONTENT:

PART I - Reactive Intermediates :

Carbenes, Nitrenes, Carbocations, Carbanions,

and arynes.

PART II - Lipids, Fats and Oils


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:


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.


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)


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.


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.


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


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


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.


Singlet and triplet nitrenes


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.


:N-:N

+ N:N:

+ N N

2. From elimination reaction:


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.


: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:


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


Dimerization reaction of Nitrenes


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.


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.


+CH2

CH2CH3

CH3

CH3CH2

CH3

+

a

b

2. Benzynes react with many nucleophiles to

form different classes of organic compounds as

shown below:


+ 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:


+

+

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.


RC

+

R

R

Stability cont’d

There are five classification of carbocations based on

their stability:

allylic 30 20 10 methyl


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


R X R++ X:

-

CH2

CH3

CH3

H+

C+

CH3

CH3

CH3

3. From Diazonium ions

R N+

N R N2+R+


Reactions of carbocations

Generally, carbocations undergo three basic types of

reaction:

1. Nucleophile capture –

2. Elimination to form a pi bond


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.


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.


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.


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


Examples are:

1.

2.

3.

CH3 COH CH3 COH+-OH

CH3

COH

COH

+ CH3 COH-OH

COH

Cinnamaldehyde


COH

HOC+

KCN

OH O

ORGANIC CHEMISTRY III (CHM 320)

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