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EpoxidesNomenclature

Synthesis and ReactionsDr. Sapna Gupta

Epoxides

• Epoxide: A cyclic ether in which oxygen is one atom of a three-membered ring.

– Simple epoxides are named as derivatives of oxirane.

– Where the epoxide is part of another ring system, it is shown by the prefix epoxy-.

– Common names are derived from the name of the alkene from which the epoxide is formally derived.

Oxirane(Ethylene oxide)

CH2H2 C

O1

2 3

cis- 2,3-Dimethyloxirane(cis- 2-Butene oxide)

1,2-Epoxycyclohexane(Cyclohexene oxide)

C

O

C

HCH3

HH3 C

H

H

O

1

2

Ethers Nomenclature, Synthesis and Reactions 2

Synthesis of Epoxides

1) From Peroxides: The most common method is oxidation of an alkene using a peroxycarboxylic acid (a peracid).

CO-

COOH

Mg2 + CH3 COOH

COOH

Cl

Magnesium

monoperoxyphthalate

(MMPP)

2

Peroxyacetic acid

(Peracetic acid)

meta-Chloroperoxy-

benzoic acid

(MCPBA)

O O

O

O

RCOOH

O

CH2Cl2O

H

H

RCOH

O

A carboxylic acid

1,2-Epoxycyclohexane (Cyclohexene oxide)

A peroxy-carboxylic acid

Cyclohexene

++ RCOOH

O

CH2Cl2O

H

H

RCOH

O

A carboxylic acid

1,2-Epoxycyclohexane (Cyclohexene oxide)

A peroxy-carboxylic acid

Cyclohexene

++

C C

H3 C

H CH3

H

RCO3 HH3 C H

H CH3

C C

O

trans-2-Butene t rans-2,3-Dimethyloxirane(a racemic mixture)

H CH3H3 C H

C C

O

+

Reagents

Example

Stereospecificty


Synthesis of Epoxides, contd…

Peroxyacid epoxidation mechanism


2) Cyclization of Halohydrin (intermolecular William synthesis)

Reactions of Epoxides

• Ethers are not normally susceptible to attack by nucleophiles.

• Because of the strain associated with the three-membered ring, epoxides readily undergo a variety of ring-opening reactions.


+ HNu C C

Nu

HO

C C

O

:

Reactions - Epoxide Ring Opening in Acid

• Trans diol formed in water solvent.


• Alkoxy alcohol formed in alcohol solvent.

H

O

H

H+, H2O

H

HO H

OH

H

O

H

H+, CH3OH

H

HO H

OCH3

• Stereochemistry of the glycols. RCO3 H

OsO4 , t-BuOOH

H

H

O

OH

OH

H+

H2 O

OH

OH

trans- 1,2-Cyclopentanediol(formed as a racemic mixture)

cis- 1,2-Cyclopentanediol(achiral)

OH

OH

+

• Mechanism

Regiochemistry of Acid Catalyzed Ring Opening of Epoxides

Nucleophile preferably adds to more substituted carbon site if primary and secondary C’s


Reactions - Epoxide Ring Opening: Acids Vs Bases

• In acid, the nucleophile attacks the protonated epoxide at the most substituted carbon

• Base attacks the least hindered carbon.

Conjugated System 8

CH3CH2OHHC CH2

CH3

OH

OCH2CH3

_

HC CH2

CH3

O

OCH2CH3OCH2CH3

HC CH2

CH3

O

HC CH2

OHH3C

OCH2CH3+

HC CH2

OHH3C

OCH2CH3H

CH3CH2OH

+

HC CH2

CH3

O

HH2O

Reactions of Epoxides, contd…

The value of epoxides is the variety of nucleophiles that will open the ring and the combinations of functional groups that can be prepared from them.


HSCH2 CHOH

CH3

H2O/ H3 O+

HOCH2CHOH

CH3

CHH2 C

O

CH3

H2 NCH2 CHOH

CH3

CCH2CHOH

CH3

N

Na+SH

-/ H2 O

NH3Na+C N

-/ H2 O

2. H2 O

1. HC C- Na

+

HC CCH2 CHOH

CH3

A-mercaptoalcohol

Methyloxirane

A -aminoalcoholA-hydroxynitrile

A -alkynylalcohol

A glycol

CH3

CH3CH2CH2CHOHChain elongation using Grignard reagent

1. CH3CH2MgBr2. H2O

NH2-

Ethylene Oxide

Ethylene oxide is a valuable building block for organic synthesis because each of its carbons may be converted to a functional group.


Na+CN

-

CH3 NH2

C C-Na

+CH3

OH

ON

OH

CH3

H

N COH

N OCH3

O

H2 SO4

H2 / M

N

OH

CH3

OH

N N-HCH3

OH

H2 N

SOCl2

NH3

N

Cl

CH3

Cl

(1)(2)

(3) (4) (6)

(8)(5) (7)

Applications of Ethers

• Most commonly – for epoxy resins. A polymer used as an adhesive to protect the surface.

Epoxy resin + hardener = polymer

• Cross linking makes the epoxy resin a good coating agent.


Key Concepts

• Epoxides

• Peracids

• Ring openings• Stereospecific


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