2. 2
• Epoxides are cyclic three member ethers containing
oxygen atom as member of ring. Epoxides are also
called oxiranes.
• The C—O—C bond angle for an epoxide must be near to
60°, a considerable deviation from the tetrahedral bond
angle of 109.5°.
• Thus, epoxides have angle strain, making them more
reactive than other ethers.
3. 3
• Epoxides can be named in three different ways — As
epoxyalkanes, oxiranes, or alkene oxides.
• To name an epoxide as an epoxyalkane, first name the
alkane chain or ring to which the O atom is attached, and
use the prefix “epoxy” to name the epoxide as a
substituent. Use two numbers to designate the location
of the carbon atoms to which the O’s is bonded.
Nomenclature of EpoxidesNomenclature of Epoxides
4. 4
• Epoxides can also be named as derivatives of oxirane,
the simplest epoxide having two carbons and one oxygen
atom in a ring.
• The oxirane ring is numbered to put the O atom at
position one, and the first substituent at position two.
• In most of cases, no number is used for a substituent in a
monosubstituted oxirane.
O O
CH3
oxirane
(epoxyethane)
2-methyloxirane
(epoxypropane)
O
CH3
O
CH3H3C
O
CH3
CH3
2-ethyloxirane
(1,2-epoxybutane)
2,3-dimethyloxirane
(2,3-epoxybutane) 2,2-dimethyloxirane
(1,2-epoxy-2-methy-
propane)
5. 5
• Epoxides are also named as alkene oxides, since they
are often prepared by adding an O atom to an alkene. To
name an epoxide in this way:
Mentally replace the epoxide oxygen with a double
bond.
Name the alkene.
Add the word oxide at the end.
H2C CH2
O
H2C
CH3
O
CH3
ethene
(ethylene)
oxirane
(ethylene oxide)
prop-1-ene
(propylene)
2-methyloxirane
(propylene oxide)
6. 6
Preparation of epoxidePreparation of epoxide
There are two main laboratory methods for the preparation of
epoxides:
Epoxidation of alkenes by reaction with peroxy acids.
Base-promoted ring closure of vicinal halohydrins.
7. 7
Epoxidation of alkenes – oxidationEpoxidation of alkenes – oxidation
Epoxides are very easy to prepare via the reaction of an alkene with a peroxy acid.
This process is known as epoxidation.
C C R O
O
O
H
O
R O
O
H+ +
alkene
peroxy acid
Epoxide
Carboxylic acid
oxidation
A commonly used peroxy acid is
-peroxyacetic acid (CH3CO2OH),
- peroxy trifluoroacetic acid, peroxy
- benzoic acid,
- peroxy m-chlorobenzoic acid, etc.
Also carried out by using hydrogen peroxide and NaOH.
8. 8
Epoxidation of alkenesEpoxidation of alkenes
Epoxidation of alkenes with peroxy acids is a syn addition to the double bond.
Substituents that are cis to each other in the alkene remain cis in the epoxide;
substituents that are trans in the alkene remain trans in the epoxide.
C
C
H
H
H3C O
O
O
H
O
H
H
H3C O
O
H+ +oxidation
Peracetic acid
acetic acid
(E)-1,2-diphenylethene
(2R,3R)-2,3-diphenyloxirane
O
H H
(2R,3S)-2,3-diphenyloxirane
Not forming isomeric -
9. 9
Epoxidation of alkenesEpoxidation of alkenes
Addition of an O atom from either side of the trigonal planar cis-alkene leads to the same
compound - an achiral meso compound that contains two stereogenic centers.
H3C O
O
H+
acetic acid
O
H
H3C CH3
H
C C
H
CH3H3C
H
(Z)-but-2-ene
H3C O
O
O
H+ oxidation
Peracetic acid O
H
CH3H3C
H
+
O is added
below the plane
O is added
above the plane
(2R,3S)-2,3-
dimethyloxirane
(2S,3R)-2,3-
dimethyloxirane
pair of achiral meso compounds are formed
Addition of an O atom from either side of the trigonal planar trans-alkene leads to the
same compound - an enantiomeric compounds.
C C
H
CH3H
H3C
H3C O
O
O
H
O
H
H3C
CH3
H+ oxidation
Peracetic acid(E)-but-2-ene
O
H
CH3
H3C
H
+ + H3C O
O
H
acetic acid
O is added
below the plane
O is added
above the plane
(2R,3R)-2,3-
dimethyloxirane
(2S,3S)-2,3-
dimethyloxirane
pair of enantiomers are formed
12. 12
• Organic compounds that contain both a hydroxy group and a halogen atom on
adjacent carbons are called halohydrins. It was synthesized by the treatment of
halogen followed by hydrolysis.
• In halohydrins, an intramolecular version of the Williamson ether synthesis can
occur to form epoxides.
From vicinal halohydrinsFrom vicinal halohydrins
C C
Alkene
C C
O
Epoxide
X2
HOH
C C
XHO
HO-
vicinal halohydrin
13. 13
From vicinal halohydrins - stereochemistryFrom vicinal halohydrins - stereochemistry
Substituents that are cis to each other in the alkene remain cis in the epoxide.
This is because formation of the bromohydrin involves anti addition, and the
ensuing intramolecular nucleophilic substitution reaction takes place with
inversion of configuration at the carbon that bears the halide leaving group.
C C
HH
C C
HH
O
Br2
HOH
HO-
vicinal halohydrin
C C H
Br
HO
H
C C H
Br
-
O
H
-Br-
(Z)-but-2-ene
cis-2,3-epoxybutane
(2R,3S)-2,3-dimethyloxirane
C C
H
H
C C
H
H
O
trans-2,3-epoxybutane
Br2
HOH
HO-
vicinal halohydrin
C C
Br
HO
H
H
C C
Br
-
O
H
H
-Br-
(E)-but-2-ene
(2R,3R)-2,3-dimethyloxirane
15. 15
Reactions of EpoxidesReactions of Epoxides
• Recall that epoxides do not contain a good leaving group.
• Epoxides do contain a strained three-membered ring with two polar
bonds.
• Nucleophilic attack opens the strained three-membered ring, making it a
favorable process even with a poor leaving group.
16. 16
• The reaction occurs readily with strong nucleophiles and with acids like HZ,
where Z is a nucleophilic atom.
17. 17
• Virtually all strong nucleophiles open an epoxide ring by a two-step reaction
sequence:
• In step 1, the nucleophile attacks an electron-deficient carbon, thus cleaving the
C—O bond and relieving the strain of the three-membered ring.
• In step 2 the alkoxide is protonated with water to generate a neutral product with
two functional groups on adjacent atoms.
• Common nucleophiles that open the epoxide ring include ¯OH, ¯OR, ¯CN, ¯SR
and NH3. With these strong nucleophiles, the reaction occurs by an SN2
mechanism.
19. 19
Let’s now consider the stereochemical consequences of the reaction of 1,2-
epoxycyclohexane with ¯OCH3.
Nucleophilic attack of ¯OCH3 occurs from the backside at either C—O bond,
because both ends are similarly substituted. Since attack at either side occurs with
equal probability, an equal amount of the two enantiomers (i.e. a racemic mixture)
is formed.
21. 21
• Acids HZ that contain a nucleophile Z also open epoxide rings by a two-step
sequence.
• HCl, HBr and HI, as well as H2O and ROH in the presence of acid, all open an
epoxide ring in this manner.
23. 23
• Ring opening of an epoxide with either a strong nucleophile
or an acid HZ is regioselective because one constitutional
isomer is the major or exclusive product.
• Note that the site selectivity of these two reactions is exactly
opposite.
24. • Epoxides can be opened by many strong nucleophiles.
• Both regioselectivity and stereoselectivity must be
considered.
Ring-opening of Epoxides
25. Question
• What is the product isolated when the epoxide below reacts with
NaOCH3
in CH3
OH?
• A) B)
• C) D)
