HETEROCYCLES OF 3, 4 AND 7 MEMBERS

cover models

The properties of three-membered rings are related to strain, due to the low bond angles. Angular strain gives rise to opening reactions to form acyclic products.


In this section we will study the following heterocycles: oxirane, thiirane, 2H-azirine, aziridine, dioxirane, oxaziridine, 3H-diazirine and diaziridine.

oxirano


Boiling point: 10.5ºC
Ring stress: 114 KJ/mol
dipole moment: 1.88 D
Characteristics: Colorless liquid, soluble in water, very poisonous in the gas phase.

tiirano


Boiling point: 55ºC
Ring stress: 83 KJ/mol
dipole moment: 1.66 D
Characteristics: Colorless liquid, partially soluble in water.

2h-azirina


Boiling point:
Ring stress: 170 KJ/mol.
dipole moment:
Characteristics: Very unstable substance, with high reactivity in the C=N bond

aziridina

Boiling point: 57ºC
Ring stress: 113 KJ/mol.
dipole moment: 1.89 D
Characteristics: Colorless liquid, soluble in water, poisonous, with an ammonia-like odor.

Physical properties: It presents an annular tension of 114 KJ/mol. The dipole moment is 1.88 D
Spectroscopic data: UV spectrum: λ=171 nm. NMR : δH= 2.54, δC= 39.7

propiedades-fisicas

Intramolecular cyclization (Williamson intramolecular): Halogenated alcohols in position 2 cyclize in basic media to form oxiranes.

sintesis williamson del oxirano

Mechanism:

Mecanismo síntesis de Williamson

Oxiranes act as Lewis bases, protonating on oxygen in acidic media. The second type of reactivity is related to ring strain that favors opening reactions.

Isomerization to carbonyl compounds: Oxiranes in the presence of Lewis acids isomerize to form aldehydes or ketones. Thus, oxirane in the presence of BF 3 is transformed into acetaldehyde

oxirano-reactividad
Example: Propose a mechanism for the following transformation:

oxirano-reactividad-02.gif


Solution - The BF 3 protonates the oxygen of the oxirane, turning it into a good leaving group and favoring the opening of the cycle. A rearrangement to a hydroxycarbocation will give the expected product.

oxirano-reactividad-03.gif