Structure and reactivity in organic chemistry
08/10/19
Alicyclic compounds contain at least one non-aromatic ring
E.g.,
Some experimental observations: the proton NMR spectrum of benzene-d5 reveals a single
resonance, regardless of temperature. Whereas, cyclohexane d11 gives one signal at 0
degrees and two signals at -90 degrees
Linking a linear molecule into a ring restricts its possible geometries.
Strain arises from non-ideal molecular geometries and can be classified as:
The total strain = bond length strain + bond angle strain + torsional strain + non-bonded
interactions
Strain can be measured by looking at experimental heat of combustion observations
Torsional strain arises from repulsion between electrons and substituents on adjacent
atoms
E.g., the strain in propane reaches a minimum at an angle of 60 degrees (staggered) and
a maximum at 0 degrees (eclipsed)
, Non-bonded interactions cause strain when groups not directly bonded to one
another are pushed too close together i.e., less than the sum of their VDW radii
E.g., cyclodecane:
Bond length strain arises from compression or stretching
Even small deviations away from ideal bond length are extremely costly
Below is a conjugated system where the observed bond lengths are different
from the typical bond lengths
The ideal sp3-C centre has a bond angle of 109.5 degrees
Bond angle deformation is less costly than changes in bond length
Most stable ring size should be 5 as:
08/10/19
Alicyclic compounds contain at least one non-aromatic ring
E.g.,
Some experimental observations: the proton NMR spectrum of benzene-d5 reveals a single
resonance, regardless of temperature. Whereas, cyclohexane d11 gives one signal at 0
degrees and two signals at -90 degrees
Linking a linear molecule into a ring restricts its possible geometries.
Strain arises from non-ideal molecular geometries and can be classified as:
The total strain = bond length strain + bond angle strain + torsional strain + non-bonded
interactions
Strain can be measured by looking at experimental heat of combustion observations
Torsional strain arises from repulsion between electrons and substituents on adjacent
atoms
E.g., the strain in propane reaches a minimum at an angle of 60 degrees (staggered) and
a maximum at 0 degrees (eclipsed)
, Non-bonded interactions cause strain when groups not directly bonded to one
another are pushed too close together i.e., less than the sum of their VDW radii
E.g., cyclodecane:
Bond length strain arises from compression or stretching
Even small deviations away from ideal bond length are extremely costly
Below is a conjugated system where the observed bond lengths are different
from the typical bond lengths
The ideal sp3-C centre has a bond angle of 109.5 degrees
Bond angle deformation is less costly than changes in bond length
Most stable ring size should be 5 as:
