1-8 structural o chem Flashcards
enantiomers
mirror images.
non superimposable
no plane of symmetry
diastomers
no plane of symmetry
not all chiral centres have change
interatomic bonding
Different interatomic distances,
so different dipole moments, melting points,
boiling points, IR and NMR spectra,
solubilities, chromatographic properties etc
chirality can arise from
a “chiral axis” where A= A’ are not equal
for chiral axis molecules what does the ability to inconvert mean?
One of the transition states is accessible, \ mirror image forms can interconvert
transition states are
achiral
steric clashes
high energy
BINAP
stable. cannot interconvert
allenes
Two double bonds with a common carbon – each bond needs a different p orbital so
planes must be orthogonal.
amines are achiral as
lone pair can interconvert to mirror image
Pyramidal N is not usually a stereogenic centre, due to rapid inversion
P groups cannot as
more core electrons. cannot inverconvert Pyramidal P can be a stereogenic centre, enantiomers can arise
how to calculate the number of potential wells
3N-6 dimensions, where N = no. of atoms
rotations give what
Rotation gives an infinite number of conformation
what is a conformer
Different shapes possible for a single molecule (usually related by rotations about single bonds)
what are conformer
conformations at the lowest energy
torsion bond
bond between two chemical bonds
syn-periplanar
maxima / eclipsed 0
anticlinal
60 lower/second minima staggered
gauche
120 eclipsed maxima
anti-periplanar
180 true mimina / eclipsed
gauche
240 eclipsed maxima
difference between different conformers
difference between conformers is
temperature.
calculate K relating to conformers
K = [anti-periplanar] /
[gauche]
= e ^ -G/RT
why does R point away from R2 in esters and amides
Steric effect between Rs and overlap between lone pairs and empty σ* (overlap between full and empty orbitals is always stabilising).
ester N is more electropositive than O therefore…
the resonance structure has a greater influence so much that it can appear in H NMR
what is prefered for amides, Z or E?
Z far more stable (steric + some electronic effect?) Only one set of signals in NMR, structures predictable
Baeyer strain.
Strain due to distorted bond angle θ
3 rings are
planar
when the internal angle is lower tha 109 what does this mean?
raised energy
talk about whether bent or planar conformations are prefered for 4 rings structures
For bent conformation, θ < 90o, so Baeyer strain increases but Planar conformation suffers from Pitzer strain
– strain due to suboptimal torsion angle . bent is then prefered. as Lower Pitzer strain compensates for increased Baeyer strain
7 membered rings have
but still some Pitzer strain even with their Most stable conformation, same with 8.
staggered conformations
Overlap of filled orbital with empty
orbital is always a stabilising factor
(lowers energy)
eclipsed conformation
Overlap of filled orbitals is always a
destabilising factor (raises energy).
what influences conformation changes ?
energy barrier if it is small there will be many interconversions
