define conformation
any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond
define a Newman projection
a way to view a molecule by looking along a carbon-carbon bond
define torsional or dihedral strain
repulsions between groups on adjacent atoms
why do alkane conformations exist?
bond rotations are possible between sp3 hybridised carbons (cylindrical symmetry of a sigma bond)
staggered conformation vs eclipsed conformation
staggered - bonds on adjacent C atoms are furthest apart (least torsional strain)
eclipsed conformation - bonds on adjacent C atoms are closest together (most torsional strain)
how does energy relate to stability of a molecule?
less stable -> higher energy
define steric strain
repulsive force that arises when atoms not bonded to each other are forced closer to one another (repulsion between electron clouds)
anti vs gauche conformation
anti: substituents are 180’ from one another -> no torsional strain + no steric strain
gauche: substituents are 60’ from one another -> no torsional train + YES steric strain
do cycloalkanes have more or less flexibility than open-chain alkanes?
cycloalkanes have less flexibility (less conformational freedom)
are rings larger than 3 atoms flat?
no; cyclic molecules adopt non-planar conformations to minimise angle and torsional strain
define angle strain
arise from bond angles that do not permit maximum orbital overlap between the atoms of a molecule (ideal=109.5’)
describe and draw cyclopropane
- 3-membered ring must be planar (flat) with bond angles of 60’
- requires that sp3-sp3 bonds are bent (reduced orbital overlap between carbons)
- all C-H bonds eclipsed (lots of torsional strain)
describe and draw cyclobutane
- if all 4 C’s were flat, there would be less angle strain than cyclopropane but more torsional strain
- torsional strain is alleviated in the butterfly conformation
describe and draw cyclopentane
- planar cyclopentane would have no angle strain but very high torsional strain
- non-planar conformation has very little ring strain and reduced torsional strain
- four carbon atoms are in a plane, fifth is above the plane: envelope conformation
describe and draw cyclohexane
- substituted cyclohexane rings occur widely in nature: free of angle strain and torsional strain
- tetrahedral angles between all carbons in the chair conformation
distinguish between axial and equatorial positions for chair conformations of cyclohexane
- axial are perpendicular to the ring plane
- equatorial are along ring plane
- every C atom has one axial and one equatorial H position
draw the axial and equatorial cyclohexane chair conformation groups
how do cyclohexane conformations have mobility?
chair conformations readily interconvert, resulting in exchange of axial and equatorial positions by a chair-flip
- all axial positions rotate to equatorial positions, while all equatorial positions rotate to axial
- each carbon has one up and one down position; up positions remain up after flip, while down positions remain down
why aren’t two conformations of any mono substituted cyclohexane equally stable?
due to 1,3-diaxial interactions: a type of steric strain between axial ring substituents
how do diaxial interactions affect the prevalence of a conformation?
a molecule will exist in the conformation where steric hindrance is minimised for longer
what impacts the magnitude of steric strain of one H-R 1,3 diaxial interaction?
- size and shape of the substituent affects the magnitude of steric strain.
- bulkier alkyl groups are more stable in the equatorial position
define stereoisomers
compounds in which the atoms are connected in the same order but differ in their spatial geometry
give an example of stereoisomers
- cycloalkanes with two or more substituents
- disubstituted cycloalkanes can exist as cis-trans stereoisomers
trans: two groups up/down on opposite face of ring
cis: two groups up/up or down/down on same face of ring
do a conformational analysis of cis-1,2-dimethylcyclohexane
