Conformations

Question 1

define conformation

Answer 1

any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond

Question 2

define a Newman projection

Answer 2

a way to view a molecule by looking along a carbon-carbon bond

Question 3

define torsional or dihedral strain

Answer 3

repulsions between groups on adjacent atoms

Question 4

why do alkane conformations exist?

Answer 4

bond rotations are possible between sp3 hybridised carbons (cylindrical symmetry of a sigma bond)

Question 5

staggered conformation vs eclipsed conformation

Answer 5

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)

Question 6

how does energy relate to stability of a molecule?

Answer 6

less stable -> higher energy

Question 7

define steric strain

Answer 7

repulsive force that arises when atoms not bonded to each other are forced closer to one another (repulsion between electron clouds)

Question 8

anti vs gauche conformation

Answer 8

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

Question 9

do cycloalkanes have more or less flexibility than open-chain alkanes?

Answer 9

cycloalkanes have less flexibility (less conformational freedom)

Question 10

are rings larger than 3 atoms flat?

Answer 10

no; cyclic molecules adopt non-planar conformations to minimise angle and torsional strain

Question 11

define angle strain

Answer 11

arise from bond angles that do not permit maximum orbital overlap between the atoms of a molecule (ideal=109.5’)

Question 12

describe and draw cyclopropane

Answer 12

• 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)

Question 13

describe and draw cyclobutane

Answer 13

• 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

Question 14

describe and draw cyclopentane

Answer 14

• 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

Question 15

describe and draw cyclohexane

Answer 15

• substituted cyclohexane rings occur widely in nature: free of angle strain and torsional strain
• tetrahedral angles between all carbons in the chair conformation

Question 16

distinguish between axial and equatorial positions for chair conformations of cyclohexane

Answer 16

• axial are perpendicular to the ring plane
• equatorial are along ring plane
• every C atom has one axial and one equatorial H position

Question 17

draw the axial and equatorial cyclohexane chair conformation groups

Question 18

how do cyclohexane conformations have mobility?

Answer 18

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

Question 19

why aren’t two conformations of any mono substituted cyclohexane equally stable?

Answer 19

due to 1,3-diaxial interactions: a type of steric strain between axial ring substituents

Question 20

how do diaxial interactions affect the prevalence of a conformation?

Answer 20

a molecule will exist in the conformation where steric hindrance is minimised for longer

Question 21

what impacts the magnitude of steric strain of one H-R 1,3 diaxial interaction?

Answer 21

• size and shape of the substituent affects the magnitude of steric strain.
• bulkier alkyl groups are more stable in the equatorial position

Question 22

define stereoisomers

Answer 22

compounds in which the atoms are connected in the same order but differ in their spatial geometry

Question 23

give an example of stereoisomers

Answer 23

• 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

Question 24

do a conformational analysis of cis-1,2-dimethylcyclohexane

Question 25

do a conformational analysis of trans-1,2-dmethylcyclohexane