2.2 Conformational Isomers

Question 1

conformational isomers (conformers)

Answer 1

are the same molecule, only at different points in their natural rotation around single (sigma) bonds

recall: single bonds are free to rotate

Question 2

Newman projections

Answer 2

a drawing that helps visualize the 3-dimensional structure of a molecule

there are drawn looking straight down a carbon-carbon bond

Question 3

name the 2 conformations of straight chain conformational isomers

Answer 3

staggered and eclipsed

Question 4

2 types of staggered conformation

Answer 4

anti conformation: the largest groups are opposite (180 degrees apart)

gauche conformation: the largest groups are close together (60 degrees apart)

Question 5

Eclipsed conformation

Answer 5

this conformation has groups directly in front of each other

totally eclipsed: highest energy state, 2 largest groups are in the same plane on the same side

Question 6

totally ecliped conformation

Answer 6

the two largest groups are directly in front of each other and strain is at a maximum

Question 7

explain the potential energy vs degree of rotation plot of butane

Answer 7

higher potential energy = less stable

Question 8

3 factors that can result in ring strain

Answer 8

angle strain, torsional strain, steric strain (non bonded strain)

Question 9

angle strain

Answer 9

results when bond angles deviate from their ideal values by being stretched or compressed

Question 10

torsional strain

Answer 10

results when cyclic molecules must assume conformations that have eclipsed or gauche interactions

Question 11

steric strain (van der waals repulsion)

Answer 11

results when nonadjacent atoms or groups compete for the same space

ex. this is the main type of strain occuring in flagpole interactions

Question 12

what is the dominant source of strain in flagpole interactions

Answer 12

non bonded strain / steric strain

Question 13

what is the most stable conformation of cyclohexane

Answer 13

the chair conformation

Question 14

what are the 3 most common conformations of cyclohexane

Answer 14

chair

boat

twist (scew-boat)

Question 15

what occurs during a chair flip

Answer 15

the chair passes through the “half-chair” conformation

all equatorial groups become axial and vice versa

all dashes and wedges _remain the sam_e (components point UP stay pointing UP)

Question 16

which position do bulky groups favour in a boat conformation

Answer 16

equatorial

Question 17

axial vs equatorial

Answer 17

axial (black): group is perpendicular to the plane of the ring (stick up or down)

equatorial (red): parallel to the plane of the ring (stick out)

Question 18

cis vs trans rings

Answer 18

cis: both groups are on the same side of the ring (up/up or down/down)
trans: both groups are on opposite side of the ring (up/down)

Question 19

how to convert between ring and chair diagrams

Answer 19

wedges = UP

dashes = DOWN

ups alternate from equatorial to axial

Question 20

drawing bonds for chair conformations

Answer 20

each carbon has an “up” and a “down”
the “up”s and “downs” of each type alternate