Week 4: stereochemistry of alkanes and cycloalkanes

Question 1

torsional strain

Answer 1

eclipsed conformer

Question 2

staggered

Answer 2

low energy, spread out, no overlap

Question 3

steric strain

Answer 3

GAUCHE: non-bonded atoms forced close

Question 4

steric strain angle

Answer 4

dihedral 60 degrees

Question 5

anticonformation

Answer 5

substituent groups 180 degrees apart, lowest energy

Question 6

gauche

Answer 6

steric strain: staggered (no torsional strain) but substituents too close (60 degrees)

Question 7

Which is higher energy?
staggered gauche vs eclipsed torsional in butane?

Answer 7

torsional

Question 8

trans

Answer 8

opposite

Question 9

cys

Answer 9

same side

Question 10

Why do we draw alkane chains as zig zags?

Answer 10

anticonformation + tetrahedral

Question 11

Why do cycloalkanes with more than 3 carbons adopt non-planar conformations?

Answer 11

minimize angle strain and torsional strain

Question 12

angle strain

Answer 12

strain from maintaining an angle other than the preferred sp3 tetrahedral 109 degrees

Question 13

Why is cyclopropane weakly bonded?

Answer 13

high torsional strain (H eclipsed) and
angle strain (60 degrees) so orbitals must bend to bond

Question 14

cyclobutane

Answer 14

paper-plane conformation
3 in plane, 1 above
lowers torsional strain, increases angle strain

Question 15

cyclopentane

Answer 15

envelope conformation
4 in plane, 1 above
lowers torsional strain, increases angle strain

Question 16

What is the strain with two large subs in a cyclohexane? What position are they in?

Answer 16

1-3 diaxial interaction
axial position

Question 17

iso

Answer 17

branched

Question 18

What position is most stable for large subs? Why?

Answer 18

equatorial
no/less 1-3 diaxial interactions