stereoisomerism in alkanes

Question 1

what is eclipsed form (ethane/propane)

Answer 1

hydrogens of one carbon is direcly behind the other, angle is 0 degrees

maximum repulsion

Question 2

what is staggered form (ethane/propane)

Answer 2

angle is 60 degrees

Question 3

what is gauche form (ethane/ propane)

Answer 3

any other angle

Question 4

is

which conformation of ethane/propane is the most and the least stable

Answer 4

most stable- staggered
least stable- eclipsed

Question 5

what is tortional barrier

Answer 5

energy difference between two extreme conformations of ethane and propane

ethane- 3kcal/mol
propane- 3.4kcal/mol

Question 6

dihedral angle in staggered (anti) of butane

Answer 6

180 degrees

Question 7

dihedral angle in eclipsed (partial) of butane

Answer 7

240 or 120 degrees

Question 8

dihedral angle in fully eclipsed of butane

Answer 8

0 degrees

Question 9

dihedral angle in skew/guache of butane

Answer 9

60 or 300 degrees

Question 10

most and least stable conformation of butane

Answer 10

most stable- staggered (anti) at 180 degrees
least stable- fully eclipsed at 0 degrees

Question 11

order of stability of conformations of butane

Answer 11

anti/staggered > guache/skew > partially eclipsed > fully eclipsed

however in some cases gauche is more stable than anti because of intermolecular H-bonding and dipole-dipole attraction

Question 12

tell the formula of

mole fraction of anti and guache conformations

Answer 12

dipole moment of compound = dipole of anti(mole frac of anti) + dipole of guache(mole fraction of guache)

Question 13

how many axial and equatorial hydrogen in chair and boat conformations of cyclohexane

Answer 13

each carbon atom has one axial and one equatorial hydrogen, so six each.

Question 14

which conformation of cyclohexane is more stable?

chair or boat?

Answer 14

chair is more stable than boat

as adjacent hydrogens are quite staggered

the energy difference between the two is very small so they havent been isolated so far

Question 15

where are the flagpole hydrogen positions in each of the conformations of cyclohexane

chair and boat

(C1 AND C4)

Answer 15

chair- trans side
boat-cis side

so steric hindrance is increased in boat and stability is decreased

Question 16

stability order of conformations of cyclohexane

Answer 16

chair> twist boat> boat> half chair

also, cyclohexane with CH3 in equatorial position is more stable than it in axial position