O-chem Exam #2

Singe bonds can _____, which results in multiple 3D structures called _____

rotate + conformations

Newman projections are good for investigating the ______ of different conformations

relative stability

If a hydrogen is on a straight line, then it is going _____

up or down

imagine you are looking at the dash wedge model from the side in which the arrow is pointing

if the arrow is pointing to the right then

• a dash goes left

• wedge goes right

if the arrow is pointing to the left

• the dash is going right

• the wedge is going left

hydrogens are going to orient themselves as far apart as possible

• this is called a dihedral angle

Dihedral angle

60 degree angle between hydrogens, which are trying to get as far apart from each other as possible

staggered conformations are more _______

stable

staggered conformations:

when hydrogens are far apart as possible and have the 60 degree dihedral angle

eclipsed conformations:

hydrogens go in front of each other

eclipsed conformations are less ____ and have more _____

less stable, more energy because of the torsional strain of twisting

Torsional Strain:

• the difference in energy between staggered and eclipsed conformations is caused by the torsional strain

An eclipsed hydrogen = ____ kj/mol

4 kj/mol

So when this happens to three hydrogens in total, it is 12 kj/mol

Energy eclipsing of Propane

• when two hydrogens are eclipsed, it is 4 kj/mol

• when a methyl and a hydrogen are eclipsed, it is 6 kj/mol

• from this you can calculate the total energy of the thing (4+4+6) = 14 kj/mol

Energy eclipsing of Butane

we have TWO ch3 now (methyl), therefore butane has torsional strain = higher energy

what is steric strain?

• this is when the steric hit each other, they bump into each other when twisting

In an ANTI conformation, methyls are _______ from each other as possible with an angle of _______

as far away from each other as possible = lowest energy

they are 180 degrees apart

when methyl groups are eclipsed, it is the _____

highest possible energy conformation

when methyls are eclipsed, it is ______ kj/mol

11 kj/mol

A Gauche conformation is when methyls are ____ to each other and have an angle of _____

next to each other ____ 60

ideal bond angle for sp3 hybridized carbon is

109.5 degrees

if cycle alines were flat, each carbon in the ring would experience

ring strain

cyclohexane is most stable because it can configure where all angles are ___

in a chair conformation

cyclohexane has no _______ in the _____ conformation

ring strain + chair conformation

in cyclohexane is in a chair formation the bond angles are at ____

109.5 degrees —- no torsional strain

____ is the least stable conformation of cyclohexane

half-chair

______ is the most stable conformation of cyclohexane

chair

each carbon in the ring/chair has two substituents:

• axial position (sticking up and down)

• equatorial position (sticking to the side)

axial substituents point:

straight up and down, alternating around the ring, clockwise around the ring

the methyl group is most stable when in the ______

equatorial position

Why is the methyl group more stable in the equatorial position?

it sticks out further away from the rest of the molecule (less strain, less steric strain)

When the methyl is in the axial position, the methyl group causes _____

steric strain that destabilizes the conformation

In other words, the equitorial position reduces _________ interactions/strain

gauche