Isomerism

Conformers and Constitutional Isomers

Different conformations of atoms in 3D space result from ___

rotation about single bonds

Newman Projections

representation of different rotation of molecules

Look left, dash

left

Look right, dash

right

wedges become dashes when rotated

180

180 degree dihedral angle

staggered confirmation

60 degree dihedral angle

gauche confirmation

0 degree dihedral angle

eclipsed confirmation

Angles / confirmations lowest to highest energy:

staggered, gauche, 120 degree, eclipsed

Confirmation is based on the dihedral angle of

largest sub on front C and largest sub on back C

steric

presence of branch(es)

branched structures have larger ____ and increased ___, and are higher

surface area, e- repulsion, energy

steric hindrance

branches disallow molecule to lessen e- repulsion

Lowest energy cycloalkanes have ___ degree bond angles, ___ hybridization, and ____ confirmation

109.5, sp³, staggered

high ring strain = ___ relative energy, ___ deviation from 109.5 bond angle

high, greater

torsional strain

how easily the ring can twist

eclipsed confirmation = ___ torsional strain

high

low torsional strain = ___ eclipsing, ___ relative energy, twists ___

less, lower, easily

Cyclopropane and cyclobutane have ___ torsional strain and __ relative energy

high, high

cyclopentane and cyclohexane have ___ torsional strain and ___ relative energy

low, low

cyclohexane is also represented using

chair structure

Chair structure has __ C pointing up and __ C pointing down

3, 3

axial

straight up or straight down bond

bond point up, axial points straight

up

bond points down, axial points straight

down

equatorial subs point ___ from the chair

away

bond point up, equatorial point

down at an angle

bond points down, equatorial points

up at an angle

chair structures DO NOT have

wedges or dashes

axial directions ___ angle direction, equatorial directions ___ angle direction

match, opposite

ring flip of chair structure — middle confirmation = ___

boat

total ring flip —

up C now point down, down C now point up

In a ring flip, axial becomes

equatorial

In a ring flip equatorial becomes

axial

In a ring flip, substituent direction (up or down)

remains the same

A value increases as number of branches ___ (steric interference)

increases

Ring structure is more stable when molecule with higher A value is

equatorial

Structural isomers have the ___ molecular formula but different

same, atom connectivity

Saturated

the highest number of hydrogens allowed are on the molecule — only single bonds no rings

Unsaturated

double bonds, triple bonds, rings — loss of hydrogens on the molecule

ring = __ unsaturations

1

double bond = __ unsaturations

1

triple bond = __ unsaturations

2

Calculate IHD

Number of unsaturations = (2(#C) + 2 - #H - #X + #N) / 2