chem 51a midterm 2

prof vanderwal

chair conformation stability

• if you have one group on the ring the more stable chair will be the one with the group in an equatorial position

• two groups best if they occupy equatorial position 

• if only one can be equatorial in either chair then the more stable will be the one with the larger group in the equatorial position 

wedge and dash

wedged = up

dash = down

*no correlation between up/down and axial/equatorial

newman

• most stable is staggered specifically anti

cis

two groups are up or down

trans

one group is up and the other is down

gauche interactions in in cyclohexane rings

• 2 guache interactions for each axial groups in cyclohexane ring

• 1 guache interaction for each equatorial in cyclohexane ring

methane

1

ethane

2

propane

3

butane

4

pentane

5

hexane

6

heptane

7

octane

8

nonane

9

decane

10

H-H eclipsing

0.9kcal/mol

H-CH3 eclipsing

1.4kcal/mol

dihedral angle

chiral

• objects that exist as enantiomers

• all molecules w/ exactly one stereogenic center are chiral

• a molecule w/ no stereogenic center are usually not chiral

• OPTICALLY ACTIVE

achiral

• molecules contain a plane of symmetry

stereogenic center

• only 3° and 4° carbons can be stereogenic

• any c with 4 diff substituents

• DONT GET FOOLED BY THE RING

drawing enantiomers 

• draw mirror image 

OR

• exchange any 2 substituents dash/wedge

meso compounds 

• molecules w/ stereogenic centers; have a plane of symmetry & mirror images are identical 

• mus thave at least two stereogenic centers

racemic mixture

• an equal amount of two enantiomers 

• optically inactive 

• no rotation (=0°) is observed b/c two enantiomers rotate PPL to an equal extent in opp directions 

roation of polarized clockwise vs counterclockwise

• clockwise = d or +

• counterclockwise = l or -

• two enantiomers rotate plane polarized light to an equal extent but in opposite directions

physical properties of isomers

• Constitutional isomers and diastereomers have different physical properties.

• enantiomers have the same physical properties 

  • can not be separated by physical properties

enantiomeric excess