OChem 8A - Ch4

isomers- arrangement of isomers in space

Isomers

compounds with the same molecular formula but different structures

falls into two main classes:

• constitutional isomers

• stereoisomers

Constitutional isomers

differ in the way their atoms are connected

ex) ethanol and dimethyl ether have same formula C2H6O but their atoms are connected differently - (the oxygen in ethanol is bonded to a carbon and to a hydrogen, whereas the oxygen in dimethyl ether is bonded to two carbons).

Cis-trans isomers / geometric isomers result from…

restricted rotation

what causes restricted rotation in cis-trans isomers?

cyclic structure or a double bond

cis isomer

compound with the substituent (in example H) on the same side of the double bond

trans isomer

compound with the substituents (H in example) on opposite sides of the double bond

cis and trans have ___ molecular formula and ___ bonded atoms but ____ configurations.

same, same, different;

differ in the way atoms are oriented in space

trans isomers have dipole moments of ____ and why?

0 because of the dipole moments their individual bonds cancel

conformations / conformers

different spatial arrangements of the same compound (ex: anti & gauche conformers)

they cannot be separated

some conformers are more stable than others

configurations/ stereoisomers

different compounds (ex. cis and trans isomers)

can be separated from each other

bonds have to be broken to interconvert compounds with different configurations

priority depends on

• atomic number - relative priority depends on the atomic numbers of the artoms bonded directly to the sp2 carbon. the greater atomic number higher priority.

• If there is a tie (same two atoms attached to sp2 carbon) consider the atomic # of atoms attached to tied atoms

• if an atom is doubly bonded to another, treat it as if it were singly bonded to two of those atoms

• if an atom is triply bonded, treat it as if it were singly bonded to 3 of those atoms

• cancel atoms that are identical in the two groups; use the remaining atoms to determine the group with higher priority

chiral

nonsuperimposable mirror image

• ex) hand, ear, seat config. in a car

achiral

object is the same as its mirror image → superimposable

a chiral molecule has…

• an asymmetric center - an atom bonded to four different groups

• (only sp3 carbons can be asymmetric centers)

enantiomers

molecules that are non superimposable mirror images of each other

enantiomers = chiral

each member of a pair of enantiomers is chiral because it has a non superimposable mirror image (like the hand, ear, car seats img)

how to see if a molcule is achiral

• mirror image is superimposable

• check by rotating the molecule clockwise

R is

• to the right - clockwise

S is

to the left - counterclockwise

(latin root → sinister for left)

if lowest priority isn’t on hatched switch it with the hatched bond and if it was moved once is the opposite (if after switch it was R it was originally S)

Recognizing pairs of enantiomers

• determine configurations

• if one is R and other is S then enantiomers

• if both R or S they’re identical compounds.

chiral compounds are what optical activity

optically active - rotates the plane of polarization

• (+)= clockwise rotation

• (-) = counterclockwise rotation

achiral compounds are what optical activity

optically inactive - does not rotate the plane of polarization

2ⁿ = max number of stereoisomers

n= number of asymmetric centers

meso compound

• has asymmetric centers but achiral, superimposable on mirror image

• has two or more asymmetric enters and a plane of symmetry

“in the case of cyclic compounds, the cis isomer will be a meso compound and the trans isomer wil be a pair of enantiomers”