Isomers

Question 1

structural (constitutional) isomers

Answer 1

share only their molecular formula

Question 2

physical properties

Answer 2

don’t change the composition of matter; i.e. melting point, boiling point, solubility, odor, and density

Question 3

chemical properties

Answer 3

have to do with the reactivity of the molecule with others and result in changes in chemical composition

Question 4

stereoisomers

Answer 4

share the same atomic connectivity; have the same structural backbone

Question 5

conformational isomers (conformers)

Answer 5

differ in rotation around single bonds

Question 6

configurational isomers

Answer 6

can be interconverted only by breaking bonds

Question 7

staggered conformation

Answer 7

no overlap of atoms along the line of sight

Question 8

anti conformation

Answer 8

the two largest groups are antiperiplanar (in the same plane but on opposite sides)

Question 9

gauche conformation

Answer 9

occurs when the two largest groups are 60 degrees apart

Question 10

eclipsed conformation

Answer 10

must pass through this to convert from anti to gauche; two methyl groups are 120 degrees apart and overlap with the hydrogen atoms on the adjacent carbon

Question 11

totally eclipsed

Answer 11

two methyl groups directly overlap each other; highest-energy state

Question 12

ring strain

Answer 12

depends on angle strain, torsional strain, and non bonded strain

Question 13

angle strain

Answer 13

results when bond angles deviate from their ideal values by being stretched or compressed

Question 14

torsional strain

Answer 14

results when cyclic molecules must assume conformations that have eclipsed or gauche interactions

Question 15

non bonded strain (steric strain)

Answer 15

results when nonadjacent atoms or groups compete for the same space

Question 16

chair conformation of cyclohexane

Answer 16

most stable because it minimizes all three types of strain

Question 17

axial hydrogens

Answer 17

perpendicular to the plane of the ring (stick up or down)

Question 18

equatorial hydrogens

Answer 18

parallel to the plane of the ring (stick out)

Question 19

chair flip

Answer 19

one chair form is converted to the other; all axial Hs become equatorial and vice versa

Question 20

conversions with large groups

Answer 20

bulky groups slow conversions; favor the equatorial position to reduce strain

Question 21

cis

Answer 21

both substituents on the same side

Question 22

trans

Answer 22

substituents on opposite sides

Question 23

optical isomers

Answer 23

different spatial arrangement of groups in these molecules affects the rotation of plane-polarized light

Question 24

chiral

Answer 24

mirror image can not be superimposed on the original object; molecule lacks an internal plane of symmetry

Question 25

achiral

Answer 25

mirror images that can be superimposed

Question 26

chiral center

Answer 26

carbon with four different substituents

Question 27

enantiomers

Answer 27

non superimposable mirror images of each other; same connectivity but differ at every chiral center; identical properties except optical activity and reactions in chiral environments

Question 28

diastereomers

Answer 28

chiral and share same connectivity but are not mirror images of each other; differ at some but not all chiral centers

Question 29

optical activity

Answer 29

refers to the rotation of plane-polarized light by a chiral molecule; enantiomers rotate light to the same magnitude but in opposite directions

Question 30

dextrorotary (d-)

Answer 30

labeled (+); rotates plane of polarized light to the right

Question 31

levorotatory (l-)

Answer 31

labeled (-); rotates plane of polarized light to the left

Question 32

specific rotation

Answer 32

(A)=Aobs/c*l
A=specific rotation in degrees
Aobs= observed rotation in degrees
c=concentration in g/ml
l=path length in dm

Question 33

racemic mixture

Answer 33

when both (+) and (-) enantiomers are present in equal concentrations; do not rotate plane polarized light

Question 34

2^n rule

Answer 34

for any molecule with n chiral centers, it can have 2^n stereoisomers

Question 35

cis-trans isomers

Answer 35

specific subtype of diastereomers in which substituents differ in their position around an immovable bond, such as a double bond or a ring structure

Question 36

meso compounds

Answer 36

a molecule with chiral centers that has an internal plane of symmetry; optically inactive

Question 37

Cahn-Ingold-Prelog priority rules

Answer 37

used to determine E/Z; priority is assigned based on the atom bonded to the double-bonded carbons; the higher the atomic number, the higher the priority

Question 38

Z alkene

Answer 38

two highest priority substituents are on the same side of the double bond

Question 39

E alkene

Answer 39

two highest priority substituents are on opposite sides of the double bond

Question 40

steps to determine absolute configuration at chiral center

Answer 40

1. assign priority by atomic number
2. Arrange molecule with lowest priority sub in the back (or invert stereochemistry by switching two subunits)
3. Draw a circle around the molecule from highest to lowest
4. Clockwise =R, Counterclockwise = S

Question 41

Fischer projection

Answer 41

horizontal lines(wedges)=bonds going out from the plane
vertical line(dashes)=bonds going into the plane