Structural Isomers

Question 1

What are isomers?

Answer 1

molecules with the same molecular formula but different chemical structure

Question 2

What are structural isomers?

Answer 2

aka “constitutional” only share the same molecular formula and not the same connectivity

Question 3

What are stereoisomers?

Answer 3

isomers that have the same connectivity as well as molecular formula

Question 4

What are enantiomers?

Answer 4

stereoisomers that are nonsuperimposable but still mirror images, also considered configurational isomers

Question 5

What are diastereomers?

Answer 5

stereoisomers that are not superimposable and also not mirror images, also considered configurational isomers, they have two or more stereogenic centers and differ at some but not all of these centers

Question 6

Confirmational vs configurational isomers

Answer 6

Conformational isomers differ in rotation around single bonds while configurational isomers can only be interconverted by breaking and reforming covalent bonds

Question 7

What are the two types of staggered conformations?

Answer 7

anti and gauche

Question 8

What is an anti conformation?

Answer 8

when the two largest groups are antiperiplanar (same plane, opposite sides)

Question 9

What is gauche conformation

Answer 9

when the two largest groups are 60 degrees apart

Question 10

What is the eclipsed conformation?

Answer 10

when the two largest groups are 120 degrees apart and overlap with hydrogen molecules on the other carbon behind them

Question 11

What is totally eclipsed conformation?

Answer 11

when the two largest groups are completely overlapped - synperiplanar

Question 12

What are the types of ring strain?

Answer 12

Angle strain - when bond angles deviate from their ideal values by being stretched or compressed
Torsional strain - when cyclic molecules must assume conformations that have eclipsed or gauche interactions
Nonbonded strain - when nonadjacent atoms or groups compete for the same space

Question 13

Axial

Answer 13

when hydrogen atoms are perpendicular to the plan of the ring (sticking up or down)

Question 14

Equatorial

Answer 14

when hydrogen atoms are parallel to the plane of the ring (sticking out), preferred by the larger group

Question 15

cis vs trans

Answer 15

cis - when both substituents are located on the same side of the ring
trans - when the substituents are on opposite sides of the ring

Question 16

Optical isomers

Answer 16

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

Question 17

What makes a molecule chiral?

Answer 17

its mirror images cannot be superimposed on the original object; molecule lacks an internal plane of symmetry, aka “handedness”

Question 18

What makes a molecule achiral?

Answer 18

its mirror images can be superimposed

Question 19

How does a polarizing compound work?

Answer 19

Ordinary light is waves vibrating in all possible planes perpendicular to its direction of propagation. After passing through a polarizer, it produces light oscillating in one direction/plane

Question 20

What does it mean for a molecule to be dextrorotatory?

Answer 20

the compound rotates the plane of light to the right/clockwise and is labeled with a D

Question 21

What does it mean for a molecule to be levorotatory?

Answer 21

the compound rotates the plane of light to the left/counterclockwise and labeled with a L

Question 22

What is the specific rotation equation?

Answer 22

[alpha] = alpha(subobs)/ c x l
[alpha] = specific rotation in degrees
alpha(subobs) = observed rotation in degrees
c = concentration in g/mL
l = path length in dm

Question 23

What is a racemic mixture?

Answer 23

when (+)/(D) and (-)/(L) enantiomers are present in equal concentrations

Question 24

For any molecule with n chiral center, there are how many possible sterioisomers?

Answer 24

2^n

Question 25

What determines optical activity?

Answer 25

the molecule must have chiral centers but also lack a plane of symmetry

Question 26

What are meso compounds?

Answer 26

molecules that have chiral centers and also an internal plane of symmetry, these are not optically active

Question 27

Cahn-Ingold-Prelog priority rules

Answer 27

priority is assigned based on the atom bonded to the double-bonded carbons

(1) higher the atomic number, higher priority
(2) if atomic numbers are equal, move outward and use rule 1 again

Question 28

What is the difference between a (E) and (Z) molecule?

Answer 28

Z is given to the molecule that has the two highest priority substituents on the same side of the double bond
E is given for the opposite

Question 29

How do you determine if a molecule is (R) or (S)?

Answer 29

(1) Figure out which substituent is highest priority and label each substient in descending priority
(2) Try inverted your visual image so lowest priority atom is in the back of the molecule (or just know that you need to flip your final answer otherwise)
(3) Draw a circle from highest priority to lowest, if circle is counterclockwise/left, it’s (S), otherwise it’s (R)
(4) Do this for each chiral center