Ochem Exam 2

Question 1

A molecule with 0 chiral centers is…
A molecule with 1 chiral center is…

Answer 1

0 - achiral
1 - chiral

Question 2

If a molecule’s chiral centers are all the same configuration, then it is a…

Answer 2

identical molecule

Question 3

Can a molecule with 1 chiral center have a diastereomer? Why or why not?

Answer 3

No, because if the R/S stays the same it is identical, but if the single R/S changes it is is an enantiomer because “all” the R/S changed.

Question 4

If a molecule has multiple chiral centers but is overall symmetric, it is a…

Answer 4

meso compound

Question 5

In a molecule with multiple chiral centers, when all the R/S stay the same then the molecules are…

Answer 5

identical

Question 6

In a molecule with multiple chiral centers, when all the R/S change then the molecules are…

Answer 6

enantiomers

Question 7

In a molecule with multiple chiral centers, when only some of the R/S change then the molecules are…

Answer 7

diastereomers

Question 8

Achiral molecules have a —— mirror image.

Answer 8

superimposable

Question 9

Chiral molecules have a ——- mirror image.

Answer 9

non superimposable

Question 10

What is a chiral center?

Answer 10

a carbon with 4 different groups

Question 11

What are the differences in properties between enantiomers and diastereomers?

Answer 11

enantiomers: some properties except how they interact with chiral things
diastereomers: different properties

Question 12

What is a racemic mixture?

Answer 12

50/50 mix of enantiomers

Question 13

When the 4th group is in the back, what direction are the R and S configurations?

Answer 13

clockwise - R
counterclockwise - S

Question 14

When the 4th group is in the front, what direction are the R and S configurations?

Answer 14

clockwise - S
counterclockwise - R

Question 15

When you prioritize groups for chiral center configurations, how do you name them?

Answer 15

highest atomic number gets highest priority
if the atom is the same, move out to the first point of difference

Question 16

What bonds are formed and broken in each of the types of reactions?

Answer 16

substitution - break sigma and form sigma
elimination - break 2 sigma and form a pi
addition - break a pi bond and form 2 sigma

Question 17

What do the thermodynamics and kinetics tell you about a reacation?

Answer 17

thermodynamics - equilibria/extent/stability
kinetics - rate/speef

Question 18

What does the Keq tell you about a reaction?

Answer 18

> 1 product favored (exo)
<1 reactant favored (endo)

Question 19

What does the dH or dG tell you about a reaction?

Answer 19

• product favored (exo)
  + reactant favored (endo)

Question 20

How do you write the rate equation for a reaction?
SN1 and SN2 specifically

Answer 20

rate=k[reactants of the rate determining step]
SN1 - rate=k[substrate]
SN2 - rate=k[substrate][nucleophile]

Question 21

The rate determining step is the step with the…

Answer 21

highest transition state

Question 22

How does activation energy affect the rate of a reaction? How is activation energy affected by catalyst and temperature?

Answer 22

higher Ea = slower
lower Ea = faster

higher temp lowers Ea and increases rate
lower temp increases Ea and slows the rate

Question 23

SN1 has (1/2) step(s) and (does/does not) have an intermediate

Answer 23

2 step, does have an intermediate

Question 24

SN2 has (1/2) step(s) and (does/does not) have an intermediate

Answer 24

1 step, does not have an intermediate

Question 25

What types of substrate are preferred by SN1 and SN2? Why?

Answer 25

SN1 - tertiary, secondary, and primary if it has resonance. Carbocation needs stabilized by the branches.
SN2 - primary, secondary, NEVER tertiary. Needs room for the “backside approach” to happen.

Question 26

How many structures can be produced from an SN1 reaction? Why?

Answer 26

2, because the Nu can bond at the front or back of the carbocation

Question 27

In an SN1 reaction, what happens if the 2 structures produced are enantiomers vs diastereomers?

Answer 27

enantiomers - 50/50 racemic mixture
diastereomers - some other mix

Question 28

What makes a good leaving group?

Answer 28

weak base, pKa less than 16 (generally I-, Br-, Cl-, but NOT F-)

Question 29

What type of nucleophile is best for a SN1 and SN2 reaction?

Answer 29

SN1 - must be weak, pKa <4.8, as strong/stronger acid form than carboxylic acid (such as O- halogens-, OAc-, acetic acid OO-)
SN2 - good nucleophile, pKa>5 (higher than carboxylic acid)

Question 30

What type of solvent is preferred by SN1 and SN2?

Answer 30

SN1 - polar protic (contain an OH)
SN2 - polar aprotic

Question 31

When does inversion occur in an SN2 reaction?

Answer 31

when the backside approach happens on a chiral center

Question 32

For any elimination reaction to happen, there MUST be…

Answer 32

beta hydrogens

Question 33

In an elimination reaction, multiple structures called ——– can be produced if…

Answer 33

regioisomers
the beta hydrogens are not equivalent

Question 34

In an elimination reaction, identical beta hydrogens will produce…

Answer 34

identical products

Question 35

tetrasubstituted alkenes are —– stable than monosubstituted/not substituted alkenes. Why?

Answer 35

more
More R groups (increased substitution) means more electrons are donated to the double bond area, so it becomes more stable.

Question 36

Are cis or trans substituted alkenes more stable? Why?

Answer 36

trans, because the groups are farther apart from eachother