Lecture Quiz 4 - Lectures 16 - 20

Question 1

halogenation can create (achiral/chiral) centers

Answer 1

halogenation can create chiral centers

Question 2

if you create a chiral center from achiral starting material, we get a (achiral/chiral) product

Answer 2

achiral start
create chiral center

=ACHIRAL product (racemic mix, 1:1 R/S 0 optic activity)

Question 3

if you react a chiral center with achiral intermediate, we get a (achiral/chiral) product

Answer 3

chiral start
ACHIRAl intermediate

=ACHIRAL product (racemic)

Question 4

an existing chiral center has influence on ?

Answer 4

transition state

Question 5

when we form diastereomers, do we get a racemic mix?

Answer 5

NO - no equal mix

Question 6

If we start with a chiral molecule that passes through a chiral transition state, we get a (achiral/chiral) product

Answer 6

chiral start
chiral TS

CHIRAL product

Question 7

Order from largest to smallest
F, cl, br, i

Answer 7

I>br>cl>f

Question 8

increase in size of an halogen will

(inc/dec) bond length
(inc/dec) bond strength

(inc/dec) boiling pt
(add/subtract) dispersion
(inc/dec) solubility

Answer 8

Size increase =
-increase Bond length and boiling pt
-decrease bond strength and solubility
-add dispersion

Question 9

What are the 2 rules of nucleophilic substitution?

Answer 9

1) mechanism steps –> constant charge
2) can’t exceed octet rule

Question 10

What is the rate law of SN2?

Answer 10

rate law = k[R-X][Nu-]

Question 11

What has better orbital overlap? front side attack or back side?

Answer 11

back side attack

Question 12

When you do backside attack, the LG and Nu- are ??? degrees apart

Answer 12

180

Question 13

When we do front side attack, what happens to the sterochemistry?

Answer 13

maintain stereo

Question 14

When we do back side attack, what happens to the stereochemistry?

Answer 14

stereochem inverse

Question 15

Why do we do backside attack for SN2?

Answer 15

b/c product inverts stereochemistry at reacting C

Question 16

When we do backside attack, we (always/sometimes/never) invert the 3D structure and we (always/sometimes/never) change the R/S

Answer 16

always invert
sometimes change

Question 17

What do we do if we have a stereocenter and we don’t want to flip it?

Answer 17

Reacting twice

Question 18

T/F All stereocenters are inverted at SN2?

Answer 18

F, we only invert at the carbon/func group that reacts

Question 19

What makes SN2 work?

Answer 19

1) LG has to leave
2) Nu- has to approach
3) Structure needs space for Nu- to approach and needs to invert
4) Solvent - faster rxn if it helps Nu- dissolve faster or helps TS

Question 20

What makes a good LG?

Answer 20

Leaving with e-
Has to be willing to take the (-) charge
Will become a conjugate base

Question 21

A good acid loses H+ and forms an anion. A good LG loses?

Answer 21

Carbon - forms anion

Question 22

The (weaker/stronger) the conjugate acid, the better the LG

Answer 22

Stronger conjugate acid

Question 23

T/F If there are 2 LGs, the one with the lower pkA will leave first

Answer 23

True = Lower pKA, stronger acid = better LG = will leave first

Question 24

What makes a good nucleophile?

Answer 24

Has an extra e- and wants to give them away

Question 25

Nucleophilicity is about (thermodynamics/kinetics)

Answer 25

Nu- = kinetics = fastest rate = best Ea

Question 26

What is a better Nu-? HO- or H2O

Answer 26

HO- because it is an anion

Question 27

What is better? NH3 or H2O?

Answer 27

NH3 = less electronegative.

Question 28

Why are molecules moving down the periodic table better nucleophiles?

Answer 28

increase in size increases orbital reach with increases polarizability as a neutral molecule easier to share e- from a large orbital since they don't care about polarizability

Question 29

Protic solvents are better nucleophiles when

Answer 29

they have a loose solvation shell because it's harder for Nu- to approach the E+ Larger ion = better Nu-

Question 30

Aprotic solvents are better nucleophiles when

Answer 30

they have more e- to share. since all of them have loose solvation shells, there is no hindrance so we don't care about the size anymore

Question 31

Steric hindrance (allows/prevents) Nu- from approaching E+

Answer 31

prevents

Question 32

Due to steric hindrance (bulkier/smaller) groups are preferred as nucleophiles

Answer 32

smaller groups = easier access = bettter u-

Question 33

List from fastest to slowest as a SN2 rxn methyl, 3*, 2*, and 1*

Answer 33

methyl = least steric hindrance 1* 2* 3* = slowest, doesn't do Sn2

Question 34

What transition state will be faster as SN2 - anti or gauche?

Answer 34

gauche = normal Sn2 access

Question 35

What is the rate law of Sn1?

Answer 35

rate law = k[R-X]

Question 36

SN1 is also a hydrolysis rxn which means

Answer 36

C-X bonds is broken with H2O

Question 37

Sn1 is also a solvolysis rxn which means

Answer 37

C-X bond is broken with a solvent

Question 38

List from fastest to slowest as a SN1 rxn methyl, 3*, 2*, and 1*

Answer 38

3* = most hyperconjugation 2* 1* methyl = slowest

Question 39

Does Nu- identity affect rate for SN1?

Answer 39

No

Question 40

When we use SN1, what type of stereochemistry do be get?

Answer 40

Mixed R/S because we don't know the stereochemistry

Question 41

If there are multiple stereocenters in a molecule under going SN1 mech, will we get anything racemic?

Answer 41

Nah, probably not because there is no change in the stereochemistry of the unreacting Carbons.

Question 42

What is the rate law of E1?

Answer 42

rate law = k[R-X]

Question 43

In the elimination process, you start with (0/1/2) molecules and get (0/1/2) molecules

Answer 43

Start with 1, get 2 moleucles

Question 44

T/F SN1 and E1 have different rate laws but they have different RDS and occur separately

Answer 44

FALSE -both have same rate law -same RDS -occur together

Question 45

For Sn1 and E1, nucleophile activity affects a) rate law b) speed of rxn c) product ratio d) all of the above

Answer 45

affects only product ratio

Question 46

If there is Nu- present, we will get (both/E1 only/SN1 only) If there is no Nu- present, we will get (both/E1 only/SN1 only)

Answer 46

Nu- present = both No Nu- = only E1

Question 47

What is resiochemistry?

Answer 47

Where does the bond go? Which direction is it going

Question 48

Which diastereomer is more preferred? cis or trans?

Answer 48

trans because the alkenes are on different sides so there is lower E

Question 49

T/F E1 prefers more R groups

Answer 49

True

Question 50

T/F Haloalkanes are good electrophiles at the carbon and good nucleophiles at the halogen.

Answer 50

F

Question 51

T/F SN2 reactions depend on both the concentration of nucleophile and the concentration of haloalkane

Answer 51

T

Question 52

T/F SN2 reactions always proceed with inversion of stereochemistry due to backside attack.

Answer 52

True

Question 53

T?F SN2 reactions invert every chiral center in the molecule.

Answer 53

F

Question 54

T/F Backside attack always converts R centers to S centers

Answer 54

F

Question 55

T/F Frontside attack aligns poorly with the orbitals in an sp3 carbon, while backside attack aligns well with the backside of the sp3 orbital

Answer 55

T

Question 56

T/F Fluoride is a better leaving group than iodide because it is more electronegative.

Answer 56

False - Iodide is a better LG because it has a stronger conjugate acid

Question 57

T/F Good leaving groups are the conjugate bases of weak acids.

Answer 57

F = they are the conjugate bases of STRONG acids

Question 58

T/F Tertiary haloalkanes do fast SN2 reactions due to stable carbocations

Answer 58

False - 3*C have more steric hindrance so they do not do Sn2 rxns

Question 59

T/F Methyl and primary haloalkanes do fast SN2 reactions due minimal steric hindrance.

Answer 59

T

Question 60

T/F Secondary haloalkanes can do SN2 but are slower than primary haloalkanes

Answer 60

T

Question 61

T/F Branches next to a primary haloalkane slow down SN2 reactions

Answer 61

T

Question 62

T/F Primary haloalkanes with CH or quaternary carbons alpha to the reacting carbon are 'hindered primary' haloalkanes.

Answer 62

T

Question 63

T/F 1-bromobutane will react much slower than 1-bromopropane, and much faster than 1-bromopentane in an SN2 reaction.

Answer 63

F b/c all of them are primary alkanes and the length of the chain does not matter as much as the 1*/2*3* steric hindrance. So they would all react pretty much the same rate

Question 64

T/F SN1 and E1 have the same rate law.

Answer 64

T

Question 65

T/F We can get SN1 reactions without any E1 side product.

Answer 65

F

Question 66

T/F We can get E1 reactions without any SN1 side product.

Answer 66

T = when there is no Nu-, we have only E1 products

Question 67

T/F Increasing the amount of good nucleophile increases the ratio of SN1:E1 product.

Answer 67

T - nucleophiles affect product ratio of SN1:E1

Question 68

T/F E1 reactions make alkenes at any carbon in the haloalkane.

Answer 68

F = most substituted alkene aka most alkyl groups on the double bond is preferred

Question 69

T/F Trans alkenes are more stable than cis alkenes.

Answer 69

True = less E, better E1, more favored, easier to form

Question 70

T/F E1 reactions prefer to form the most substituted alkene because it's most stable.

Answer 70

T

Question 71

T/F Unimolecular mechanisms go through a chiral transition state and have predictable stereochemistry in the products.

Answer 71

False = ACHIRAL carbocation intermediate and less predictable b/c the intermediate is flat

Question 72

What is the rate law of E2?

Answer 72

rate law = k[R-X][base] or rate law = k[R-X][Nu-]

Question 73

How many steps do E2 mechanism have? Describe the TS. Is the stereochemistry predicatable/random?

Answer 73

E2 = single step, definite TS, predictive stereochem

Question 74

What does antiperiplanar transition state mean?

Answer 74

The H and LG must be 180* apart/anti-rotamer

Question 75

E1 mechanism forms (0/1/2) diastereomers and E2 mech forms (0/1/2) diastereomers.

Answer 75

E1 = forms 2 diastereomers E2 = forms only 1

Question 76

According to regioselectivity, which constitutional isomer is preferred for E1. What about E2?

Answer 76

E1 = most substituted alkene E2 = can pick the alkene depending on the base

Question 77

(Unhindered/Hindered) base gets the BEST product. This rule is known as? (Kinetically/Thermodynamically) favored. Does it prefer (less/more) substituted

Answer 77

Unhindered (1* RO-) = best product = Saytzev's Rule thermodynamically favored Less substituted

Question 78

(Unhindered/Hindered) base gets the EASIEST product. This rule is known as? (Kinetically/Thermodynamically) favored. Does it prefer (less/more) substituted alkenes?

Answer 78

Hindered (2* RO-) base = easiest product = Hofmann's rule kinetically favored More substituted

Question 79

If we have a poor Nu- , what mechanism would we use for different reacting carbons

Answer 79

Poor Nu- NO RXN for methyl, 1* = SN1/E1 for 2* and 3*

Question 80

If we have a weakly basic Nu-, what mechanism would we use for different reacting carbons?

Answer 80

Weakly Basic = SN2 for methyl, 1*, 2* = SN1/E1 for 3*

Question 81

If we have an unhindered strong Nu- what mechanism would we use for different reacting carbons?

Answer 81

Unhind Nu- = SN2 for methyl and unhind 1* = E2 (terminal) for hindered 1* = E2 (Saytzev) for 2* and 3*

Question 82

If we have a strong base Nu-, what mechanism would we use for different reacting carbons?

Answer 82

Strong Base = SN2 for methyl = E2 (terminal) for 1* = E2 (Hofmann) for 2* and 3*

Question 83

E2 Reactions use ??? transition states where the leaving group and H are ??? degrees apart

Answer 83

antiperiplanar 180