Exam 4 Flashcards
(70 cards)
1
Q
substitution vs elimination
A
- substitution: one group exchanged for another
- elimination: formation of a pi bond
2
Q
A substitution reaction can occur when:
A
a suitable electrophile is treated with a nucleophile
3
Q
substrate
A
- leaving group
- electrophile
4
Q
Good leaving group must
A
- Electronegative leaving group creates a partial charge on the site of attack to attract the negative nucleophile
- Be able to stabilize the electrons it leaves with
5
Q
common leaving groups
A
halogens
6
Q
Alkyl halides
A
carbon group (alkyl) bonded to a hallide (F, Cl, Br, I)
7
Q
Halide abbreviations
A
- fluoro
- chloro
- bromo
- iodo
8
Q
A
- Primary
- 1 C off alpha
9
Q
A
- secondary
- 2 C off alpha
10
Q
A
- Tertiary
- 3 C off alpha
11
Q
All substitution reactions involve
A
- nucleophilic attack
- loss of a leaving group
- may also include p+ transfer or rearrangement
12
Q
SN2 nucleophile attacks from
A
- backside
- electron density repels the attacking nucleophile from the front-side
13
Q
inversion of configuration
A
- configuration of chirality center changes
- SN2
14
Q
stereospecific
A
a rxn in which configuration of product depends on the configuration of the starting material
15
Q
SN2 stereospecificity
A
stereospecific
16
Q
____________________________ electrophiles react more readily under SN2 conditions
A
- less sterically hindered
- lower substituted
17
Q
relative energy of the _______ determines rate of the reaction
A
transition state
18
Q
Steric hinderance on the beta position
A
can also decrease the rate of the reaction
19
Q
SN1
A
- stepwise process
- loss of a leaving group followed by a nucleophilic attack
20
Q
rate determining step of SN1
A
loss of a leaving group
21
Q
reactivity of SN1
A
- rate is higher with a higher substituted carbocation because more stable
- more stable= smaller Ea
22
Q
a carbocation forms during
A
SN1
23
Q
stereochemistry of SN1
A
- inversion and retention of configuration
- racemization
- carbocations are planar, nucleophile can attack from either side with equal liklihood
- slight preference to inversion b/c leaving group blocks one side of the carbocation
24
Q
OH is a _______ leaving group
H2O is a ________ leaving group
A
BAD
EXCELLENT
25
Proton transfer required during ___ when \_\_\_
* SN1
* OH is leaving group
26
antime the attacking nucleophile is neutral, \_\_\_\_\_\_\_\_\_
proton transfer required at the end of the mechanism
27
Carbocation rearrangement occurs when
* it will generate a more stable carbocation
* before nucleophilic attack
* forms two products
* rearranged generally favored---\> nucleophilic attack is intermolecular and carbocation is intramolecular. Intramolecular processes occur more rapidly in general
28
oxonium ions are
generally lower in energy than carbocations
29
4 factors that determine SN1 or SN2
* substrate--sterics and stability
* strength of nucleophile
* quality of leaving group
* slovent
30
substrate:
* primary : SN2
* tertiary : SN1
* allylic/benzylic halides: either
* secondary either
31
nucleophile:
* [Nuc-] only affects rate in SN2
* strong nucleophile favors SN2
* Weak nucleophile disfavors SN2, therefore favoring SN1
* strength determined by polarizability
32
weak nucleophiles
* F-
* H2O
* ROH
33
Strong Nucleophiles
* I- HS- HO-
* Br- H2S RO-
* Cl- RSH N=--C-
34
leaving group
* highly stabilized for SN1
* Good leaving groups are conjugate acids of strong bases
* HI + H2O ⥂ I - + H3O+
35
common leaving groups
* Halogens (better than)
* sulfonate ions
36
Protic solvent
* at least 1 hydrogen atom capable of H-Bonding
* used for SN1
37
Polar Aprotic solvent
* no hydrogen atoms capable of H-bonding
* SN2
38
why use polar protic solvent for SN1?
* solvent stabilizes the full and partial charges that form during SN1
39
Why use Polar Aprotic for SN2?
* nucleophile doesnt get stabilized by the solvent, leaving it open to attack the electrophilic carbon
40
in cyclic alkenes w/ less than 8 atoms, only ___ alkenes are stable
cis
41
bredt's rule
not possible for bridgehead carbon to possess a c-c double bond if it involves a *trans* pi bond
42
Assigning E or Z to stereoisomers
* prioritize groups attached to C=C based on atomic number
* Z= zame zide of double bond
* E= opposite sides
43
*trans* isomers are generally more stable because
they have less steric strain
44
General elimination rxn
H+ atom and leaving group are eliminated
45
* All elimination reactions involve:
* loss of a leaving group
* proton transfer
46
Concerted elimination
base abstracts a proton and leaving group leaves simultaneously
47
stepwise process
leaving group leaves then base abstracts a proton
48
E2 is the ______ mechanism
concerted
49
elimination occurs when
reagent functions as a base and abstracts a proton
50
why can tertiary substrates react in E2 but not SN2?
steric hinderance prevents the reagent from functioning as a nucleophile in SN2, but the reagent can still function as a base in E2 w/o much steric hindrance
51
why do tertiary substrates react faster in E2?
for tertiary, the transition state exhibits a partial double bond that is more highly substituted and therefore lower in energy (Ea)
52
what molecules react in E2?
Primary, tertiary, and secondary.
tertiary just faster
53
regioselective
A reaction that can produce two or more constitutional isomers but produces one as the major product
54
zaitsev product
the more substituted alkene
55
hoffman product
less substituted alkene
56
the regiochemical outcome of an E2 product can often be controlled by \_\_\_\_\_\_\_\_\_\_
* carefully choosing the base
* sterically hindered bases often favor hofmann product
57
why is the hofmann product favored for sterically hindered bases?
destabilizing steric effects of the Zaitsev transition state start to outweigh the stabilizing effect of forming the more substituted double bond
58
The 3° substrate should (for E2) \_\_\_\_\_\_\_\_\_\_
proceed through a more stable transition state and form a more stable product
59
If there are \_\_\_\_\_\_\_\_\_\_\_, multiple products are possible
multiple reactive sites or regions on a molecule
60
stereoselective
a reaction in which one substrate produces two stereoisomers in unequal amounts
61
stereospecific
a rxn in which the configuration of the product is dependent on the configuration of the starting material
62
why is *anti*-coplanar favored?
* *anti*-coplanar conformation is staggered where *syn*-coplanar is eclipsed.
* *syn*-coplanar conformation would involve a transition state of higher energy as a result of eclipsed geometry
* therefore elimination occurs more rapidly via *anti*-coplanar
63
periplanar
conformation in which H and leaving group are almost coplanar
64
stereoisomeric product of E2 depends on:
configuration of the starting alkyl halide
65
β proton must be arranged ________ to the leaving group in order\_\_\_\_\_\_\_\_
*anti*-periplanar
for the rxn to occur
66
what factors affect product distribution of a stereoselective E2 reaction?
the more stable isomeric alkene will predominate
67
the E1 mechanism is a \_\_\_\_\_\_\_\_\_
step-wise process
68
Because E1 and SN1 proceed by \_\_\_\_\_\_\_\_, E1 rxn will result in \_\_\_\_\_\_\_\_
* forming a carbocation intermediate
* a mixture of products
69
E1 reactions generally produce the _____ product predominantly. WHY?
* zaitsev
* zaitsev intermediate is lower higher substituted and therefore lower in energy due to lower steric strain
70
E1 rxns are \_\_\_\_\_\_\_
* Stereoselective--preference for the formation of the *trans* stereoisomer
