Test 2 Flashcards
(155 cards)
1
Q
Concerted reactions occur without an ?
A
intermediate
2
Q
woodward-hoffman rule
A
conservation of orbital symmetry
3
Q
three types of concerted pericyclic rxns
A
cycloaddition (ie. diels), electrocyclic, sigmatropic
4
Q
cycloaddition rxns involve
A
combination of two molecules to form a ring
5
Q
cycloaddition reactions characterized by ?
A
number of pi bonds involved
6
Q
two main types of cycloaddition rxns
A
diels alder, 1,3-dipolar
7
Q
three approaches based on MO theory
A
orbital correlation, transition state aromaticity, frontier MO
8
Q
frontier MO approach
A
overlap of diene homo with dienophile lumo
9
Q
antarafacial
A
addition to opposite faces of pi system
10
Q
suprafacial
A
face to face addition of pi system
11
Q
transition state aromaticity
A
transition state has even number of nodes: huckel, odd: mobius. Then use aromaticity rules (4n+2 for huckel)
12
Q
orbital correlation diagrams
A
draw orbitals for reactants and products and determine symmetry. list each according to energy. pair orbitals of same symmetry
13
Q
loss of stereospecificity in DA rxn a result of?
A
strongly electrophilic + strongly nucleophilic
14
Q
alder rule
A
in addition of dienophiles to cyclopentadiene, large substituents prefer to take on endo conformation (opposite face of bridge) even though it is more sterically congested
15
Q
1-ERG diene + EWG dienophile prefers ortho/para?
A
ortho
16
Q
2-ERG diene + EWG dienophile prefers ortho/para?
A
para
17
Q
1-EWG diene + ERG dienophile prefers ortho/para?
A
ortho
18
Q
2-EWG diene + ERG dienophile prefers ortho/para?
A
para
19
Q
2-EWG diene + ERG dienophile prefers ortho/para?
A
para
20
Q
solvent effects in DA rxns
A
typically nonpolar organic solvents are used. Water and other polar solvents can accelerate through “enforced hydrophobic interactions”
21
Q
solvent effects in DA rxns
A
typically nonpolar organic solvents are used. Water and other polar solvents can accelerate through “enforced hydrophobic interactions”
22
Q
2+2 cycloadditions require ?/? topology
A
supra/antara
23
Q
1,3 dipolar cycloaddition orientation is favored by that giving?
A
strongest frontier orbital interaction
24
Q
conrotatory
A
substituents rotate in the same direction
25
in DA rxns, endo product is formed faster when dienophile substituent has pi electrons because?
secondary orbital overlap with formed double bond when in endo orientation
26
an electrocyclic reaction product has one fewer ? and one more ? (or viceversa)
pi bond and one more ring
27
woodward-hoffman rules for electrocyclization
even number of pi bonds = asymmetric = conrotatory
28
woodward-hoffman rules for electrocyclization
even number of pi bonds = asymmetric = conrotatory
29
cycloadditions that form 4, 5, or 6 membered rings involve suprafacial/antarafacial bond formation
supra (geometric constraints)
30
cycloadditions that form 4, 5, or 6 membered rings involve suprafacial/antarafacial bond formation
supra (geometric constraints)
31
woodward-hoffman rules for cycloaddition
even number of pi bonds = antarafacial (thermal conditions)
32
TE-AC
thermal and even = antarafacial and conrotatory
33
TE-AC
thermal and even = antarafacial and conrotatory
34
Stereospecific
reaction in which the stereochemistry of the reactant completely determines the stereochemistry of the product without any other option
35
Stereoselective
Stereoselective
36
SN1 reactions favored by polar/nonpolar solvent in case of ionization by neutral substrate
polar
37
SN1 reactions favored by polar/nonpolar solvent in case of ionization by cationic substrate. why?
nonpolar. reactants are more strongly solvated by polar solvent
38
SN2 rxn leaving group is primarily affected by? secondarily?
C-X bond strength. stability of anion.
39
1,3 dipolar cycloaddition rxns are generally concerted [?pis + ? pis] and highly stereospecific
[2 pis + 4 pis]
40
4n systems undergo electrocyclization by con/disrotatory closure
conrotatory
41
sigmatropic shift where migrating group stays on the same face
suprafacial
42
1,3-sigmatropic hydrogen shift prefers supra/anatra
antara
43
1,5-sigmatropic hydrogen shift prefers supra/anatra
supra
44
1,3-sigmatropic alkyl shift prefers supra/anatra retention/inversion
supra and inversion
45
1,5-sigmatropic alkyl shift prefers supra/anatra retention/inversion
supra and retention
46
3,3 sigmatropic shift is considered supra/antara
supra
47
why is 3,3 sigmatropic considered stereospecific
EE and ZZ form syn product
48
why is 3,3 sigmatropic considered stereoselective
favors E isomer
49
3,3 sigmatropic TS conformation
usually chair like but sometimes boat like
50
? factors can make a boat TS preferable
steric
51
cope rearrangements can occur at lower temperatures if ?
strain is relieved
52
allyl ethers of phenols undergo ? rearrangements
claisen
53
? group speeds up 3,3 sigmatropic
3-amino
54
? substituents accelerate claisen
methoxy or other donor at carbon 2
55
irland-claisen rearrangement
OTMS group present
56
SN1 rate determining step is endo/exo thermic
endo
57
SN1 has early/late TS
late
58
SN1 is favored by polar/nonpolar solvent with neutral substrate
polar
59
SN1 is favored by polar/nonpolar solvent with cationic substrate
nonpolar
60
SN1 is not affected by ?
nucleophile
61
SN1 stereochemisry
generally not selective. but usally inverted
62
SN2 involves ? nucleophile - ? CX interaction
nonbonding nucleophile - antibonding CX interaction
63
SN2 stereochemistry
inverted
64
rehybridization in SN2 rxn
sp3 to p to sp3
65
SN1 limit
there is no covalent interaction between the nucleophile and the reactant
66
SN2 limit
bond formation is concerted with bond breaking
67
substituiton, Three states of reactant to which nucleophile can react
contact ion pair (inversion), solvent-separated (mostly inversion), dissociated (racemization)
68
internal return
ion pair can collapse with predominant retention of configuration
69
evidence for internal return
k_ex/k_rac = 2.3 in experiment
70
effect of stronger nucleophile on internal return experiment
solvent separated cation is attacked by stronger nucleophile before racemization can occur. k_ex doesn't change.
71
substituiton, energy relationship between three ion pair species
equivalent in energy
72
positive rho value
reaction is aided by EWG groups. forms cation in TS.
73
positive sigma value
electron withdrawing group
74
if rho^+ correlates better than normal rho...
evidence that a cation is formed in the TS that is in conjugation with benzene
75
coupled displacement
positive rho value, but bimolecular (second order)
76
strong solvation lowers/raises energy of an anionic nucleophile relative to the TS. Results in increased/decreased E_a
lowers. increased
77
electronegativity of nucleophile
high electronegativity disfavored as electron density must be donated to antibonding orbital on leaving group
78
polarizability of nucleophile
high polarizability is favored, makes better nucleophile
79
more stable lone pair is more/less basic
less
80
sulfonate ester use as leaving group
good leaving group and preparation of reactant without change in configuration of carbon center
81
types of sulfonate esters
triflates, tosylates, nosylate, mesylate
82
make a leaving group better
coordinate with electrophile. protonate and alcohol to make stable water leaving group. OR lewis acid coordination of halides
83
adjacent carbonyl groups accelerate SN1/SN2
SN2
84
alkoxy substituents stabilize SN2 TS that is cationic/anionic
cationic. donates pi electrons
85
? groups stabilize either anionic or cationic
vinyl and phenyl
86
neighboring group participation
involvement of nearby nucleophilic substituents in substitution
87
assessment of carbocation stability (3)
hydride affinity, pKr of triarylmethyl cations, thermodynamic data for ionization of alkyl chlorides
88
direct observation of carbocations
NMR and x-ray crystallography of carbocation generated from magic acid
89
exo brosylate carbocation winstein interpretation
assisted by C1 and C6 bonding electrons to stabilize non classical carbocation
90
exo brosylate carbocation HC brown interpretation
rapid conversion between different classical secondary carbocations
91
bridged structures are ...
most likely to be stable when a strained bond can participate or when solvation of positive charge is difficult. proximity to anions favors classical. likely TSs
92
Ad_E 3
termolecular electrophilic addition. involves two moles of electrophile. concerted. anti addition
93
addition reactant that is most likely to form carbocation intermediate
proton ( hard )
94
when electrophile becomes harder/softer bridged intermediates become important
softer (ex. halonium)
95
HBr addition regioselectivity
free radical chain addition competes with ionic mechanism to form anti-markovnikov
96
HI >/
HI > HCl
97
In the presence of ?, HBr undergoes exclusive ionic addition
silica or alumina adsorbents
98
For addition of HBR or HCl to alkenes, the overall rate is ? order. exceptions?
third. second order with respect to halide and first with respect to alkene. If stable carbocation is formed, overall second order
99
For addition of HBR or HCl to alkenes, the additon is syn/anti. why? exceptions?
anti. concerted attack oh hydrogen and attack by halide. If stable carbocation is formed, overall second order
100
? solvents can compete with halides in addition reaction
nucelophilic
101
explanation for evidence of carbocations but third order
activation of an electrophile by another one
102
why are acid catalyzed hydrations of alkene accelerated by ERG?
increase electrons density at alkene and stabilize carbocation formed. (ex. alkyl, methoxy)
103
strained alkenes show enhanced reactivity toward acid-catalyzed hydration, why?
higher ground state energy
104
reactivity of halogens in addition rxns
F2(highly exothermic, difficult to control) >Cl2>Br2>I2(reversible)
105
mechanism and kinetics of bromine addition
formation of alkene bromine complex. formation of ionic intermediate. complex rate (three terms)
106
chlorine addition is usually ? order
second
107
Z vs E 2-butene, more reactive in halide addition
Z-2-butene
108
two types of bromination intermediates and preferred stereochmeistry
brominium ion (anti) and beta-bromination (nonstereospecific unlcess ion intermediate collapses before rotation can occur, syn addition predominates)
109
reversibility of Bromination
brominium ion can be released to form free bromine if a cyclopentene is present to react with free bromine.
110
pyridinium bromide tribromide
supplies tribromide. reacts through Br2-alkene complex. strong anti preference
111
increase epoxide reactivity
ERG in alkenes, EWG in peroxy acid
112
effect of hydroxy groups on epoxide rxns
epoxidation occurs on side of double bond closest to hydroxyl group due to H-bonding
113
affect of allylic hydroxyl group
h-bonds with epoxide and stabilizes addition to same side.
114
steric control in epoxide ring opening
attack at less substituted carbon, anti-markovnikov
115
electronic control in epoxide ring opening
substituents stabilize positive charge and attack occurs at more substituted carbon. markovnikov
116
solvemercuration and oxymercuration are anti/markovnikov
markovnikov
117
rate determining step in oxymercuration
nucleophilic addition
118
If there is little steric hindrance in oxymercuration, syn/anti is preferred
anti
119
argenation
formation of silver complexes for use in analytical methods such as TLC, HPLC, GLC
120
hydroboration is anti/markovnikov
anti
121
hydroboration is syn/anti
syn
122
hydroboration is thermally reversible/irreversible
reversible
123
why does Z-2-butene have higher enantioselectivity in hydroboration that E-2-butene
less steric effect in TS
124
IcpBh2
borane with high enantiomeric purity
125
alkynes are more/less reactive toward electrophilic addition. why?
less. unstable vinyl cation. more so unstable for bridged
126
aryl substituted acetylene additon of HCl
mainly syn product with Cl attached to aryl carbon
127
affect of increasing bromide concentration in aryl acetylene addition
greater AD_E 3 character. anti and anti-markovnikov
128
alkynes can be hydrated in aqueous acid solutions to give ? product
ketone
129
chlorination of aryl substituted alkynes
vinyl cation intermediate. not stereoselective
130
alkyl monosubstituted alkynes give syn/anti addition
syn
131
alkyl disubstituted alkynes give syn/anti addition
anti (bridged TS)
132
bromination of aryl substituted alkynes substituent effects
EWG on arly group withdraws electrons and destabilizes cation. more stereospecifically anti
133
bromination of aryl substituted alkynes bromide salt effects
enhances anti addition
134
electrophilic addition to allenes occurs at sp or sp2 carbon
sp
135
strong organic bases used in elimination rxns
dbn and dbu
136
E2 elimination
concerted.
137
E1 elimination
leaving group leaves and carbocation is formed, electrons bound to hydrogen are used to remove carbocation
138
E1cb elimination
strong base removes Hydrogen to form anion. anion electrons shift and kick off hydrogen
139
two types of E1cb
reversible (k1,k-1 > k2) and irreversible(k1
140
variable E2 transition state theory
many types of beta-elimination mechanisms intermediate between limiting types
141
important factors in Eliminations
1) nature of LG 2) electronin and steric effects of the substituents 3) nature of base 4) solvent effect
142
protic solvent stabilizes/destabilizes carbocation
stabilizes
143
E1 TS should resemble ?
carbocation intermediate
144
E!cb regiochemistry
proton abstracted from more substituted carbon
145
saytzeff rule
elimination resulting in more substituted alkene
146
hoffman rule
E2 rxns with poor LGs give less substituted alkene (quaternary ammonium salts)
147
for cyclic systems, there is a preference for syn/anti in eliminations. why?
anti. maximizes orbital overlap. avoids eclipsing present in syn
148
syn elimination is extensive for ?
quaternary ammonium salts
149
equatorial/axial preference of leaving groups in cyclohexyl eliminations. why?
axial. antiperiplanar to Hydrogens
150
syn/anti elimination prevalent in rigid rings
syn
151
cyclobutatone is more/less reactive than cyclopentanone and cyclohexanone
more
152
cyclohexanone is more/less reactive than cyclopentanone
more
153
why is cyclopentanone less reactive
makes partially eclipsed conformation
154
Protonation or Lewis acid-complexation increases/decreases the electrophilicity of the carbonyl group
increases
155
felkin ahn
place largest group perpendicular to carbonyl. smaller group is further away from carbonyl. attack occurs opposite of large group
