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Chapter 9 Flashcards

1
Q

nucleophilic substitution refers to any reaction in which an electron rich … replaces a …

A

nucleophile; leaving group

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2
Q

the leaving group can be a …. or another … group that can form a … or another stable species

A

halide; electronegative group; stable anion

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3
Q

if the nucleophile is negatively charged (e.g. OH-), in a substitution rxn, the atom donating the pair of electrons (i.e. Nu) becomes … in the product

A

neutral

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4
Q

if the nucleophile is uncharged (e.g. NH3) in the sub rxn, the atom donating the pair of e- becomes … in the initial product

A

positively charged

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5
Q

a fundamental difference between SN2 and SN1 rxns is the timing of … between and the leaving group and of … between C and the nucleophile

A

bond breaking; bond forming

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6
Q

for SN2, bond breaking and bond forming occur … Thus departure of the leaving group is … by the incoming nucleophile

A

simultaneously; assisted

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7
Q

S in SN2 stands for …, n for … and 2 for a …

A

substitution; nucleophilic; biomolecular rxn

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8
Q

SN2 is bimolecular because the haloalkane and nucelophile are involved int he

A

rate-determining step

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9
Q

mech for SN2 rxn: make a new bond between a nu and an electrophile and simultaneously break a bond to give stable molecules/ions: the nucleophile attacks the reactive center from the side … the leaving group; that is, an SN2 rxn involves … attack of the nu.

A

opposite; backside

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10
Q

backside attack by the nu is facilitated through the … of the C-Lv bond, by which the C has a … charge and therefore attracks the electron rich nu. Secondly, the electron density of the nucleophile entering from the backside assists in …, thereby helping the Lv to leave

A

polarization; partially positive; breaking the C-Lv bond

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11
Q

the electron density of the nu attacking from the backside can be thought of as populating the … of the C-Lv bond, weakening that bond as the new C-Nu sigma bond becomes … Therefore, upon collision with a nu, the most effective way to fill this orbital is by collision from the … of the C, which … the C-Lv bond on the other side of the C

A

antibonding molecular orbital; stronger; backside; breaks

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12
Q

for an Sn1 rxn, bond breaking between the C and Lv is entirely … before bond forming with the nucleophile begins

A

completed

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13
Q

in the designation Sn1, 1 stands for a … rxn. This is because only the … is invovled in the rate-determining rate

A

unimolecular; haloalkane

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14
Q

For SN1, the nucleophile is also the …, hence the name …

A

solvent; solvolysis

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15
Q

mech for SN1:

  1. break a bond to give stable molecules/ions: ionization of the C-Lv bond forms a … intermediate. because no nu is assisting the departure of the halide anion, this is the relatively …, … step of the rxn
  2. make a new bond between a nucleophile and an electrophile: rxn of the carbocation with the nucleophile occurs, and can yield a positively charged species. attack of the nu occurs with approximately equal probability from either … of the planar carbocat. intermediate
  3. take a … away to make the species neutral
A

carbocation; slow; rate-determining; face; proton

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16
Q

in an sn1 rxn, the rate-determining step is the cleavage of the … to form a carbocation intermediate

A

C-Lv bond

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17
Q

when one of these collisions between the solvent molecules and the haloalkane is of high enough E to distort the haloalkane into a geometry in which the bond to the leaving group is almost …, the transition state for departure of the leaving group can be achieved and the Sn1 mech is enabled

A

completely broken

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18
Q

an Sn2 rxn involves a … step and therefore has no …

A

single; intermediates

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19
Q

an Sn1 rxn has … steps (or … steps when a … as the last step), each with a … Importantly, an intermediate carbocat is formed

A

two; three; proton is removed; transition state

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20
Q

those rxns whose rate is dependent only on the concentration of haloalkane are classified as …; those rxns whose rate is dependent on the concentration of both haloalkane and nucleophile are classified as …

A

Sn1; Sn2

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21
Q

because the transition state for formation of the carbocation intermediate in an SN1 mechanisms invovles only the haloalkane and not the nucleophile and this step isr ate-determining, it is a … process. the result is a … rxn. In this instance, the rate of rxn is expressed as the rate of … of the starting material. the rate has no dependence on the concentration of the

A

unimolecular; first-order; disappearance; nucleophile

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22
Q

we can conclude that any substitution rxn whose rate depends only upon [R-Lv] proceeds via an

A

Sn1 mechanism

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23
Q

For Sn2, both species must collide and are present in the transition state; that is, the rxn is … and is …

A

biomolecular; second order

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24
Q

when the rate of a substitution rxn depends on both [R-Lv] and [Nu-], we conclude that an … mechanism is occurring

A

Sn2

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25
Q

although examples of complete racemization for Sn1 have been observed, it is common to find only …, with the predominant product being the one with … of configuration at the chriral center. Although bond breaking between C and the Lv is complete, the Lv remains associated for a short time with the carbocation in an ion pair

A

partial racemization; inversion

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26
Q

to the extent hat the laving group remains associated with the carbocation as an ion pair, it … approach of the nucleophile from that side of the carbocation. The result is that somewhat moe than 50% of the product is formed by attack of the nucleophile from the side of the carbocation … that of the leaving group. whenever we observe partial to complete racemization of stereochemistry, we conclude that an … mechanism is operative

A

hinders; opposite; Sn1

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27
Q

every sn2 rxn proceeds with … attack by the nucleophile and therefore .. of configuration

A

backside; inversion

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28
Q

whenever complete inversion of configuration is found in a substitution rxn, we conclude that an … mechanism is occurring

A

Sn2

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29
Q

the rates of sn1 rxns are governed mainly by … factors, namely the relative .. of … intermediates. the rates of sn2 rxns, on the other hand, are governed mainly by … factors, and their transition states are particularly sensitive to … at the site of rxn. The ability of groups, because of their size, to hinder access to a rxn site within a molecule is called …

A

electronic; stabilities; carbocation; steric; bulky groups; steric hindrance

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30
Q

allylic carbocations are considerably more stable than comparably substituted … because … is associated with the .. interaction between the positively charged carbon and the adjacent pi bond

A

alkyl carbocations; delocalization; resonance interaction

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31
Q

benzylic carbocations show approximately the same stability as … carbocations. both are stabilized by … of the positive charge due to adjacent pi bonds. benzylic carbocations can be written as …

A

allylic; resonance delocalization; C6H5-CH2+

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32
Q

Sn1 rxns rarely occur with … C and never on … carbons. The carbocations derived from these C bonds are too … to form

A

sp2; sp; unstable

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33
Q

tertiary haloalkanes react by an … mechanism because tertiary carbocations are relatively … and tertiary haloalkanes are protected against … In fact, tertiary haloalkanes are never observed to react by an … mechanism

A

Sn1; stable; backside attack; Sn2

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34
Q

Halomethanes and primary haloalkanes are never observed to react by an … mechanism. they have little crowding around the reaction site and react by an … mechanism because methyl and primary carbocations are …

A

SN1; SN2; unstable

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35
Q

secondary haloalkanes may react by either … or … mechanisms, depending on the … and …

A

Sn1; Sn2; nucleophile; solvent

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36
Q

the carbon bearing the halogen in a haloalkane is called the ..-C and the next C is called the …-C

A

alpha; beta

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37
Q

For all practical purposes, primary halides with … do not undergo Sn2 reactions. these screen the corresponding carbon, hindering nucleophilic approach

A

beta branches

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38
Q

SN2 rxns never occur on … or … hybridized C. Hence, you should never consider performing a substation on a … halide, an … halide, or an … halide

A

sp2; sp; vinyl; aryl (benzylic); alkynyl

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39
Q

in the transition state for nucleophilic substitution on a haloalkane, the leaving group develops a … in both sn1 and sn2 rxns; therefore, the ability of a group to function as a leaving group is related to how … it is as an … The most stable anions, and therefore the .. leaving groups are the … of …

A

partial negative charge; stable; anion; best; weak conjugate bases; strong acids

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40
Q

the best leaving groups in the series in the txtbk are .., .., and …
…, … ion (…), and … ion (…) are such poor leaving groups that they rarely, if ever, are displaced in nucleophilic aliphatic substitution

A

I-; Br-; Cl-; OH-; methoxide (CH3O-); amide (NH2-)

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41
Q

the common protic solvents for nucleophilic substitution rxns are …., low molecular weight …, and low molecular weight …

A

water; alcohols; carboxylic acids

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42
Q

protic solvents solvate ionic substances by …

A

hydrogen bonding

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43
Q 
common protic solvents: 
.. 
... acid (...) 
... (...)
... (...)
... acid (...)
A

water; formic; HCOOH; methanol; CH3OH; ethanol; CH3Ch2OH; acetic; CH3COOH

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44
Q

polar aprotic solvents:

… (… abbrev. not structure), … (…), … (…abbrev.) , …

A

dimethyl sulfoxide; DMSO; acetonitrile; CH3C=_N (triple bond); N,N-dimethylformamide; DMF; acetone

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45
Q

aprotic solvent: a solvent that cannot serve as a

A

hydrogen bond donor

46
Q

the rate of Sn1 rxns depends on both the ability of the solvent to keep opposite charges … and its ability to … both … and … sites by solvation.

A

separated; stabilize; positive; negative

47
Q

the solvents that best solvate charges are … such as .water; … such as methanol and ethanol; and, to a lesser degree, … such as formic acid and acetic acid

A

polar protic solvents; low-molecular-weight alcohols; low-molecular-weight carboxylic acids

48
Q

the stronger the solvation of the nucleophile, the … the energy required to remove the nucleophile from its solation shell to reach the trasnition state and hence the .. the rate of the Sn2 rxn

A

greater; lower

49
Q

polar aprotic solvents can solvate … very well, but they solvate … relatively poorly, bc they cannot donate a H bond to an .., as can a protic solvent. For this reason, nucleophiles are … and more … in polar aprotic solvents than in protic solvents; so the rates of SN2 rxns are dramatically …

A

cations; anions; anion; freer; reactive; accelerated

50
Q

you should realize that the SN2 mechanism is viable in all solvents that … the … and … Only … solvents are viable for SN1

A

dissolve; nucleophile; electrophile; polar protic

51
Q

nucleophilicity is a … property measured by relative ..

A

kinetic; rates of rxn

52
Q

the more rapidly a nucleophile reacts with a substrate in an SN2 rxn, the more … it is

A

nucleophilic

53
Q 
good nucleophiles include: 
..., ... 
..., ... 
..., ..., ..., ..., 
..., ..., ... 
(ones at top are best)
A

Br-; I-
CH3S-; RS-;
HO-; CH3O-; RO-; R-C=_C- (triple bond)
CN-; N3-; H2N-

54
Q 
moderate nucleophiles include (ones at top are best): 
..., ... 
..., ...
..., ..., ..
.., ..., ..., ...
A

Cl-; F-;
CH3COO-, RCOO- (have carbonyl groups)
CH3SH; RSH; R2S
NH3; RNH2; R2NH; R3N

55
Q

poor nucleophiles include (ones at top are best):
..
…, …
…, ….

A

H2O;
CH3OH; ROH
CH3COOH, RCOOH (carboxylic acids)

56
Q

basicity and nucleophilicity are often related because they both involve a … making a bond to another atom

A

lone pair of electrons

57
Q

in general, … bases are good nucleophiles

A

sterically unhindered strong bases

58
Q

… anions (…) are classified as moderate nucleophiles

A

carboxylate; RCO2-

59
Q

neutral oxygen species are … nucleophils

A

weak

60
Q

nucelophiles that have conjugate acids with pKas above 11 are …, around 11 are .., and below 11 are …

A

strong; moderate; weak

61
Q

all other factors being equal, the stronger the interaction of the nucleophile with the solvent, the … its nucleophilicity

A

solvent

62
Q

because anions are only … solvated in polar aprotic solvents, they are freer and participate … in nucleophilic substitution rxns. In these solvents, their relative nucleophilicities … their relative basicitiies. The relative nucleophilicities in polar aprotic solvents for halides is …

A

poorly; readily; parallel; F->Cl->Br->I-

63
Q

in polar protic solvents, iodide ion, the least basic of the halide ions, has the … nucleophlicity

A

greatest

64
Q

in polar aprotic solvents, anions are only … solvated and therefore relatively free to participate in nucleophliic sub rxns and … dictates nucleophilicity

A

weakly; basicity

65
Q

in polar protic solvents, anions are … solvated by … with solvent molecules and are therefore less free to participate in nucleophilic sub rxns and … dictates nucleophilicity

A

highly; H-bonding; polarizability

66
Q

because there is little or no carbocation character at the substitution center, SN2 rxns are free of … In contrast, SN1 rxns often proceed with …

A

rearrangement; rearrangement

67
Q

for methyl halides:

… is favored, … does not occur bc the cation is so … that it is never observed in solution

A

Sn2; Sn1; unstable

68
Q

for primary halides:
… is favored, … rarely occurs bc the cation is so unstable that it’s not formed in solution (… and … are the exceptions)

A

SN2; SN1; allylic; benzylic

69
Q

secondary halides:
… is favored in aprotic solvents with good nucleophiles
… is favored in protic solvents with poor nucleophiles

A

SN2; SN1

70
Q

tertiary halides:
… does not occur becasue of … around the rxn center
… is favored bc of the ease of formation of tertiary carbocats

A

SN2; steric hindrance; Sn1

71
Q

substitution at a chiral center for SN2 results in … of config, whereas for SN1 .. is favored bc the carbocat intermediate is … and attack of the nu occurs with more or less equal probability from either side

A

inversion; racemization; planar

72
Q

… is the competing process to substituion

A

beta elimination

73
Q

beta elimination is the combination of take a … away and … to give stable molecules/ions

A

proton; break a bond

74
Q

… promote B-elim rxns

A

strong bases

75
Q

examples of strong bases: …, …, …, and … anions

A

OH-; OR-; NH2-; acetylide

76
Q

the solvent used for B-elim rxns is commonly the … of the base used

A

conjugate acid

77
Q

when the more substitued alkene is the dominant product for elim, the reaction is said to follow …, or to undergo …

A

Zaitsev’s rule; Zaitsev elim

78
Q

E1: breaking of the C-Lv bond to give a carbocation is … before any rx occurs with the base to lose a H and form the double bond

A

complete

79
Q

for E1, e tands for … and 1 stands for …

A

elim; unimolecular

80
Q

E1 mechanism:

  1. break a bond to give stable molecules/ions: rate-determining … of the C-Lv bond gives a …
  2. take a proton away: proton transfer from the … to … gives the alkene
A

ionization; carbocat intermediate; carbocat; solvent

81
Q

in an E1 mech., one transition state exists for the … and a second for the …

A

formation of the C+; loss of H

82
Q

E1 competes with …, but they mostly occur together

A

SN1

83
Q

For Sn2: proton transfer to the base, formation of the C=C, and ejection of the X ion occur ..; all bond-breaking and bond-formings teps are …

A

simultaneously ;concerted

84
Q

the transition state for SN2 has considerable

A

double bond character

85
Q

E2 mech: take a proton away and simultaneously break a bond to give stable molecules/ions–bond breaking and bond forming are …;
E stands for …
2 stands for …: both the … and the … are involved in the transition state for rate-determining step

A

concerted; elimination; bimolecular; haloalkane; base

86
Q

one simplifying aspect of the competition between sub and elim is to consider an E2 pathway only when …, …, …, and … anions are used

A

hydroxide; alkoxides (RO-); acetylides; amide ions

87
Q

the rate determining step in an E1 is … to form a …

thus the rxn is unimolecular and follows … kinetics

A

ionization of the leaving group; carbocation; first–order

88
Q

only one step occurs in an E2 mechanism, and the transition state is bimolecular. the rxn is …: … in haloalkane and … in base

A

second order; first order; first order

89
Q

the major product in E1 rxns is the …

… is possible

A

more stable alkene; rearrangement

90
Q

For E2 rxns the major regioisomeric product tends to be formed following …
… double bonds predominate over … double bonds when either is possible
with larger, sterically hindered bases, the major product is often the … bc rxn occurs primarily at the most accessible H

A

Zaitsev’s rule; trans; cis; less substituted alkene

91
Q

the lowest-energy transition state of an E2 rxn is commonly the one in which the … and … are oriented .. and … to each other. It allows for proper orbital overlap between the base, the proton being removed, and the leaving group

A

H; Lv; anti; coplanar

92
Q

the required anti and coplanar transition state geo can also be used to predict the regiochem of E2 elim in halocyclohexanes. In these molecules, anti and coplanar correspond to … and …

A

trans; diaxial

93
Q

primary halides for elim:
… not observed. carbocat unstable
… is favored, but usually requires … base

A

E1; E2; sterically hindered strong base

94
Q

secondary halides for elim:
… is the main rxn with weak bases
… is the main rxn with strong bases

A

E1; E2

95
Q

tertiary halides for elim:
… main rxn with weak bases
… rxn with strong bases

A

E1; E2

96
Q

For E2 a … and … arrangement is still possible

A

syn; coplanar

97
Q

it is difficult to predict the ratio of substition to elimination for first order rxns. the majority of the time, tho, … predominates when weak bases are used

A

SN1

98
Q

For Sn2 vs E2, the guiding principles are:

  1. branching at the alpha or beta carbons increases … and retards … rxns, but increases the rate of … bc of the increased stability of the … product
  2. the greater the … of the attacking reagent, the greater the SN2/E2 ratio. the greater the … of the reagent,t he greater the E2/SN2 ratio
A

steric hindrance; SN2; E2; alkene; nucleophilicity; basicity

99
Q

if a nucleo/base has a conjugate acid with a pka …. and is a … nu, then an SN2 will dominate. If the pka is …, the .. character will usually outcompete the other and an … will dominate

A

below 11; good; above 11; basic

100
Q

in general, higher temps result in increasing extents of … at the expense of …

A

elimination; substitution

101
Q

whent he alkyl group is methyl the only possible outcome is an …, SN1 doesnt happen cuz C+ is too unstable, and there’s only one C so elim can’t happen either

A

SN2 rxn

102
Q

for methyl halides only … occurs

for primary, SN2 occurs with …./…, E2 occurs with …, … , SN1/E1 … occurs

A

SN2; good nu; weak bases; strong, bulky; rarely

103
Q

for secondary, SN2 occurs with bases/nu where pka of conj acid is … or less, E2 occurs when pka of conjugate acid is … or greater, SN1/E1 is common in rxns with … in … solvents

A

11; 11; weak nu; polar protic

104
Q

for tertiary:
e2 occurs with ..
sn1/e1 occurs with …/… if the solvent is ..
SN2 … occurs

A

strong bases; poor nu; weak bases; never

105
Q

if the nuc/base is a strong base and sterically hindered, it will … nucleophile and … is the major pathway

A

not be a good; E1

106
Q

withany of the basic or weakly basic good/moderate nu, products predominantly form from … pathways

A

Sn2

107
Q

primary: if the nuc/base is neither good/moderate nu, we are likely to get SN2/E2 in ratio that is difficult to predict/no rx. examples are…, …, and … acids

A

water; alcohols; carboxylic

108
Q

secondary: if nuc/base is strong, … will dominate, despite whether it is hindered or not
with a weak base that is good/moderate nu, … will dominate
when nuc isn’t good, if the solvent is …, we can induce Sn1/E1
if the solvent is neither polar nor protic and the nuc/base is neither strong/good, … are possible

A

E2; SN2; polar protic; all four pathways

109
Q

tertiary: if nuc/base is strong, … will dominate
if the solvent is polar and protic, we can induce … and … often by applying heat
if nuc/base isn’t good, and solvent isn’t plar and protic … are possible

A

E2; SN1, E1; E1, SN1, and E2

110
Q

when there is an internal nu, it participates in the .. to give an intermediate, which then reacts with an … to complete the rxn. thus, although this rxn has kinetic charcteristics of an .., it actually involves two succesisve … displacement rxns;

A

departure of the Lv; external nu; Sn1; SN2

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