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exam #3 Flashcards

1
Q

most common bases used in elimination reactions are

A

alkoxides

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2
Q

common bases used in dehydration

A

sodium hydroxide, potassium hydroxide, sodium methoxide, sodium ethoxide, potassium tert-butoxide

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3
Q

alkenes are classified by

A

number of carbon atoms bonded to the carbons of the double bond

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4
Q

stability of an alkene increases as the number of

A

R groups bonded to the double bond carbons increases

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5
Q

higher s character =

A

how much easier an atom accepts electron density

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6
Q

trans isomer

A

when R groups are on opp sides

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7
Q

cis isomer

A

when R groups are on same side

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7
Q

stereoisomers on C=C bond is possible when

A

two groups on each C must be different

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7
Q

what is more stable? trans or cis alkenes?

A

TRANS b/c groups bonded to double bond carbons are further apart (reducing steric interactions)

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8
Q

E2

A

bimolecular elimination

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9
Q

E1

A

unimolecular eliminiation

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10
Q

E2 and E1 differ in

A

timing of bond cleavage and formation (analogous to SN2 and SN1)

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11
Q

most common mechanism for dehydrohalogenation?

A

E2

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12
Q

rate = k[(CH3)3CBr][-OH]

A

all bonds are broken and formed in a single step

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13
Q

E2 generally has

A

strong, neg charged bases (-OH, -OR)

uses DBN and DBU

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14
Q

what are DBN and DBU?

A

sterically hindered nitrogen basese

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15
Q

base appears in (x) so rate of E2 rxn (y) as strength of (z)

A

rate equation

increases

base increases

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16
Q

when the base strength increases so does the

A

E2 reaction rate

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17
Q

the better the leaving group, the

A

faster the E2 rxn

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18
Q

polar aprotic solvents

A

increase E2 rxn rates

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19
Q

SN2 and E2 mechanisms differ in

A

how the R group affects the rxn rate

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20
Q

as the number of R groups on the carbon with the leaving group increases,

A

rate of E2 rxn increases

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21
Q

when alkyl halides have two or more different Beta-carbons,

A

more than one alkene product is formed

one of the product usually predominates when this happens

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22
Q

major product is more

A

STABLE (has more substituted double bond)

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23
Q

zaitsev rule

A

formation of major and minor product when alkyl halides have 2+ different beta-carbons

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24
Q

zaitsev rule:

A

the major product in beta-elimination has the more substituted double bond

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25
Q

regioselective:

A

when it yields predominantly or exclusively one constitutional isomer when more than one is

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26
Q

when a mixture of stereoisomers is possible from a dehydrohalogenation…

A

the major product is the more stable stereoisomer

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27
Q

stereoselective:

A

when rxn forms predominantly / exclusively one stereoisomer when 2+ are possible

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28
Q

e2 rxn is stereoselective bc

A

one stereoisomer is formed preferentially

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29
Q

rate of E1 rxn increases as

A

number of R groups on carbon w/ leaving group increases

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30
Q

strength of the base determines

A

whether a rxn is E1 or E2

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31
Q

strong bases favor

A

E2

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32
Q

weaker bases favor

A

E1

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33
Q

E1 rxns are

A

regioselective

  • favor formation of the more substituted, more stable alkene

zaitsev’s rule applies

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34
Q

SN1 and E1 have the same

A

first step (formation of a carbocation)

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35
Q

SN1 and E1 differ in

A

what happens to the carbocation

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36
Q

E1 competes w/

A

SN1

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37
Q

E1 rxns of alkyl halides are much less

A

useful than E2 rxns

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38
Q

transition state of an e2 rxn consists of

A

four atoms from an alkyl halide

one H atom

two C atoms

leaving group (x)

all aligned in a plane

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39
Q

two ways for C–H and C–X to be

A

coplanar

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40
Q

syn periplanar

A

H and X are on same side

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41
Q

anti periplanar

A

H and X are on opp sides

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42
Q

E2 elimination require the

A

anti periplanar geometry

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43
Q

anti periplanar arrangement has a

A

staggered conformation

two electron-rich groups are far apart

PREFERRED GEOMETRY

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44
Q

syn periplanar arrangement has an

A

eclipsed formation

two electron-rich groups are close

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45
Q

strength of base is the

A

MOST important factor in determining mechanism for elimination

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46
Q

a single elimination rxn produces a

A

pi bond of an alkene

two consecutive elimination rxns produce two pi bonds of an alkene

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47
Q

alkynes are prepared by

A

two successive dehydrohalogenation rxns

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48
Q

two elimination rxns are needed to

A

remove two moles of HX from a dihalide substrate

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49
Q

vicinal dihalide or a geminal dehalide –>

A

two different starting materials can be used in elimination rxns that remove two moles of HX from a dihalide substrate

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50
Q

to synthesize alkenes,

A

stronger bases are needed

typical bases used -NH2 (amide)

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51
Q

good nucleophiles that are weak bases favor

A

substitution over eliminiation

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52
Q

why do good nucleophiles favors sub over elimination?

A

certain anions generally give products of substitution b/c they are good nucleophiles/weak bases

e.g. I-, Br-, HS-, -CN, CH3COO-

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53
Q

bulky non-nucleophilic bases favor

A

elimination over substitution

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54
Q

KOC(CH3)3, DBU, DBN are

A

too sterically hindered to attack tetravalent carbon but are able to remove a small proton

55
Q

alcohols contain

A

hydroxy group (OH) bonded to an sp3 hybridized carbon

56
Q

epoxides are

A

ethers having O atom in a three-membered ring

57
Q

epoxides are also called

A

oxiranes

58
Q

an epoxide is a special type of

A

ether

59
Q

when an OH group is bonded to a ring,

A

the ring is numbered beginning with the OH group

60
Q

b/c the functional group is at C1,

A

the 1 is usually omitted from the name

61
Q

the ring is then numbered in a clockwise/counterclockwise to

A

give the next substituent the lowest number

62
Q

how to name alcohols (common names)

A

1) name all carbon atoms of the molecule as a single alkyl group

2) add word ‘alcohol’, separating words with a space

63
Q

simple ethers (naming them)

A

name both alkyl groups bonded to the O, arrange names alphabetically, add word ‘ether’

for symmetrical ethers, name alkyl group and add “di-“ prefix

64
Q

more complex ethers are named using IUPAC system using following rules

A

name simple alkyl group (–yl to —oxy)

name remaining alkyl group as an alkane

65
Q

cyclic ethers have an

A

O in the ring

common example: THF

66
Q

alcohols/ethers/epoxides exhibit

A

dipole-dipole interactions b/c they have a bent structure w/ two polar bonds

67
Q

alcohols are capable of

A

intermolecular H bonding

68
Q

alcohols are more polar than

A

ethers and epoxides

69
Q

what affects H bonding?

A

steric hindrance

70
Q

preparation of ethers by

A

williamson ether synthesis

71
Q

an alkoxide salt is

A

needed to make an ether

72
Q

alkoxides can be prepared from alcohols by a

A

bronsted-lowry acid-base rxn

73
Q

sodium ethoxide is prepared by

A

treating ethanol w/ NaH

74
Q

NaH is a very good base for forming alkoxide b/c

A

the by-product of the rxn, H2, is a gas that just bubbles out of the rxn mixture

75
Q

halohydrins:

A

organic compounds that contain both a hydroxy group and a halogen atom on adjacent carbons

76
Q

in halohydrins, an intramolecular version of the

A

williamson ether synthesis can occur to form epoxides

77
Q

OH group in alcohols is a

A

POOR leaving group

78
Q

halogen atom serves as a

A

GOOD leaving group in alkyl halides

79
Q

for an alcohol to undergo nucleophilic substitution, OH must be

A

converted into a better leaving group

by using acid, -OH can become H20 (good leaving group)

80
Q

dehydration:

A

B-elimination rxn in which OH and H are removed from alpha and beta carbon atoms respectively

81
Q

dehydration is typically carried out using

A

H2SO4 + other strong acids

POCl3 (in presence of amine base)

82
Q

typical acids used for alcohol dehydration

A

H2SO4

TsOH

83
Q

more substituted alcohols (x) more easily

A

DEHYDRATE

increasing reactivity

84
Q

when an alcohol has 2-3 beta-carbons, dehydration is

A

regioselective and FOLLOWS zaitsev’s rule

85
Q

the more substituted alkene is the major product when

A

a mixture of constitutional isomers is possible

86
Q

secondary and tertiary alcohols react by

A

e1 mechanism

87
Q

primary alcohols react by

A

e2 mechanism

88
Q

primary carbocations are highly

A

unstable (need carbocation intermediate to complete dehydration rxn)

89
Q

primary alcohols undergo

A

HYDRATION and then e2 mechanism

90
Q

rearrangement

A

when carbocation intermediates will be converted into a more stable carbocation by a shift of H or an alkyl group

91
Q

1,2 shift can convert

A

a less stable carbocation into a more stable carbocation

92
Q

rearrangements are not unique to

A

dehydration rxns

can occur whenever a carbocation is formed as a reactive intermediate

93
Q

dehydration can also be done via

A

POCl3 and pyridine (an amine base) in place of H2So4 / TsOH

94
Q

POCl3 serves the same role as

A

a strong acid does in acid-catalyzed dehydration

converts poor LG (-OH) into a good LG

then dehydration –> e2

95
Q

substitution reactions do NOT occur with alcohols unless

A

-OH is converted into a good leaving group

96
Q

reaction of alcohols w/ HX (X = Cl, Br, I) is a general method to prepare

A

primary, secondary, tertiary alkyl halides

97
Q

more substituted alcohols usually react more rapidly with

A

HX

98
Q

order of reactivity can be rationalized by considering the

A

rxn mechanisms involved

depends on R group structure

99
Q

hydrogen halides reactivity increases with

A

INCREASING acidity

100
Q

b/c Cl- is a poorer nucleophile than

A

Br- or I-, the rxn of primary alcohols with HCl occurs only when an additional lewis acid catalyst (usually ZnCl2) is added

101
Q

complexation of ZnCl2 with the O atom of the alcohol makes a

A

very good leaving group that facilitates the SN2 rxn

102
Q

when a primary or secondary alcohol is treated with SOCl2 and pyridine,

A

an alkyl chloride is formed with HCl and So2 as by products

103
Q

treatment of a primary or secondary alcohol with PBr3 forms

A

an alkyl halide

104
Q

alcohols can be converted into

A

alkyl tosylates

105
Q

alkyl tosylate is composed of

A

alkyl group R (derived from an alcohol)

tosylate is a good LG

106
Q

tosyl group (CH3C6H4SO2-) is known as

A

Ts

107
Q

through TsCl, alcohols are converted to

A

tosylates in the presence of pyridine

converts poor LG (-OH) into a good one (-OTs)

108
Q

tosylate is a good LG b/c of its

A

conjugate acid, p-toluenesulfonic acid is a STRONG ACID

109
Q

alkyl tosylates have good

A

LGs b/c they undergo nuc substitution and beta-elimination

110
Q

alkyl tosylates are treated with

A

STRONG nucs (SN2) and bases (E2)

111
Q

for ethers to undergo sub/elimination, their poor LG must be

A

converted into a good LG by rxn with strong acids (HBr and HI)

112
Q

HBr & HI can provide

A

nucleophiles, Br- and I-

113
Q

H2SO4 does not have a

A

nucleophile

add H20 for hydrolysisw

114
Q

when ehters react with HBr or HI,

A

both C–O bonds are cleaved and two alkyl halides are formed as products

115
Q

mechanisms of ether cleavage

A

SN1 or SN2 (depends on R’s identity)

116
Q

when secondary or tertiary alkyl groups are bonded to the ether oxygen,

A

C–O bond is cleaved by an Sn1 mechanism (involves carbocation)

117
Q

with methyl or primary R groups,

A

C–O bonds are cleaved by SN2

118
Q

negatively charged nucleophiles attack

A

SN2-like (less hindered) 1>2>3

119
Q

epoxides in acids use

A

SN1-like mechanism

120
Q

reactions of epoxides

A

ring opening of an epoxides (either with a strong nuc or acid) is regioselective

one constitutional isomer is the major or exclusive product

site selectivity of these 2 rxns is exactly opposite

121
Q

nucleophile attacks a

A

carbon atom –> substitution product

122
Q

bronsted-lowry base removes a proton to

A

form a pi bond –> elimination product

123
Q

nucleophiles that are weak bases

A

-SH
Br-
-CN
I-
CH3CO2-

sub is favored over elimination

124
Q

strong bulky bases

A

-OC(CH3)3
DBU
DBN

e2 elimination favored

125
Q

strong nucs and bases

A

-OH
-OR

SN2 and E2 favored

126
Q

weak nucs and bases

A

H20
ROH

SN1 and E1 favored

127
Q

how to classify alkenes

A

count the number of R groups bonded to the C=C

with 2 groups on the C=C, alkene is cis or trans

128
Q

alkene stability increases

A

when # of R groups bonded to double bond carbons increases

129
Q

trans or cis alkenes are more stable?

A

trans

130
Q

how to find products/major product of an elimination rxn

A

1) identify alpha and beta carbons

2) remove halogen and substitute carbons

3) more substituted product is favored

131
Q

more substituted alkenes are favored in

A

E2/E1 reactions (Zaitsev rule)

132
Q

PRIMARY ALKYL HALIDE reacts with

A

strong nuc (SN2)

strong bulky base (E2)

133
Q

SECONDARY alkyl halide reacts with

A

strong base and nuc (SN2 + E2)

strong bulky base (E2)

weak base and nuc (SN1+E1)

134
Q

TERTIARY alkyl halide reacts w/

A

weak base and nuc (SN1 + E1)

strong base (E2)

135
Q

hydride shift (1,2-H shift):

A

less stable carbocations rearrange to more stable carbocations by the shift of an H atom

136
Q

alkyl shift

A

LESS stable carbocations rearrange to more stable carbocations by the shift of an alkyl group

orgo (6 decks)

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