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organic chem > lecture 7: elimination > Flashcards

lecture 7: elimination Flashcards

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1
Q

explain why trans is more stable than cis

A

beacuse in cis isomers there is steric hinderance (more crowding)

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2
Q

the more subsituted an alkene is, the more or less stable is it

A

more stable

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3
Q

why are more subsititued alknes more stable

A

because added alkyl groups donate electrons to the sp2 carbon because of hyperconj and inductition

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4
Q

rings of cyclos contained 5 C or fewer can only exist in what form

A

cis form

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5
Q

why can cyclo alkenes 5c or less only excist in cis form

A

because of bond strain

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6
Q

in elimination do you make or remove bonds

A

make bonds

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7
Q

what two reactions compete

A

elimination and subsition

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8
Q

if the reactor is nucleophillic is it S or E

A

S

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9
Q

if the reactor is basic is it S or E

A

E

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10
Q

what is another name for dehydrohalogenation

A

beta elimination, 1-2 elimination

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11
Q

explain dehydrohalogenation

A

synthesiszing alkenes via the elimination of H-X bond from adjacent atoms of alkyl halides

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12
Q

the carbon atom that bears the LG is called what

A

the alpha carbon

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13
Q

the carbon atom adjacent to the C that bears the LG is called what

A

the beta carbon

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14
Q

is the e2 bimol or uni

A

bimol

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15
Q

explain tthe rate for e2

A

in the rate determining step, the reaction reatet is deermined by the concentrtatiton of substrtatte and of base

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16
Q

what tis the general mechniams of e2

A

in e2 rxn, a base removes a b hydrogen from b carbon as the double bond forms and a LG departs from alpha carbon

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17
Q

explain e2 mechamisn

A

1) a base regioselecttively removes an H+ from a carbon that is adjacent (beta) to the halogen carbon (alpha)
2) as the H+ is removed, e tthat is shared w carbon moves ttowards adjacent halogen bonded c atom
3) as the e moves to the alpha carbon, the halogen LG leaves and takes the e pair with it (hetero)

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18
Q

know the e2 mechanism

A
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19
Q

what is the difference between syn and anti eliminatiton

A

in anti, tthe base atttacks the b hydrogen on the oPPOSITE side of the LG

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20
Q

why is anti more sttable

A

since less steric hinderance

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21
Q

what is zaitzevs rule

A

an elimination that occurs to give the most stable and highly subsituted alkene

(removes H from the beta position thtat is most subsituted/least amount of carbons)

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22
Q

is a more internal or external bond more stable

A

internal

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23
Q

formation of more subsituted alkene is favoured with a large or small base

A

small

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24
Q

formation of a less substituted alkene is favoured with what type of base

A

a bulky base

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25
Q

explain the affectt of base size on formation of alekens

A

if you use a small base, the major product will be the highly subsituted alkene (because there is no steric hinderance)

if you use a bulky sterically hindered base, it has a hard time removing an internal hydrogen (crowding) therefore it will remove an external H to form a less subsituted

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26
Q

the e2 recation produces mostly what isomer and why

A

thee E isomer (because it does antti elim)

=steric interactions prefer bulky groups on opposite sides form the most stable bond)

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27
Q

in a cyclohexane, e2 rection can only occur when the groups are in what positions

A

can only occur when the LG is axial and the H is axial

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28
Q

true or false: for cyclohexane e2 reactions, the LG and the H mus tbe in the equitorial position

A

false, axial

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29
Q

why do the halogen and hydrogen need to be axial from each otther

A

they need to be in the antiperiplanar position from the leaving group

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30
Q

when NU attacks b hydrogen, what rxn occurs

A

elim

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31
Q

when NU attacks the c bearing the LG, what rxn happens

A

substitutuon

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32
Q

en2 and sn2 prefer what nucleophiles and solvents

A

strong nu

aprotic solvents

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33
Q

explain the affect of classificaiton of the alkyl halide on elim vs substituon

A

primary= favours sn2 due to less steric hinderance (can easily reach C)

ttertiary= favours e2 due to more stable tertiaty carbocation (hard to reach center C)

34
Q

explain affect of termperate on what type of reaction

A

w/o heating= substituion

w/ heating= elim

35
Q

true or false: increase temp faours subsitution

A

false, elimination

36
Q

strong sterically hindered bases favour what rxn

A

elimination

37
Q

give example of bulky base

A

tbut

38
Q

why do small bases (ex: CH3O) favour sub

A

has easy time rection the C

39
Q

why do bulky bases prefer elimination

A

they cannot get tclose to the center C due to steric hinderance

40
Q

hydroxide, amides icnrease likelyhood or what

A

elimination

41
Q

cl-, ch3co2- increase likelyhood or what

A

sub

42
Q

if there is a negative charge on the oxygen that makes a good or bad base

A

strong base

43
Q

if solvent acts as a BASE and removes a hydrogen, what rxn is it

A

elim

44
Q

if solvent acts as a NU what rxn

A

sn

45
Q

explain the e1 recation mechanism

A

the alkyl halide dissocates forming a carbocation (similar to sn1)
2) removal of h by the base

46
Q

how is leaving group able to leave on its own in e1 reactions

A

it is aided by the polar solvent

47
Q

e1 reactions are favoured by substrates that form stable carbocations true or false

A

true

48
Q

list the relative reactivities of alkyl halides in e1 reaction

A 
3 benzylic (most stable=resonance)
3 allylic
2 benxylic
2 allylic
3
1 benzylic
1 allylic
2
1
vinyl
49
Q

what is the reaction of alcohols when heated with strong acid

A

most alcohols undergo dehydration b=rxn (loss of h20) to form an alkene when heated with a strong acid

50
Q

2 and 3 degree alcohols favour what type of reaction

A

e1

51
Q

primary alcs favour what type of recatiton

A

e2 with high temps

52
Q

true or false: 2 and 3 degree alchols are most diffuclt to dehydrtea==ate

A

false, primayr==ry

53
Q

what is the relative ease with which alcohols undergo dehydration

A

3 better than 2 than 1

54
Q

know the mechanism for dehydration of alcohols (2.3)

A
55
Q

what is the general mechanism for dehydration of 2 3 alchols in elimination

A

1) alcholol acceots a protton from the acid in the fast step
2) the protonated alcohol loses a molucule of water (LG) to become a carbocattion=rate determining
3) carbocation loses a porton to a base (base may be another alcohol, waater etc) and formation of alkene

56
Q

e1 and sn1 reactons take place via carbocation intermediates which means what is possibor

A

rearragnements

57
Q

sn2 prefers protic or aprotic

A

aprotic

58
Q

sn2 reacvtity

A

me
1
2

59
Q

sn2 nucleophile needed

A

good (ex: OH-)

60
Q

sn2 tetmperated needed

A

low (25)

61
Q

sterics of sn2

A

low (OH-)

62
Q

what is the sterochem of sn2

A

incversion of configuration

63
Q

what is the solvent of sn1

A

polar protic (h20, etoh)

64
Q

what is the recatiivty for sn1

A

3
2
1

65
Q

what is the nu for sn1 reaction

A

weak (h20)

66
Q

what is the temperate for sn1

A

low (25)

67
Q

what are the stterics for sn1

A

low

68
Q

what is the sterochemistry of sn1

A

makes racemic mixtures

69
Q

what is the solvent for e2

A

polar aprotic (DMSO, DMF)

70
Q

what is the reacivtiy of e2

A

3
2
1

71
Q

what is the nu for e2

A

good (OH-)

72
Q

what is the teermpeaerted for e2

A

high (80)

73
Q

what is the sterics of NU for E2

A

high

74
Q

what is the sterochem of e2 recaetions

A

cis trans

internal bond favoured

75
Q

what is the solvent for e1

A

polar protic

76
Q

what is the reacivtiy for e1

A

3
2
1

77
Q

what is the nu for e1

A

poor (h2)

78
Q

what is the temperated for e1

A

high

79
Q

what is the sterics for nu for e1

A

high (T\HotBU)

80
Q

what are the sterochemi for e1

A

cis trans internal bond favoured

organic chem (4 decks)

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