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Orgo 1 > Ch 8 - Alkenes and Elimination Reactions > Flashcards

Ch 8 - Alkenes and Elimination Reactions Flashcards

1
Q

elimination reactions are a common type in compounds possessing

A

a leaving group

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2
Q

beta elimination(1,2 elimination)

A

a proton from the beta(B) position is removed with the leaving group forming a double bond

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3
Q

dehydrohalogenation

A

specific beta elimination of a leaving group which is a halide

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4
Q

dehydration

A

specific beta elimination of a leaving group which is H2O

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5
Q

alkene

A

a C=C bond in the compound

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6
Q

acylic compound

A

compounds that do not contain a ring

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7
Q

4 steps of nomenclature of Alkenes

A
  • identify the parent
    - identify the substituents
    - assign a locant to each substituent
    - Arrange the substituents alphabetically
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8
Q

the pie bond should receive the lowest number possible despite

A

the presence of alkyl substituents

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9
Q

degree of substitution

A

alkenes can have up to 4 R(alkyl) groups around the double bond

	- monosubstituted
	- disubstituted
	- trisubstituted
	- tetrasubstituted
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10
Q

a double bond is composed of

A

a pie and sigma bond

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11
Q

the sigma bond is due to

A

overlapping sp2 hybridize orbits

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12
Q

the pie bond is due to

A

overlapping p orbitals

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13
Q

cycloalkenes comprised of fewer than seven carbon atoms cannot

A

accommodate a trades pie bond

- there can be a pie bond in cis configuration

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14
Q

a seven ring structure can

A

accommodate a pie bond in trans configuration BUT it is unstable at room temperature

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15
Q

An 8-membered ring is the smallest ring that can

A

accommodate a trans double bond(pie bond) and be stable at room temperature

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16
Q

bredt’s rule

A

states it is not possible for a bridgehead carbon of a bicyclic system to possess a C=C doubt bond if it involves a trans pie bond being incorporated in a small ring
- bicyclic compounds can only exhibit a double bond at a bridgehead if one of the rings has at least 8 carbon atoms

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17
Q

cis and trans designations only work for similar groups

A
  • E and Z are used for nonsimilar groups
    - E = opposite side
    - Z – same side
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18
Q

priority of E and Z is determined by

A

the same rules as chirality centers but you look at the atoms in the vinylic positions by the C=C double bond

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19
Q

in general, a cis alkene will be less stable than its stereoisomeric trans alkene

A
  • cis will have higher steric strain
    - heats of combustion reflect this with cis being slight higher even though both cis and trans can yield the same product
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20
Q

the degree of substitution will affect alkene stability

A
  • the greater the delocalization the greater the stability

- monosubstituted

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21
Q

proton transfers and loss of a leaving group will

A

eliminate a group

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22
Q

ALL elimination reactions exhibit proton transfer and loss of a leaving group

A

some elimination reactions can exhibit nucleophilic attack and rearrangement

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23
Q

elimination can occur as a

A

concerted mechanism or stepwise

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24
Q

in a concerted mechanism the proton transfer and the loss of the leaving group

A

occur simultaneously

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25
Q

in a stepwise mechanisms the leaving group

A

leaves generating an intermediate carbocation which is then deprotonated by a base to produce an alkene

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26
Q

concerted process for elimination

A

abase abstracts a proton and the leaving group leaves simultaneously

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27
Q

stepwise process for elimination

A

first the leaving group leaves and then a base abstracts a proton

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28
Q

E2 rate =

A

k[substrate][base]

- second order kinetics

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29
Q

E2 – bimolecular elimination

A

two chemical entities in a concerted mechanism

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30
Q

tertiary substrates work rapidly with E2 because it is acting as a base to abstract a proton

A

unlike Sn2 and steric restrictions to get to the carbon

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31
Q

E2 reactivity:

A
  • tertiary > secondary > primary

- the transition state is lowest in energy when a tertiary substrate is used and therefore the Ea is lower

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32
Q

most primary substrates readily undergo E2 reactions

A

(tertiary substrates react more rapidly)

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33
Q

regiochemistry

A

an elimination reaction can produce more than one possible product if the B positions are not identical

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34
Q

regioselective

A

both products of regiochemistry are formed but the more substituted alkene is generally observed
- higher stability

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35
Q

Zaitsev product

A

the more substituted alkene

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36
Q

Hofmann product

A

the less substituted alkene

37
Q

the regiochemical outcome of an E2 reaction can often be controlled

A

by carefully choosing the base

38
Q

stereoselective

A

the substrate produces two stereoisomers in unequal amounts during an E2 reaction
- trans is typically less energy than cis

39
Q

coplanar

A

the atoms which must lie in the same plane when only one proton is present for an E2 reaction
- the proton at the B position, the leaving group, and the two carbon atoms which will bear the pie bond(must line up the p orbitals)

40
Q

Anti-coplanar

A

referring to the relative positions of the proton and leaving group in context of newman projections
- anti

41
Q

syn – coplanar

A

referring to the relative positions of the proton and leaving group in context to the newman projection
- eclipsed

42
Q

elimination will occur more rapidly via the anti-coplanar conformation because

A

the transition state is lower energy(lower Ea)

43
Q

periplanar

A

describes when a proton and a leaving group are nearly coplanar(178-179 degree angle)
- still creates enough orbital overlap for an E2 reactions to occur

44
Q

anti-periplanar

A

significant enough overlap occurs that it will suffice like an anti-coplanar requirement for an E2 reactions

45
Q

the stereoisomeric product of an E2 process depends on the configuration of the starting alkyl halide

A
  • it is ABSOLUTELY WRONG to say that the product will always be the trans isomer
    - must draw the newman projection and then determine which stereoisomeric product is obtained
46
Q

the stereospecificity of an E2 reaction is only relevant when the B position has only one proton

A
  • B position must be arranged anti-periplanar
    - results in both stereoisomeric products being obtains
    - the more stable isomeric alkene will predominate
47
Q

in a stereoselective E2 reaction

A

the substrate itself is not necessarily stereoisomric; nevertheless this substrate can produce two stereoisomeric products and it is found that one stereoisomeric product is formed in higher yield

48
Q

in a stereospecific E2 reaction

A
  • the substrate is stereoisomeric, and the stereochemical outcome is dependent on which stereoisomeric substrate is used
49
Q

the requirement for an anti-periplanar conformation demands that an E2 reaction can

A

only occur from the chair conformation in which the leaving group occupies an axial position

50
Q

when the leaving group is equatorial it can not be anti-periplanar with any of its neighboring hydrogen atoms

A
  • must be axial
51
Q

an E2 reaction can only take place when the leaving group and the proton are on

A

opposite sides of the ring(one on a wedge and the other on a dash)

52
Q

the chair conformation in which the compound spends its time in will determine the rate of the reaction

A
  • an E2 reaction will be time dependent

- If in the wrong conformation(higher energy conformation) the reaction will occur more slowly

53
Q

two major factors to consider before drawing the products of an E2 reaction

A
  • regiochemistry

- stereochemistry

54
Q

E1 rate = k[substrate]

A
  • first order reaction

- stepwise

55
Q

E1 – unimolecular elimination reaction

A
  • E = elimination

- 1 = unimolecular

56
Q

E1 reaction rate is very sensitive to the nature of the starting alkyl halide

A
  • tertiary halides reacting most readily

- identical to the trend in Sn1 reactions

57
Q

Primary is least reactive and tertiary is

A

most reactive E1 process

58
Q

like an Sn1 process the first step in an E1is the loss of the leaving group to form a carbocation intermediate

A

generally in competition with each other

59
Q

if the substrate is an alcohol a strong acid will be required in order to protonate the OH group

A

like an Sn1 reaction

60
Q

In an E1 mechanism the more substituted alkene(Zaitsev product) is the

A

major product

61
Q

the regiochemical outcome of an E1 process cannot be controlled

A

major difference from an E2 process which can be controlled by the choice of base(more or less steric hinderence)

62
Q

E1 reactions are not stereospecific

A

do not require and anti-periplanarity

63
Q

E1 reactions are stereoselective

A

when cis and trans are both possible there is a favoring of the formation of the trans stereoisomer

64
Q

E1 core steps

A

Loss of LG followed by PT

65
Q

E1 additional steps

A
  • proton transfer before the core steps(LG followed by PT)

- Carbocation rearrangement between the two core steps(LG followed by PT)

66
Q

a PT is required before the E1 core steps(LG followed by PT) for the same reason as Sn1 reaction

A
  • necessary when the leaving group is an OH or other bad leaving group
    - an acid is required to make thi reaction happen
67
Q

carbocation rearrangement can occur via a methyl or hydride shift between the E1 core steps(LG followed by PT)

A

will occur if a more stable carbocation can be formed

68
Q

when carbocation rearrangement occurs in an E1 reaction both products from rearrangement and those without rearrangement will appear

A

can be more than 2 total products

69
Q

E2 is concerted and rarely occurs with

A

additional steps to the mechanism

70
Q

A carbocation is NEVER formed in an E2 reaction so

A

carbocation rearrangement is not possible

71
Q

E2 generally require a strong base so bad leaving groups are not possible thus no PT needed

A

possible but rare

72
Q

substitution and elimination tend to be in competition with each other

A

there is typically NOT a clear winner and both processes occur

73
Q

3 steps to determining all products and to predict which products are major and minor

A
  • determine the function of the reagent
    - analyze the substrate and determine the expected mechanism(s)
    - consider any relevant regiochemical and stereochemical requirements
74
Q

the first goal is to determine the function of the reagent

A
  • is it a strong or weak nucleophile?
    - kinetic function for rate of reaction
    - is it a strong or weak base?
    - theremodynamic function for equilibrium
    - nucleophilicity and basicity do not always parallel each other
75
Q

the greater the polarizability of the nucleophile the stronger the nucleophile will be

A
  • larger size with many electrons distant from the nucleus will be strong nucleophile
    - even if a charge is lacking can still be quite strong
76
Q

in a proton transfer process the equilibrium will favor the

A

weaker base

77
Q

4 categories of reagents

A
  • nucleophile only(strong nucleophile and weak base
  • Base Only(bad nucleophile and good base)
  • Strong nucleophile and Strong Base
  • Weak Nucleophile and Weak Base
78
Q

4 categories of reagents

nucleophile only(strong nucleophile and weak base)

A
  • Cl-, Br-, I-, HS-, H2S, RS-, RSH
79
Q

4 categories of reagents

Base Only(bad nucleophile and good base)

A
  • H-(or NaOH etc), DBN, DBU
80
Q

4 categories of reagents

Strong nucleophile and Strong Base

A
  • HO-, MeO-, EtO-
    - hydroxides(HO-) and alkoxide(RO-) ions
    - generally used for bimolecular processes(Sn2 and E2)
81
Q

4 categories of reagents

Weak Nucleophile and Weak Base

A
  • H2O, MeOH, EtOH
    - water, alcohols(ROH)
    - used for unimolecular processes(Sn1 and E1)
82
Q

always determine whether a reagent is a strong or weak nucleophile by looking for charge and polarizability

A

then determine whether or not the reagent is a strong base using either a quantitative or qualitative method

83
Q

Outcomes for reagents that function only as nucleophiles

A
  • ONLY substitution reactions will occur(no elimination)
    - SH-, Br- etc
    - Primary, secondary, and tertiary substrates
    - Primary substrate -> Sn2
    - Secondary Substrate -> Sn2 and Sn1
    - Tertiary substrate -> Sn1
84
Q

Outcomes for reagents that function only as bases

A
  • Only elimination reactions will occur(no substitution)
    - strong bases resulting in E2 process
    - H-, DBN etc
    - Primary, secondary, and tertiary substrates
    - Primary substrate -> E2
    - Secondary Substrate -> E2
    - Tertiary substrate -> E2
85
Q

Outcomes for reagents that are strong bases and strong nucleophiles

A
  • bimolecular mechanisms will dominate(Sn2 and E2)
    - the rates of Sn2 and E2 are affected differently by the substrate
    - Sn2 dominates primary substrate
    - E2 dominates secondary
    - only E2 is present in tertiary(Sn2 is too sterically hindered)
    - E2 not affected by steric hindrances
    - Primary, secondary, and tertiary substrates
    - Primary substrate -> E2(minor) + Sn2(major)
    - Secondary Substrate -> E2(Major) + Sn2(minor)
    - Tertiary substrate -> E2
86
Q

outcomes for reagents that are weak bases and weak nucleophiles

A
  • primary and secondary substrates are not practical
    - RO,H2O etc
    - tertiary substrate is practical -> Sn1 + E1
    - Sn1 is generally favored but E1 can predominate over Sn1 at elevated temperatures
87
Q

review page 389 for outcomes for reagents

A

review page 389 for outcomes for reagents

88
Q

Substitution vs Elimination

Predicting the products

Remember

A
  • determine the function of the reagent
    - analyze the substrate to determine the expected mechanism(s)
    - consider any relevant regiochemical and stereochemical requirements

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