Ch 8 - Alkenes and Elimination Reactions Flashcards
(88 cards)
1
Q
elimination reactions are a common type in compounds possessing
A
a leaving group
2
Q
beta elimination(1,2 elimination)
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a proton from the beta(B) position is removed with the leaving group forming a double bond
3
Q
dehydrohalogenation
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specific beta elimination of a leaving group which is a halide
4
Q
dehydration
A
specific beta elimination of a leaving group which is H2O
5
Q
alkene
A
a C=C bond in the compound
6
Q
acylic compound
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compounds that do not contain a ring
7
Q
4 steps of nomenclature of Alkenes
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- identify the parent
- identify the substituents
- assign a locant to each substituent
- Arrange the substituents alphabetically
8
Q
the pie bond should receive the lowest number possible despite
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the presence of alkyl substituents
9
Q
degree of substitution
A
alkenes can have up to 4 R(alkyl) groups around the double bond
- monosubstituted - disubstituted - trisubstituted - tetrasubstituted
10
Q
a double bond is composed of
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a pie and sigma bond
11
Q
the sigma bond is due to
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overlapping sp2 hybridize orbits
12
Q
the pie bond is due to
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overlapping p orbitals
13
Q
cycloalkenes comprised of fewer than seven carbon atoms cannot
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accommodate a trades pie bond
- there can be a pie bond in cis configuration
14
Q
a seven ring structure can
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accommodate a pie bond in trans configuration BUT it is unstable at room temperature
15
Q
An 8-membered ring is the smallest ring that can
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accommodate a trans double bond(pie bond) and be stable at room temperature
16
Q
bredt’s rule
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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
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
18
Q
priority of E and Z is determined by
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the same rules as chirality centers but you look at the atoms in the vinylic positions by the C=C double bond
19
Q
in general, a cis alkene will be less stable than its stereoisomeric trans alkene
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- 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
20
Q
the degree of substitution will affect alkene stability
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- the greater the delocalization the greater the stability
- monosubstituted
21
Q
proton transfers and loss of a leaving group will
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eliminate a group
22
Q
ALL elimination reactions exhibit proton transfer and loss of a leaving group
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some elimination reactions can exhibit nucleophilic attack and rearrangement
23
Q
elimination can occur as a
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concerted mechanism or stepwise
24
Q
in a concerted mechanism the proton transfer and the loss of the leaving group
A
occur simultaneously
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in a stepwise mechanisms the leaving group
leaves generating an intermediate carbocation which is then deprotonated by a base to produce an alkene
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concerted process for elimination
abase abstracts a proton and the leaving group leaves simultaneously
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stepwise process for elimination
first the leaving group leaves and then a base abstracts a proton
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E2 rate =
k[substrate][base]
| - second order kinetics
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E2 – bimolecular elimination
two chemical entities in a concerted mechanism
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tertiary substrates work rapidly with E2 because it is acting as a base to abstract a proton
unlike Sn2 and steric restrictions to get to the carbon
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E2 reactivity:
- 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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most primary substrates readily undergo E2 reactions
(tertiary substrates react more rapidly)
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regiochemistry
an elimination reaction can produce more than one possible product if the B positions are not identical
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regioselective
both products of regiochemistry are formed but the more substituted alkene is generally observed
- higher stability
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Zaitsev product
the more substituted alkene
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Hofmann product
the less substituted alkene
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the regiochemical outcome of an E2 reaction can often be controlled
by carefully choosing the base
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stereoselective
the substrate produces two stereoisomers in unequal amounts during an E2 reaction
- trans is typically less energy than cis
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coplanar
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)
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Anti-coplanar
referring to the relative positions of the proton and leaving group in context of newman projections
- anti
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syn – coplanar
referring to the relative positions of the proton and leaving group in context to the newman projection
- eclipsed
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elimination will occur more rapidly via the anti-coplanar conformation because
the transition state is lower energy(lower Ea)
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periplanar
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
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anti-periplanar
significant enough overlap occurs that it will suffice like an anti-coplanar requirement for an E2 reactions
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the stereoisomeric product of an E2 process depends on the configuration of the starting alkyl halide
- 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
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the stereospecificity of an E2 reaction is only relevant when the B position has only one proton
- B position must be arranged anti-periplanar
- results in both stereoisomeric products being obtains
- the more stable isomeric alkene will predominate
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in a stereoselective E2 reaction
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
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in a stereospecific E2 reaction
- the substrate is stereoisomeric, and the stereochemical outcome is dependent on which stereoisomeric substrate is used
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the requirement for an anti-periplanar conformation demands that an E2 reaction can
only occur from the chair conformation in which the leaving group occupies an axial position
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when the leaving group is equatorial it can not be anti-periplanar with any of its neighboring hydrogen atoms
- must be axial
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an E2 reaction can only take place when the leaving group and the proton are on
opposite sides of the ring(one on a wedge and the other on a dash)
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the chair conformation in which the compound spends its time in will determine the rate of the reaction
- an E2 reaction will be time dependent
| - If in the wrong conformation(higher energy conformation) the reaction will occur more slowly
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two major factors to consider before drawing the products of an E2 reaction
- regiochemistry
| - stereochemistry
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E1 rate = k[substrate]
- first order reaction
| - stepwise
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E1 – unimolecular elimination reaction
- E = elimination
| - 1 = unimolecular
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E1 reaction rate is very sensitive to the nature of the starting alkyl halide
- tertiary halides reacting most readily
| - identical to the trend in Sn1 reactions
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Primary is least reactive and tertiary is
most reactive E1 process
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like an Sn1 process the first step in an E1is the loss of the leaving group to form a carbocation intermediate
generally in competition with each other
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if the substrate is an alcohol a strong acid will be required in order to protonate the OH group
like an Sn1 reaction
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In an E1 mechanism the more substituted alkene(Zaitsev product) is the
major product
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the regiochemical outcome of an E1 process cannot be controlled
major difference from an E2 process which can be controlled by the choice of base(more or less steric hinderence)
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E1 reactions are not stereospecific
do not require and anti-periplanarity
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E1 reactions are stereoselective
when cis and trans are both possible there is a favoring of the formation of the trans stereoisomer
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E1 core steps
Loss of LG followed by PT
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E1 additional steps
- proton transfer before the core steps(LG followed by PT)
| - Carbocation rearrangement between the two core steps(LG followed by PT)
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a PT is required before the E1 core steps(LG followed by PT) for the same reason as Sn1 reaction
- necessary when the leaving group is an OH or other bad leaving group
- an acid is required to make thi reaction happen
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carbocation rearrangement can occur via a methyl or hydride shift between the E1 core steps(LG followed by PT)
will occur if a more stable carbocation can be formed
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when carbocation rearrangement occurs in an E1 reaction both products from rearrangement and those without rearrangement will appear
can be more than 2 total products
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E2 is concerted and rarely occurs with
additional steps to the mechanism
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A carbocation is NEVER formed in an E2 reaction so
carbocation rearrangement is not possible
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E2 generally require a strong base so bad leaving groups are not possible thus no PT needed
possible but rare
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substitution and elimination tend to be in competition with each other
there is typically NOT a clear winner and both processes occur
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3 steps to determining all products and to predict which products are major and minor
- determine the function of the reagent
- analyze the substrate and determine the expected mechanism(s)
- consider any relevant regiochemical and stereochemical requirements
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the first goal is to determine the function of the reagent
- 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
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the greater the polarizability of the nucleophile the stronger the nucleophile will be
- larger size with many electrons distant from the nucleus will be strong nucleophile
- even if a charge is lacking can still be quite strong
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in a proton transfer process the equilibrium will favor the
weaker base
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4 categories of reagents
- nucleophile only(strong nucleophile and weak base
- Base Only(bad nucleophile and good base)
- Strong nucleophile and Strong Base
- Weak Nucleophile and Weak Base
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4 categories of reagents
nucleophile only(strong nucleophile and weak base)
- Cl-, Br-, I-, HS-, H2S, RS-, RSH
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4 categories of reagents
Base Only(bad nucleophile and good base)
- H-(or NaOH etc), DBN, DBU
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4 categories of reagents
Strong nucleophile and Strong Base
- HO-, MeO-, EtO-
- hydroxides(HO-) and alkoxide(RO-) ions
- generally used for bimolecular processes(Sn2 and E2)
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4 categories of reagents
Weak Nucleophile and Weak Base
- H2O, MeOH, EtOH
- water, alcohols(ROH)
- used for unimolecular processes(Sn1 and E1)
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always determine whether a reagent is a strong or weak nucleophile by looking for charge and polarizability
then determine whether or not the reagent is a strong base using either a quantitative or qualitative method
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Outcomes for reagents that function only as nucleophiles
- 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
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Outcomes for reagents that function only as bases
- 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
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Outcomes for reagents that are strong bases and strong nucleophiles
- 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
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outcomes for reagents that are weak bases and weak nucleophiles
- 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
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review page 389 for outcomes for reagents
review page 389 for outcomes for reagents
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Substitution vs Elimination
Predicting the products
Remember
- determine the function of the reagent
- analyze the substrate to determine the expected mechanism(s)
- consider any relevant regiochemical and stereochemical requirements
