Ch 8 - Alkenes and Elimination Reactions Flashcards
elimination reactions are a common type in compounds possessing
a leaving group
beta elimination(1,2 elimination)
a proton from the beta(B) position is removed with the leaving group forming a double bond
dehydrohalogenation
specific beta elimination of a leaving group which is a halide
dehydration
specific beta elimination of a leaving group which is H2O
alkene
a C=C bond in the compound
acylic compound
compounds that do not contain a ring
4 steps of nomenclature of Alkenes
- identify the parent
- identify the substituents
- assign a locant to each substituent
- Arrange the substituents alphabetically
the pie bond should receive the lowest number possible despite
the presence of alkyl substituents
degree of substitution
alkenes can have up to 4 R(alkyl) groups around the double bond
- monosubstituted - disubstituted - trisubstituted - tetrasubstituted
a double bond is composed of
a pie and sigma bond
the sigma bond is due to
overlapping sp2 hybridize orbits
the pie bond is due to
overlapping p orbitals
cycloalkenes comprised of fewer than seven carbon atoms cannot
accommodate a trades pie bond
- there can be a pie bond in cis configuration
a seven ring structure can
accommodate a pie bond in trans configuration BUT it is unstable at room temperature
An 8-membered ring is the smallest ring that can
accommodate a trans double bond(pie bond) and be stable at room temperature
bredt’s rule
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
cis and trans designations only work for similar groups
- E and Z are used for nonsimilar groups
- E = opposite side
- Z – same side
priority of E and Z is determined by
the same rules as chirality centers but you look at the atoms in the vinylic positions by the C=C double bond
in general, a cis alkene will be less stable than its stereoisomeric trans alkene
- 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
the degree of substitution will affect alkene stability
- the greater the delocalization the greater the stability
- monosubstituted
proton transfers and loss of a leaving group will
eliminate a group
ALL elimination reactions exhibit proton transfer and loss of a leaving group
some elimination reactions can exhibit nucleophilic attack and rearrangement
elimination can occur as a
concerted mechanism or stepwise
in a concerted mechanism the proton transfer and the loss of the leaving group
occur simultaneously
in a stepwise mechanisms the leaving group
leaves generating an intermediate carbocation which is then deprotonated by a base to produce an alkene
concerted process for elimination
abase abstracts a proton and the leaving group leaves simultaneously
stepwise process for elimination
first the leaving group leaves and then a base abstracts a proton
E2 rate =
k[substrate][base]
- second order kinetics
E2 – bimolecular elimination
two chemical entities in a concerted mechanism
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
E2 reactivity:
- tertiary > secondary > primary
- the transition state is lowest in energy when a tertiary substrate is used and therefore the Ea is lower
most primary substrates readily undergo E2 reactions
(tertiary substrates react more rapidly)
regiochemistry
an elimination reaction can produce more than one possible product if the B positions are not identical
regioselective
both products of regiochemistry are formed but the more substituted alkene is generally observed
- higher stability
Zaitsev product
the more substituted alkene
Hofmann product
the less substituted alkene
the regiochemical outcome of an E2 reaction can often be controlled
by carefully choosing the base
stereoselective
the substrate produces two stereoisomers in unequal amounts during an E2 reaction
- trans is typically less energy than cis
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)
Anti-coplanar
referring to the relative positions of the proton and leaving group in context of newman projections
- anti
syn – coplanar
referring to the relative positions of the proton and leaving group in context to the newman projection
- eclipsed
elimination will occur more rapidly via the anti-coplanar conformation because
the transition state is lower energy(lower Ea)
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
anti-periplanar
significant enough overlap occurs that it will suffice like an anti-coplanar requirement for an E2 reactions
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
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
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
in a stereospecific E2 reaction
- the substrate is stereoisomeric, and the stereochemical outcome is dependent on which stereoisomeric substrate is used
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
when the leaving group is equatorial it can not be anti-periplanar with any of its neighboring hydrogen atoms
- must be axial
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)
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
two major factors to consider before drawing the products of an E2 reaction
- regiochemistry
- stereochemistry
E1 rate = k[substrate]
- first order reaction
- stepwise
E1 – unimolecular elimination reaction
- E = elimination
- 1 = unimolecular
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
Primary is least reactive and tertiary is
most reactive E1 process
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
if the substrate is an alcohol a strong acid will be required in order to protonate the OH group
like an Sn1 reaction
In an E1 mechanism the more substituted alkene(Zaitsev product) is the
major product
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)
E1 reactions are not stereospecific
do not require and anti-periplanarity
E1 reactions are stereoselective
when cis and trans are both possible there is a favoring of the formation of the trans stereoisomer
E1 core steps
Loss of LG followed by PT
E1 additional steps
- proton transfer before the core steps(LG followed by PT)
- Carbocation rearrangement between the two core steps(LG followed by PT)
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
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
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
E2 is concerted and rarely occurs with
additional steps to the mechanism
A carbocation is NEVER formed in an E2 reaction so
carbocation rearrangement is not possible
E2 generally require a strong base so bad leaving groups are not possible thus no PT needed
possible but rare
substitution and elimination tend to be in competition with each other
there is typically NOT a clear winner and both processes occur
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
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
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
in a proton transfer process the equilibrium will favor the
weaker base
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
4 categories of reagents
nucleophile only(strong nucleophile and weak base)
- Cl-, Br-, I-, HS-, H2S, RS-, RSH
4 categories of reagents
Base Only(bad nucleophile and good base)
- H-(or NaOH etc), DBN, DBU
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)
4 categories of reagents
Weak Nucleophile and Weak Base
- H2O, MeOH, EtOH
- water, alcohols(ROH)
- used for unimolecular processes(Sn1 and E1)
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
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
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
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
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
review page 389 for outcomes for reagents
review page 389 for outcomes for reagents
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
