4- E2 Flashcards
what are elimination reactions
- Elimination reactions consist of the removal of Hydrogen and Leaving Group from adjacent carbons to form π bond.
- start with a sigma bond, end with a pi bond
what is needed for an E2 reaction
a base that will take away the proton, form a pi bond, kick the leaving group
what kind of a mechanism does E2 proceed through
concerted mechanism, bimolecular
what are the detailed steps for E2 reaction mechanisms
- base that removes a proton from the beta carbon
- pi bond forms between the alpha carbon and the beta carbon
- because of the pi bond forming, the leaving group gets kicked off
what do strong bulky bases act as
weak nucleophiles, because steric congestion comes with them. with these bases, you cannot do SN2 (cannot act as good nucleophile)
what are examples of strong bulky bases
t-BuOK; LDA, DBU (11 atoms), DBN (9 atoms))
what do strong small bases act as
good nucleophile (can be SN2 or SN1)
what are examples of strong small bases
–OH, -OCH3, -OC2H5, -NH2, -H
why are -H (hydrides) special in E2
they are not nucleophiles, only act as bases (no SN2 possible)
what does an energy diagram for E2 look like
- one transition state
- both reagents at the transition state (because both are involved in the RDS)
what is the rate reaction for E2, what order is the reaction
rate = k[Base][R-X]
second-order reaction
explain what happens what happens when there are different beta protons in the substrate
- we will have a major and a minor
- need to discuss regioselectivity when both carbon betas are different
- size of the base is the criteria behind regioselectivity
- if you have small base → one major product
- if you have large base → two products, the opposite will be major
what do bulky bases prefer and what do small bases prefer
bulky bases prefer less steric hinderance (wherever it is easier to rip a proton off of)
small bases prefers the most stable product
what is the most stable alkene
Most stable alkene is the most substituted alkene (more alkyl substituents on π bond)
what is the Zaitsev product
- two carbons attached to the pi bond (di-substituted)
- the major product for small bases
what is the Hofmann product
- one carbon attached to the pi bond (mono-substituted alkene)
- the major product for bulky bases
why is the zaitsev’s product faster
More stable transition state with more substitution leads to faster reaction
explain the energy diagram for E2: zaitsev’s rule
- zaitsev’s product will have a lower transition state (indicating major product)
explain how alkenes are stabilized via hyperconjugation
- overlap between sigma orbitals and pi* orbitals = stabilizing pi orbital
- electrons from the sigma orbital go into the empty pi* orbital (creates an extended molecular orbital between the sigma and the pi orbital)
- the more substituted the pi bond, the more the sigma orbital and pi* orbital will overlap = more overall stability
- more substituted alkenes = more stable
explain how alkenes are stabilized via resonance
- more delocalization = more stable alkene
what is the reversal of selectivity, exception to zaitsev’s rule
- Bulky bases favour the less substituted alkene (less stable)
- Bulky bases can only reach the most accessible hydrogens.
- the one that has the least carbons interfering with it
what is the only stereochemistry in which E2 can proceed in acyclic newman projections?
E2 can only proceed if C─H and C─LG lie in an antiperiplanar conformation.
what way do you represent acyclic substrates? what way do you represent cyclic substrates?
- Newman projection is used for acyclic substrates.
- Chair conformation is used for cyclic substrates
what product will be created when you have one reactive conformation? what product will be created when you have two reactive conformation?
one: you can have either E or Z major. it depends on the final conformation of your newman projection
two: you will always have E (major) and Z (minor)
what is the relationship between number of protons on the beta carbon and the number of products
one proton = one product
two protons = two products
what is the only stereochemistry in which E2 can proceed in cyclic chair conformations?
you need to put the C alpha-LG and C beta-H axial and anti (to get the anti-periplanar)
what should the pKa of a conjugate acid be for something to be considered a strong base?
strong base needs to have a conjugate acid with a pKa > 11
what are the three steps for differentiating between E2, SN2 and SN1
- To compare, start with determining if the substrate (electrophile) is primary, secondary or tertiary
- 1) look at electrophile (1º, 2º, 3º)
- 2) nucleophile / base comparison
- 3) reaction conditions (solvent or temperature)
what are the steps for a primary substrate (SN2 vs E2)
- Major is SN2 product unless bulky base which is a weak Nucleophile was used
- look at electrophile first, then move to the criteria of comparing the nucleophile and the base, then move to the criteria of the reaction conditions
- SN2 will always predominate unless the reagent is a strong bulky base or a hydride base
what are the steps for a tertiary substrate (SN1 vs E2)
- It depends on the Nucleophile/Base used.
- If weak base/strong Nucleophile, SN1 would be favoured.
- If strong base/weak nucleophile used, E2 is favoured
- if you have a reagent that only acts as a nucleophile → SN1 (pKa of CA is <11)
- -CN, -N3, X-, RS-, ROH
- if you have a reagent that is a strong bas (pKa CA > 11) → E2
- RO-, RNH-
what are the steps for a secondary substrate (SN1 vs SN2 vs E2)
- SN1 = weak nucleophile, less heat, polar protic solvent
- SN2 = good nucleophile, less heat, polar aprotic solvent
- E2 = more basic nucleophile, more heat, polar protic solvent
- weak base/Nu= no SN2 and no E2 → has to be SN1
- strong base/Nu = SN1 is the option you can eliminate
- SN2 vs E2:
- if bulky base or hydride → E2
- if reagent only acts as strong nucleophile (X-, CN-, N3- RS-)
- SN2 vs E2:
- third criteria: what if my reagent is strong but it can react as a strong base and a strong nucleophile?
- look at reaction conditions: temperature, solvent
- if there is heating → E2 predominates
- if the solvent is polar protic → E2 predominates
- if the solvent is aprotic → SN2 predominates
- look at reaction conditions: temperature, solvent
what are the steps for a stabilized primary substrate (SN2 vs SN1 vs E2)
- if the nucleophile/base is weak → SN1
- if the nucleophile/base is strong → SN2 vs E2
- if the reagent is only a nucleophile → SN2
- if the reagent is only a base → E2
- if the reagent is a strong base/Nu → check temperature and solvent
- if heating → E2
- if protic → E2
- if aprotic → SN2
list of only Nu
- X- (Br-, Cl-, I-)
- -CN
- -N3
- RS-
- HS-
- RNH2 (only in 1º and 2º. if 3º and heating→ E2, if no heating → SN1)
- RCOO-
- RSH (moderate nucleophile if 1º)
list of only base
- LDA
- tBuO-
- DBN
- DBU
- NaH
- KH
list of strong Nu/base
- RO-
- HO-
- -C≡CR
- -NR2
- -NH2
list of weak Nu/base
- ROH
- water
- carboxylic acid (RCOOH)
- RSH (depends on the substrate → 2º, 3º)
