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rena conquers organic chemistry > 4- E2 > Flashcards

4- E2 Flashcards

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1
Q

what are elimination reactions

A
  • 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
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2
Q

what is needed for an E2 reaction

A

a base that will take away the proton, form a pi bond, kick the leaving group

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3
Q

what kind of a mechanism does E2 proceed through

A

concerted mechanism, bimolecular

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4
Q

what are the detailed steps for E2 reaction mechanisms

A
  • 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
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5
Q

what do strong bulky bases act as

A

weak nucleophiles, because steric congestion comes with them. with these bases, you cannot do SN2 (cannot act as good nucleophile)

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6
Q

what are examples of strong bulky bases

A

t-BuOK; LDA, DBU (11 atoms), DBN (9 atoms))

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7
Q

what do strong small bases act as

A

good nucleophile (can be SN2 or SN1)

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8
Q

what are examples of strong small bases

A

–OH, -OCH3, -OC2H5, -NH2, -H

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9
Q

why are -H (hydrides) special in E2

A

they are not nucleophiles, only act as bases (no SN2 possible)

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10
Q

what does an energy diagram for E2 look like

A
  • one transition state

- both reagents at the transition state (because both are involved in the RDS)

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11
Q

what is the rate reaction for E2, what order is the reaction

A

rate = k[Base][R-X]

second-order reaction

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12
Q

explain what happens what happens when there are different beta protons in the substrate

A
  • 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
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13
Q

what do bulky bases prefer and what do small bases prefer

A

bulky bases prefer less steric hinderance (wherever it is easier to rip a proton off of)
small bases prefers the most stable product

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14
Q

what is the most stable alkene

A

Most stable alkene is the most substituted alkene (more alkyl substituents on π bond)

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15
Q

what is the Zaitsev product

A
  • two carbons attached to the pi bond (di-substituted)

- the major product for small bases

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16
Q

what is the Hofmann product

A
  • one carbon attached to the pi bond (mono-substituted alkene)
  • the major product for bulky bases
17
Q

why is the zaitsev’s product faster

A

More stable transition state with more substitution leads to faster reaction

18
Q

explain the energy diagram for E2: zaitsev’s rule

A
  • zaitsev’s product will have a lower transition state (indicating major product)
19
Q

explain how alkenes are stabilized via hyperconjugation

A
  • 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
20
Q

explain how alkenes are stabilized via resonance

A
  • more delocalization = more stable alkene
21
Q

what is the reversal of selectivity, exception to zaitsev’s rule

A
  • 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
22
Q

what is the only stereochemistry in which E2 can proceed in acyclic newman projections?

A

E2 can only proceed if C─H and C─LG lie in an antiperiplanar conformation.

23
Q

what way do you represent acyclic substrates? what way do you represent cyclic substrates?

A
  • Newman projection is used for acyclic substrates.

- Chair conformation is used for cyclic substrates

24
Q

what product will be created when you have one reactive conformation? what product will be created when you have two reactive conformation?

A

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)

25
Q

what is the relationship between number of protons on the beta carbon and the number of products

A

one proton = one product

two protons = two products

26
Q

what is the only stereochemistry in which E2 can proceed in cyclic chair conformations?

A

you need to put the C alpha-LG and C beta-H axial and anti (to get the anti-periplanar)

27
Q

what should the pKa of a conjugate acid be for something to be considered a strong base?

A

strong base needs to have a conjugate acid with a pKa > 11

28
Q

what are the three steps for differentiating between E2, SN2 and SN1

A
  • 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)
29
Q

what are the steps for a primary substrate (SN2 vs E2)

A
  • 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
30
Q

what are the steps for a tertiary substrate (SN1 vs E2)

A
  • 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-
31
Q

what are the steps for a secondary substrate (SN1 vs SN2 vs E2)

A
  • 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-)
  • 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
32
Q

what are the steps for a stabilized primary substrate (SN2 vs SN1 vs E2)

A
  • 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
33
Q

list of only Nu

A
  • 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)
34
Q

list of only base

A
  • LDA
  • tBuO-
  • DBN
  • DBU
  • NaH
  • KH
35
Q

list of strong Nu/base

A
  • RO-
  • HO-
  • -C≡CR
  • -NR2
  • -NH2
36
Q

list of weak Nu/base

A
  • ROH
  • water
  • carboxylic acid (RCOOH)
  • RSH (depends on the substrate → 2º, 3º)

rena conquers organic chemistry (6 decks)

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