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CH10136 > Elimination- Base > Flashcards

Elimination- Base Flashcards

1
Q

Which elimination is affected by the base

A
  1. E2
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2
Q

What type of bases can be used for E1 reactions

A
  1. Unaffected by the base
  2. Very weak bases (have strong conjugate acid)
  3. Ideally the base will not act as a nucleophile to avoid competing substitution
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3
Q

Give 2 examples of good bases to use in E1 reactions

A
  1. HSO4- and H2PO4-
  2. If needed H+ from conjugate acid can protonate alcohol groups to make a better leaving group
  3. HSO4-/H2PO4- are very weak bases and poor nucleophiles so there is no competing substitution
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4
Q

What type of bases favour E2 reactions

A
  1. More basic nucleophiles
  2. High concentration of base will accelerate E2 elimination
    3.
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5
Q

What is the difference between nucleophilic substitution and elimination

A
  1. Nucleophilic attack at carbon leads to substitution whereas attack at hydrogen leads to elimination
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6
Q

What is the strength of the conjugate acid of a strong base

A
  1. Weak conjugate acid
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7
Q

What is basicity

A
  1. It is a measure of the ability of a species to accept a proton
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8
Q

How can you determine basicity of a species

A
  1. Use the acidity of the protonated form of a base (the conjugate acid) to quantify how basic a species is
  2. Measure the pKa of the conjugate acid of a base (pKaH)
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9
Q

Name 6 common anionic oxygen bases and give their pKaH in H2O

A
  1. Carbonate- 10
  2. phenoxide- 10
  3. Hydroxide- 14
  4. Methoxide- 16
  5. Ethoxide- 16
  6. tert-Butoxide- 17
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10
Q

Name 8 neutral nitrogen bases and give their pKaH

A
  1. Pyridine- 5.2
  2. Imidazole- 6.9
  3. Ammonia- 9.2
  4. Triethylamine- 11
  5. Piperidine- 11
  6. DBU- 12
  7. DBN- 13
  8. TBD- 15
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11
Q

What does a lower pKa value indicate

A
  1. Indicates a stronger acid

2. So strong bases have higher pKaH

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12
Q

What is pKaH

A
  1. pKa of the conjugate acid of a base
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13
Q

What affect does steric demand of the nucleophile/base have

A
  1. B-hydrogen atoms are more accessible than the carbon alpha to the leaving group
  2. Sterically demanding bases favour elimination
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14
Q

Describe why the size of nucleophile affects if substitution or elimination

A
  1. If small it can approach the sigma* of C-X bond so substitution is favoured
  2. If large it cannot interact with sigma* C-X bond so favours more accessible beta-hydrogen so elimination
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15
Q

Describe properties of DBU and DBN

A
  1. They are sterically demanding amidine bases that can be used to perform E2 elimination reactions
  2. Strong bases as the conjugate acid is stabilised by delocalisation of the positive charge
  3. Bulky bases, so don’t undergo substitution reactions
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16
Q

If it is a strong sterically demanding base what reaction does it prefer

A
  1. E2 elimination

CH10136 (30 decks)

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