Elimination- Base

Question 1

Which elimination is affected by the base

Answer 1

1. E2

Question 2

What type of bases can be used for E1 reactions

Answer 2

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

Question 3

Give 2 examples of good bases to use in E1 reactions

Answer 3

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

Question 4

What type of bases favour E2 reactions

Answer 4

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

Question 5

What is the difference between nucleophilic substitution and elimination

Answer 5

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

Question 6

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

Answer 6

1. Weak conjugate acid

Question 7

What is basicity

Answer 7

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

Question 8

How can you determine basicity of a species

Answer 8

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)

Question 9

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

Answer 9

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

Question 10

Name 8 neutral nitrogen bases and give their pKaH

Answer 10

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

Question 11

What does a lower pKa value indicate

Answer 11

1. Indicates a stronger acid

2. So strong bases have higher pKaH

Question 12

What is pKaH

Answer 12

1. pKa of the conjugate acid of a base

Question 13

What affect does steric demand of the nucleophile/base have

Answer 13

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

Question 14

Describe why the size of nucleophile affects if substitution or elimination

Answer 14

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

Question 15

Describe properties of DBU and DBN

Answer 15

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

Question 16

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

Answer 16

1. E2 elimination