Acidity, Basicity, pKa: Leaving Group Ability

Question 1

effect of hybridisation on pKa

Answer 1

carbon centre: sp3 —> sp2 —> sp
H attached to these carbon centres pKa decreases –>

s orbitals closer to nucleus than p orbitals
so s orbital electrons are lower in energy and thus more stable
if orbital has more s character electrons are more tightly held

sp = 50% s character (lower in energy than);
sp2 = 33% s character
sp3 = 25% s character (highest energy e-)

Question 2

acid chloride –> ester

Answer 2

Cl- best LG as conjugate acid is strongest

Question 3

ketone + grignard

Answer 3

tetrahedral intermediate has no sensible LGs attached (C-C bonds strong, resulting anions are unstable as conjugate acids aren’t strong), so water additions leads to production of an ALCOHOL

Question 4

mathematically define pH

pH scale not fit for quantifying LG ability because:

Answer 4

pH = -log[H3O+]

pH is a only a measure of ACIDITY OF THAT SOLUTION, not strength of acid compared to others
pH depends on acid [ ] and STRUCTURE (how dissociated acid is so how many H3O+ ions it produces, i.e. how many hydronium ions it contains)

Question 5

Brønsted definition of acids and bases

Answer 5

Acid = species with tendency to lose a proton
base = species with tendency to gain a proton

Question 6

quantifying LGA: pKa scale

what makes the best LG?

Answer 6

must look @ eqbm const. to measure acid strength relative to water (efficacy of proton donation, not [acid])

lower pKa = stronger acid = larger Ka

partial/less dissociation happens when conjugate base is stronger

stronger acids (HA) have weaker conjugate bases (A-)
stronger bases (A-) have weaker conjugate acids (HA)
LOWER pKaH (protonated form of LG) = BETTER LEAVING GROUP

Question 7

anion stability 3 factors

Answer 7

anions can be stabilised through several pathways:
- electronegative elements STABILISE NEGATIVE CHARGE/anion so increased LGA

• negative charge delocalisation
  (resonant forms drive charge over atoms) –> in resonance curly arrows, NO ATOMS MOVE, NO σ BONDS BROKEN/FORMED, just electron movement in π bonds
• A-H bond strength
  (down vs. across periodic table)
• hybridisation
  sp3–>sp2–>sp pKa decreases

Question 8

HCl pKa

Answer 8

-7

Question 9

tosic acid pKa & structure

Answer 9

-1.3

Me-benzene-S(=O)₂-O⁻⁻

Question 10

carboxylic acid pKa

Answer 10

4.8

Question 11

phenol pKa

Answer 11

10

Question 12

1,3-diester pKa and structure

Answer 12

12

RO–(C=O)–CH₂–(C=O)–OR

Question 13

water pKa

Answer 13

15.7

Question 14

ketone pKa and conjugate base structure

Answer 14

20

==(–O⁻⁻)–

Question 15

alkyne pKa

Answer 15

24

Question 16

ammonia/amine pKa

Answer 16

33-36

Question 17

alkane pKa

Answer 17

48-60