Leaving group strength and pKa

Question 1

Which study provided proof of the tetrahedral intermediate. Give the mechanism for the hydrolysis reaction that demonstrated this.

Answer 1

Bender 1951. Used active O16 species to show the transformation from the O18 ester to the O16 ester

Question 2

What does pKa refer to

Answer 2

The ease of proton loss of the species. Not the ease of the formation of the species through proton loss

Question 3

Equation for the pKa(H) of ammonia

Answer 3

NH4+ –> NH3 + H+

Question 4

6 factors that contribute towards conjugate base strength (hence acid strength)

Answer 4

1. Where negative charge resides
2. Delocalisation of charge
3. Size of the conjugate base
4. Inductive effects
5. Solvation
6. Hybridization

Question 5

Where negative charge resides explanation

Answer 5

The negative charge is more stable on more electronegative species. This makes F- a better conjugate base than OH- for example.

Question 6

Delocalisation explanation

Answer 6

Charge is inherently stabilised through delocalisation. The overlap of orbitals lowers the energy of the ‘orbital’ in which the charge resides. The more delocalisation, the better the conjugate base

Question 7

Size explanation

Answer 7

Single atom conjugate bases are stronger when larger. The charge is more diffuse and the bond to the proton is weaker

Question 8

Inductive effects explanation

Answer 8

Electronegative species delocalised charge to an extent, and therefore stabilise the conjugate base

Question 9

Solvation explanation

Answer 9

The more a conjugate base interacts with a solvent, the more stable it is. Bulky substituents reduce the number of orientations that the solvent can interact with the charge. Organic solvents are also worse at solvating anions

Question 10

Hybridisation explanation

Answer 10

the more s character an atom has, the closer the -ve charge is to the nucelus, and hence the stronger the conjugate base

Question 11

What does high pKa of the acid mean regarding its conjugate base

Answer 11

Conjugate base is a better leaving group and a worse nucelophile

Question 12

Starting at 0-30 in increments of 5, what is the general rule for the pKa of organic species

Answer 12

halides, carboxylic acids, phenols = amines, alcohols, enols, amides (35 not 30)

Question 13

How can you use known pKa to find unknown pKas

Answer 13

Rearrange the Keq to find it in terms of the Ka of each acid involved. Convert to pKa, should be:
pKeq = pKa(start acid) - pKa(final acid)

Question 14

What determines the electrophilicity of carbonyl derivatives

Answer 14

The degree of electron delocalization between the X group (OH, Cl, NH3 etc) and the pi system of the carbonyl group. More electronegative species will incur more delocalization making weaker C=O bonds. The pi* orbital is increased in energy by delocalization, which makes the species less electrophilic as this orbital is less accessible