Properties of Aromatic Compounds (Acid/Base Chemistry)

Question 1

What is pKₐ?

Answer 1

pKₐ describes how much of the acid has been deprotonated (and now exists in its conjugate base form)
It is a quantitative measure of acid strength in solution

Question 2

What is a Bronsted acid

Answer 2

is a species with a tendency to lose a proton

Question 3

What is a Bronsted base

Answer 3

is a species with a tendency to accept a proton

Question 4

The lower the pKₐ….

Answer 4

the strong the acid

Question 5

How do you work out the pKₐ from Ka

Answer 5

Question 6

How can you work out Ka

Answer 6

Using the concentration of [H⁺]² in the solution over the concentration of the acid

Question 7

What are the 3 main factors which affect the strength of an acid

Answer 7

1) The stability of the conjugate base A¯
2) The H-A bond strength
3) The solvent

Question 8

What two factors effect th stability of the conjugate base A¯

Answer 8

1) how electronegative A is
2) inductively and mesomerically (resonance) electron-withdrawing substituents stabilise A¯ (making HA a stronger acid)

Question 9

What factor effect the H-A bond strength?

Answer 9

H-A bond strength decreases down the periodic table
(size/energy mismatch of atomic orbitals) making it easier for the bond to break

Question 10

How does the solvent effect the strength of an acid?

Answer 10

The better the solvent is at solvating/stabilising ions, the easier it is for the acid to be deprotonated

Question 11

How is the strength of a base traditionally measured

Answer 11

pKₐₕ: the pKₐ of its conjugate acid

Question 12

a lower pKₐₕ means

Answer 12

a stronger acid, hence a weaker base

Question 13

How to you work out Kₐₕ

Answer 13

by dividing the concentration of the [base] over the concentration of [H⁺]²

Question 14

Any factor that increases the strength of an acid usually decreases the strength of the corresponding conjugate base
Hence what are the main factors which affect the strength of a base?

Answer 14

Is the availability of its lone pair of electron
The stability of the conjugate acid (the more stable the conjuate acid, the stronger the base)

Question 15

What two factors effect the availablity of a bases lone pair of electrons

Answer 15

• How electronegative it is
• Inductively and mesomerically electron-withdrawing substituents make B a weaker base
  Therefore the lone pair is less available for reaction with H⁺

Question 16

Which is the weaker base and why?

Answer 16

Cyclohexylamine: pKₐₕ = 10.7
aniline: pKₐₕ = 4.6 (weaker base)
Lower basicity of aniline is due to the partial delocalisation (sp³) of the Nitrogen LP into armatic ring

Question 17

what are the pKₐₕ values of the para-substitueted anilines

Answer 17

the NO₂ is electron withdrawing, therefore LP is less available = lower pKₐₕ
Chlorine is highly electronegative, therefore LP is less available = lower pKₐₕ
Methoxy pushes electrons into the ring making it more electron rich, increasing availability of LP = higher pKₐₕ

Question 18

which out of cyclohexanol and phenol is more acidic

Answer 18

Cyclohexanol: pKₐ = 16
Phenol: pKₐ = 10
Higher acidicity of phenol is due to resonance stabilisation of the conjugate base

Question 19

What is the acidity of methane like, relative to toluene and diphenylmethane

Answer 19

methane: pKₐ = 50
toluene: pKₐ = 40
diphenylmethane: pKₐ = 33
the methyl anion is stabilised by delocalisation resulting in methyl groups attached to aromatic rings more acidic than simple aliphatic methyl groups

Question 20

What is the acidity of ethanoic acid like relative to benoic acid

Answer 20

ethanoic acid: pKₐ = 4.76
benzoic acid: pKₐ = 4.2 (only a little more acidic)
This is because it is not possible to stabilise the benozoic acid anion by conjugation into the ring

Question 21

How is it possible to affect the acidicity of benzoic acid

Answer 21

by adding subsituents
adding electron-withdrawing substituents will increase the acidicity (positive σ value)
adding electron-donating substituents will decrease the acidicity (negative σ value)

Question 22

What is the substituent constant (σ) realtive to benzoic acid

Answer 22

positive σ-values are more acidic (lower pKₐ than benzoic acid)
negative σ-values are less acidic (higher pKₐ than benzoic acid)

Question 23

Using the Hammett reationship, how would you work out the substituent constant-σ (e.g. for benzoic acid)

Answer 23

Question 24

What does the reaction constant ρ tell us

Answer 24

what effect electron-withdrawing or electron-donating substitents have on reaction rate
hence giving us an idea about the mechanism of the reaction

Question 25

you can work out the reaction constant (ρ) from the gradient of substituent constant-σ over rate constant
a positive ρ value would mean?

Answer 25

a positve ρ value means that electron-withdrawing substiuents increase the reaction rate
i.e. there is a build-up of negative charge in the transition state of the RDS which is stabilised by the electron-withdrawing group

Question 26

you can work out the reaction constant (ρ) from the gradient of substituent constant-σ over rate constant
a negative ρ value would mean?

Answer 26

a negative ρ value means that electron-donating substituents increase the reaction rate
i.e. there is a build-up of positive charge in the transition state of the rate determining step
which is stabilised by the electron-donating group

Question 27

What do the Hammett parameters describe?

Answer 27

how the structure of aromatic compounds affect their reactivity

Question 28

What is a flaw with the Hammett approach

Answer 28

The effect of the substituents is purely electronic
The Hammett appraoch does not work for orth-substitents due to steric effects