Properties of Aromatic Compounds (Acid/Base Chemistry) Flashcards
What is pKₐ?
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
What is a Bronsted acid
is a species with a tendency to lose a proton
What is a Bronsted base
is a species with a tendency to accept a proton
The lower the pKₐ….
the strong the acid
How do you work out the pKₐ from Ka
How can you work out Ka
Using the concentration of [H⁺]² in the solution over the concentration of the acid
What are the 3 main factors which affect the strength of an acid
1) The stability of the conjugate base A¯
2) The H-A bond strength
3) The solvent
What two factors effect th stability of the conjugate base A¯
1) how electronegative A is
2) inductively and mesomerically (resonance) electron-withdrawing substituents stabilise A¯ (making HA a stronger acid)
What factor effect the H-A bond strength?
H-A bond strength decreases down the periodic table
(size/energy mismatch of atomic orbitals) making it easier for the bond to break
How does the solvent effect the strength of an acid?
The better the solvent is at solvating/stabilising ions, the easier it is for the acid to be deprotonated
How is the strength of a base traditionally measured
pKₐₕ: the pKₐ of its conjugate acid
a lower pKₐₕ means
a stronger acid, hence a weaker base
How to you work out Kₐₕ
by dividing the concentration of the [base] over the concentration of [H⁺]²
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?
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)
What two factors effect the availablity of a bases lone pair of electrons
- 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⁺
Which is the weaker base and why?
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
what are the pKₐₕ values of the para-substitueted anilines
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
which out of cyclohexanol and phenol is more acidic
Cyclohexanol: pKₐ = 16
Phenol: pKₐ = 10
Higher acidicity of phenol is due to resonance stabilisation of the conjugate base
What is the acidity of methane like, relative to toluene and diphenylmethane
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
What is the acidity of ethanoic acid like relative to benoic acid
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
How is it possible to affect the acidicity of benzoic acid
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)
What is the substituent constant (σ) realtive to benzoic acid
positive σ-values are more acidic (lower pKₐ than benzoic acid)
negative σ-values are less acidic (higher pKₐ than benzoic acid)
Using the Hammett reationship, how would you work out the substituent constant-σ (e.g. for benzoic acid)
What does the reaction constant ρ tell us
what effect electron-withdrawing or electron-donating substitents have on reaction rate
hence giving us an idea about the mechanism of the reaction
you can work out the reaction constant (ρ) from the gradient of substituent constant-σ over rate constant
a positive ρ value would mean?
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
you can work out the reaction constant (ρ) from the gradient of substituent constant-σ over rate constant
a negative ρ value would mean?
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
What do the Hammett parameters describe?
how the structure of aromatic compounds affect their reactivity
What is a flaw with the Hammett approach
The effect of the substituents is purely electronic
The Hammett appraoch does not work for orth-substitents due to steric effects
