CM2002 Acidity and Basicity
Reactions: Bond formation
Most reactions involve electron rich molecules forming bonds to electron deficient molecules, ie. Nucleophiles forming bonds to electrophiles.
Classification of reactions:
1. Acid/Base reactions
2. Functional Group transformations
3. Carbon-Carbon bond formation
Acid/Base reactions lead to formation of a salt.
Mechanisms: Describe how a reaction takes place by showing what is happening to valence electrons during formation and breaking of bonds.
Reaction of a hydroxide ion and a proton to form water. OH− is a nucleophile and H+ is an electrophile. The reaction takes place between nucleophilic centre (oxygen) and electrophilic centre (hydrogen) with the formation of water.
Mechanism:
Lone pair of electrons on the oxygen atom used to form bond. Oxygen ‘loses’ 1 electron and hydrogen ‘gains’ 1 electron, therefore oxygen loses negative charge and hydrogen loses positive charge.
Curly arrows:
1. Shows movement of electron pair.
2. Start from lone pair of electrons on an atom or middle of bond to be broken.
3. Point to an atom if electrons will end up as lone pair on atom.
4. Otherwise point to where the new bond will form.
Bronsted-Lowery Acids and Bases:
Bronsted- Lowry definition of an acid is a molecule which can provide a proton.
Bronsted-Lowry definition of a base is a molecule which can accept that proton.
Mechanism:
Bronsted-Lowry Acids: - provides a proton.
Molecule which contains acidic hydrogen. To be an acidic hydrogen it must be slightly positive or electrophilic – if hydrogen is attached to electronegative atom like a halogen, oxygen or nitrogen.
Later we will look at hydrogens attached to carbon which are acidic.
Bronsted-Lowry Bases: - accepts a proton.
Molecule which can form a bond to a proton, e.g. negatively charged ions with lone pairs of electrons.
Acid Strength
Acids – can be strong or weak. pKa is a measure of the strength of an acid.
Keq: Equilibrium constant (extent of dissociation of the acid).
Stronger acid – greater concentration of products, i.e. Ka is large.
pKa = − log10Ka. The pKa for CH3CO2H is 4.76.
The stronger the acid the lower the pKa.
Scale -10 to +50 (Strong to weak acids). HClO4 (pKa -10) to CH4 (pKa +50). There is a 1060 difference in reactivity.
Deprotonation may be favourable thermodynamically (pKa) but kinetically slow – may be the rate-limiting step. Heteroatom deprotonation is usually rapid.
Factors influencing acidity of H-A:
1. Strength of H-A bond – relatively unimportant
2. Electronegativity of A
3. Factors which favour formation of A− relative to H-A:
(a) Resonance effects
(b) Inductive effects
(c) Steric effects
4. Solvent effects
Electronegativity of A:
O and S are more electronegative than C. Thiol is 106 more acidic than alcohol due to longer S-H bond. Also due to easier salvation.
Resonance Effects:
1. The carbonyl group is inductively electron-withdrawing (E.W.G), and therefore weakens the O-H bond.
This effect is relatively minor.
2. The anion that is formed after deprotonation is stabilised by delocalisation of electrons.
Delocalisation without charge separation: two canonical forms identical in energy.
Delocalisation with charge separation- much less effective in stablising structure.
Delocalisation of negative charge stabilises the structure. It also eases solvation.
In the acetate the responsibility for the negative charge is shared equally between the two O atoms and this leads to a resonance hybrid as shown above.
Stabilisation is less efficient than in the acetate as the valence bond structures are not equivalent. When the electron density is localized on Carbon the energy is higher than when it is on the Oxygen.
Therefore the A is the major contributor to the resonance hybrid and structures B-D are minor.
Carbon Acids:
Carbonyl group- increases the acidity of adjacent protons:
Nitro group has a strong effect on the adjacent protons.
Inductive Effects:
These are usually smaller than resonance effects and are a result of electronegativity differences.
Hybridisation as Inductive effect:
As the s character in orbitals increase, the orbitals become more spherically symmetrical. The carbon atom has a stronger pull on the electron density. We also get additional solvation effects.
The CH3 group is inductively electron donating, this is a small effect.
The main difference is easier salvation of HCO2- relative to CH3CO2-.
Carboxylic acid- electron withdrawing group (E.W.G). This increases the acidity of the other group.
Deprotonation of the 2nd group is more difficult as removal of H+ from an anion is difficult.
Phenols
Additional resonance effect with 2 (ortho) or 4 (para) substituted compounds. Otherwise we have the same effects as in carboxylic acids.
Carbon Acids:
- Important reactive intermediates in organic chemistry, therefore its important to know under which conditions they will form.
Carbanions are sp3 hydridised and have a tetrahedral geometry:
Electronegativity:
Inductive Effects:
Resonance Effects:
As seen above, e.g Ketone.
Stabilisation through aromatic structures:
Sulfur substituents:
Very efficient at stabilizing carbanions