Extra acids & bases

Question 1

Is basicity/acidity the same as nucleophilicity/electrophilicity?

Answer 1

No. Consider both with their distinct criteria.

Question 2

What is the behavior of water as an acid/base?

Answer 2

Water is a very weak acid and base. It dissociates to a very small extent. Ka = Kb = Kw = [H+][OH-] = 1.0 * 10^-14 at 25C.

Question 3

Name the common examples of strongly basic anions. Why are they strongly basic?

Answer 3

Common strongly basic anions are OH-, OCH₃-, O²-.

OCH₃- is a is the conjugate base of methanol (CH3OH).
-OH is the conjugate base of water (H2O).
Both CH3OH and H2O are very weak acids. Thus, as the conjugate base of a very weak acid is a strong base, both of these are strongly basic.

The O²- anion is extremely reactive and readily accepts a proton to form the more stable OH- anion. Thus it is strongly basic by the bronsted definition.

Note: Uncommon but H⁻ accept proton readily to form much more stable H₂ gas.
Note: A primary alkyne is acidic but less acidic than alcohols or water, thus its conjugate base is a strong base.

Question 4

Why are group 1 and 2 salts of strongly basic anions strong bases?

Answer 4

Group 1 and 2 salts of strongly basic anions are strong bases themselves. This is as the bonding of the salt would be ionic and dissociate easily and fully in water to form strongly basic anions (eg. OH-). This creates a strongly basic solution.

Note: This is indeed true for most metals, with exceptions for complexes and highly polarised salts.

Question 5

Name common strong base salts.

Answer 5

Common strong base salts include oxides and hydroxides of group 1 and group 2 metals.

Note: Group 1 hydrides (eg, NaH) also form H⁻

Question 6

How does the basicity of an atom change if the atom is lower down the group, or further right across a period? Explain your answer, and provide an example.

Answer 6

Anions lower down the group have a larger atomic radius due to the extra electron shell. Thus, charge density is lower, and the anion is more stable and less willing to accept a proton.

Examples: OH- > OS- (not strong), CH3O- > CH3S- (not strong).

Anions further to the right of a period are more electronegative, and thus hold their electrons more strongly, are less willing to donate them and thus are less basic. This effect of electronegativity is weaker than the group trend.

Note: Basicity depends on charge density/stability of the anion. Nucleophiles depend on how tightly the electron pair is held.

Question 7

What is the criteria for an acidic H?

Answer 7

H atoms bonded in a polar bond with F, O, N, halogens or S are common acidic H.

H atoms bonded to C are also very slightly acidic, but less compared to H bonded to O. Alkane H < alkene H < alkyne H.

Molecules where the conjugate base is stabilised (eg. Phenol, CF3CH2OH) are stronger acids. This factor is what causes the electronegativity trend.

Note for 1: H has little electron density, and the polar bond reduces this electron density even further. This extremely low electron density weakens the electrostatic bond from the molecule to the proton. Thus, it is easy for the proton (H+) to dissociate, and the H is acidic.

Note for 3: Higher s character of orbital with lone pair → more electronegative → better able to stabilise negative charge → more stable. This outweighs the bond being stronger.

Question 8

What is the most common strongly acidic cation?

Answer 8

The most common strongly acidic cation is the H+ hydrogen ion (aka proton).

Question 9

Name the common strong acids.

Answer 9

Common strong acids:
- Oxyacids
- hydrohalides (excluding HF as the small atomic radius of F causes a very short, strong bond)

These are strong acids as they dissociate fully in water to form acidic H+ cations.

Note: oxyacids are strong acids because the conjugate base is resonance stabilised

Question 10

What are criteria for a basic atom?

Answer 10

Atoms which are electron rich with lone pairs and not being withdrawn from (eg. thru polar bond) are willing to accept H+.

Eg. O in —OH, N in —NH2.

When the conjugate acid is more stabilised, it is more basic.