Acidity And Basicity

Question 1

Describe the three factors that affect the strength of an acid

Answer 1

1. Stability of conjugate base - by spreading the negative charge or having an electronegative atom bonded nearby (electron-withdrawing group), the more electronegative the atom is the more stable the conjugate base is and the less basic it is so the stronger the acid.
2. The bond strength of H-A - the easier it is to break the bond the stronger the acid
3. The solvent - the better the solvent is at stabilising the ions formed the easier it is for the reaction to occur

Question 2

Why does having an electron-withdrawing group nearby help to stabilise the conjugate base?

Answer 2

As the sigma bond is polarised and sigma electrons will be unevenly distributed and more towards the electronegative atom, but this becomes less significant as the EWG is further away from the negative charge

Question 3

How does hybridisation affect acid strength?

Answer 3

Sp hybridised molecules are more acidic than sp2 and sp3 as they have greater s character and so the electrons are held more tightly towards the nucleus, and it is better at stabilising the negative charge of the conjugate base

Question 4

How does the spread of charge in a molecule affect basicity?

Answer 4

The stronger the base the more charge is localised to a certain point on the molecule

Question 5

Name two factors that affect the strength of the a base

Answer 5

How accessible the lone pair of electrons are (affected by energy, usually HOMO of molecule, the higher the energy the more reactive it is and the stronger the base. If the electrons are on an EWG or can be delocalised then the base is weak), and what extent the resultant positive charge formed can be stabilised by the solvent or delocalisation

Question 6

How can basicity and pKa be increased on a nitrogen atom in a molecule?

Answer 6

Increasing its electron density which makes the lone pair higher in energy and more available for protonation by attaching an EWG or conjugation by adding an electron donating group e.g. alkyl chain

Question 7

Describe the effects that can decrease electron density on nitrogen and hence makes amines less basic (all amines have a pKa higher than ammonia)

Answer 7

1. N atom bonded to EWG - inductive effect decreases as EWG is bonded further away
2. Lone pair is sp or sp2 hybridised - lone pair held more tightly
3. Lone pair conjugated with EWG
4. Lone pair involved in maintaining aromaticity of molecule

Question 8

Why can amides not be protonated?

Answer 8

As delocalisation ‘ties up’ lone pair as amide is planar with sp2 N and lone pair in p orbital so can overlap effectively with carbonyl group

Question 9

Where can amides be protonated?

Answer 9

On the oxygen atom as carbon is electron-deficient and it would be energetically unfavourable

Question 10

Why are amidines stronger bases than amines and amides?

Answer 10

As they contain two N atoms that can be protonated and electron density increased by conjugation of one sp2 N with other sp3 N, and protonation occurs on sp2 N. (Similar rules apply for neutral oxygen bases)