Acids and Bases

Question 1

How are pKa and pKb related?

Answer 1

pKa+pKb=14 (at 298K)

Question 2

How do conformations relate with acidity?

Answer 2

Sometimes the anion cannot be conjugated as the conformation prevents overlap with the p orbitals meaning some cyclic/spiro ketones are less acidic than originally thought

Question 3

How do the free energy changes related to acid strength?

Answer 3

Positive ΔG means a very weak acid
ΔG around 0, mildly acidic, favouring dissociation and reassociation equally
-ΔG, strongly acidic, favouring disassociation

Question 4

How does acidity change for cis and trans dicarboxylic acids and why?

Answer 4

For the first disassociation, the cis isomer is more acidic as hydrogen bonds can form between the carboxyl and carboxylate, helping to stabilise the conjugate base
For the second disassociation, the trans isomer is more acidic. The cis isomer experiences greater repulsion between the negatively charged carboxylates, destabilising the conjugate base

Question 5

How does basicity change across a period and why?

Answer 5

Basicity decreases across the period
This is because the electronegativity increases, resulting in the lone pairs of an atom laying more closely to the positively charged nucleus
As a result, the lone pairs are less exposed and less available for protonation

Question 6

What are aromatic compounds and how do they relate to acidity?

Answer 6

Compounds with enhanced stability as a result of them being flat, cyclic, fully conjugated molecules with 4n+2 pi electrons (Huckel’s rule)
If an anion is an aromatic compound, the conjugate base is stabilised increasing acidity

Question 7

What Gibbs equation is related to pKa?

Answer 7

ΔG= nrt x pKa

Question 8

What happens to acidity across a period?

Answer 8

The electronegativity increases across a period resulting in the dipole of the hydrogen becoming more positive
This also results in a more stable anion, so acidity increases

Question 9

What happens to acidity of the halogens down the group and why?

Answer 9

Electronegativity decreases
Down the group, Zeff increases, meaning their is greater orbital contraction of the inner shell, however outer shells become more diffuse, meaning the charge distribution over the conjugate base is greater increasing stability
This larger size also reduces lone pair-lone pair repulsion in the conjugate base increasing acidity
Additionally, the difference in size between the 1s orbital and increase size of the halogen p orbital means less effective orbital overlap down the group, resulting in weaker longer sigma bonds and so greater acidity
And larger size of I- means greater solvation as smaller reduction in entropy as negative charge more greatly distributed

Question 10

What is a bronsted-acid and base?

Answer 10

Acid=Proton donor
Base=Proton acceptor

Question 11

What is the general order of acidity for the main functional groups?

Answer 11

Carboxylic acids>Phenol> Malonates/Thiols>Alcohols>Ketones>Nitriles>Esters/Alkynes>Amides>Amines>Alkenes>Alkanes

Generally, hydrocarbons will be the least acidic, with alkynes the most, and resonance stabilisation within the hydrocarbon helping stabilise slightly
Heteroatoms with to help with resonance is next acidic
Then heteroatoms directly bonded to the proton without resonance
And the most acidic are heteroatoms bonded to the proton with an anion that can be resonance stabilised

Question 12

What is the rough pKa for carboxylic acids, phenols, alcohols, alkynes, ketones, alkenes, alkanes, and benzyl compounds?

Answer 12

Carboxylic= 4
Phenols=10
Alcohols=17
Ketones=20-24
Alkynes=25=Ester
Alkenes=40
Alkanes=50
Benzene around alkenes, and benzyl only slightly lower

Question 13

Why are alkynes more acidic than alkanes?

Answer 13

Alkynes are sp hybridised whilst alkanes are sp3
The sp orbital has greater s character, resulting in the sp orbital being lower in energy and closer to the positively charged nucleus
Therefore the lone pair of the conjugate pair is more greatly stabilised, resulting in greater acidity

Question 14

Why are amines better bases than amides?

Answer 14

Amides experience resonance and delocalisation, within the conjugate acid
This reduces electron density and increasing stability, lost upon accepting a proton, reducing basicity

Question 15

Why is 4-nitrophenol more acidic than 3-nitrophenol?

Answer 15

4-nitro has resonance structures which involves delocalisation to include the nitro group, increasing stability by increasing the delocalisation
3-nitro does not delocalise through the nitro group

Question 16

Why is ammonia a stronger base than a nitrile?

Answer 16

Ammonia is sp3 hybridised whilst nitriles are sp
The sp3 orbital has less s character, resulting in it being higher in energy are further away from the nucleus, resulting in the least stable lone pair
This results in the conjugate acid being more stable
Additionally, the positive charge of the conjugate acid is further away from the positive nucleus reducing repulsion

Question 17

Why is an etheraniline more basic than nitroaniline?

Answer 17

The ether is electron donating and nitro electron withdrawing. The stronger base has a higher electron density to more greatly accept a proton.

Question 18

Why is Cl3COOH more acidic than CH3COOH? Compare EWG and EDG?

Answer 18

Cl is an electron withdrawing group
The negative charge of the conjugate base is better stabilised via inductive effects, through polarisation through sigma bonds, by helping to partially share the negative charge
Electron Donating groups donate electrons towards the already negative molecule so destabilise the conjugate base
EWG generally a carbon bonded to an electronegative element e.g aldehyde, nitrile, halogen, nitro
EDG have lone pairs directly bonded e.g ethers, alcohols, amines

Question 19

Why is phenol less acidic than a carboxylic acid?

Answer 19

Although both are resonance stabilised and phenol does have more resonance structures, only one has no charge on the carbons, and the aromaticity is lost for 3 structures
The carboxylic acid has fewer structures but of higher quality, with the negative charges on the oxygens and no charge on the carbons, therefore more greatly stabilising the conjugate base increasing acidity

Question 20

Why is the basicity of phenol less than an alcohol?

Answer 20

The lone pair of electrons of the oxygen in phenol is delocalised within the aromatic ring, helping stabilise the acid and lowering energy
This means a weaker base, whilst delocalisation does not occur within the alcohol

Question 21

Why is the order of acidity for ketones, amides, and esters? Why?

Answer 21

Ketones>Esters>Amides
Esters and Amides experience resonance as the conjugate acid, stabilising the molecule and therefore reducing acidity
Esters are more acidic than amides as the inductive effects of the ester group are greater electron withdrawing than amide, helping to stabilise the negative charge of the conjugate base more effectively. Additionally, the delocalisation of the lone pair of nitrogen in the conjugate acid is more effective than oxygen, meaning the amide is more stable than the ester

Question 22

Why is toluene more acidic than benzene?

Answer 22

Toluene’s most acidic protons are part of the alkyl group, with the negative charge of the remaining anion delocalised in and out of the ring, resonance stabilised
Benzene’s protons must only be removed from the ring itself, but the lone pair cannot be delocalised as no further p orbital overlap can occur due to the differing geometry of the already delocalised system
This localised lone pair is high in energy so lower in acidity

Question 23

Why is diphenylmethane more acidic than phenylmethane and triphenylmethane?

Answer 23

2 phenyl groups are the optimum number to enable delocalisation of the lone pair of the conjugate base into both pi systems
3 phenyl groups in the same plane would cause too much steric hinderance so delocalisation within the conjugate base can only be possible with two of the phenyls. This means the pKa is similar in both
Having 2 phenyl rings would enable greater delocalisation of electrons within the conjugate base than only one so greater stability of the base and greater acidity

Question 24

Why is the proton next to a nitro group particularly acidic?

Answer 24

The positive charge of the nitrogen means their is greater electron polarity/ electron withdrawing ability of the nitro group to stabilise the conjugate base
There is also resonance stabilisation the negative charges on the electronegative oxygen resulting in greater stability of the conjugate base

Question 25

Why are intramolecular hydrogen bonds more stabilising than intermolecular?

Answer 25

As the molecules are already in place, there is no change in alignment required for intramolecular hydrogen bonding, meaning no decrease in entropy For intermolecular, hydrogen bonding resultings in a more ordered structure, reducing entropy and so decreasing the stabilising effect

Question 26

How can solvation affect acidity?

Answer 26

Polar solvents, especially protic solvents which can H-bond, can stabilise the conjugate base via solvation, increasing acidity However, solvation reduces entropy so is not always favourable. If there are EWG or a way to spread the negative charge, there are multiple points of solvation, increasing efficiency of solvation and so stability of the conjugate base, so also acidity Same with size, so I- larger than F-, I- easier to solvate

Question 27

Why do the methylamines increase in basicity with more methyls in gas phase but N(Me)3 is not the most basic in water?

Answer 27

Gas Phase: The alkyl groups stabilise the conjugate acid via inductive effects, increasing the basicity of the conjugate base. Water: More alkyl groups increase steric hinderance, hindering stabilisation of the conjugate acid via solvation and thus reducing basicity, meaning as a compromise N(Me)2H is the most basic

Question 28

What is pKah?

Answer 28

Using the conjugate acid of a base to measure basicity e.g NH4+---/ NH3 + H+ Lower PkaH stronger acid, so weaker base

Question 29

Why is Cl3N less basic than NH3?

Answer 29

EWGs like Cl reduce the availability of the lone pair of the conjugate base via inductive effects, reducing protonation Additionally, the EWG will destabilise the conjugate acid via electron polarisation

Question 30

Why is a 2-methylaniline with N(Me)2 more basic than 4-methylaniline?

Answer 30

The methyl groups and methylamine are in close proximity with the ortho position, and if they lay in the same plane, they would cause a high steric hinderance Instead, the amine is not planar, and therefore the lone pair of the amine is not delocalised into the pi system as it is not conjugated, reduced mesomeric effects This increases the availability of the lone pair, increasing basicity For 4, the methyl is far enough away to enable the amine to remain planar, and so conjugated, with the mesomeric effects reducing basicity

Question 31

How do amides act as bases?

Answer 31

As there is delocalisation of the electrons of the amide, the proton is accepted at the oxygen to maintain resonance in the conjugate acid Amides remain planar

Question 32

Why are bridge-head amides more basic than expected?

Answer 32

The bridge-head prevents the nitrogen from adopting an sp2 structure and is instead sp3. As a result, there cannot be overlap of the p orbitals with the carbonyl, meaning no delocalisation Therefore, protonation of the nitrogen can occur, with the electron donating inductive effects of the alkyl increasing the stability of the positively charged conjugate base

Question 33

Why are imines more basic than amides?

Answer 33

Protonation at a nitrogen instead of an oxygen, as both molecules still have resonance stabilisations within the conjugate acid Nitrogen more readily accepts protons as the lone pair is more available due to a lower electronegativity

Question 34

Why are nitroalkanes more acidic than ketones?

Answer 34

Although both exhibit resonance stabilisation of the conjugate base, the nitro group is more greatly electrn-withdrawing due the electronegative nitrogen and oxygen of the nitro, stabilising more strongly via inductive and mesomeric effects

Question 35

Why are esters more acidic than amides?

Answer 35

The inductive effects of the ester is greater than amides, as oxygen is more electronegative Greater stabilisation of the anion