COOH

Question 1

state how the pKa values affect acidity

Answer 1

1. smaller pKa value, larger Ka value
2. larger extent of dissociation of H+ ions
3. stronger acid

Question 2

Effect of X (eg. F, Cl) on acidity acids (eg, CF3COOH, CCl3COOH)

Answer 2

(*X makes acid more acidic)

1. F is more electronegative than Cl
2. F has a stronger e- withdrawing effect & disperses -ve charge on O atom of conjugate base to a larger extent
3. conjugate base is more stable
4. more H+ ions are dissociated
5. stronger acid

Question 3

state how acidities of methanol, phenol, ethanoic acid compare with e/o

Answer 3

acidity: ethanoic acid > phenol > methanol
stability of conjugate base: CH3COO- > C6H5O- > CH3O-

CH3O- :
1. electron-donating -CH3 grp intensifies -ve charge on O of anion
2. destabilise anion
3. least H+ dissociated

C6H5O- :
1. -ve charge on O delocalised into benzene ring
2. stabilise anion
3. more H+ ions dissociated

CH3COO- anion:
1. -ve charge on O2 delocalised over O-C-O bond in 2 equivalent resonance structures
2. more stable than C6H5O-
3. most H+ dissociated

Question 4

Effect of distance of X on acidity (CH2FCH2COOH + CH3CHFCOOH)

Answer 4

acidity: CH3CHFCOOH > CH2FCH2COOH

1. Both have same no. of e- withdrawing F grp
2. (CH3CHFCOO-): F is nearer to COO-
3. e- withdrawing effect is stronger
4. extent of dispersion of -ve charge is stronger
5. anion is more stable
6. more H+ ions are dissociated

Question 5

Why is CH3COCl more acidic than CH2ClCOOH? Explain effect of adding silver nitrate.

Answer 5

1. CH3COCl is hydrolysed by H2O to form CH3COOH + HCl
2. strong acid HCl which fully dissociates will account for low pH
3. Cl- from HCl form white ppt with Ag+
4. CH2ClCOOH will not form white ppt
5. Cl is covalent bonded to C
6. no free Cl- in solution

Question 6

explain why monoanion of trans- (acid) is unable to form intramolecular H bonds unlike cis-? suggest how does it affect affect acid strength.

Answer 6

1. restricted rotation about C=C bond
2. LP of e- on O- & H of COOH are on opposite sides of mlc & are too far away to interact
3. intramolecular H bond stabilises conjugate base
4. more H+ dissociated
5. cis (acid) is stronger than

Question 7

explain ease of hydrolysis/reactivities of halogenocompounds with warm NAOH(aq)

Answer 7

ease of hydrolysis (nucleophilic sub):
- acyl chloride > alkyl chloride > aryl chloride
- alkyl chloride: R-I > R-Br > R-Cl

acyl chloride (eg. CH3CH2COCl)
1. carbonyl C is bonded to 2 electronegative atoms O + Cl
2. C is highly e- deficient
3. more readily attacked by Nu-
4. Cl atom connected to C=O is more easily substituted

alkyl (eg. (CH3)3CBr/I)
1. C is bonded to only 1 electronegative atom
2. C is less e- deficient
3. attract Nu- less strongly

4. Br has smaller atomic size than I
5. orbital overlap btw C & Br is more effective
6. C-Br bond strength > C-I

aryl chloride:
1. delocalisation of LP e- on Cl atoms into benzene ring
2. C-Cl has partial double bond character
3. bond is harder to break