Suggested Solutions to Discussion Questions Carboxylic

Suggested Solutions to Discussion Questions (Tutorial 17) 1. (a) Ethyl ethanoate <

Ethanol

<

Phenol

<

ethanoic acid <

benzoic acid [1]

Consider a weak acid, HA H+

HA



+

A

More stable A , greater dissociation, stronger acid. No acidic proton electron donating benzene ring present. ethyl group present allows intensifies delocalisation of of  cannot act as negative charge negative charge on an acid! and destabilizes Oalkoxide anion O of  into benzene ring stabilizing the phenoxide ion

negative charge is dispersed over 2 electronegative oxygen atoms in 2 equivalent resonance structures causing the anion to be greatly stabilised

Presence of benzene enables delocalisation of negative charge in the anion into the benzene ring, ring, stabilising the anion

(b) CH3CH2CH2CH2COOH < CH3COOH < HOCH2COOH < CCl3COOH H+

HA



+

A

More stable A , greater dissociation, stronger acid. Compare stability of A , taking CH 3COO  as reference: alkyl groups are elctron donating by inductive effect. It intensifies intensifies the negative  charge & destabilises the anion. CH 3CH2CH2CH2COO   is only slightly less stable than CH3COO  due to longer alkyl chain (effect of alkyl chain length is minimal) OH is electron withdrawing by inductive effect. It helps to disperse the negative  charge & stabilises the anion. HOCH 2COOH is more acidic than CH 3COOH. the presence of 3 Cl atoms (electron withdrawing by inductive effect) disperses the  negative charge extensively. CCl3COO   is greatly stabilised & CCl 3COOH is the strongest acid among the 4. 









2.

 All three compounds compounds react with with Na(s) to give effervescence effervescence of of H 2(g) and the + corresponding Na salts which are soluble in water. E

CH2OH

H3C

+

+

OH

Na (s)

COOH

+

F

-

Na(s)

+

CH2O Na  + 1/2 H2

-

H3C

+

O Na  + 1/2 H2

-

Na (s)

+

COO Na  + 1/2 H2

G Both E and F have no reaction with aqueous Na 2CO3. G reacts with aq. Na 2CO3 to give effervescence of CO 2(g) and Na + salt which is soluble in water.

2

COOH

+ Na2CO3 (aq)

2

-

+

COO Na  + CO2

G 1

E has no reaction with aqueous NaOH. Both F and G react with NaOH to give the corresponding Na + salts which are soluble in water.

+

H3C

OH

-

H3C

NaOH (aq)

+

O Na  + H2O

F -

+ NaOH (aq)

+

COO Na  + H2O

COOH

G  Add aqueous Na2CO3 to the sample. If effervescence of CO 2 is detected, then the sample is G. If not, then the sample is either E or F, which can be differentiated by adding aqueous NaOH. Only F is soluble in aqueous NaOH. 3. C 6H  and C 6H  [acid strength]   5 COOH   5OH   Test: Add Na2CO3 (aq) [or NaHCO3 (aq)] Observations: Effervescence observed. No effervescence observed Colourless gas (CO 2) evolved formed white ppt with aq. Ca(OH) 2 Equation: 2 CH3COOH + Na2CO3  2 CH3COO Na+ + CO2 + H2O 

(b) HOCH 2CH  and ClCH 2 CH 2COOH [acyl chloride vs acid]   2COCl     Test: Add AgNO3(aq) Observations: White fumes No white fumes White ppt (AgCl) No ppt observed Equation: HOCH2CH2COCl + H2O  HOCH2CH2COOH + HCl Cl (aq) + Ag+(aq)  AgCl (s) 

(c)C 6H  and C 6H  and C 6H    5COCl     5COBr     5 CH 2 Cl [acyl chloride vs acyl bromide vs alkyl halide] Test: Add AgNO3(aq) Observations: White fumes White fumes White fumes White ppt (AgCl) Off-white ppt (AgBr) White ppt (AgCl) only when heated Equation: C6H5COX + H2O  C6H5COOH + HX C6H5CH2Cl + H2O  + C6H5CH2OH + HCl X (aq) + Ag (aq)  AgX (s) X= Cl, Br Cl (aq) + Ag +(aq)  AgCl (s) 



(d) CH3COOCH2CH2CH3 and CH3COOCH(CH3)2 Test: 1) Heat with NaOH (aq) 2) Add aq. I2 Observations: No yellow ppt formed yellow ppt formed Equation: CH3COOCH2CH2CH3 +OH  CH3COO  + HOCH2CH2CH3 CH3COOCH(CH3)2 +OH  CH3COO + HOCH(CH3)2 HOCH(CH3)2 + 4I2 + 6OH  CH3COO  + CHI3 + 5I  + 5H2O 









4.

(a)





dilH SO , aq.KMnO

2 4 CH3CH2OH + [O]     CH3COOH + H2O heat

CH3COOH

SOCl2 room temperature

  

 CH3COCl  + SO2 + HCl

2

 O      =

(b)

(c)

CH3CH2OH + CH3CH2COCl  (from a)

CH3CH2OH + [O]

C

H

CH3C-O-CH2CH3

K2Cr 2O7(aq), dil H2SO4, heat with immediate distillation

CH3CHO + H2O

O CH3

room temperature

  

+

OH

KCN

HCN

CH3

C

CN

H

OH CH3

C CN H

OH

dil HCl + 2H2O + H+

(d) CH3CH2CH2OH + 2 [O]

CH3

C COOH

heat

+ NH4+

H

KMnO4 (aq), dil H2SO4, heat

CH3CH2COOH + H2O

room temperature

CH3CH2COOH + SOCl2

CH3CH2COCl + SO2 + HCl O

+CH3CH2COCl + Na O

5.

C O

+ NaCl

COCl

COOH

+

room temperature SOCl2

COCl

+ 6.

CH3CH2

NH2CH2COOH

room temperature

+

HCl

+

CONHCH 2COOH

+

SO2

HCl

F undergoes neutralization with NaOH(aq)

 F

contains an acidic fn gp  F is either a phenol or a carboxylic acid F undergoes condensation reaction with 2,4-DNPH but does not undergo oxidation with Fehling’s solution  F is a carbonyl compound that is not an aliphatic aldehyde  F is either an aromatic aldehyde or a ketone F undergoes electrophilic substitution with Br 2 water to give G.

3

 F

contains a strongly activated benzene ring  F is a phenol Since G has 2 H atoms substituted with 2Br atoms  G is a 2- or 4-substituted phenol F gives a positive iodoform test to yield H  F is a methyl ketone  H is a carboxylic acid with 1 C less H undergoes neutralization with both NaOH(aq) and Na2CO3(aq).  H is a carboxylic acid (confirmed) H is obtained by alkaline hydrolysis of J followed by acidification.  H is 1,2-disubstituted. OH

O C OH

 H

is OH

OH

O

Br 

C

O C

OH CH3

O C OH

CH3 Br 

F

G

H

[1] per correct structure

Equations: [F + NaOH] OH

O-Na+ O

O C

CH3

C

+ NaOH

CH3

+ H2O

[F + 2,4-DNPH] OH

C

OH

O2N

CH3

CH3 C

O

+

H2NNH

NO2

O2N

NNH

NO2 + H2O

[F + Br 2(aq)] OH

O C

OH CH3

Br 

O C

+ 2Br 2

CH3

+ 2HBr 

Br 

[F + aq. alkaline I2]

4

OH

O-

O C

O C

CH3 + 3I2 + 5OH-

O-

+ CHI3 + 3I- + 4H2O

[acidification after iodoform test] [acidification after J + NaOH] O-

OH

O C

O-

O C

+

+ 2H

OH

[J + NaOH] OH

O-

O C

OCH3

O C

+ 2OH

-

O-

+ H2O

+ CH3OH

[J + Na2CO3] OH 2

OH

O C

OH

+ Na2CO3

2

O C

O-Na+

+ CO2 + H2O

5