Chap 14: Hydroxy Compounds (-OH)

Question 1

What are the conditions for reduction of alcohols?

Answer 1

[R]
Na (s), room temp

obv: effervescence
H2(g) evolved gives a pop sound with a lighted splint

Question 2

What are the conditions for the formation of esters?

Answer 2

condensation (nucleophilic acyl substitution)

alcohol + carboxylic acid ⇌ ester + H2O
carboxylic acid, conc. H2SO4, heat under reflux

or

alcohol + acyl chloride –> ester + HCl
anhydrous acyl chloride at room temperature

Question 3

What are the conditions for oxidation of alcohol?

Answer 3

(a) K2Cr2O7, H2SO4 (aq), heat under reflux
(b) KMnO4, H2SO4 (aq), heat under reflux

1° alc -> carboxylic acid
2° alc -> ketone

controlled [O]
K2Cr2O7, H2SO4 (aq), heat with immediate distillation

obv: orange K2Cr2O7 turns green or purple KMnO4 decolourises

Question 4

Conditions for dehydration of alcohols

Answer 4

Elimination
(a) excess conc. H2SO4 at 170°C
(b) heat with Al2O3 to 350°C

alcohol —> alkene
ethanol —> alkene

note: more substituted product will be formed

Question 5

What are the conditions for halogenation of alcohols?

Answer 5

N.S.

Chlorination
(a) SOCl2 at room temp.
(b) PCl3 or PCl5 at room temp.
(c) HCl (g)

obv: for SOCl2 and PCl5 (steamy white fumes or HCl observed)

Bromination
(a) PBr3 at room temp
(b) HBr (g)

Iodination
(a) PI3
(b) HI (g)

Question 6

What are the conditions for tri-iodinemethane test?

Answer 6

[O]
NaOH (aq), I2 (aq), warm

obv: yellow pot of CHI3 formed
- presence of methyl carbonyl (4655 for balancing eqn)

Question 7

Compare the acidities of ethanol, water and phenol

Answer 7

Ethanol, water and phenol are Brønsted-lowry-Lowry acids as they are proton donors.

The ethoxide ion (CH3CH2O-) is the least stable. the -ve charge on O atom is intensified by the e- donating alkyl group, hence destabilising the conjugate base. thus the POE for dissociation of ethanol lies more to the left, and ethanol donates a proton less readily than water

The phenoxide ion is the most stable as the p orbital of O overlaps with the pi e- cloud of benzene, resulting in the lone pair of e- on O delocalising into the benzene ring. the -ve charge on O atom is dispersed over the benzene ring, hence stabilising the conjugate base. therefore, phenol is the most acidic and POE for dissociation of phenol lies more to the right and phenol donates a proton more readily than water.

Question 8

What are the reactions of the -OH group in phenol?

Answer 8

i) Reaction w base: acid-base rxn
NaOH (aq) at room temp.
phenoxide ion C6H5O- ion is generated

ii) Reaction w sodium: [R]
Na (s) at room temp.
obv: effervescence: H2(g) evolved gives a pop sound with lighted splint

iii) Formation of esters: condensation (nucleophilic acyl sub)
Anhydrous acyl chloride, room temp.
obv: sweet-smelling ester produced as an immiscible layer from water, steamy fumes of HCl produced

Question 9

What are the reactions of the benzene ring in phenol?

Answer 9

(i) nitration (E.S.)
monosub pdt: dilute HNO3 (aq), room temp.
multisub pdt: conc. HNO3, room temp.

(ii) bromination (E.S.)
monosub pdt: Br2 in CCl4 at room temp.
obv: orange-red Br2 decolourises and steamy white fumes of HBr (g) formed

multisub pdt: Br2 (aq), room temp.
obv: orange Br2 decolourises and white ppt of 2,4,6-tribromophenol formed. steamy fumes of HBr (g) may also be observed

(iii) Reaction w FeCl3
neutral FeCl3 (aq) at room temp.
obv: purple colouration