Hydroxy Compounds

Question 1

Why do alcohols have higher boiling point than alkanes of comparable Mr?

Answer 1

Larger amount of energy is needed to overcome the stronger hydrogen bonds between the alcohol molecules than the weaker id-id interactions between the alkane molecules.

Question 2

Descibe the hybridisation of the carbon in aliphatic and aromatic alcoholc.

Answer 2

Aliphatic alcohols contain an -OH group bonded to a sp3 hybridised carbon atom.
Aromatic alcohols contain an -OH group bonded to a sp2 hybridised carbon atom of a benzene ring.

Question 3

Why does branching of the carbon skeleton lower boiling point?

Answer 3

With branching, molecules become more spherical. Compaed with straight chain isomers, there is less surface area of contact and hence less extensive instantaneous dipole-induced dipole interactions between molecules.

Question 4

Describe the solubility of alcohols in water.

Answer 4

Alcohols are relatively more soluble in water than slammed due to the presence of the -OH group which enables them to form hydrogen bonds with water molecules. Solubility decreases as the number of carbon atoms increases as the bulky non polar aryl/alkyl groups in the higher alcohols hinder the formation of effective hydrogen bonds between alcohol and water molecules.

Question 5

State the reagents and conditions to prepare alcohols from alkenes.

Answer 5

Reaction: Electrophilic Addition
(Laboratory method)
1. concentrated H2SO4, cold
2. H2O, heat

(Industrial method)
steam, conc H3PO4 catalyst and heat

Question 6

State the reagents and conditions to prepare alcohols from halogenoalkanes.

Answer 6

Reaction: Nucleophilic Substitution
Reagent and conditions: aqueous NaOH/KOH, heat

Question 7

State the reagents and conditions to prepare alcohols from aldehydes and ketones.

Answer 7

Reaction: Reduction
Reagents and conditions: LiAlH4 in dry ether, r.t OR NaBH4 in ethanol, r.t OR H2, Ni/Pt/Pd catalyst, r.t

NaBH4 can only reduce ketones and aldehydes and LiAlH4 can only reduce carboxylic acids, ketones, nitriles and aldehydes.

Question 8

State the reagents and conditions to prepare alcohols from carboxylic acids.

Answer 8

Reaction: Reduction
Reagent and Conditions: LiAlH4 in dry ether, r.t

Question 9

State the three sites of reactivity in aliphatic alcohols.

Answer 9

1. Due to the electronegative O atom attached to the C atom, the C atom becomes electron-deficient and is susceptible to nucleophilic attack, resulting in the heterolytic fission of the C-O bond. ROH acts as an electrophile
2. The O atom of the -OH group has lone pairs of electrons and a high electron density. This makes it electron rich and therefore acts as a nuclephile.
3. The O-H bond is polarised such that deprotonation may occur and therefore acts as a Bronsted Lowry acid.

Question 10

State the reagents and conditions needed to convert alcohols to alkenes.

Answer 10

Reaction: Elimination
Reagent and Conditions: excess conc H2SO4, heat (more common) OR Al2O3 catalyst, heat OR conc H3PO4 catalyst, heat

Question 11

State the reagents and conditions of all nucleophilic substitution reactions alcohols undergo.

Answer 11

1. Conc HCl, ZnCl2 catalyst, heat OR HBr(g), r.t OR HI(g), r.t Products: R-X + H2O(l)
2. Anhydrous PCl3/PBr3/PI3, heat Products: R-X + H3PO3(l)
3. Anhydrous PCl5, r.t Products R-Cl + POCl3(l) + HCl(g)
4. Anhydrous SOCl2, heat Products: R-Cl + SO2(g) + HCl(g)
   For 3 and 4, white fumes of HCl are produced, which turns damp blue litmus paper red

Question 12

State the reagents and conditions of all oxidation reactions primary alcohols can undergo.

Answer 12

1. K2Cr2O7(aq), H2SO4(aq), immediate distill Products: Aldehyde + H2O
2. K2Cr2O7(aq)/KMnO4(aq), H2SO4(aq), heat Products: carboxylic acid + H2O
   Observations:
   Purple KMnO4(aq) is decolorised
   Orange K2Cr2O7(aq) turns green

Question 13

State the reagents and conditions needed to oxidise secondary alcohols.

Answer 13

Reagents and conditions: K2Cr2O7(aq)/KMnO4(aq), H2SO4(aq), heat
Products: Ketone + H2O
Observations:
Purple KMnO4(aq) is decolorised
Orange K2Cr2O7(aq) turns green

Question 14

Why are tertiary alcohols resistant to oxidation?

Answer 14

This is due to the absence of alpha hydrogens in tertiary alcohols.

Question 15

State the reagents and conditions required for the tri-idomethae/iodoform reaction.

Answer 15

Reagents and conditions: Iodine, aqueous NaOH (alkaline aqeuous iodine), heat
Observations: Brown I2 decolourises and a pale yellow CHI3 ppt is formed

Question 16

State all the reagents and conditions needed for alcohols to undergo a condensation reaction.

Answer 16

1. Carboxylic acid + alcohol, conc H2SO4 catalyst + heat under reflux (reversible)
2. Acyl chloride + r.t

Question 17

Why does 2-nitrophenol have a lower boiling point than 4-nitrophenol?

Answer 17

In 2-nitrophenol the proximity of the -OH and -NO2 groups allows intramolecular hydrogen bonding to occur. Thus , intermolecular hydrogen bonding is less extensive, resulting in a lower boiling point than 4-nitrophenol.

Question 18

Describe the structure of phenol.

Answer 18

The oxygen atom of the OH group in phenol can be taken to be sp2 hybridised, same as the six carbon atom of the benzene ring. The 2p orbital on the oxygen atom of the -OH group, carrying a lone pair of electrons can have side on overlap with the pi orbitals of the benzene ring. As a result, the lone pair of electrons on the oxygen atom is delocalised into the benzene ring.

Question 19

State the four sites of reacitivity in phenols.

Answer 19

1. O atom of the -OH group with lone pairs of electrons lone pair delocalised into benzene ring acts as a weak nucleophile.
2. Delocalisation of the lone pair of electrons on O into the benzene ring disperses neative charge on the phenoxide ion, , stabilising the conjugate base and hence making ArOH a stronger Bronsted Lowry acid than ROH.
3. Delocalisation of lone pair into the benzene ring imparts partial double bond character to the C-O, strengthening the C-O bond, rendering ArOH a very poor electrophile
4. Benzene ring is activated towards electrophilic substitution due to delocalisation, rendering the benzene ring with a higher electron density.

Question 20

State the reagents and conditions required for phenol to undergo a condensation reaction.

Answer 20

1. Phenol in NaOH, r.t
2. ROCOCl, r.t

Phenol is first reacted with NaOH in an acid base reaction and is converted to the negatively charged phenoxide ion which is a stronger nucleophile than phenol.

Question 21

State all the reagents and conditions of all the electrophilic substitution reaction phenol can undergo.

Answer 21

1. Br2(aq), r.t Product: 2,4,6-tribromophenol (white ppt)
2. Br2 in CCl4, r,t Product: 2/4-bromophenol
3. Conc HNO3(aq). r.t Product: 2,4,6-trinitrophenol
4. Dilute HNO3(aq), r.t Product: 2/4-nitrophenol

Question 22

Why is ethanol a weaker acid than water?

Answer 22

The electron donating CH3CH2- group intensifies the negative charge, destabilising the ethoxide ion, CH3CH2O-, relative to the hydroxide ion. Hence water is a stonger acid.

Question 23

Why is phenol a stronger acid than alcohol and water?

Answer 23

The 2p orbital of the oxygen atom of the O- group, carrying a lone pair of electrons can have a side on overlap with the pi orbitals of the benzene ring. As a result the pair of electrons is delocalised into the benzene ring, which disperses the negative charge, stabilisng the phenoxide ion, making it resonance stabilised.

Question 24

State how the stability of the phenoxide and hydroxide ions formed are affected by other groups.

Answer 24

Electron withdrawing groups: Disperses negative charge, stabilising the phenoxide/hydroxide ion. (Eg. NO2)
Electron donating groups: Intensifies the negative charge, destabilising the phenoxide/hydroxide ion. (E.g CH3O)

Question 25

State the identification tests for phenol.

Answer 25

1. Add Br2(aq) to sample at r.t Observation: Orange Br2 decolorises and white ppt of 2,4,6-tribromophenol is formed 2. Add neutral FeCl3 to sample at r.t Obsevation: Violet colouration is observed

Question 26

State the identification tests for aliphatic alcohols.

Answer 26

1. Add anhydrous PCL5(s) to the sample Observation: White fumes of HCl(g) will be evolved 2. Add a small piece of Na(s) to sample Observation: Effervesence of H2(g) which gives a 'pop' sound with a lighted splint 3. [For primary and secondary] Add K2Cr2O7(aq)KMnO4(aq) and H2SO4(aq) and heat using a hot water bath Observation: Orange K2Cr2O7 turns green/Purple KMnO4 is decolourised 4. [Alcohols with the CH3CHOH group] Add I2 and NaOH(aq) to the sample and heat with hot water bath Observations: Brown I2 decolorises, pale yellow ppt of CHI3 is formed

Question 27

Explain why phenol does not undergo nucleophilic substitution.

Answer 27

(Basically the whole delocalisation of the lone pair of electrons into benzene ring explanation) + This leads to the partial double bond character in the C-O bond, strengthening the bond and making it more difficult to break. + Electron rich bezene ring of phenol will repel an incoming nucleophile

Question 28

Explain why phenols can undergo electrophilic substitution.

Answer 28

(Basically the whole delocalisation of the lone pair of electrons into benzene ring explanation) + This makes the bezene ring more electron rich and hence moe susceptible to electrophilic substitution.

Question 29

State alcohol and phenol's reactivity with sodium and bases.

Answer 29

1. Both alcohol and phenol reacts with sodium to form hydrogen gas. However alcohols are much weaker acids than water and reacts less vigorously with Na 2. Only phenol reacts with NaOH as alcohols are weaker acids 3. Only phenol reacts with Na2CO3 to form NaHCO3