Phenols

Question 1

What are phenols?

Answer 1

• AROMATIC compounds
• Hydroxyl group, -OH, is attached directly to the benzene ring

Question 2

What happens when a PHENOL is dissolved in water?

Answer 2

• Loses the H+ from the -OH group
• The NEGATIVE CHARGE on the OXYGEN can be stabilised via its DELOCALISATION around the benzene ring
• Aqueous phenol forms a WEAK acidic solution
• Position of equilibrium ~ LEFT
• Concentration of protons in the mixture ~ LOW
• Phenol is sparingly soluble in water

C6H5OH + aq (reversible sign) C6H5O- + H+
(phenoxide ion)

Question 3

Why is phenol only slightly soluble in water?

Answer 3

• The -OH group forms hydrogen bonds with water
• However, the benzene ring can only form weak London forces with water
• The benzene rings of phenol form London forces with each other
• This causes layers to form

Question 4

What happens when an ALCOHOL is added to water?

Answer 4

• Does NOT form an acidic solution
• The negative charge on the oxygen CANNOT be stabilised via delocalisation when the proton is lost
• The O- ion strongly attracts the H+ ion and the alcohol is REFORMED

Question 5

Reaction of phenol with NaOH

• Equation
• Observations

Answer 5

acid + alkali —– salt + water

C6H5OH + NaOH —- C6H5O-Na+ + H2

Salt ~ Sodium Phenoxide
Bonding ~ IONIC

OBSERVATION:
- Crystals dissolve more readily in NaOH(aq) than water
- Phenol is soluble in NaOH(aq)
- The reaction is reversed to produce solid
phenol

Question 6

Reaction of phenol with Na

• Equation
• Observations

Answer 6

acid + metal —- salt + hydrogen

C6H5OH + Na —- C6H5O-Na+ + 1/2H2

Salt ~ Sodium phenoxide

SODIUM:
- loses an electron (Na —- Na+ + e-)
- Reduces H+(aq) to produce 1/2H2(aq)

OBSERVATIONS:
- effervescence
- solid dissolves

Question 7

The Acidity of functional groups

• Strong base ~ NaOH
• Weak base ~ Na2CO3

Answer 7

Strong Base :

H+(aq) + OH-(aq) ——- H2O(l)

Alcohol ~ no reaction
Phenol ~ reaction takes place
Carboxylic acid ~ reaction takes place

Weak base :

2H+(aq) + CO32-(aq) —— CO2(g) + H2O(l)

Alcohol ~ no reaction
Phenol ~ no reaction
Carboxylic acid ~ reaction takes place

Question 8

How to distinguish between a carboxylic acid and a phenol?

Answer 8

UI INDICATOR ~ confirms their acidity

CARBONATE ~
Carboxylic acid ~ effervescence
Phenol ~ NO effervescence , not acidic to react with carbonates

Question 9

Br2(aq) & Phenol

Answer 9

• Phenol DECOLOURISES aqueous bromine
• colour change from ORANGE to COLOURLESS
• WHITE PPT of 2,4,6-tribromophenol
• Phenol can react with bromine directly without the need of a halogen carrier

Question 10

Which structural feature in phenol is responsible for its greater reactivity in comparison with benzene?

Answer 10

• The lone pairs of electrons on the oxygen in the phenol group
• Each lone pair is located in a separate p-orbital

Question 11

Why does bromine react more readily with phenol in comparison with benzene?

Answer 11

• Phenol has a lone pair of electrons on the oxygen atom which is DELOCALISED into the benzene ring
• This INCREASES the electron density
• Phenol is able to POLARISE the bromine more than benzene can
• Bromine is attracted more strongly to the ring in phenol compared with benzene
• NO halogen carrier required

Question 12

Electrophilic Substitution in phenol

Answer 12

• OH group in phenol directs SUBSTITUTIONS to the 2,4 and 6 positions in the benzene ring
• Occurs due to the DELOCALISATION of the oxygen lone pair into the benzene ring produces the highest electron density at these positions

Question 13

Substitutions in phenylamine

Answer 13

• Like phenol, phenylamine has a lone pair of electrons but on the NITROGEN instead
• It is DELOCALISED into the benzene ring via p-orbital overlap
• Electron density is INCREASED, polarising bromine sufficiently
• Bromine is attracted more strongly to the ring as compared with benzene

Question 14

Why do the second and third substitutions in phenylamine occur at faster rates than those in phenol?

Answer 14

The lone pair on the nitrogen is:

• HIGHER IN ENERGY
• More readily donated to the benzene ring

Question 15

Activating groups ~ Role

Answer 15

• DONATE electron density to the ring
• ACTIVATE ring towards electrophilic attack
• DIRECT substitutions to the 2- and 4- positions (& 6- thereafter)

Question 16

Examples of :

Activating groups/ 2- & 4-directing

Answer 16

-OH (phenol) *

-R (alkyl) *

-NH2 (amines) *

-OR (ether)

-C6H5 (phenyl)

-F, -Cl, -Br, -I (halogens)

-NHR (amides)

Question 17

Deactivating Groups ~ Role

Answer 17

• WITHDRAWS electron density to the ring
• DEACTIVATE ring towards electrophilic attack
• DIRECT substitutions to the 3-position (& 5-position thereafter)

Question 18

Examples of :

Deactivating groups/ 3-directing groups

Answer 18

-NO2 (nitro group) *

-CHO (aldehyde)

-RCOR (ketone)

-COOH (carboxylic acid)

-COOR (ester)

-CN (nitrile)

-NR3+ (alkylammonium ion)

-SO3H (sulfonic group)