Phenol

Draw the following structures

1. Phenol

2. 2-ethylphenol

3. 3,4,4-trichlorobiphenyl

4. 2-phenylethanol

Extension:

Why is no. 4 not a phenol?

Learning Outcomes

Explain what PCBs are Describe the reactions of phenol Discuss the uses of phenols

PCB’s (Polychlorinated biphenyls)

3,4,4-trichlorobiphenyl

Learning Outcomes

Explain what PCBs are Describe the reactions of phenol Discuss the uses of phenols

PhenolPhenol cannot be made directly from

benzene, the benzene ring is inert to attack by the nucleophile OH-.

This is called the ‘cumene’ process

+CH3CH=CH2

AlCl3

heat

CH(CH3)2

i)O2

ii)H+

OH

+CH3COCH3

benzene propene(propan-2-yl) benzene‘cumene’

propanonephenol

Diploma

PHENOLPHENOL

Structure phenol is an aromatic alcohol with formula C6H5OH

the OH group is attached directly to the benzene ringit is an almost colourless crystalline solid

Preparation You cannot put an OH group directly onto a benzene ring byelectrophilic substitution, so phenol has to be synthesised ina multi-stage process

Step 1 - Nitration of benzene

reagents conc. nitric acid and conc. sulphuric acid (catalyst)

conditions reflux at 55°C

equation C6H6 + HNO3 ——> C6H5NO2 + H2O

mechanism electrophilic substitution

1

PHENOLPHENOL

Structure phenol is an aromatic alcohol with formula C6H5OH

the OH group is attached directly to the benzene ringit is an almost colourless crystalline solid

Preparation You cannot put an OH group directly onto a benzene ring byelectrophilic substitution, so phenol has to be synthesised ina multi-stage process

Step 2 - Reduction of nitrobenzene

reagents tin and conc. hydrochloric acid

conditions reflux

equation C6H5NO2 + 6 [H] ——> C6H5NH2 + 2H2O

mechanism reduction

2

PHENOLPHENOL

Structure phenol is an aromatic alcohol with formula C6H5OH

the OH group is attached directly to the benzene ringit is an almost colourless crystalline solid

Preparation You cannot put an OH group directly onto a benzene ring byelectrophilic substitution, so phenol has to be synthesised ina multi-stage process

Step 3 - Diazotisation of phenylamine

reagents nitrous acid and hydrochloric acid (use sodium nitrite)

conditions keep below 10°C

equation C6H5NH2 + HNO2 + HCl ——> C6H5N2+ Cl¯ +

2H2O

reaction type diazotisation

3

PHENOLPHENOL

Structure phenol is an aromatic alcohol with formula C6H5OH

the OH group is attached directly to the benzene ringit is an almost colourless crystalline solid

Preparation You cannot put an OH group directly onto a benzene ring byelectrophilic substitution, so phenol has to be synthesised ina multi-stage process

Step 4 - Substitution of diazo group by OH

reagents water

conditions warm above 10°C

equation C6H5N2+ Cl¯ + H2O ——> C6H5OH + N2 +

HCl

reaction type hydrolysis / substitution

4

PHENOL - PHENOL - REACTIONSREACTIONS OF THE OH GROUP

Water phenol is a weak acidit dissolves very slightly in water to form a weak acidic solutionit is a stronger acid than aliphatic alcoholsthe ring helps weaken the O-H bond and stabilises the resulting anion

C6H5OH(aq) C6H5O¯(aq) + H+(aq)

NaOH phenol reacts with sodium hydroxide to form a salt - sodium phenoxideit is ionic and water soluble

C6H5OH(aq) + NaOH(aq) ——> C6H5O¯ Na+(aq) + H2O(l)

Sodium phenol reacts with sodium to form an ionic salt - sodium phenoxidehydrogen is also producedthis reaction is similar to that with aliphatic alcohols such as ethanol

2C6H5OH(s) + 2Na(s) ——> 2C6H5O¯ Na+(s) + H2(g)

ELECTROPHILIC SUBSTITUTION

Ease the OH group is electron releasing

it increases the electron density of the delocalised system

it makes substitution much easier compared to benzene

a p orbital on the oxygen overlaps with the p orbitals in benzene

PHENOL - PHENOL - REACTIONSREACTIONS OF THE AROMATIC RING

p orbitals in the systemthe p orbital on the O overlaps with the p orbitals in the ring

ELECTROPHILIC SUBSTITUTION

Bromine the OH group is electron releasing

it increases the electron density of the delocalised system

it makes substitution much easier compared to benzene

the electron density is greatest at the 2,4 and 6 positions

substitution takes place at the 2,4 and 6 positions

phenol reacts readily with bromine water WITHOUT A CATALYST

it is so easy that multiple substitution takes place

other electrophiles such as NO2+ react in a similar way

PHENOL - PHENOL - REACTIONSREACTIONS OF THE AROMATIC RING

The OH group is electron releasing Electron pair donation takes place from a p orbital on

oxygen It increases the electron density of the delocalised

system It makes substitution much easier compared to benzene The electron density is the greatest at the 2,4 and 6

positions Substitution takes place at the 2,4 and 6 positions Phenol reacts readily with bromine water without a

catalyst It is so easy that multiple substitution takes place. Other electrophiles such as NO2

+ react in a similar way

Learning Outcomes

Explain what PCBs are Describe the reactions of phenol Discuss the uses of phenols

Summary sheet

Phenol summary sheet.doc

Exam questions

benzene and phenols.rtf