ROUTESROUTESTO PHENOLTO PHENOL

A guide for A level studentsA guide for A level students

KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING20152015

SPECIFICATIONSSPECIFICATIONS

INTRODUCTION

This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.

Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available.

Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...

www.knockhardy.org.uk/sci.htm

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CONTENTS

• Prior knowledge

• Synthesis from benzene

• Reactions of the OH group

• Reactions of the ring

• Benzene diazonium chloride - preparation

• Benzene diazonium chloride - reactions

PHENOLPHENOL

Before you start it would be helpful to…

• know the functional groups found in organic chemistry

• know the arrangement of bonds around atoms

• recall and explain electrophilic substitution of aromatic rings

PHENOLPHENOL

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

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

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)

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)

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

BENZENE DIAZONIUM CHLORIDEBENZENE DIAZONIUM CHLORIDE

Structure has the formula C6H5N2+ Cl¯

a diazonium group is attached to the ringthe aromatic ring helps stabilise the ion

BENZENE DIAZONIUM CHLORIDEBENZENE DIAZONIUM CHLORIDE

Structure has the formula C6H5N2+ Cl¯

a diazonium group is attached to the ringthe aromatic ring helps stabilise the ion

PREPARATIONreagents phenylamine, nitrous acid and hydrochloric acid

conditions keep BELOW 10°C

equation C6H5NH2 + HNO2 + HCl ——> C6H5N2+ Cl¯ + 2H2O

notes nitrous acid is unstable and is made in situ from sodium nitrite

C6H5NH2 + NaNO2 + 2HCl ——> C6H5N2+ Cl¯ + NaCl +

2H2O

the solution is kept cold to slow down decompositionof the diazonium salt

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

DIAZONIUM SALTS - DIAZONIUM SALTS - REACTIONSREACTIONS

Benzene diazonium chloride undergoes two main types of reaction

SUBSTITUTION OFTHE DIAZONIUM GROUP nitrogen (N2) expelled

COUPLING REACTIONS nitrogen atoms are retained

DIAZONIUM SALTS - DIAZONIUM SALTS - SUBSTITUTIONSUBSTITUTION

OHreagents water (HYDROLYSIS)

conditions warm above 10°C

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

use the only reasonably simple way to substitute OH phenol is an antiseptic and is used to make polymers

DIAZONIUM SALTS - DIAZONIUM SALTS - SUBSTITUTIONSUBSTITUTION

OHreagents water (HYDROLYSIS)

conditions warm above 10°C

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

use the only reasonably simple way to substitute OH phenol is an antiseptic and is used to make polymers

Ireagents potassium iodide solutionconditions warm equation C6H5N2

+ Cl¯ + KI ——> C6H5I + KCl + N2

DIAZONIUM SALTS - DIAZONIUM SALTS - COUPLINGCOUPLING

reagents phenol and sodium hydroxide

conditions alkaline solution below 10°C

equation C6H5N2+Cl¯ + C6H5OH + NaOH ——> C6H5-N=N-C6H4OH + NaCl +

H2O

(4-hydroxyphenol)azobenzene YELLOW

use making azo dyes

THE AZO GROUP

SOME AZO DYESSOME AZO DYES

(4-hydroxyphenol)azobenzene YELLOW

4-phenylazophenylamine YELLOW

The first azo dye to be made

methyl orange

YELLOW IN ALKALIRED IN ACID

© 2015 JONATHAN HOPTON & KNOCKHARDY PUBLISHING© 2015 JONATHAN HOPTON & KNOCKHARDY PUBLISHING

ROUTESROUTESTO PHENOLTO PHENOL

THE ENDTHE END