ROUTES TO PHENOL A guide for A level

ROUTES TO PHENOL A guide for A level

ROUTES TO PHENOL A guide for A level students KNOCKHARDY PUBLISHING KNOCKHARDY PUBLISHING PHENOL 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.argonet.co.uk/users/hoptonj/sci.htm Navigation is achieved by... either clicking on the grey arrows at the foot of each page or using the left and right arrow keys on the keyboard PHENOL CONTENTS Prior knowledge Synthesis from benzene Reactions of the OH group Reactions of the ring Benzene diazonium chloride - preparation Benzene diazonium chloride - reactions

Revision check PHENOL 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 PHENOL Structure phenol is an aromatic alcohol with formula C6H5OH the OH group is attached directly to the benzene ring it is an almost colourless crystalline solid PHENOL Structure

phenol is an aromatic alcohol with formula C6H5OH the OH group is attached directly to the benzene ring it is an almost colourless crystalline solid Preparation You cannot put an OH group directly onto a benzene ring by electrophilic substitution, so phenol has to be synthesised in a multi-stage process PHENOL Structure phenol is an aromatic alcohol with formula C6H5OH the OH group is attached directly to the benzene ring it is an almost colourless crystalline solid Preparation

You cannot put an OH group directly onto a benzene ring by electrophilic substitution, so phenol has to be synthesised in a multi-stage process 1 Step 1 - Nitration of benzene reagents conc. nitric acid and conc. sulphuric acid (catalyst) conditions reflux at 55C equation C6H6 + HNO3 > C6H5NO2 + H2O mechanism

electrophilic substitution PHENOL Structure phenol is an aromatic alcohol with formula C6H5OH the OH group is attached directly to the benzene ring it is an almost colourless crystalline solid Preparation You cannot put an OH group directly onto a benzene ring by electrophilic substitution, so phenol has to be synthesised in a multi-stage process 2 Step 2 - Reduction of nitrobenzene

reagents tin and conc. hydrochloric acid conditions reflux equation C6H5NO2 + 6 [H] > C6H5NH2 mechanism reduction + 2H2O PHENOL Structure phenol is an aromatic alcohol with formula C6H5OH the OH group is attached directly to the benzene ring it is an almost colourless crystalline solid

Preparation You cannot put an OH group directly onto a benzene ring by electrophilic substitution, so phenol has to be synthesised in a multi-stage process 3 Step 3 - Diazotisation of phenylamine reagents nitrous acid and hydrochloric acid (use sodium nitrite) conditions keep below 10C equation C6H5NH2 + HNO2 + HCl > C6H5N2+ Cl + 2H2O reaction type

diazotisation PHENOL Structure phenol is an aromatic alcohol with formula C6H5OH the OH group is attached directly to the benzene ring it is an almost colourless crystalline solid Preparation You cannot put an OH group directly onto a benzene ring by electrophilic substitution, so phenol has to be synthesised in a multi-stage process 4 Step 4 - Substitution of diazo group by OH

reagents water conditions warm above 10C equation C6H5N2+ Cl + H2O > C6H5OH reaction type hydrolysis / substitution + N2 + HCl PHENOL - REACTIONS OF THE OH GROUP Water phenol is a weak acid

it dissolves very slightly in water to form a weak acidic solution it is a stronger acid than aliphatic alcohols the ring helps weaken the O-H bond and stabilises the resulting anion C6H5OH(aq) C6H5O(aq) + H+(aq) PHENOL - REACTIONS OF THE OH GROUP Water phenol is a weak acid it dissolves very slightly in water to form a weak acidic solution it is a stronger acid than aliphatic alcohols the ring helps weaken the O-H bond and stabilises the resulting anion

C6H5OH(aq) NaOH C6H5O(aq) + H+(aq) phenol reacts with sodium hydroxide to form a salt - sodium phenoxide it is ionic and water soluble C6H5OH(aq) + NaOH(aq) > C6H5O Na+(aq) + H2O(l) PHENOL - REACTIONS OF THE OH GROUP Water phenol is a weak acid

it dissolves very slightly in water to form a weak acidic solution it is a stronger acid than aliphatic alcohols the ring helps weaken the O-H bond and stabilises the resulting anion C6H5OH(aq) NaOH C6H5O(aq) + H+(aq) phenol reacts with sodium hydroxide to form a salt - sodium phenoxide it 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 phenoxide

hydrogen is also produced this reaction is similar to that with aliphatic alcohols such as ethanol 2C6H5OH(s) + 2Na(s) > 2C6H5O Na+(s) + H2(g) PHENOL - REACTIONS OF THE AROMATIC 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 BENZENE DIAZONIUM CHLORIDE Structure has the formula C6H5N2+ Cl a diazonium group is attached to the ring the aromatic ring helps stabilise the ion

PREPARATION reagents conditions equation phenylamine, nitrous acid and hydrochloric acid keep below 10C 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 decomposition of the diazonium salt C6H5N2+ Cl + H2O >

C6H5OH + HCl + N2 DIAZONIUM SALTS - REACTIONS Benzene diazonium chloride undergoes two main types of reaction SUBSTITUTION OF THE DIAZONIUM GROUP nitrogen expelled

COUPLING REACTIONS nitrogen atoms are retained DIAZONIUM SALTS - SUBSTITUTION OH reagents conditions equation use I reagents conditions equation

water (hydrolysis) warm above 10C C6H5N2+ Cl + H2O > C6H5OH + HCl + the only reasonably simple way to substitute OH phenol is an antiseptic and is used to make polymers potassium iodide solution warm C6H5N2+ Cl + KI >

C6H5I + KCl + N2 N2 DIAZONIUM SALTS - COUPLING reagents conditions equation phenol and sodium hydroxide alkaline solution below 10C (4-hydroxyphenol)azobenzene

YELLOW use making dyes REVISION CHECK What should you be able to do? Recall the structures of phenol and benzene diazonium chloride Explain and understand the difference in reactivity between phenol and benzene Recall the reagents and conditions used in the steps to convert benzene into phenol Recall the different types of reaction undergone by benzene diazonium chloride CAN YOU DO ALL OF THESE? YES NO

You need to go over the relevant topic(s) again Click on the button to return to the menu WELL DONE! Try some past paper questions ROUTES TO PHENOL THE END 2004 JONATHAN HOPTON & KNOCKHARDY PUBLISHING

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