ROUTES TO PHENOL A guide for A level students KNOCKHARDY PUBLISHING 2008 SPECIFICATIONS

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... Navigation is achieved by... either clicking on the grey arrows at the foot of each page orusing the left and right arrow keys on the keyboard PHENOL KNOCKHARDY PUBLISHING

CONTENTS Prior knowledge Synthesis from benzene Reactions of the OH group Reactions of the ring Benzene diazonium chloride - preparation Benzene diazonium chloride - reactions 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

PHENOL Structurephenol is an aromatic alcohol with formula C 6 H 5 OH the OH group is attached directly to the benzene ring it is an almost colourless crystalline solid

PHENOL Structurephenol is an aromatic alcohol with formula C 6 H 5 OH the OH group is attached directly to the benzene ring it is an almost colourless crystalline solid PreparationYou 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 Structurephenol is an aromatic alcohol with formula C 6 H 5 OH the OH group is attached directly to the benzene ring it is an almost colourless crystalline solid PreparationYou cannot put an OH group directly onto a benzene ring by electrophilic substitution, so phenol has to be synthesised in a multi-stage process Step 1 - Nitration of benzene reagentsconc. nitric acid and conc. sulphuric acid (catalyst) conditionsreflux at 55°C equationC 6 H 6 + HNO 3 ——> C 6 H 5 NO 2 + H 2 O mechanismelectrophilic substitution 1

PHENOL Structurephenol is an aromatic alcohol with formula C 6 H 5 OH the OH group is attached directly to the benzene ring it is an almost colourless crystalline solid PreparationYou cannot put an OH group directly onto a benzene ring by electrophilic substitution, so phenol has to be synthesised in a multi-stage process Step 2 - Reduction of nitrobenzene reagentstin and conc. hydrochloric acid conditionsreflux equationC 6 H 5 NO [H] ——> C 6 H 5 NH 2 + 2H 2 O mechanismreduction 2

PHENOL Structurephenol is an aromatic alcohol with formula C 6 H 5 OH the OH group is attached directly to the benzene ring it is an almost colourless crystalline solid PreparationYou cannot put an OH group directly onto a benzene ring by electrophilic substitution, so phenol has to be synthesised in a multi-stage process Step 3 - Diazotisation of phenylamine reagentsnitrous acid and hydrochloric acid (use sodium nitrite) conditionskeep below 10°C equationC 6 H 5 NH 2 + HNO 2 + HCl ——> C 6 H 5 N 2 + Cl¯ + 2H 2 O reaction typediazotisation 3

PHENOL Structurephenol is an aromatic alcohol with formula C 6 H 5 OH the OH group is attached directly to the benzene ring it is an almost colourless crystalline solid PreparationYou cannot put an OH group directly onto a benzene ring by electrophilic substitution, so phenol has to be synthesised in a multi-stage process Step 4 - Substitution of diazo group by OH reagentswater conditionswarm above 10°C equationC 6 H 5 N 2 + Cl¯ + H 2 O ——> C 6 H 5 OH + N 2 + HCl reaction typehydrolysis / substitution 4

PHENOL - REACTIONS PHENOL - REACTIONS OF THE OH GROUP Waterphenol 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 C 6 H 5 OH(aq) C 6 H 5 O¯(aq) + H + (aq)

PHENOL - REACTIONS PHENOL - REACTIONS OF THE OH GROUP Waterphenol 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 C 6 H 5 OH(aq) C 6 H 5 O¯(aq) + H + (aq) NaOHphenol reacts with sodium hydroxide to form a salt - sodium phenoxide it is ionic and water soluble C 6 H 5 OH(aq) + NaOH(aq) ——> C 6 H 5 O¯ Na + (aq) + H 2 O(l)

PHENOL - REACTIONS PHENOL - REACTIONS OF THE OH GROUP Waterphenol 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 C 6 H 5 OH(aq) C 6 H 5 O¯(aq) + H + (aq) NaOHphenol reacts with sodium hydroxide to form a salt - sodium phenoxide it is ionic and water soluble C 6 H 5 OH(aq) + NaOH(aq) ——> C 6 H 5 O¯ Na + (aq) + H 2 O(l) Sodiumphenol 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 2C 6 H 5 OH(s) + 2Na(s) ——> 2C 6 H 5 O¯ Na + (s) + H 2 (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 - REACTIONS PHENOL - REACTIONS OF THE AROMATIC RING p orbitals in the system the 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 NO 2 + react in a similar way PHENOL - REACTIONS PHENOL - REACTIONS OF THE AROMATIC RING

BENZENE DIAZONIUM CHLORIDE Structurehas the formula C 6 H 5 N 2 + Cl¯ a diazonium group is attached to the ring the aromatic ring helps stabilise the ion

BENZENE DIAZONIUM CHLORIDE Structurehas the formula C 6 H 5 N 2 + Cl¯ a diazonium group is attached to the ring the aromatic ring helps stabilise the ion PREPARATION reagentsphenylamine, nitrous acid and hydrochloric acid conditionskeep BELOW 10°C equationC 6 H 5 NH 2 + HNO 2 + HCl ——> C 6 H 5 N 2 + Cl¯ + 2H 2 O notesnitrous acid is unstable and is made in situ from sodium nitrite C 6 H 5 NH 2 + NaNO 2 + 2HCl ——> C 6 H 5 N 2 + Cl¯ + NaCl + 2H 2 O the solution is kept cold to slow down decomposition of the diazonium salt C 6 H 5 N 2 + Cl¯ + H 2 O ——> C 6 H 5 OH + HCl + N 2

DIAZONIUM SALTS - REACTIONS Benzene diazonium chloride undergoes two main types of reaction SUBSTITUTION OF THE DIAZONIUM GROUPnitrogen (N 2 ) expelled COUPLING REACTIONSnitrogen atoms are retained

DIAZONIUM SALTS - SUBSTITUTION OH reagentswater (HYDROLYSIS) conditionswarm above 10°C equationC 6 H 5 N 2 + Cl¯ + H 2 O ——> C 6 H 5 OH + HCl + N 2 usethe only reasonably simple way to substitute OH phenol is an antiseptic and is used to make polymers

DIAZONIUM SALTS - SUBSTITUTION OH reagentswater (HYDROLYSIS) conditionswarm above 10°C equationC 6 H 5 N 2 + Cl¯ + H 2 O ——> C 6 H 5 OH + HCl + N 2 usethe only reasonably simple way to substitute OH phenol is an antiseptic and is used to make polymers I reagentspotassium iodide solution conditionswarm equationC 6 H 5 N 2 + Cl¯ + KI ——> C 6 H 5 I + KCl + N 2

DIAZONIUM SALTS - COUPLING reagentsphenol and sodium hydroxide conditionsalkaline solution below 10°C equation C 6 H 5 N 2 + Cl¯ + C 6 H 5 OH + NaOH ——> C 6 H 5 -N=N-C 6 H 4 OH + NaCl + H 2 O (4-hydroxyphenol)azobenzene YELLOW usemaking azo dyes THE AZO GROUP

SOME AZO DYES (4-hydroxyphenol)azobenzene YELLOW 4-phenylazophenylamine YELLOW The first azo dye to be made methyl orange YELLOW IN ALKALI RED IN ACID

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