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methylbenzene ethylbenzene NO2 HO O benzoic acid NO2 Phenol L.O. To explore the effect of an OH group on the reactivity of the benzene ring. To be able to describe and explain the key reactions of phenol. OH Phenol Structure: Phenol is an aromatic alcohol with formula C6H5OH - OH group is attached directly to the benzene ring - Almost colourless crystalline solid Properties: The OH group changes the reactivity of the benzene ring and the benzene ring changes the reactivity of the OH group. • Hydrogen bonding from OH group = stronger intermolecular forces resulting in phenol being solid at room temperature. • Acting as an ACID: OH group also hydrogen bonds with water. Therefore phenol dissolves in water and alkalis (forming soluble ionic salts). • Weak acid so does not fully ionise in water and so does not react with carbonates to produce CO2. What are the products when phenol reacts with the following? Water NaOH Na Phenol – reactions of the OH group Water -Phenol is a weak acid and dissolves only slightly in water forming a weakly acidic solution -Stronger acid than aliphatic alcohols -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 -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 (similar to sodium reacting with aliphatic alcohols such as ethanol) 2C6H5OH(s) + 2Na(s) 2C6H5O¯ Na+(s) + H2(g) Phenol Preparation: An OH group cannot be directly added to a benzene ring by electrophilic substitution. - Instead, phenol is synthesised in a multi-stage process 1 Step 1 - Nitration reagents conditions equation mechanism of benzene conc. nitric acid and conc. sulphuric acid (catalyst) reflux at 55°C C6H6 + HNO3 C6H5NO2 + electrophilic substitution H2O Phenol 2 Step 2 - Reduction of nitrobenzene reagents tin and conc. hydrochloric acid conditions reflux equation C6H5NO2 + 6 [H] mechanism reduction C6H5NH2 + 2H2O 3 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 Phenol 4 Step 4 - Substitution of diazo group by OH reagents water conditions warm above 10°C equation C6H5N2+ Cl¯ + H2O reaction type hydrolysis / substitution C6H5OH Naming derivatives of phenol + N2 + HCl What does the OH group do to the reactivity of the benzene group? The OH groups ACTIVATES the benzene ring. Group ELECTRON DONATING Example(s) Electron density of ring Ease of substitution Position of substitution OH, CH3 Increases Easier 2,4,and 6 ONE LONE PAIR of electrons on the oxygen of OH interacts with the delocalised electrons of the benzene ring, aiding E+ attack. This results in MILDER CONDITIONS for phenol reactions compared to the corresponding benzene reactions. Phenol - reactions of the aromatic ring Electrophilic substitution Bromine - OH group is electron releasing - Increases the electron density of the delocalised system, so making substitution much easier compared to benzene - Electron density is greatest at the 2,4 and 6 positions, so substitution takes place at the 2,4 and 6 positions - Phenol reacts readily with bromine water WITHOUT A CATALYST - In fact so easy, multiple substitution takes place Nitric acid - Dilute HNO3 reacts rapidly with phenol at room temperature. - Resulting product is a BROWN MIXTURE. - Main products are 2 and 4-nitrophenol. Compare this to the conditions required for nitration of a benzene ring when the ring is NOT activated. Uses of phenol • Production of antiseptics and disinfectants • Antiseptics – chemicals that kill microorganisms, but can be used safely on the skin. • Disinfectants – chemicals that kill microorganisms. Very powerful reagent and as such is now not used on skin or living tissue as too dangerous. • Phenol not used as an antiseptic as it burns the skin. • Substituted phenols can however be used as an antiseptic – TCP, Dettol. • Thermosetting plastics (resins) • Thermoplastic polymers – soften on heating and can be moulded into different shapes. These shapes remain on cooling (e.g. polyethene, pvc) • Thermosetting polymers – set hard on formation and CAN NOT be softened or moulded (BAKELITE) Polyethene and pvc Thermoplastic polymers Bakelite Thermosetting polymer N.B: Phenol is only partially soluble and forms 2 layers with water at r.t. OH OH Br Br O- NaOH (aq) 3Br2 (aq) Colourless solution + H20 + 3HBr (aq) Br CH3COCl White ppt formed (NO catalyst needed) O O Ester formation = phenylethanoate + HCl