Phenol

Draw the following structures Phenol 2-ethylphenol 3,4,4-trichlorobiphenyl 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

Diploma Phenol Phenol cannot be made directly from benzene, the benzene ring is inert to attack by the nucleophile OH-. CH(CH3)2 OH AlCl3 i)O2 +CH3CH=CH2 +CH3COCH3 ii)H+ heat benzene propene (propan-2-yl) benzene ‘cumene’ propanone phenol This is called the ‘cumene’ process

PHENOL 1 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 55°C equation C6H6 + HNO3 ——> C6H5NO2 + H2O mechanism electrophilic substitution

PHENOL 2 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 + 2H2O mechanism reduction

PHENOL 3 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 10°C equation C6H5NH2 + HNO2 + HCl ——> C6H5N2+ Cl¯ + 2H2O reaction type diazotisation

PHENOL 4 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 10°C equation C6H5N2+ Cl¯ + H2O ——> C6H5OH + N2 + HCl reaction type hydrolysis / substitution

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) NaOH 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)

ELECTROPHILIC SUBSTITUTION PHENOL - REACTIONS OF THE AROMATIC RING 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 the p orbital on the O overlaps with the p orbitals in the ring p orbitals in the system

ELECTROPHILIC SUBSTITUTION 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

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