7.9 Write the equations involved in the following reactions: (i) Reimer - Tiemann reaction (ii) Kolbe’s reaction 213 Alcohols, Phenols and Ethers Reprint 2025-26 7.57.57.57.57.5 SomeSomeSomeSomeSome Methanol and ethanol are among the two commercially important alcohols. CommerciallyCommerciallyCommerciallyCommerciallyCommercially ImportantImportantImportantImportantImportant 1. Methanol AlcoholsAlcoholsAlcoholsAlcoholsAlcohols Methanol, CH3OH, also known as ‘wood spirit’, was produced by destructive distillation of wood. Today, most of the methanol is produced by catalytic hydrogenation of carbon monoxide at high pressure and temperature and in the presence of ZnO – Cr2O3 catalyst. Methanol is a colourless liquid and boils at 337 K. It is highly poisonous in nature. Ingestion of even small quantities of methanol can cause blindness and large quantities causes even death. Methanol is used as a solvent in paints, varnishes and chiefly for making formaldehyde. 2. Ethanol Ethanol, C2H5OH, is obtained commercially by fermentation, the oldest method is from sugars. The sugar in molasses, sugarcane or fruits such as grapes is converted to glucose and fructose, (both of which have the formula C6H12O6), in the presence of an enzyme, invertase. Glucose and fructose undergo fermentation in the presence of another enzyme, zymase, which is found in yeast. In wine making, grapes are the source of sugars and yeast. As Ingestion of ethanol acts grapes ripen, the quantity of sugar increases and yeast grows on the on the central nervous outer skin. When grapes are crushed, sugar and the enzyme come in system. In moderate contact and fermentation starts. Fermentation takes place in amounts, it affects anaerobic conditions i.e. in absence of air. Carbon dioxide is released judgment and lowers during fermentation. inhibitions. Higher concentrations cause The action of zymase is inhibited once the percentage of alcohol nausea and loss of formed exceeds 14 percent. If air gets into fermentation mixture, the consciousness. Even at oxygen of air oxidises ethanol to ethanoic acid which in turn destroys higher concentrations, the taste of alcoholic drinks. it interferes with Ethanol is a colourless liquid with boiling point 351 K. It is used spontaneous respiration as a solvent in paint industry and in the preparation of a number of and can be fatal. carbon compounds. The commercial alcohol is made unfit for drinking by mixing in it some copper sulphate (to give it a colour) and pyridine (a foul smelling liquid). It is known as denaturation of alcohol. Nowadays, large quantities of ethanol are obtained by hydration of ethene (Section 7.4). Chemistry 214 Reprint 2025-26 7.67.67.67.67.6 EthersEthersEthersEthersEthers 7.6.1 Preparation 1. By dehydration of alcohols of Ethers Alcohols undergo dehydration in the presence of protic acids (H2SO4, H3PO4). The formation of the reaction product, alkene or ether depends on the reaction conditions. For example, ethanol is dehydrated to ethene in the presence of sulphuric acid at 443 K. At 413 K, ethoxyethane is the main product. Diethyl ether has been used widely as an inhalation anaesthetic. But due to its slow effect and an unpleasant recovery The formation of ether is a nucleophilic bimolecular reaction (SN2) period, it has been involving the attack of alcohol molecule on a protonated alcohol, as replaced, as an indicated below: anaesthetic, by other compounds. Acidic dehydration of alcohols, to give an alkene is also associated with substitution reaction to give an ether. The method is suitable for the preparation of ethers having primary alkyl groups only. The alkyl group should be unhindered and the temperature be kept low. Otherwise the reaction favours the formation of alkene. The reaction follows SN1 pathway when the alcohol is secondary or tertiary about which you will learn in higher classes. However, the dehydration of secondary and tertiary alcohols to give corresponding ethers is unsuccessful as elimination competes over substitution and as a consequence, alkenes are easily formed. Can you explain why is bimolecular dehydration not appropriate for the preparation of ethyl methyl ether? 2. Williamson synthesis It is an important laboratory method for the preparation of Alexander William symmetrical and unsymmetrical ethers. In this method, an alkyl Williamson (1824–1904) halide is allowed to react with sodium alkoxide.was born in London of – +Scottish parents. In R–X + R’–O Na R–O–R’ + Na X 1849, he became Professor of Chemistry Ethers containing substituted alkyl groups (secondary or tertiary) at University College, may also be prepared by this method. The reaction involves SN2 attack London. of an alkoxide ion on primary alkyl halide. 215 Alcohols, Phenols and Ethers Reprint 2025-26 O Na + CH 3–Br Better results are obtained if the alkyl halide is primary. In case of secondary and tertiary alkyl halides, elimination competes over substitution. If a tertiary alkyl halide is used, an alkene is the only reaction product and no ether is formed. For example, the reaction of CH3ONa with (CH3)3C–Br gives exclusively 2-methylpropene. It is because alkoxides are not only nucleophiles but strong bases as well. They react with alkyl halides leading to elimination reactions. ExampleExampleExampleExampleExample 7.67.67.67.67.6 The following is not an appropriate reaction for the preparation of t-butyl ethyl ether. (i) What would be the major product of this reaction ? (ii) Write a suitable reaction for the preparation of t-butylethyl ether. SolutionSolutionSolutionSolutionSolution (i) The major product of the given reaction is 2-methylprop-1-ene. It is because sodium ethoxide is a strong nucleophile as well as a strong base. Thus elimination reaction predominates over substitution. (ii) Phenols are also converted to ethers by this method. In this, phenol is used as the phenoxide moiety. Chemistry 216 Reprint 2025-26 7.6.2 Physical The C-O bonds in ethers are polar and thus, ethers have a net dipole Properties moment. The weak polarity of ethers do not appreciably affect their boiling points which are comparable to those of the alkanes of comparable molecular masses but are much lower than the boiling points of alcohols as shown in the following cases: Formula CH3(CH2)3CH3 C2H5-O-C2H5 CH3(CH2)3-OH n-Pentane Ethoxyethane Butan-1-ol b.p./K 309.1 307.6 390 The large difference in boiling points of alcohols and ethers is due to the presence of hydrogen bonding in alcohols. The miscibility of ethers with water resembles those of alcohols of the same molecular mass. Both ethoxyethane and butan-1-ol are miscible to almost the same extent i.e., 7.5 and 9 g per 100 mL water, respectively while pentane is essentially immiscible with water. Can you explain this observation ? This is due to the fact that just like alcohols, oxygen of ether can also form hydrogen bonds with water molecule as shown: 7.6.3 Chemical 1. Cleavage of C–O bond in ethers Reactions Ethers are the least reactive of the functional groups. The cleavage of C-O bond in ethers takes place under drastic conditions with excess of hydrogen halides. The reaction of dialkyl ether gives two alkyl halide molecules. Alkyl aryl ethers are cleaved at the alkyl-oxygen bond due to the more stable aryl-oxygen bond. The reaction yields phenol and alkyl halide. Ethers with two different alkyl groups are also cleaved in the same manner. The order of reactivity of hydrogen halides is as follows: HI > HBr > HCl. The cleavage of ethers takes place with concentrated HI or HBr at high temperature. 217 Alcohols, Phenols and Ethers Reprint 2025-26 MechanismMechanismMechanismMechanismMechanism The reaction of an ether with concentrated HI starts with protonation of ether molecule. Step 1: The reaction takes place with HBr or HI because these reagents are sufficiently acidic. Step 2: Iodide is a good nucleophile. It attacks the least substituted carbon of the oxonium ion formed in step 1 and displaces an alcohol molecule by SN2 mechanism. Thus, in the cleavage of mixed ethers with two different alkyl groups, the alcohol and alkyl iodide formed, depend on the nature of alkyl groups. When primary or secondary alkyl groups are present, it is the lower alkyl group that forms alkyl iodide (SN2 reaction). When HI is in excess and the reaction is carried out at high temperature, ethanol reacts with another molecule of HI and is converted to ethyl iodide. Step 3: However, when one of the alkyl group is a tertiary group, the halide formed is a tertiary halide. CH3 CH3 CH3 C O CH3 +HI CH3OH +CH 3 C I CH3 CH3 It is because in step 2 of the reaction, the departure of leaving group (HO–CH3) creates a more stable carbocation [(CH3)3C+], and the reaction follows SN1 mechanism. In case of anisole, methylphenyl CH3 CH3 + slow + oxonium ion, is CH3 C O CH3 CH3 C + CH3 OH H CH3 CH3 formed by protonation of ether. The bond between O–CH3 is weaker CH3 CH3 than the bond between O–C6H5 + – fast CH3 C + I CH3 C I because the carbon of phenyl group is sp2 hybridised and there CH3 CH3 is a partial double bond character. Chemistry 218 Reprint 2025-26 Therefore the attack by I– ion breaks O–CH3 bond to form CH3I. Phenols do not react further to give halides because the sp 2 hybridised carbon of phenol cannot undergo nucleophilic substitution reaction needed for conversion to the halide. Give the major products that are formed by heating each of the following ExampleExampleExampleExampleExample 7.77.77.77.77.7 ethers with HI. (i) (ii) (iii) (i) (ii) SolutionSolutionSolutionSolutionSolution (iii) 2. Electrophilic substitution The alkoxy group (-OR) is ortho, para directing and activates the aromatic ring towards electrophilic substitution in the same way as in phenol. (i) Halogenation: Phenylalkyl ethers undergo usual halogenation in the benzene ring, e.g., anisole undergoes bromination with bromine in ethanoic acid even in the absence of iron (III) bromide catalyst. It is due to the activation of benzene ring by the methoxy group. Para isomer is obtained in 90% yield. 219 Alcohols, Phenols and Ethers Reprint 2025-26 (ii) Friedel-Crafts reaction: Anisole undergoes Friedel-Crafts reaction, i.e., the alkyl and acyl groups are introduced at ortho and para positions by reaction with alkyl halide and acyl halide in the presence of anhydrous aluminium chloride (a Lewis acid) as catalyst. (iii) Nitration: Anisole reacts with a mixture of concentrated sulphuric and nitric acids to yield a mixture of ortho and para nitroanisole. IntextIntextIntextIntextIntext QuestionsQuestionsQuestionsQuestionsQuestions 7.10 Write the reactions of Williamson synthesis of 2-ethoxy-3-methylpentane starting from ethanol and 3-methylpentan-2-ol. 7.11 Which of the following is an appropriate set of reactants for the preparation of 1-methoxy-4-nitrobenzene and why? (i) (ii) Chemistry 220 Reprint 2025-26

7.7 Predict the major product of acid catalysed dehydration of (i) 1-methylcyclohexanol and (ii) butan-1-ol

7.13 Show how will you synthesise: (i) 1-phenylethanol from a suitable alkene. (ii) cyclohexylmethanol using an alkyl halide by an SN2 reaction. (iii) pentan-1-ol using a suitable alkyl halide?