Subj ect :- Chemist r y Submitted by: Abdul Noor XII Science(201213)
Ch. 11 Alcohols, Phenols, and EthersAlcohols and phenols are formed when a hydrogen atom in a hydrocarbon, aliphatic and aromatic respectively, is replaced by –OH group.An alcohol contains one or more hydroxyl (OH) group(s) directly attached to carbon atom(s), of an aliphatic system (CH3OH) while a phenol contains –OH group(s) directly attached to carbon atom(s) of an aromatic system (C6H5OH).The substitution of a hydrogen atom in a hydrocarbon by an alkoxy or aryloxy group (R–O/Ar–O) yields another class of compounds known as ‘ethers’, for example, CH3OCH3 (dimethyl ether).
11.1 Classification11.1.1 Mono, Di, Tri or Polyhydric Compounds
(i) Compounds containing C sp3 − OH bond:Primary, secondary and tertiary alcohols:Allylic alcohols:
Benzylic alcohols:(ii) Compounds containing C sp2 − OH bond:
11.1.2 EthersEthers are classified as simple orsymmetrical, if the alkyl or aryl groupsattached to the oxygen atom are the same,and mixed or unsymmetrical, if the twogroups are different. Diethyl ether,C2H5OC2H5, is a symmetrical etherwhereas C2H5OCH3 and C2H5OC6H5 areunsymmetrical ethers.
11.2 Nomenclature(a) Alcohols:The common name of an alcohol is derived from thecommon name of the alkyl group and adding the wordalcohol to it. For example, CH3OH is methyl alcohol.Accordingto IUPAC system (Unit 12, Class XI), the name of analcohol is derived from the name of the alkane fromwhich the alcohol is derived, by substituting ‘e’ of alkanewith the suffix ‘ol’. The position of substituents areindicated by numerals. For this, the longest carbon chain(parent chain) is numbered starting at the end nearest tothe hydroxyl group. Cyclic alcohols are named using the prefix cycloand considering the —OH group attached to C–1.
(b) Phenols:The simplest hydroxy derivative of benzeneis phenol. It is its common name and also anaccepted IUPAC name. As structure ofphenol involves a benzene ring, in itssubstituted compounds the terms ortho (1,2-disubstituted), meta (1,3-disubstituted) andpara (1,4-disubstituted) are often used in thecommon names.
(c) Ethers:Common names of ethers are derived from the names ofalkyl/ aryl groups written as separate words in alphabeticalorder and adding the word ‘ether’ at the end. For example,CH3OC2H5 is ethylmethyl ether.
11.3 Structures of Functional GroupsInalcohols, the oxygen of the –OH group is attached tocarbon bya sigma (σ ) bond formedby the overlap of a sp3 hybridised orbital of carbonwith a sp3 hybridised orbital of oxygen. Fig. depictsstructural aspects of methanol, phenol andmethoxymethane.
11.4 Alcohols and Phenols11.4.1 Preparation of Alcohols 1.From alkenes (i) By acid catalysed hydration: Alkenes react with water in the presence of acid as catalyst to form alcohols. In case of unsymmetrical alkenes, the addition reaction takes place in accordance with Markovnikov’s rule
(ii) By hydroboration–oxidation:Diborane (BH3)2 reacts with alkenes to givetrialkyl boranes as addition product. This isoxidised to alcohol by hydrogen peroxide in thepresence of aqueous sodium hydroxide.
(ii) By reduction of carboxylic acids and esters:Carboxylic acids are reduced to primary alcohols inexcellent yields by lithium aluminium hydride, astrong reducing agent.
11.4.2 Preparation of Phenols1. From haloarenesChlorobenzene is fused with NaOH at 623Kand 320 atmospheric pressure. Phenol isobtained by acidification of sodium phenoxideso produced.
2. From benzenesulphonic acidBenzene is sulphonated with oleum andbenzene sulphonic acid so formed is convertedto sodium phenoxide on heating with moltensodium hydroxide. Acidification of the sodiumsalt gives phenol.
3. From diazonium saltsA diazonium salt is formed by treating anaromatic primary amine with nitrous acid(NaNO2 + HCl) at 273-278 K. Diazoniumsalts are hydrolysed to phenols by warmingwith water or by treating with dilute acids
4. From cumenePhenol is manufactured from the hydrocarbon,cumene. Cumene (isopropylbenzene) is oxidised inthe presence of air to cumene hydroperoxide. It isconverted to phenol and acetone by treating it withdilute acid. Acetone, a by-product of this reaction, isalso obtained in large quantities by this method.
2. From carbonyl compounds (i) By reduction of aldehydes and ketones:Aldehydes and ketones are reduced to the correspondingalcohols by addition of hydrogen in the presence of catalysts(catalytic hydrogenation). The usual catalyst is a finelydivided metal such as platinum, palladium or nickel. It is alsoprepared by treating aldehydes and ketones with sodiumborohydride (NaBH4) or lithium aluminium hydride(LiAlH4). Aldehydes yield primary alcohols whereas ketonesgive secondary alcohols.
11.4.3 Physical PropertiesReplacement of hydrogen with a hydroxyl group greatly changes properties.Not as homologous as alkanes.Higher boiling points.Due to strong intermolecular attractions.Solubility:-The more compact the molecule is, the more soluble it is.4-5 carbons or less—soluble in water.
11.5 Some Commercially Important Alcohols1.MethanolMethanol, CH3OH, also known as ‘woodspirit’, was produced by destructive distillationof wood. Today, most of the methanol isproduced by catalytic hydrogenation of carbonmonoxide at high pressure and temperature andin the presence of ZnO – Cr2O3 catalyst.
2. EthanolEthanol, C2H5OH, is obtained commercially byfermentation, the oldest method is from sugars. Thesugar in molasses, sugarcane or fruits such as grapesis converted to glucose and fructose, (both of whichhave the formula C6H12O6), in the presence of anenzyme, invertase. Glucose and fructose undergofermentation in the presence of another enzyme,zymase, which is found in yeast.
11.6 Ethers11.6.1 Preparation of Ethers1. By dehydration of alcoholsAlcohols undergo dehydration in the presence of protic acids(H2SO4, H3PO4). The formation of the reaction product,alkene or ether depends on the reaction conditions. Forexample, ethanol is dehydrated to ethene in the presence ofsulphuric acid at 443 K. At 413 K, ethoxyethane is the mainproduct.
2. Williamson synthesisIt is an important laboratory method for thepreparation of symmetrical and unsymmetricalethers. In this method, an alkyl halide isallowed to react with sodium alkoxide.
11.6.2 Physical PropertiesThe C-O bonds in ethers are polar and thus,ethers have a net dipole moment.The miscibility of ethers with waterresembles those of alcohols of the samemolecular mass.