Title 2A-alcohols, ethers & phenols_FINAL_06-03-14 ( 71 - 104 ).pdf ✅

SOLUTIONS AND COLLIGARIVE PROPERTIES JEE ADVANCED-VOL - VIII RELATIONS ALCOHOLS, ETHERS & PHENOLS NISHITH Multimedia India (Pvt.) Ltd., 71 1. What are Alcohols Alcohols are aliphatic hydroxyl compounds and can be regarded as derivatives of alkanes where one or more hydrogens have been replaced by – OH groups. Structures and geometry of alcohols can be explained both on the basis of valence bond and molecular orbital theories C O H R R" R' The oxygen atom of hydroxyl group has two non-bonding electron pairs, apart from two bond pairs. The oxygen atom is sp3 hybridized and so C—O—H bond, angle is little less than 109o 28’. 2. Nomenclature and Classification There are two ways, alcohols are classified. One of them is on the basis of number of —OH groups. Monohydric alcohol : R—CH2 OH Dihydric alcohol : R CH OH CH OH R' Poly-hydric alcohol : R CH R' OH n The other is based on the kind of carbon to which —OH is attached. Primary alcohol : R—CH2—OH Secondary alcohol : CH OH R R' Tertiary alcohol : C OH R R" R' Nomenclature of alcohols can be done in various ways. a) Traditional method H3C OH Ethyl alcohol CH3 H3C HO iso propryl alcohol H3C H3C OH Iso-butyl alcohol OH H3C CH3 H3C tert-butyl alcohol HO OH ethylene glycol CH2OH benzyl alcohol ALCOHOLS, ETHERS & PHENOLS
JEEASD MAINS - VOL - I ALCOHOLS, ETHERS & PHENOLS JEE ADVANCED-VOL - VIII 72 NISHITH Multimedia India (Pvt.) Ltd., b) IUPAC Method According to this method, alcohols are named as alkanols. Each name is derived by replacing the terminal –e of the corresponding alkane by – ol. H3C OH methanol H3C CH3 OH butan-2-ol H3C OH CH3 2-methylpropan-1-ol H3C OH CH3 H3C 2-methylpentan-2-ol CH OH CH3 1 phenyl ethanol If an organic compound has a functional group of higher priority then, the presence of —OH group is prefixed as — hydroxyl. H3C CH COOH OH : Lactic acid: common name : 2 Hydroxypropanioc acid : IUPAC name H3C C OH O 1-Hydroxypropanone c)Carbinol method Alcohols are considered to be derivative of methanol (CH3OH), the simplest alcohol. The other name of methanol is carbinol and so in the nomenclature an alcohol’s name is derived as a carbinol. H3C OH methyl carbinol O H3C H3C H dimethyl carbinol CH2OH cyclopentyl carbinol 3. Preparation of Alcohols i) Hydrolysis of alkyl halides: Haloalkanes on hydrolysis gives alcohols either by SN1 or by SN2 pathway. R CH X R CH OH      2 2 The reagent used is aqueous KOH or moist silve oxide. This method of preparation is not satisfactory as alkenes are formed as side products when strong bases are used. ii)Acid catalysed hydration of alkenes: Alkenes add water in the presence of an acid catalyst to yield alcohol. The addition takes palce according to markovnikov regioselectivity. The reaction is reversible and the mechanism for the acid-catalysed hydration of an alkene is simply the reverse of that for the dehydration of an alcohol. -  A H H O + H H H O H  HA Alcohol alkene HA   2 2 H O H O A      
SOLUTIONS AND COLLIGARIVE PROPERTIES JEE ADVANCED-VOL - VIII RELATIONS ALCOHOLS, ETHERS & PHENOLS NISHITH Multimedia India (Pvt.) Ltd., 73 Acid catalysed hydration of alkene has limited synthetic utility because carbocation is the intermediate and rearrangement may take place. Thus a mixture of isomeric alcohols may result. iii)Oxymercuration Demercuration: Alkenes react with mercuric acetate in a mixture of water and tetrahydrofuran (THF) to produce (hydroxyalkyl) mercury compounds. These can be reduced to alcohols with sodium borohydride and water. a) Oxymercuration THF C C H O Hg(OCCH ) — C — C — CH COOH      2 3 2 3 O HO Hg — O — C — CH3 O b)Demercuration HO Hg — O — C — CH3 O HO H —C — C — OH NaBH — C — C — Hg 4     The net addition of H— and —OH takes place with markovnikov regioselectivity and takes place without the complications of rearrangements. iv)Hydroboration-oxidation: An alkene reacts with BH3 : THF or diborane to produce an alkylborane. Oxidation and hydrolysis of the alkylborane with hydrogen peroxide and base yields an alcohol. H CH3 BH : THF 3 Anti Markovnikov & syn addition  B CH3 H H H H + enantiomer + dialkyl & trialkyl borane B CH3 H H H H H O /OH 2 2 Boron gr. is replaced with retention of configuration   OH CH3 H H + enantiomer H3C 3 2 2 BH /THF H O /OH  H CH3 OH H
JEEASD MAINS - VOL - I ALCOHOLS, ETHERS & PHENOLS JEE ADVANCED-VOL - VIII 74 NISHITH Multimedia India (Pvt.) Ltd., The net addition of H— and —OH occurs with anti-markonikov regio-selectivity and syn stereoselectivity. v)Hydroxylation of alkenes  KMnO4 — C — C — RCO H3 — C — C — — C — C — H O/H 2    HO OH O HO OH syn-hydroxylation anti--hydroxylation 4 3 2 or OsO NaHSO , H O Alkenes can be converted to diols when they are reacted with KMnO4 , OsO4 and peroxocarboxylic acid. H2C CH2 OH OH MnO2  H O2 OH  H2C CH2 1 K MnO4 Cold    Mn O O O O H2C CH2 This reaction is a syn addition. Similarly hydroxylation in presence of OsO4 results in the formation of di-ol where the addition occurs in a syn way. H3C C H C H3C H    OsO H O 4 2 CH3 H OH H OH CH3 meso compound vi)Reduction of carboxylic acid and esters: Direct reduction of carboxylic acid is possible with a strong reducing agent like LiAlH4 . R C OH O   LiAlH R — CH OH 4 2 . The product initially formed is an alkoxide which is hydrolysed to give a primary alcohol. Esters can be conveniently reduced by reagents like sodium and ethanol or lithium aluminium hydride to give mixture of alcohols. CH2 C OR' O R 4 LiAlH R — CH OH R OH 2    Esters may also react with Grignard’s reagent to give alcohols. The details of the process is discussed later in this chapter.