Title SP-3_Ch-17_Alcohols, Phenols and Ethers.pdf ✅

Chapter Contents Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 ALCOHOLS AND PHENOLS : CLASSIFICATION Alcohols and phenols may be classified as mono, di, tri-or polyhydric compounds depending upon number of hydroxyl groups respectively in their structures as given below: Monohydric : CH3OH, C2H5OH, OH Dihydric : CH OH CH OH 2 2 OH OH Trihydric : CH OH CHOH CH OH 2 2 OH OH OH Monohydric alcohols may be further classified according to the hybridisation of the carbon atom to which the hydroxyl group is attached. Introduction This group of compounds reflects its importance in a large segment of our daily utilities. Ethanol is the main component of alcoholic drink, Glycerol which is a trihydric alcohol is used in various cosmetic formulations. Phenols are largely used in antiseptic formulations and ethers are basic ingredient for fragrances due to their higher volatility. Interestingly, in the view of chemical bonding all these molecules have same pattern of bonding as in water, all of them possess sp3 hybridised oxygen as central atom, this is the reason to put these different functional groups together.  Introduction  Alcohols and Phenols : Classification  Nomenclature  Alcohols and Phenols  Preparation of Alcohols  Preparation of Phenols  Some Commercially Important Alcohols  Ethers  Preparation of Ethers Chapter 17 Alcohols, Phenols and Ethers
188 Alcohols, Phenols and Ethers NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 (i) Compounds Containing C OH 3  sp bond : (a) Primary, secondary and tertiary alcohols : In these three types of alcohols, the –OH group is attached to 1oC, 2oC and 3oC atom respectively. CH OH (1o) 2 CH – OH (2o) C – OH (3o) (b) Allylic alcohols : In these alcohols, the –OH group is attached to a sp3 hybridized carbon next to the carbon-carbon double bond. CH = CH – CH – OH 2 2 Primary CH = CH – C – OH 2 H CH3 Secondary CH = CH – C – OH 2 CH3 CH3 Tertiary (c) Benzylic alcohols : In these alcohols, the –OH group is attached to a benzylic carbon atom. CH OH 2 Primary C – OH Secondary Tertiary H CH3 C – OH CH3 CH3 (ii) Compounds Containing C OH 2  sp bond : These alcohols contain –OH group bonded to a carbon- carbon double bond i.e., vinylic carbon or to an aryl carbon. Ex. : Vinylic alcohol : CH2 = CH – OH Phenols : OH NOMENCLATURE (a) Alcohols : The common name of an alcohol is derived from the common name of the alkyl group and adding the word alcohol to it. Ex. : CH3OH : Methyl alcohol CH3CH2OH : Ethyl alcohol CH – CH – OH 3 CH3 : Isopropyl alcohol CH – C – CH – OH 3 2 CH3 CH3 : Neopentyl alcohol IUPAC-name of an alcohol is derived from the name of the alkane and adding suffix ‘ol’ while the letter ‘e’ of the alkane is dropped CH3 – OH Methane + ol = Methanol CH3CH2 – OH Ethane + ol = Ethanol 4321 CH CH CH CH OH 3222  Butane + ol = Butan-1-ol (n-butylalcohol)
NEET Alcohols, Phenols and Ethers 189 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 CH – C – CH 3 3 OH CH3 3 21 2-methylpropane + ol = 2-Methylpropan-2-ol (tert-butylalcohol) For naming of polyhydric alcohols, the ‘e’ of alkane is retained and the ending ‘ol’ is added. The number of –OH groups is indicated by adding the multiplicative prefix, di, tri, etc. before ‘ol’. Ex. : HO – CH2 – CH2 – OH ethane-1, 2-diol (Ethylene glycol) OH CH – CH – CH 2 2 OH OH 3 21 propane-1, 2, 3-triol (Glycerol) Cyclic alcohols are named using the prefix cyclo and considering the –OH group attached to C–1 OH CH3 2-Methylcyclopentanol OH Cyclopentanol (b) Phenols : The simplest hydroxy derivative of benzene is phenol OH CH3 CH3 OH OH Phenol 2-Methylphenol 3-Methylphenol Dihydroxy benzene derivatives are named as 1, 2-, 1, 3- and 1, 4-benzenediol OH OH OH OH OH (Catechol) Benzene-1, 2-diol (Resorcinol) Benzene-1, 3-diol (Hydroquinone or quinol) Benzene-1, 4-diol OH Meta trihydroxy benzene derivative is known as phlorogucinol OH HO OH Structures of functional groups : In alcohols, the oxygen of –OH group is attached to carbon by a  bond formed by the overlap of a sp3 hybridized orbital of carbon with a sp3 hybridized orbital of oxygen. C H H O H 108.9o O 109o H H 142 pm 136 pm 96 pm The bond angle in alcohol is slightly less than the tetrahedral angle. It is due to the repulsion between the unshared electron pairs of oxygen. The carbon-oxygen bond length in phenol is slightly less than that in methanol. This is due to (i) Partial double bond character (ii) sp2 hybridised state of carbon to which oxygen is attached
190 Alcohols, Phenols and Ethers NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 ALCOHOLS AND PHENOLS Preparation of Alcohols 1. From Alkenes : (A) By acid reflux followed by hydrolysis : Alkenes react with water in the presence of acid as catalyst. The addition reaction takes place in accordance of Markovnikov’s rule. Ex. RCH CH2 RCH CH + 3 RCH CH OH 2 2 OH (major) (ii) H O2 (i) H SO 2 4 (minor) Mechanism : Step I : Protonation of alkene H2O + H+  H3O+ CH3 H C = CH + H – O – H 2 + H CH – C – C 3 CH – C – C 3 H H H H H H + + H O2 H H (Less stable) (More stable) Step II : Nucleophilic attack of H2O on carbocation CH – C – CH + H O 3 32 H (more stable) CH – C – CH 3 3 H O H H Step III : Deprotonation to form an alcohol CH – C – O 3 + H O2 H CH – C – OH 3 H CH3 H H CH3 + H O3 + Major product comes from more stable intermediate. (B) By hydroboration-oxidation : Diborane reacts with alkenes to give trialkyl boranes, which is oxidised to alcohol by H2O2 in alkaline medium to give alcohol. RCH CH2 RCH CH OH 2 2 (i) B H /TH F 2 6 (ii) H O /OH 2 2 – Mechanism : RCH CH2 RCH – CH2 H – BH2 RCH2CH BH 2 2 RCH CH2 (R CH ) 3R CH CH OH + B(OH) 2 3 2 CH B 2 3 2 H O /OH 2 2 – (R CH ) CH BH 2 2 2 -Complex (syn addition) (major) BH3 trialkyl borane RCH = CH2  Here anti-Markovnikov addition product is major product. Addition to diborane to double bond is syn selective  In this reaction alcohol is obtained in excellent yield.  Less hindered double bonds can be selectively reduced by this method.