Chapter-5 Alcohols, Ethers and Phenols A) Alcohols: Defination: The hydroxyl derivatives of alkanes are called as alcohols, in which one or more hydrogen atoms of alkane are replaced by hydroxyl groups. It is represented as R-OH. Classification of alcohols: Depending upon the number of hydroxyl groups present in the molecule, alcohols are classified as below. Monohydric alcohols: They contain only one hydroxyl group and having general formula CnH2n+1-OH or R- OH where R is an alkyl group. Monohydric alcohols further classified on the basis of carbon atom to which hydroxyl i.e. –OH group is attached. 1) Primary alcohols (10): When –OH gr. is attached to primary carbon atom then it is called as primary alcohols.e.g.- Hence primary alcohols contain –CH2OH group. 2) Secondary alcohols (20): When –OH gr. is attached to secondary carbon atom then it is called as secondary alcohols. e.g.- 3) Tertiary alcohols (30): When –OH gr. is attached to tertiary carbon atom then it is called as tertiary alcohols. E.g.- IUPAC Nomenclature: 1) First select longest carbon chain of parent alkane containing –OH gr. 2) Numbering given in such way that hydroxyl carbon should get lowest number. 3) Last letter ‘e’ of corresponding alkane is changed to ‘old’. Consider following examples- Physical Properties: 1) Alcohols are neutral in nature. 2) Lower members are liquids (C1-C12) & higher members are solid. 3) Solubility: Lower alcohols soluble in water due to formation of hydrogen bonding of alcohols with water. But as we increase molecular weight (higher member) of alcohols its solubility decreases because of increasing hydrocarbon character (non-polar nature). 4) M.P.and B.P.: 1) As carbon chain increases boiling point increases. E.g. Methyl alcohol B.P. 64.5 0C while n-pentyl alcohol boils at 138 0C . 2) As branching increases boiling point of alcohol decreases because intermolecular force of attractions between the molecules decreases. E.g. n- butyl alcohol B.P. 118 0C while t-butyl alcohol B.P. is 83 0C. CH3 H3C OH H3C OH CH 0C 3 n-butyl alcohol B.P. 118 0 t-butyl alcohol B.P. 83 C 3) Boiling point of alcohols higher than hydrocarbons of comparable same molecular weight. E.g. n-pentane (Mol. Wt. 72) B.P. 36 0C, while n-butyl alcohol (Mol. Wt. 74) has B.P. 118 0C . H3C OH 0C n-pentane B.P. 36 n-butyl alcohol B.P. 118 This is because of alcohol molecules associated together strongly by intermolecular hydrogen bonding. Hence alcohol boils at high temp. But incase of alkanes (Hydrocarbons) intermolecular force of attraction weak therefore hydrocarbons has lower B.P. Preparation of Alcohols: We can prepare alcohols by using following methods. 1) Using Grignard reagent: ( R-Mg-X) By using Grignard reagent we can prepare primary, secondary & tertiary alcohols. During this preparation aldehydes or ketones treated with Grignard reagent (RMgX) in presence of ether as a solvent first it forms Grignard complex, which on acid hydrolysis we get alcohol. a) Primary alcohol: e.g. preparation of ethyl alcohol. Only formaldehyde reacts with Grignard reagent (e.g. CH3-Mg-Br) followed by hydrolysis we get primary alcohol ethyl alcohol. b b) Secondary alcohol: e.g. preparation of propan-2-ol or isopropanol / isopropyl alcohol. Acetaldehyde on reacted with methyl magnesium bromide as a Grignard reagent we get isopropanol. H3C CH3 CH3 CH3-Mg-Br H2O O H3C C OMgBr H3C C OH + Mg(OH)Br Ether H+ H H H Acetaldehyde Complex Propan-2-ol / Isopropanol c) Tertiary alcohol: e.g. Preparation of t-butyl alcohol / 2-methyl propan-2-ol. All ketones reacted with Grignard reagent we get tertiary alcohol. When acetone treated with methyl magnesium bromide we get t- butyl alcohol. H3C CH3 CH3 CH3-Mg-Br H2O O H3C C OMgBr H3C C OH + Mg(OH)Br Ether H+ H C 3 CH CH Acetone 3 3 Complex 3- mehtyl Propan-2-ol / t- butanol d) Primary alcohol from ethylene oxide: Grignard reagent Ph-Mg-Br reacts with ethylene oxide (epoxide) it forms primary alcohol 2-phenyl ethanol or β-phenyl ethyl alcohol. e) From ester: Ethyl format treated with excess Grignard reagent we get secondary alcohol. e.g. Preparation of isopropanol. 2) Catalytic Reduction of aldehydes/ketones: Aldehyde or ketone on reduction using H2 in presence of catalyst like Pt, Pd or Raney Ni we get corresponding alcohol. Aldehyde gives primary alcohol while ketone gives secondary alcohol. 3) Reduction with sodium amalgam & water/acid: Na-Hg & water can form nascent hydrogen & which reduces aldehydes to primary alcohol and ketone to secondary alcohol. e.g. 4) Reduction with Lithium Aluminum Hydride (LiAlH4): LiAlH4 is a very specific & stronger reducing agent. It reduces saturated/ unsaturated aldehydes or ketones into saturated/ unsaturated alcohols. LiAlH4 does not reduce carbon carbon double bond. In this reaction aldehydes/ketones treated with LiAlH4 in ether as a solvent, first it forms complex & which on treated with dil.HCl to form alcohol. e.g. 5) Reduction of carboxylic acids: LiAlH4 + Carboxylic acid we get complex & which on acid hydrolysis get alcohol. Reactions of alcohols: Two types of reactions take place by alcohol. A) Reactions involving replacement of –OH group: 1. Formation of Alkyl halide (R-X): Alcohol is converted into alkyl halides using reagents like H-X, PX3 & SOCl2 as below. a) Using H-X: Alcohol heated with haloacids like HCl, HBr or HI in presence of anhydrous ZnCl2 we get corresponding alkyl halides. Lucas reagent: Lucas reagent is nothing but a mixture of HCl & anhydrous ZnCl2. Reactivity of H-X decreases in order of HI > HBr > HCl and among the alcohols reactivity order is tertiary > secondary > primary alcohol. b) Using phosphorous halides: (PX3) Alcohol reacts with phosphorous trihalides like PCl3, PBr3 PI3 to get alkyl halides. c) Using Thionyl Chloride ( SOCl2): When alcohol treated with thionyl chloride (SOC2) in presence of pyridine as base we get alkyl halides. Pyridine CH -CH -Cl + SOCl + HCl CH2- CH2- OH + SOCl2 3 2 2 Ethyl chloride Ethyl alcohol d) Iodoform test: Only those alcohols which containing structure as below gives positive iodoform test when it is treated with I2 & NaOH (sodium hypoiodite NaOI). H R OH CH3 e.g. Ethyl alcohol, isopropyl alcohol. 2-pentanol, 1-phenyl ethanol gives positive iodoform test. While methyl alcohol, t-butyl alcohol, 3-pentanol, 2-phenyl ethanol give negative iodoform test. B) Reactions involving replacement of ‘H’atom of –OH group: 1. Reactions with active metals (formation of alkoxides): Alcohols react with active metals like Na, K, Mg, Al etc. it forms alkoxide with liberation of H2 gas. The order of reactivity decreases from CH3OH > primary > secondary > tertiary alcohol. e.g. These alkoxides are used to prepare symmetrical as well as unsymmetrical ethers by Williamson’s synthesis. Williamson’s synthesis: In Williamson’s synthesis alkyl halides are reacted with alkoxide we get ether. Williamson’s synthesis used to prepare symmetrical as well as unsymmetrical ethers. Note: Tertiary alkyl alcohol can not use for preparation of ether because they easily undergo elimination reaction & give alkene instead of ether. H CH3 CH3 Elimination + R-OH + HBr R-O H2C OH H2C CH alcohol CH3 3 alkoxide Alkene t-butyl bromide 2. Esterification: Alcohols react with organic acids in presence of dehydrating agents like conc. H2SO4 or HCl to form ester. This process called as esterification. Reactivity order of alcohols for esterification is CH3OH > primary > secondary > tertiary alcohol. 3. Oxidation of alcohols: Alcohols can oxidize by various following oxidizing agents. The nascent oxygen brings about oxidation. a) Oxidation by PCC (Pyridinium chlorochromate): PCC is special mild oxidizing agent for conversion of primary alcohol to aldehyde. Preparation of PCC: PCC prepared from chromium trioxide (CrO3) + HCl + Pyridine. e.g. Oidation of benzyl alcohol to benzaldehyde. b) Oxidation by alkaline KMnO4: Primary alcohol is oxidized to carboxylic acids in presence of hot KMnO4. When oxidation reaction is complete we get potassium salt of acid & brown ppt. of MnO2 is formed and purple colour of KMnO4 disappears. c) Oxidation by acidic K2Cr2O7 : Using potassium dichromate we can carry out oxidation of primary & secondary alcohols. 1. Primary alcohol oxidation: When primary alcohol treated with K2Cr2O7 / di. H2SO4 first it form aldehyde, further aldehyde again get oxidized by same oxidizing reagent to form carboxylic acid. e.g. Oxidation of ethyl alcohol into acetic acid. 2. Secondary alcohol oxidation: Secondary alcohol on oxidation to form ketone. 3. Tertiary alcohol oxidation: For oxidation of tertiary alcohol we require stronger oxidizing agent like chromium trioxide (CrO3). Tertiary alcohols first oxidized into ketone with less no. of carbon atoms, further ketone again get oxidized into carboxylic acids. e.g. Oxidation of t-butyl alcohol into acetic acid. 4. Oxidation with conc. HNO3 : Oxidation of benzyl alcohol into benzoic acid is carried out by using conc. HNO3. O OH OH Conc. HNO3 + NO2 Benzoic acid Benzyl alcohol 5. Oppenauer Oxidation: Secondary alcohol treated with excess of acetone in presence of aluminum tertiary butoxide catalyst we get ketone. 6. Oxidation of Diols: Pinacol-Pinacolone rearrangement: In this rearrangement migration of an atom or group of atom from one site to another with in same species. e.g. When pinacol ( 2,3-dimethyl-butane 2,3-diol) is treated with dil. H2SO4, we get pinnacolone (ketone). C) Ethers: Ethers are organic compounds containing -C-O-C- linkage. The general formula of ethers CnH2nO. Ether represented by formula R-O-R or Ar-O-Ar or R-O-Ar. Classification of ethers: Two class of ethers. 1. Symmetrical / Simple ether: When both the alkyl groups or both the aryl groups are same it is called symmetrical ether.