Organic Compounds Oxygen - I

Introduction

     Alcohols, phenols, ethers, aldehydes, ketones and carboxylic acids are compounds with functional groups containing oxygen. An alcohol contains one or more hydroxyl (OH) groups(s) attached to aliphatic carbon atom(s). For e.g., ethyl alcohol formula CH₃, CH₂, OH is an alcohol.

Classification of Alcohols and Ethers

     Alcohols end phenols are classified as mono-, di- and trihydric alcohols according to the number of -OH grounds contained in their molecules.
     

Nomenclature of Alcohols

     In the common system, alcohols are named as alkyl alcohols. The word alcohol is added after the name of the alkyl group to which the hydroxyl group is attached. For e.g., CH₃OH is methyl alcohol.

Nomenclature of Phenols

     Phenols are named as derivatives of the simplest compound of this class i.e., phenol.
     Examples:
                          

Nomenclature of Ethers

     Common names of ethers follow after the names of alkyl / aryl groups written as separate words in alphabetical order. The word ether is added at the end.

Structure of Functional Groups

     In alcohol, the oxygen of the -OH group is attached to sp³ hybridized carbon by a sigma (s) bond formed by the overlap of sp³ hybrid orbital of carbon with an sp³ hybrid orbital of oxygen.

Isomerism in Alcohols and Ethers

     Alcohols with four or more carbon atoms exhibit this type of isomerism in which the carbon skeleton is different.
              

Isomerism in Aliphatic Ethers

     Ethers with the same formula and having different carbon chain skeletons are called chain isomers.
     Examples:
     

Preparation of Alcohols

     Aldehydes and ketones are reduced to the corresponding alcohols by
     a) Addition of hydrogen in the presence of catalysts like finely divided platinum, palladium, nickel and ruthenium.
     b) Treatment with chemical reagents such as sodium borohydride (NaBH₄) or Lithium aluminium hydride (LiAlH₄).

Preparation of Alcohols (Continued)

     Alkenes undergo hydration (addition of water across C=C bond) in the presence of dilute H₂SO₄ to produce alcohols. The alkyl hydrogen sulphate is formed which on hydrolysis with hot water gives alcohol.

Preparation of Phenols

     In the early nineteenth century, phenol was selected from coal tar by destructive distillation. Now phenol is commercially produced synthetically.

Physical Properties of Alcohols and Phenols

     Alcohols and Phenols consist of two parts, an alkyl/aryl group and a hydroxyl group. The properties of alcohols and phenols are due to the -OH group.

Boiling Point of Alcohols and Phenols

     Boiling point of alcohols are much higher than those of alkenes, halo alkenes or ethers of comparable molecular masses. This is because in alcohols intermolecular hydrogen bonding exists due to which a large amount of energy is required to break these bonds.

Chemical Properties of Alcohols and Phenols

     In alcohols and phenols, -OH group is the functional group. Thus the chemical properties of alcohols generally involve the reactions of -OH group. They can undergo substitution as well as elimination reaction.

Acidity of Phenols

     Phenols turn blue litmus red and react with metals liberating hydrogen. However they do not react with carbonates or bicarbonates.

Phenoxide

     Phenoxide is more stabilised by resonance than phenol. In phenol, three contributing structures involve charge separation whereas in case of phenoxide ion there is no charge separation.

Esterification of Alcohols and Phenols

     Alcohols and Phenols react with carboxylic acids, acid chlorides and acid anhydrides to form esters. The reactions between alcohols and acids are carried out in the presence of a small amount of concentrated sulphuric acid. H₂SO₄ acts as a protonating agent as well as a dehydrating agent. This reaction is called esterification.

Reaction Involving Cleavage of Carbon-Oxygen (C-O) Bond

     Alcohols react with hydrogen halides according to the following equations.
     

Dehydration

     Alcohols undergo dehydration to form alkenes (removal of a molecule of water on treating them with a protonic acid).

Oxidation

     Oxidation of alcohols involves the formation of a carbon oxygen double bond with cleavage of an O-H and C-H bonds.
     

Dehydrogenation

     When the vapours of a primary or secondary alcohol are passed over copper heated at 573 K, an aldehyde or ketone is formed.

Electrophlic Aromatic Substitution

     The -OH group attached to the benzene ring in phenol activates, it towards electrophilic substitution. It directs the incoming group to ortho and para positions in the ring as these positions become electron rich due to electronic effect (also mesomeric effect) caused by -OH group.
     

Halogenation

     On treating phenol with bromine, different reaction products are formed. When the reaction is carried out in a solvent of low polarity such as CHCl₃ or CS₂ and at low temperature, monobromo phenols are formed.
     

Kolbe's Reaction

     On reaction sodium salt of phenol with carbon dioxide gas, ortho hydroxy benzoic acid is formed as the main product. The temperature is 400 K and a pressure of 4-7 atm is required. Sodium salicylate is formed which an acidification yields salicylic acid (ortho hydroxy benzoic acid).

Reimer - Tiemenn Reaction

     On treating phenol with chloroform in presence of sodium hydroxide at 340 K, a - CHO group is introduced at ortho position of benzene ring. This reaction is known as Reimer - Tiemenn reaction. This results in the formation of O - hydroxybenzaldehyde (salicylaldehyde) and para-hydroxybenzaldehyde, the ortho isomer being the major product.

Fries Rearrangement

     Esters of phenols yields phenolic ketones on treatment with anhydrous aluminium chloride.
     
     

Preparation of Ethers

     This method involves heating of excess of primary alcohol with concentrated sulphuric acid at 413 K to get symmetrical ether.

Physical Properties

     Dimethylether and ethyl methyl ether are gases at ordinary temperature. The other lower homologes are colourless, pleasant smelling, volatile liquids with typical ether smell.

Chemical Properties

     Ethers are relatively inert compounds inspite of the presence of oxygen atom carrying two lone pairs of electrons in their molecules. These are not easily attacked by alkalis, dilute mineral acids, metallic sodium, PCl₅ etc. under ordinary conditions. It is because of this reason that these are used as solvents.

Methanol

     Methanol CH₃OH also known as wood alcohol can be produced by destructive distillation of wood.

Ethanol

     Ethanol (C₂H₅OH) is obtained commercially by fermentation.

Ethane-1,2 diol (Ethylene glycol)

     A dihydric alcohol, ethanediol is prepared commercially by acid hydrolysis of epoxy ethane (ethylene oxide).
     

Propane-1,2,3-trial (Glycerol)

     A trihydric alcohol, glycerol is a constituent of oils and fats. Oils and fats are tri esters of long chain fatty acids. On hydrolosys in presence of an alkali, the tri esters yield glycerol and the fall of the carboxylic acids.
     

Phenol

     A trihydric alcohol, glycerol is a constituent of oils and fats. Oils and fats are tri esters of long chain fatty acids. On hydrolosys in presence of an alkali, the tri esters yield glycerol and the fall of the carboxylic acids.
     

Ethers

     The chemically uncreative nature of ethers and the low cost of ethoxythane make it an important ether. Ethoxy ethane is a solvent for oils, gums, resins etc and is used as an inhalation anaesthetic. But now it is widely replaced due to unpleasant effects.

Summary

     Alcohols and Phenols are compounds containing -OH as the functional groups. In alcohols, the -OH group is attached to sp³ hybridised carbon. In phenols, it is attached to sp² hybridised carbon of an aromatic ring. In ethers, oxygen atom is attached to two carbons atoms by two separate sigma bonds.