Alcohols, Phenols and Ethers
Preparation of Alcohols
Alcohols are the hydroxy derivatives of aliphatic hydrocarbons.
Alcohols are classified as mono–, di–, tri- or polyhydric compounds depending on the basis of presence of one, two, three or many hydroxyl groups respectively in them.
On the basis of the hybridisation of the carbon atom to which the hydroxyl group is attached, the monohydric alcohols can be primary, secondary, tertiary, allylic, benzylic and vinylic.
In allylic alcohols, the —OH group is attached to a sp3 hybridised carbon next to the carbon-carbon double bond (an allylic carbon).
In common system, monohydric alcohols are named as alkyl alcohols. In the IUPAC system, monohydric alcohols are named as alkanols.
Alcohols are polar covalent molecules.
Alcohols can be prepared from alkenes (by hydration of alkene in presence of acid or by hydroboration oxidation), from carbonyl compounds (by reduction of aldehydes and ketones or by reduction of carboxylic acids and esters) and by the action of Grignard reagent on the aldehyde or ketone.
Properties of Alcohols
The boiling points of alcohols increase with increase in the number of carbon atoms. This is due to the increase of van der waals forces of attraction. The boiling points of alcohols are higher than corresponding hydrocarbons of comparable molecular mass because the –OH group in alcohols is involved in intermolecular hydrogen bonding which facilitates the association of molecule leading to increase in boiling points.
Alcohols are soluble in water due to their ability to form hydrogen bonds with water molecules. The solubility of alcohols in water decreases with increase in size of alkyl (hydrophobic) groups.
Alcohols act both as nucleophiles and electrophiles. On the basis of cleavage of bond, the reactions of alcohols can be divided into two groups: reactions involving breaking of O–H bond and the reactions involving cleavage of C–O bond in alcohols
When the bond between O–H breaks, alcohols react as nucleophiles and when the bond between C–O breaks, alcohols react as electrophiles.
Chemical properties of alcohols involve dehydration to form alkenes when treated with a protic acid e.g., concentrated H2SO4, oxidation to form carbonyl compounds, esterification reactions to form esters, catalytic dehydrogenation of primary or a secondary alcohol to form aldehydes or ketones (tertiary alcohols undergo dehydration)
and nucleophilic substitution with hydrogen halides to yield alkyl halides.
Mechanism of dehydration of alcohols involves formation of protonated alcohol, formation of carbocation and formation of ethane by elimination of a proton.
Tertiary alcohols do not undergo oxidation reaction. With strong oxidising agents (KMnO4) and at elevated temperatures, cleavage of various C–C bonds takes place and a mixture of carboxylic acids containing lesser number of carbon atoms is formed.
The acidic nature of alcohols is due to the polarity of the O–H bond. An electron-releasing group increases the electron density on oxygen atom, which decreases the polarity of O-H bond. This decreases the acid strength.
Alcohols are weaker acids than water.
Alcohols react with Grignard reagent to form alkanes.
Primary, secondary and tertiary alcohols can be distinguished by the Oxidation test, Dehydration test, and Lucas test (conc. HCl and ZnCl2).
Methanol and Ethanol
Methanol and ethanol are the chemical compounds whose molecular formulae are CH3OH and C2H5OH respectively.
Methanol is also known as ‘wood spirit’ and is produced by catalytic hydrogenation of carbon monoxide or water gas at high pressure and temperature whereas ethanol is also known as grain alcohol and is obtained commercially by fermentation of cane sugar juice.
Methanol is highly poisonous and its intake even in small quantities can cause blindness and when consumed in large quantities it can even lead to death.
Methanol is used as a solvent in paints, varnishes and chiefly for making formaldehyde. Similarly, ethanol is used as a solvent in paint industry and in the preparation of many aliphatic carbon compounds.
Ethanol is drinking alcohol and it is made unfit for drinking by mixing methanol, copper sulphate (to impart it a colour) and pyridine (a foul smelling liquid). This process is known as denaturation of alcohol.
Phenols are hydroxy derivatives of aromatic hydrocarbons in which the hydroxyl (–OH) group is directly attached to the carbon atom of an aromatic ring.
In phenols, the –OH group is attached to sp2 hybridised carbon of aromatic ring. The carbon– oxygen bond length in phenol is slightly less than that in alcohol due to the partial double bond character.
Phenol is also known as carbolic acid. 1,2 substituted compound is called ortho, 1,3 as meta and 1,4 as para derivative of phenol.
Phenols are stronger acids than alcohols and water.
Phenol is prepared from haloarenes, Benzenesulphonic acid, Diazonium salts and Cumene (isopropyl benzene).
Due to the presence of intermolecular hydrogen bonding, boiling point of phenols is higher than the corresponding aromatic hydrocarbons and haloarenes.
Solubility of phenols is lower than that of alcohols due to the larger hydrocarbon part, i.e., benzene ring (water repelling). The solubility of phenols in water is due to their ability to form hydrogen bonds with water molecules.
Phenols undergo electrophilic aromatic substitution reactions such as Nitration and Halogenation.
2,4,6-trinitrophenol is called picric acid.
On treating phenoxide ion with CO2 (weak electrophile), salicylic acid is formed. This is called Kolbe’s Reaction.
When phenol is treated with chloroform in the presence of sodium hydroxide, a –CHO group is introduced at ortho position in benzene ring. This is called Reimer-Tiemann reaction.
Reaction of phenol with zinc dust produces benzene. When phenol is oxidized with chromic acid benzoquinone is produced.
Ethers are the organic compounds having general formula R–O–R’. Here, R= alkyl/aryl group. Ethers are obtained by substitution of a hydrogen atom in a hydrocarbon (aliphatic or aromatic) by an alkoxy or aryloxy group. Structure of an ether molecule is similar to that of water molecule.
On the basis of alkyl/aryl groups present on both sides of the divalent oxygen atom are same or different, ethers are classified in two categories, Simple or symmetrical ethers and Unsymmetrical ethers.
The common names of ethers are written as separate words in alphabetical order and adding the word ether at the end.
In IUPAC system, ethers are named as alkoxy alkane. In this case larger alkyl group is named as parent hydrocarbon.
Ethers have four electron pairs, the two bond pairs and two lone pairs of electrons on oxygen arranged almost in a tetrahedral arrangement.
Ethers can be prepared either by dehydration of alcohols or by Williamson synthesis.
In Williamson synthesis, alkyl halide is treated with sodium alkoxide to form ether.
Unsymmetrical ethers can also be prepared by Williamson synthesis. Better yield of ether are obtained if the alkyl halide is primary.
Absence of intermolecular hydrogen bonding in between ether molecules accounts for their lower boiling points than those of isomeric alcohols.
Ethers are relatively less reactive than alcohols and phenols. The cleavage of C-O bond in ethers takes place under drastic conditions with excess of hydrogen halides.
The alkoxy group (-OR) is ortho, para directing and it activates the aromatic ring towards electrophilic substitution. Ethers undergo Halogenation, Friedel-crafts reaction and Nitration.