1. Introduction: Alcohols and phenols are the compounds containing one or more hydroxyl groups (—OH). The alcohols contain the —OH group attached to the alkyl group whereas in phenols, the —OH group is attached to the aromatic ring.
2. Common and IUPAC names of some alcohols:
Table 11.1: Common and IUPAC Names of Some Alcohols
Common Name | Structural Formula | IUPAC Name |
Ethyl alcohol | CH3—CH2—OH | Ethanol |
n-Butyl alcohol | CH3—CH2—CH2—CH2—OH | Butan-1-ol |
sec-Butyl alcohol |
| Butan-2-ol |
Isobutyl alcohol |
| 2-Methylpropan-1-ol |
tert-Butyl alcohol |
| 2 -Methylpropan-2-ol |
neo-Pentyl alcohol |
| 2, 2-Dimethylpropan-1-ol |
tert-Pentyl alcohol |
| 2-Methylbutan-2-ol |
Some more examples of alcohols are given below:
In IUPAC system, dihydric alcohols are named as alkanediols and trihydric alcohols are named as alkanetriols.
The simplest hydroxy derivative of benzene is phenol. The substituted phenols are named as derivatives of phenol.
Methyl phenols are known as cresols.
Some dihydric and trihydric phenols are given below:
3. Structure of R—OH: In alcohols, both the carbon atoms of R, e.g., —CH3 and oxygen atom of —OH groups are sp3 hybridised. The structure of CH3—OH is shown below.
The C—OH bond angle is slightly less than tetrahedral angle (109°28′) due to repulsions between lone pairs of electrons of oxygen atom. The C—O and O—H bonds are polar because of high electronegativity of oxygen atom.
In phenols, the –OH group is attached to sp2 hybridised carbon of an aromatic ring. The carbon-oxygen bond length (136 pm) in phenol is slightly less than that in methanol. This is due to (i) partial double bond character on account of the conjugation of unshared electron pair of oxygen with the aromatic ring and (ii) sp2 hybridised state of carbon to which oxygen is attached.
4. Methods of Preparation
Alcohols are prepared by the following methods:
(a) From alkenes
(i) By acid catalysed hydration:
Addition occurs according to Markovnikov’s rule.
(ii) By hydroboration oxidation:
Addition occurs in accordance with anti-Markovnikov’s rule.
(b) From carbonyl compounds
(i) By reduction of aldehydes and ketones:
Common reducing agents used are lithium aluminium hydride (LiAlH4), sodium borohydride (NaBH4), H2 in the presence of Ni or Pt. Aldehydes on reduction give 1° alcohols whereas ketones on reduction give 2° alcohols.
(ii) By reduction of carboxylic acids and esters:
(c) From Grignard’s reagents
The reaction of Grignard’s reagents with formaldehyde produces a primary alcohol, with other aldehydes it produces secondary alcohols and tertiary alcohols with ketones.
5. Physical Properties of Alcohols:
(a) Boiling points: Boiling points of alcohols are much higher than those of alkanes, haloalkanes or ethers of comparable molecular masses. This is because in alcohols strong intermolecular hydrogen bonding exists due to which a large amount of energy is required to break these bonds.
Among isomeric alcohols, the boiling point decreases with increase in branching in the alkyl group (due to decrease in van der Waals forces with decrease in surface area). For isomeric alcohols, the boiling points generally follow the order:
Primary alcohol > Secondary alcohol > Tertiary alcohol.
(b) Solubility of alcohols: The first three members are completely miscible with water. The solubility rapidly decreases with increase in molecular mass. The higher members are almost insoluble in water but are soluble in organic solvents like ether, benzene, etc. The solubility of lower alcohols in water is due to their ability to form hydrogen bonds with water molecules.
The solubility of alcohols in water decreases with increase in molecular mass because with increase in molecular mass the non-polar alkyl group becomes predominant and masks the effect of polar —OH group. In addition, among the isomeric alcohols the solubility increases with branching of chain. It is because the surface area of non-polar part in the molecule decreases, thus enhancing the solubility.
6. Chemical Properties of Alcohols:
(a) Reactions involving cleavage of oxygen–hydrogen (O—H) bond
Acidity of alcohols: The acidic character of alcohols is due to the polar nature of O—H bond. An electron releasing group (alkyl group) increases electron density on oxygen tending to decrease the polarity of O—H bond. This decreases the acid strength. For this reason, the acid strength of alcohols decreases in the following order:
(i) Reaction with metals:
(ii) Esterification:
(b) Reactions involving cleavage of carbon–oxygen (C—O) bond
In such type of reactions, the order of reactivity of alcohols is
3°Alcohol > 2°Alcohol > 1°Alcohol
(i) Reaction with hydrogen halides:
(ii) Reaction with phosphorus halides:
(iii) Reaction with thionyl chloride:
(iv) Dehydration: The ease of dehydration follows the following order:
3° Alcohol > 2° Alcohol > 1° Alcohol
(v) Oxidation: Oxidation of alcohols involves the formation of a carbon–oxygen double bond with the cleavage of an O—H and C—H bond.
1° alcohols on oxidation give aldehydes which on further oxidation give carboxylic acids with the same number of carbon atoms.
2° Alcohols are oxidised to ketones by CrO3.
3° alcohols do not undergo oxidation reaction as they do not have a-hydrogens. However, when oxidation is carried under strong reaction conditions and elevated temperature, they undergo oxidation with the cleavage of C—C bond.
(vi) Dehydrogenation
Primary alcohols are dehydrogenated to aldehydes.
Secondary alcohols are dehydrogenated to ketones.
Tertiary alcohols undergo dehydration to give alkenes.
7. Preparation of Phenols:
(i) From aryl halides
(ii) From benzenesulphonic acid
(iii) From diazonium salts
(iv) From Cumene. Cumene is oxidised in air to cumene hydroperoxide. It is converted to phenol and acetone by treating it with dilute acids.
8. Reactions of Phenol:
Acidity of Phenol: The acidity of phenols is due to its ability to lose hydrogen ion to form phenoxide ions In a phenol molecule, the sp2 hybridised carbon atom of the benzene ring attached directly to the hydroxyl group acts as an electron-withdrawing group. This sp2 hybridized carbon atom of a benzene ring attached directly to the hydroxyl group has higher electronegativity in comparison to the hydroxyl group. Due to the higher electronegativity of this carbon atom in comparison to the hydroxyl group attached, electron density decreases on the oxygen atom. The decrease in electron density increases the polarity of O—H bond and results in the increase in ionization of phenols. Thus, the phenoxide ion is formed. The phenoxide ion formed is stabilized by the delocalization of negative charge due to the resonance in the benzene ring. Phenoxide ion has greater stability than phenols, as in the case of phenol charge separation takes place during resonance The resonance structures of phenoxide ions explain the delocalization of negative charge In the case of substituted phenols, the acidity of phenols increases in the presence of the electron-withdrawing group. This is due to the stability of the phenoxide ion generated. The acidity of phenols further increases if these groups are attached at ortho and para positions. This is due to the fact that the negative charge in phenoxide ion is mainly delocalised at ortho and para positions of the attached benzene ring. On the other hand, the acidity of phenol decreases in the presence of electron-donating groups as they prohibit the formation of phenoxide ion.
9. Some Commercially Important Compounds:
(a) Methyl alcohol: It is produced by catalytic hydrogenation of carbon monoxide at high pressure and temperature in presence of ZnO-Cr2O3 catalyst.
(b) Ethyl alcohol: It is obtained commercially by fermentation.
(i)
(ii)
10. Ethers: Ethers are the compounds with general formula of CnH2n+2O (same as monohydric alcohols). These are represented by general structure, R—O—R′.
The groups R and R′ in ether may either be same or different. In case these groups are same, the compounds are known as simple ethers or symmetrical ethers. On the other hand, if R and R′ groups are different, the compounds are called mixed ether or unsymmetrical ethers.
Nomenclature: According to IUPAC system, ethers are named as alkoxyalkanes. The larger alkyl group forms the part of parent chain while smaller alkyl group constitutes the alkoxy group.
Table 11.2: Common and IUPAC Names of Some Ethers
Common Name | Structural Formula | IUPAC Name |
Dimethyl ether | CH3—O—CH3 | Methoxymethane |
Diethyl ether | CH3—CH2—O—CH2—CH3 | Ethoxyethane |
Methyl isopropyl ether | 2-Methoxypropane | |
Methyl tert.-butyl ether |
| 2-Methoxy-2-methyl propane |
Anisole |
| Methoxybenzene |
Phenetole |
| Ethoxybenzene |
Phenyl isopentyl ether |
| 3-Methyl-butoxybenzene |
CH3—O—CH2—CH2—O—CH3
1, 2-Dimethoxyethane
Structure of ROR: In ethers, the four electron pairs, i.e., the two bond pairs and two lone pairs of electrons on oxygen are arranged approximately in a tetrahedral arrangement. The bond angle is slightly greater than the tetrahedral angle due to the repulsive interaction between the two bulky (—R) groups. The C—O bond length (141 pm) is almost the same as in alcohols.
11. Preparation of Ethers:
By dehydration of alcohols
This method is suitable for the preparation of ethers having primary alkyl groups only.
12. Physical Properties:
1. Boiling points: Ethers have much lower boiling points as compared to isomeric alcohols. Unlike alcohols, ether molecules are not associated by hydrogen bonds. The interparticle forces existing in their liquid states are weak dipole–dipole forces.
2. Solubility: The solubility of ethers is comparable to those of corresponding alcohols. The solubility of ethers is due to the ability of their molecules to form hydrogen bond with water molecules.
However, solubility of ethers in water decreases from lower members to higher members. This is because of the relative increase in the hydrocarbon portion of the molecule which decreases the tendency of H-bond formation. Ethers are appreciably soluble in organic solvents like alcohol, benzene, acetone, etc.
13. Chemical Reactions: Ethers are relatively inert compounds in spite of the presence of oxygen atom carrying two lone pairs of electrons in their molecules. It is because of this reason that these are used as solvents. They undergo chemical reactions under specific conditions. Some of the reactions of ethers are being described as follows:
(a) Cleavage of C—O bond: Carbon oxygen bond in ethers can be cleaved by the use of reagents like halogen acids, sulphuric acid and phosphorus pentachloride, etc.
Cleavage with halogen acid: Ethers can be cleaved by the use of hydroiodic acid or hydrobromic acid to give alkyl halide and alcohol.
(X = Br, I)
In case excess of HI is used, the alcohol formed reacts further with HI to form alkyl iodide. The overall reaction can be written as
If one group is methyl and other group is tertiary alkyl group, the main product is methyl alcohol and tertiary alkyl halide. It is because the departure of leaving group (CH3—OH) creates a more stable tertiary carbocation.
In case of anisole, the products formed are phenol and methyl iodide.
The bond between O—CH3 is weaker than the bond between O—C6H5 because the carbon of phenyl group is sp2 hybridised and there is a partial double bond character. Therefore, the attack of I – ion breaks O—CH3 bond to form CH3I.
(b) Ring substitution in aromatic ethers: The alkoxy group (—OR) attached to aromatic ring activates the ring towards electrophilic substitution and directs the incoming electrophile to ortho and para positions.
The presence of negative charge at ortho and para positions indicates that electron density is more at these positions. Therefore, electrophile is likely to attack on these positions resulting in the formation of ortho and para substituted products.
14. Uses of Ethers
Ethers are used in several ways:
1. Dimethyl ether is used as refrigerant and as a solvent at low temperature.
2. Diethyl ether is used as a solvent for organic reactions and also as an industrial solvent for oils, gums, resins, etc. It is also used as an extracting solvent.
15. Some Important Name Reactions
(i) Kolbe’s reaction: When sodium phenoxide is heated with CO2 at 400 K under a pressure of 4 –7 atm, the resulting product on acidification yields salicylic acid.
Salicylic acid is the starting material for the manufacture of 2-acetoxybenzoic acid (aspirin).
(ii) Reimer–Tiemann reaction: Treatment of phenol with chloroform in the presence of sodium hydroxide followed by hydrolysis of resulting product gives o-hydroxybenzaldehyde (salicylaldehyde) as a major product.
(iii) Williamson synthesis: It consists of reacting an alkyl halide with sodium alkoxide or sodium phenoxide to form ether.
It is a convenient method for the preparation of symmetrical as well as unsymmetrical ethers.
16. Distinction between Primary, Secondary and Tertiary Alcohols
(a) Lucas test: In this test, the alcohol is treated with Lucas reagent which is an equimolar mixture of HCl and ZnCl2. Alcohols are soluble in Lucas reagent and form a clear solution. On reaction, alkyl chlorides are formed which being insoluble result in turbidity in the solution.
If turbidity appears immediately, tertiary alcohol is indicated.
If turbidity appears within five minutes, secondary alcohol is indicated.
If turbidity appears only upon heating, primary alcohol is indicated.
(b) Iodoform test: When ethyl alcohol or any alcohol containing the group 
or
(c) Ferric chloride test for phenols: Phenol gives a violet-coloured water soluble complex with ferric chloride.
In general, all compounds containing enolic group 
respond to this test.
However, the colours of complexes are different such as green, blue, violet, etc., and depend upon the structure of phenols.
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