Alcohols Phenols and Ethers Notes
Phenols: Nomenclature, methods of preparation, physical and chemical properties, acidic nature of phenol, electrophilic substitution reactions, uses of phenols.
Ethers: Nomenclature, methods of preparation, physical and chemical properties, uses.
Alcohols: Nomenclature, methods of preparation, physical and chemical properties (of primary alcohols only), identification of primary, secondary and tertiary alcohols, mechanism of dehydration, and uses with special reference to methanol and ethanol.
Explain why p-nitrophenol is more acidic than phenol?
Nitro group of phenol produces – I and – R effect. Because of these two effects —NO2 group is electron withdrawing in nature. So, the electron density in the O—H bond of p-nitrophenol decreases relative to the O—H bond of phenol.
The decrease in electron density of the O—H bond of p-nitrophenol, the polarity of O—H bond is decrease and in turn make it more acidic than phenol.
Explain why alcohols and ethers of comparable molecular mass have different boiling points?
Boiling point depends upon the strength of intermolecular forces of attraction. Higher these forces of attraction, more will be the boiling point. Alcohols undergo intermolecular hydrogen bonding. So, the molecules of alcohols are held together by strong intermolecular forces of attraction.
But in ethers no hydrogen atom is bonded to oxygen. Therefore, ethers are held together by weak dipole-dipole forces, not by strong hydrogen bond. Since, lesser amount of energy is required than to break weak dipole-dipole forces in ethers than to break strong hydrogen bonds in alcohol.
Explain why low molecular mass alcohols are soluble in water?
Solubility of alcohols and phenols in water is due to their ability to form hydrogen bonds with water molecule. The hydrocarbon part methoxy methane (i.e., R group) tends to prevent the formation of hydrogen bonds. Alcohols with lower molar mass will have smaller hydrocarbon part and therefore tendency to form hydrogen bonding is more and they are more soluble in water.
CH3CH2OH > CH3CH2CH2OH
(More soluble) (Less soluble)
Why is the reactivity of all the three classes of alcohols with conc. HCl and ZnCl2 (Lucas reagent) different?
The reaction of alcohols with Lucas reagent (conc. HCl and ZnCl2) follow SN1 mechanism. SN1 mechanism depends upon the stability of carbocations (intermediate). More stable the intermediate carbocation, more reactive is the alcohol.
Tertiary carbocations are most stable among the three classes of carbocations and the order of the stability of carbocation is 3° > 2° > 1°. This order, inturn, reflects the order of reactivity of three classes of alcohols i.e., 3° > 2° > 1°.
Thus, as the stability of carbocations are different so the reactivity of all the three classes of alcohols with Lucas reagent is different.
Explain why nucleophilic substitution reactions are not very common in phenols?
Resonance is an important factor in phenols. During resonance -OH group in phenol gives its electrons to the benzene ring. As a result of this, the electron density on benzene ring is very high. This increased electron density repels nucleophiles.
Therefore, nucleophiles cannot attack the benzene ring and phenols usually do not give nucleophilic substitution reaction.
What is denatured alcohol?
Alcohol is used in large quantities in the manufacture of alcoholic liquors. Its continuous use damages the various vital organs. Therefore, to refrain the people from drinking alcohol, heavy excise duty is levied on the sale of alcoholic beverages. But, it is used in various industries as it is a very good solvent.
Therefore, industrial alcohol must be cheap. Thus, to provide cheaper alcohol to industries and to refrain people from drinking alcohol, it is mixed with some copper sulphate, pyridine, methyl alcohol or acetone.
Alcohol is made unfit for drinking by mixing some quantity of any of these substances in it. This is called denatured alcohol.
