Example 7.3

Example . Alcohols are versatile compounds. They react both as nucleophiles and electrophiles. The bond between O–H is broken when alcohols react as nucleophiles. . . Chemical Reactions Alcohols as nucleophiles (i) (ii) The bond between C–O is broken when they react as electrophiles. Protonated alcohols react in this manner. Protonated alcohols as electrophiles Based on the cleavage of O–H and C–O bonds, the reactions of alcohols and phenols may be divided into two groups: (a) Reactions involving cleavage of O–H bond . Acidity of alcohols and phenols (i) Reaction with metals : Alcohols and phenols react with active metals such as sodium, potassium and aluminium to yield corresponding alkoxides/phenoxides and hydrogen. In addition to this, phenols react with aqueous sodium hydroxide to form sodium phenoxides. Sodium phenoxide + H O O Na + Na The above reactions show that alcohols and phenols are acidic in nature. In fact, alcohols and phenols are Brönsted acids i.e., they can donate a proton to a stronger base (B:). (ii) Acidity of alcohols: The acidic character of alcohols is due to the polar nature of O–H bond. An electron-releasing group (–CH , –C H ) 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: Alcohols are, however, weaker acids than water. This can be illustrated by the reaction of water with an alkoxide. This reaction shows that water is a better proton donor (i.e., stronger acid) than alcohol. Also, in the above reaction, we note that an alkoxide ion is a better proton acceptor than hydroxide ion, which suggests that alkoxides are stronger bases (sodium ethoxide is a stronger base than sodium hydroxide). Alcohols act as Bronsted bases as well. It is due to the presence of unshared electron pairs on oxygen, which makes them proton acceptors. (iii) Acidity of phenols: The reactions of phenol with metals (e.g., sodium, aluminium) and sodium hydroxide indicate its acidic nature. The hydroxyl group, in phenol is directly attached to the