Chemistry Physical Properties and Chemical Reactions-1

### Topics Covered :

● Physical Properties
● Boiling Points
● Solubility
● Chemical Reactions
● Reactions Involving Cleavage of O-H Bond
● Acidity of Alcohols and Phenols
● Esterification

### Physical Properties :

=> Alcohols and phenols consist of two parts, an alkyl/aryl group and a hydroxyl group.

=> The properties of alcohols and phenols are chiefly due to the hydroxyl group.

=> The nature of alkyl and aryl groups simply modify these properties.

### Boiling Points :

=> The boiling points of alcohols and phenols increase with increase in the number of carbon atoms (increase in van der Waals forces).

=> In alcohols, the boiling points decrease with increase of branching in carbon chain (because of decrease in van der Waals forces with decrease in surface area).

=> The color{red}(–OH) group in alcohols and phenols is involved in intermolecular hydrogen bonding as shown in fig.1.

color{red}("Note ") : Boiling points of alcohols and phenols are higher in comparison to other classes of compounds, namely hydrocarbons, ethers, haloalkanes and haloarenes of comparable molecular masses.

color{red}("Example " ) : Ethanol and propane have comparable molecular masses but their boiling points differ widely. The boiling point of methoxymethane is intermediate of the two boiling points. See fig.2.

=> The high boiling points of alcohols are mainly due to the presence of intermolecular hydrogen bonding in them which is lacking in ethers and hydrocarbons.

### Solubility :

=> Solubility of alcohols and phenols in water is due to their ability to form hydrogen bonds with water molecules as shown in fig.

=> The solubility decreases with increase in size of alkyl/aryl (hydrophobic) groups.

=> Several of the lower molecular mass alcohols are miscible with water in all proportions.
Q 3042056833

Arrange the following sets of compounds in order of their increasing boiling points:
(a) Pentan-1-ol, butan-1-ol, butan-2-ol, ethanol, propan-1-ol, methanol.
(b) Pentan-1-ol, n-butane, pentanal, ethoxyethane.

Solution:

(a) Methanol, ethanol, propan-1-ol, butan-2-ol, butan-1-ol, pentan-1-ol.
(b) n-Butane, ethoxyethane, pentanal and pentan-1-ol.

### Chemical Reactions :

=> Alcohols are versatile compounds.

=> They react both as nucleophiles and electrophiles.

=> The bond between color{red}(O–H) is broken when alcohols react as nucleophiles.

=> Based on the cleavage of color{red}(O–H) and color{red}(C–O) bonds, the reactions of alcohols and phenols may be divided into two groups :

### Reactions involving cleavage of color{red}(O–H) bond of Alcohols and Phenols :

Following reactions are shown by cleavage of color{red}(O-H) bond :

### Acidity of alcohols and phenols :

(i) color{green}("Reaction with Metals ") : Alcohols and phenols react with active metals such as sodium, potassium and aluminium to yield corresponding alkoxides/phenoxides and hydrogen. See fig.1.

color{red}(2R - O - H +2Na → undersettext(Sodium alkoxide)(2R - O - Na+H_2))

● In addition to this, phenols react with aqueous sodium hydroxide to form sodium phenoxides. See fig.2.

● 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:). See fig.3.

(ii) color{green}("Acidity of Alcohols ") : The acidic character of alcohols is due to the polar nature of color{red}(O–H) bond.

● An electron-releasing group color{red}(–CH_3, –C_2H_5) increases electron density on oxygen tending to decrease the polarity of color{red}(O-H) bond. This decreases the acid strength.

● For this reason, the acid strength of alcohols decreases in the following order as shown in fig.4.

● Alcohols are, however, weaker acids than water. This can be illustrated by the reaction of water with an alkoxide. See fig.5.

● 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) color{green}("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 color{red}(sp^2) hybridised carbon of benzene ring which acts as an electron withdrawing group.

● Due to this, the charge distribution in phenol molecule, as depicted in its resonance structures, causes the oxygen of color{red}(–OH) group to be positive. See fig.6.

● The reaction of phenol with aqueous sodium hydroxide indicates that phenols are stronger acids than alcohols and water.

=> Let us examine how a compound in which hydroxyl group attached to an aromatic ring is more acidic than the one in which hydroxyl group is attached to an alkyl group. The ionisation of an alcohol and a phenol takes place as shown in fig.7.

● Due to the higher electronegativity of color{red}(sp^2) hybridised carbon of phenol to which color{red}(–OH) is attached, electron density decreases on oxygen. This increases the polarity of color{red}(O–H) bond and results in an increase in ionisation of phenols than that of alcohols.

=> Now let us examine the stabilities of alkoxide and phenoxide ions. In alkoxide ion, the negative charge is localised on oxygen while in phenoxide ion, the charge is delocalised.

● The delocalisation of negative charge (structures I-V) makes phenoxide ion more stable and favours the ionisation of phenol.

● Although there is also charge delocalisation in phenol, its resonance structures have charge separation due to which the phenol molecule is less stable than phenoxide ion. See fig.8.

=> In substituted phenols, the presence of electron withdrawing groups such as nitro group, enhances the acidic strength of phenol. This effect is more pronounced when such a group is present at ortho and para positions. It is due to the effective delocalisation of negative charge in phenoxide ion.

● On the other hand, electron releasing groups, such as alkyl groups, in general, do not favour the formation of phenoxide ion resulting in decrease in acid strength. Cresols, for example, are less acidic than phenol.
Q 3062056835

Arrange the following compounds in increasing order of their acid strength: Propan-1-ol, 2,4,6-trinitrophenol, 3-nitrophenol, 3,5-dinitrophenol, phenol, 4-methylphenol.

Solution:

Propan-1-ol, 4-methylphenol, phenol, 3-nitrophenol, 3,5-dinitrophenol, 2,4, 6-trinitrophenol.

### Esterification :

=> Alcohols and phenols react with carboxylic acids, acid chlorides and acid anhydrides to form esters.

color{red}(Ar // RO-H +R' -COOH overset(H^+)⇄ Ar//ROCOR' +H_2O)

color{red}(Ar//R-OH + (R' CO)_2O overset(H^(+))⇄ Ar//ROCOR'+R'COOH)

color{red}(R//ArOH + R' COCl oversettext(Pyridine)→ R//ArOCOR' +HCl)

=> The reaction with carboxylic acid and acid anhydride is carried out in the presence of a small amount of concentrated sulphuric acid.

● The reaction is reversible, and therefore, water is removed as soon as it is formed.

● The reaction with acid chloride is carried out in the presence of a base (pyridine) so as to neutralise color{red}(HCl) which is formed during the reaction. It shifts the equilibrium to the right hand side.

● The introduction of acetyl color{red}(CH_3CO) group in alcohols or phenols is known as acetylation.

● Acetylation of salicylic acid produces aspirin. See fig.

● Aspirin possesses analgesic, antiinflammatory and antipyretic properties.