Alcohols Phenols and Ethers Test - 7

Carboxylic acids are reduced to primary alcohols in excellent yields by lithium aluminium hydride, a strong reducing agent.

However, LiAlH₄ is an expensive reagent, and therefore, used for preparing special chemicals only. Commercially, acids are reduced to alcohols by converting them to the esters followed by their reduction using hydrogen in the presence of catalyst (catalytic hydrogenation).

RCHO + H₂ → RCH₂OH

• Aldehydes and ketones are most readily reduced with hydride reagents.
• The reducing agents LiAlH₄ and NaBH₄ act as a source of 4 x H^- (hydride ion)
• Overall 2 H atoms are added across the C=O to give H-C-O-H
• Hydride reacts with the carbonyl group, C=O, in aldehydes or ketones to give alcohols.
• The substituents on the carbonyl dictate the nature of the product alcohol.
• Reduction of methanal (formaldehyde) gives methanol.
• Reduction of other aldehydes gives primary alcohols.
• Reduction of ketones gives secondary alcohols.

Monochlorination of toluene gives benzylchloride.
C₆H₅CH₃ + Cl₂ + hv →→ C₆H₅CH₂Cl
benzyl chloride on reaction with aq. NaOH will give benzyl alcohol by substitution reaction.
C₆H₅CH₂Cl + NaOH(aq) →→ C₆H₅CH₂OH

In alkanes, the only intermolecular forces are van der Waals dispersion forces. Hydrogen bonds are much stronger than these and therefore it takes more energy to separate alcohol molecules than it does to separate alkane molecules. That's the main reason that the boiling points are higher.

Fehling's Solution Test: When acetaldehyde is heated with Fehling solution, red ppt. of cuprous oxide are formed. But benzaldehyde does not give red ppt. with fehling solution.

Reaction are

CH₃CHO + 2Cu²⁺ + 5OH^- →→CH₃COO^- + Cu₂O + 3H₂O

C₆H₅CHO + 2Cu²⁺ + 5OH^- →→ No red ppt.

• The oxygen atom of an alcohol is sp³ hybridised and has two non bonding pairs of electrons.
• The O-H bond of alcohols is strongly polarised and hydrogen bonding occurs in much the same way as in water molecules
• As a consequence, alcohols have relatively high boiling points compared to other organic compounds of a similar molecular weight.

Lucas reagent is a solution of anhydrous zinc chloride (Lewis acid) in concentrated hydrochloric acid. It is used as a reagent to test alcohols and classify them in accordance to their reactivity. The reaction is a substitution reaction where the chloride of the zinc chloride gets replaced by the hydroxyl group of the alcohol.