ALCOHOLS

Introduction

• Aliphatic alcohols are hydroxyl derivatives of alkanes.• -OH (functional group)• R-OH (general formula)• Reactions involve the breaking of the C---OH, with

the removal of the-OH group; or the O--H bond, with the removal of –H.

• The type of reaction involves the substitution, in which a group replaces the –OH or –H, or elimination, in which double bond is formed.

1. Replacement of Hydrogen by Sodium/Reaction with Active Metals

• In the removal of H atom from the –OH bond, the ease of reaction is determine by the relative polarization of the O-H bond. Groups which tend to increase the electron density on the oxygen, such as alkyl groups bonded to the carbonyl carbon, would strengthen the O—H bond.

• The reaction with alkali metals which results in the formation of alkoxides.

• R –O—H + Na R – O –Na + ½ H2

Reactivity of R-OH: 1o>2o>3o

2. Replacement of Hydroxyl with Halogen (Lucas Test)

• In reactions involving the replacement of the OH group, has been effected by passing anhydrous hydrogen chloride or by employing the Lucas test.

• Tertiary alcohols react immediately to form an immiscible upper layer of alkyl chloride.

• The alkyl chloride is insoluble in the medium causes the solution to become cloudy before separates as a distinct layer.

• With secondary alcohols, the cloudiness appears after 5 minutes; with primary alcohols do not react under these conditions.

• Lucas reagent ( anhydrous ZnCl2 & HCl) R R | ZnCl2 |R – C – OH + HCl R – C – Cl + H2O | | R R

• Reactivity of R-OH: 3o>2o>1o

3. Oxidation• Carried out with chromic acid (sodium

dichromate/ potassium dichromate and sulfuric acid)

• Primary alcohols are oxidized to aldehydes and further oxidation to acids with the same number of carbon atoms as the original alcohols.

• R –CH2OH [O] R – CHO +Cr3+ (green)

• Secondary alcohols are oxidized to ketones with about the same ease as primary alcohols.

• The oxidations of primary and secondary alcohols are indicated by the change in color of the solution from orange to green.

• R – CHOH [O] R – C=O + Cr 3+ (green) | | R R

• Tertiary alcohols are resistant to oxidation as this involve the rupture of the carbon to carbon bond in the alcohol.

• Not readily oxidized if so, it will form acids with lower carbon content.

• The oxidizing agents maybe CuO, acid dichromate or alkaline or acid permanganate.

• R3-OH no reaction

4. Esterification

• Primary and secondary alcohols react with carboxylic acids to form esters.

• Mineral acid like sulfuric acid is usually added to speed the reaction.

• The process is called esterification. • The product is detected by the onset of a sweet

fruity odor.• R – OH+R-COOH R – COOR + H2O

ESTER

5. Test for methyl alcohol

• Primary and secondary alcohols can be oxidized to aldehydes and ketones through dehydrogenation.

• Carried out using copper catalyst at 300 o without addition of oxidizing agent.

• CH3 – OH Cu HCH=O + H2 300 o formaldehyde

• Results in the formation of formaldehyde which gives a pink-red ring when treated with resorcinol and sulfuric acid.