Alcohols IB Chemistry Topic 10.4

10.4 Alcohols Asmt. Stmts 10.4.1 Describe, using equations, the complete combustion of alcohols. 10.4.2 Describe, using equations, the oxidation reactions of alcohols. 10.4.3 Determine the products formed by the oxidation of primary and secondary alcohols.

Properties of Alcohols Alcohols have the general formula: CnH2n+1OH The physical properties of alcohols are similar to those of both water and hydrocarbons The shorter chain alcohols such as methanol and ethanol are similar to water, in general they have higher boiling points than hydrocarbons but lower than water dissolve in water to some degree are more polar than hydrocarbons but less polar than water 7

Reactions of Alcohols Alcohols undergo several types of reactions including: Combustion Oxidation Esterification Dehydration Reactions with active metals We will focus on the first 2 reactions… for now. 8

Describe, using equations, the complete combustion of alcohols. 10.4.1 Describe, using equations, the complete combustion of alcohols.

Reactions of Alkenes: The general mechanism Alcohols burn in an excess supply of oxygen to form carbon dioxide and water in an exothermic reaction: 2CH3OH(l) + 3O2(g) → 2CO2(g) + 4H2O(l) ∆H = -726kJmol-1 2C5H11OH(l) + 15O2(g) → 10CO2(g) + 12H2O(l) ∆H = -3330kJmol-1

Describe, using equations, the oxidation reactions of alcohols. 10.4.2 Describe, using equations, the oxidation reactions of alcohols.

Oxidation Alcohols are oxidized to alkanals (aldehydes) or alkanones (ketones) 9

Oxidizing Agents The most common oxidizing agents are KMnO4 in basic solution, K2Cr2O7 in acidic solution, or oxygen from the air. [O] over the yields arrow indicates an oxidizing agent The most commonly used for organic reactions is acidified potassium dichromate (VI) H+/Cr2O7 is bright orange When heated, it oxidizes the alcohol and changes color, as Cr+7 is reduced to Cr+3 The reduced form of chromium is green Figure 10.55

Primary Alcohols are Oxidized to Alkanals Primary alcohols are oxidized to become carboxylic acids in a two-step process. Step 1: alcohol → aldehyde Ex. Ethanol is oxidized to become ethanal 10

Primary Alcohols are Oxidized to Alkanals Primary alcohols are oxidized to become carboxylic acids in a two-step process. Step 2: aldehyde → carboxylic acid Ex. Ethanal is further reduced to become ethanoic acid 10

Mechanism of the Oxidation of Primary Alcohols

Oxidation of Primary Alcohols Experimental conditions may be altered to yield the desired product… Aldehydes can be removed by distilling it as it forms (due to its lower boiling point) If the carboxylic acid is the desired product, then the aldehyde should be exposed to [O] for a long period of time and heated under reflux See Fig. 10.56

Secondary Alcohols are oxidized to Alkanones 11

Secondary Alcohols are oxidized to Alkanones Since secondary alcohols only have one hydrogen attached to the carbon atom, the only possible product is a ketone 11

Oxidation of Secondary Alcohols Propan-2-ol will oxidize into propanone

Tertiary Alcohols are not easily oxidized 12

10.4.3 Determine the products formed by the oxidation of primary and secondary alcohols.

Testing Solutions 2,4-dinitrophenylhydrazine solution tests for the presence of aldehydes and ketones by forming orange crystals To differentiate between the two classes: Fehling’s solution starts blue and will turn orange-brown in the presence of aldehydes. It will remain blue for ketones. Tollens’ reagent produces a silver mirror effect on the inside of the test tube in the presence of aldehydes, but will remain clear for ketones. See Fig. 10.58

Testing Solutions Fehling’s Solution Tollen’s Reagent Aldehyde is produces the “silver mirror” Ketone is clear (unreacted) Fehling’s Solution Aldehyde is orange Ketone is blue (unreacted)