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.
Alcohols have the general formula: C n H 2n+1 OH 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 Properties of Alcohols
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.
10.4.1 Describe, using equations, the complete combustion of alcohols.
10.4.1 Reactions of Alkenes: Combustion Alcohols burn in an excess supply of oxygen to form carbon dioxide and water in an exothermic reaction: 2CH 3 OH(l) + 3O 2 (g) → 2CO 2 (g) + 4H 2 O(l) ∆H = -726kJmol -1 2C 5 H 11 OH(l) + 15O 2 (g) → 10CO 2 (g) + 12H 2 O(l) ∆H = -3330kJmol -1
10.4.2 Describe, using equations, the oxidation reactions of alcohols.
Primary, Secondary & Tertiary Draw and name a 5-carbon… Primary alcohol Secondary alcohol Tertiary alcohol
10.4.2 Oxidation Alcohols are oxidized to alkanals (aldehydes) or alkanones (ketones)
10.4.2 Oxidizing Agents The most common oxidizing agents are KMnO 4 in basic solution, K 2 Cr 2 O 7 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), written as H + /Cr 2 O 7 H + /Cr 2 O 7 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
10.4.2 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
10.4.2 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
10.4.2 Mechanism of the Oxidation of Primary Alcohols
10.4.2 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
10.4.3 Determine the products formed by the oxidation of primary and secondary alcohols.
10.4.3 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 colourless for ketones. See Fig. 10.58
10.4.3 Testing Solutions Fehling’s Solution Aldehyde is orange Ketone is blue (unreacted) http://www.youtube.com/watch?v=WmwTRbQLIVo Tollen’s Reagent Aldehyde is produces the “silver mirror” Ketone is colourless (unreacted) http://www.youtube.com/watch?v=F-Emzzls6Io