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.
•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
Properties of Alcohols
Reactions of Alcohols
Alcohols undergo several types of reactions including:
CombustionOxidationEsterificationDehydrationReactions 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:
2CH3OH(l) + 3O2(g) → 2CO2(g) + 4H2O(l)
∆H = -726kJmol-1
2C5H11OH(l) + 15O2(g) → 10CO2(g) + 12H2O(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 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), written as H+/Cr2O7 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
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
Distill to obtain an aldehyde
water out
water in
condensed aldehyde out
boling solution
aldehyde vapor condensing tube
Heat under reflux to obtain a carboxylic acid
water out
water in
boling solution
vapor condenses and falls back into flask
condensing tube
10.4.2 Secondary Alcohols are oxidized to Alkanones
10.4.2 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
10.4.2 Oxidation of Secondary Alcohols
Propan-2-ol will oxidize into propanone
10.4.2 Tertiary Alcohols are not easily oxidized
10.4.2 Esterification
Alcohol + Carboxylic acids Esters
A quick note…
OH
O+ OH
O
Oac id
acetic acid propan-1-ol ethyl acetate
OH
O
OH O
O
propionic acid
propan-1-olpropyl propionate
+a c id
+
+
OH2
OH2
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