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