A-Level organic chemistry exam revision notes on alcohols

Part 4. The chemistry of ALCOHOLS

4.2 Synthesis methods and manufacture of alcohols - conditions, reagents and equations

Part 4. 2 The Laboratory Synthesis and Industrial Manufacture of Alcohols

Sub-index for this alcohol chemistry page 4.2 on the synthesis and manufacture of alcohols

4.2.1 Hydrolysis of halogenoalkanes - a laboratory synthesis

4.2.2 The acid catalysed hydration of alkenes

4.2.3 Industrial manufacture of alcohols

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4.2.1 The hydrolysis of halogenoalkanes to alcohols

You must know the structures of primary, secondary and tertiary halogenoalkanes (haloalkanes)

You must know the structures of primary, secondary and tertiary alcohols

(a) The reaction between a halogenoalkane and the hydroxide ion and reagents and method

Both these soluble strong bases (alkalis) are source of a powerful nucleophile, the hydroxide ion (OH^–).

This is a nucleophilic substitution reaction because the attacking reagent is a nucleophile and the halogen functional group/atom is replaced by the hydroxyl functional group (alcohol):

RX  +  OH^-  ===> ROH  +  X^-   (R = alkyl, X = Cl, Br or I)

The hydrolysis usually takes faster with an alkali than pure water because water is a weaker nucleophile than the hydroxide ion.

See Part 3.4 The substitution reaction of halogenoalkanes (haloalkanes) with sodium/potassium hydroxide to give alcohols

Strictly speaking all the reactants and products should be suffixed by (aq) apart from water (l).

The reaction is usually carried out by refluxing the halogenoalkane with aqueous sodium hydroxide (NaOH) and potassium hydroxide (KOH) (left diagram above)

The cold water cooled Liebig vertical condenser prevents the loss of volatile molecules e.g. solvent or product.

The alcohol can be distilled from the mixture using fractional distillation (right diagram above).

When an organic compounds like halogenoalkanes are not very soluble in water, the reaction with an aqueous reagent is very slow because of a much reduced chance of reactant particle interactions e.g. compared to a homogenous mixture where the haloalkane dissolves in the reagent solvent.

So, even under reflux, the hydrolysis reaction can be quite slow because halogenoalkanes are insoluble in water, but using aqueous ethanol solvent increases their solubility and speed of the reaction, BUT ...

... NaOH, and in particular KOH, are very strong bases, and if pure ethanol is used as the solvent, you can get significant quantities of an alkene via an elimination reaction.

See Part 3.7 The elimination reactions of halogenoalkanes (haloalkanes) with potassium hydroxide to give alkenes

 

(b) bromoethane  +  sodium hydroxide  ===>  ethanol  +  sodium bromide

  +  NaOH      +  NaBr

since hydroxide and bromide are free ions the equations are better written as ...

bromoethane  +  hydroxide ion  ===>  ethanol  +  bromide ion

  +  OH^–      +  Br^–   (displayed formula equation)

  +  OH^–      +  Br^–   (structured formula ionic equation)

  +  OH^–      +  Br^–   (skeletal formula equation)

An example of primary alcohol formation.

 

(c) 1-bromopropane  +  sodium hydroxide  ===>  propan-1-ol  +  sodium bromide

  +  NaOH      +  NaBr

since hydroxide and bromide are free ions the equations are better written as ...

1-bromopropane  +  hydroxide ion  ===>  propan-1-ol  +  bromide ion

  +  OH^–      +  Br^–

  +  OH^–      +  Br^–

Another example of primary alcohol formation.

 

(d) bromocyclohexane  +  hydroxide ion  ===>  cyclohexanol  +  bromide ion

  +  OH^–    +  Br^–

An example of secondary alcohol formation.

 

(e) 2-bromopropane  +  sodium hydroxide  ===> propan-2-ol  + sodium bromide

CH₃CHBrCH₃  +  OH^-  ===>  CH₃CH(OH)CH₃  +  Br^-

Another example of secondary alcohol formation.

 

(f)  2-chloro-2-methylpropane  + sodium hydroxide  ===>  2-methylpropan-2-ol  + sodium chloride

(CH₃)₃CCl  +  OH^-  ===>  (CH₃)₃COH  +  Cl^-

Another example of tertiary alcohol formation.

 

(g) 1,3-dichlorobutane  + sodium hydroxide  ===> butane-1,3-diol  +  sodium bromide

+  2OH^-  ===>     +  2Cl^-

The diol product is both a primary and secondary alcohol (reverse one of the images to match up!)

 

See also Parts 3.3 and 3.4

 Reactivity trends of halogenoalkanes - introduction to their nucleophilic substitution reactions

The substitution reaction of halogenoalkanes (haloalkanes) with sodium/potassium hydroxide to give alcohols

and in the mechanism sections

 Nucleophilic substitution by water/hydroxide ion [S_N1 or S_N2 mechanisms, hydrolysis to give alcohols]

TOP OF PAGE and indexes

4.2.2 The acid catalysed hydration of alkenes to alcohols

Concentrated sulfuric acid adds to an alkene to produce an alkyl hydrogensulfate salt (i), which is hydrolysed on warming with water to give the alcohol (ii) e.g.

(a) (i) ethene  + sulfuric acid  ===> ethyl hydrogensulfate

H₂C=CH₂  + H₂SO₄  ===> CH₃CH₂OSO₂OH

(ii) ethyl hydrogensulfate  +  water  ===>  ethanol  + sulfuric acid

 CH₃CH₂OSO₂OH  +  H₂O  ===> CH₃CH₂OH  +  H₂SO₄

There is only one product with a symmetrical alkene

(b) (i) propene  + sulfuric acid  ===> propyl hydrogensulfates

H₂C=CH₂CH₃  + H₂SO₄  ===> CH₃CH₂CH₂OSO₂OH  or  CH₃CH(OSO₂OH)CH₃

(ii) propyl hydrogensulfates  +  water  ===> propanol alcohols + sulfuric acid

There are two isomeric alcohols from an unsymmetrical alkene, because there are two possible alkylhydrogensulfates derived from propene.

CH₃CH₂CH₂OSO₂OH  +  H₂O  ===> CH₃CH₂CH₂OH  +  H₂SO₄

giving the primary alcohol propan-1-ol

CH₃CH(OSO₂OH)CH₃  +  H₂O  ===> CH₃CH(OH)CH₃  +  H₂SO₄

giving the isomeric secondary alcohol propan-2-ol

For (b)(ii), propan-2-ol is the major product, which you can work out from the Markownikoff (Markownikov) Rule.

There will always be two isomeric alcohol products with an unsymmetrical alkene

For details see Part 2.6 The reaction of alkenes with steam - addition of water - synthesis of alcohols and the reaction of alkenes with concentrated sulfuric acid

TOP OF PAGE and indexes

4.2.3 Industrial manufacture of alcohols

Examples of how alcohols are manufactured and synthesised on a large scale.

(a) The industrial manufacture of methanol

Methanol can be manufactured by mixing hydrogen and carbon monoxide gases and passing them over a zinc oxide and chromium oxide catalyst.

2H₂(g)  +  CO(g)  ===> CH₃OH(g)

The methanol can be condensed and purified by fractional distillation.

 

(b) Methods of manufacturing ethanol ('alcohol')

The production of ethanol (i) from sugar fermentation or (ii) synthesis from ethene:

(i)  C₆H₁₂O_6(aq) ====> 2C₂H₅OH_(aq) + 2CO_2(g)

(in the brewing and wine industry)

All the manufacturing processes are fully described on the Ethanol manufacture page

(ii)  ===>

(acid catalysed hydration of alkene synthesis of alcohols in the chemical industry)

For details see Part 2.6 The reaction of alkenes with steam - addition of water - synthesis of alcohols and the reaction of alkenes with concentrated sulfuric acid

Electrophilic addition mechanism of adding water to alkenes [acid catalyst] to form alcohols

 

(c) Other alcohols and other methods

Most of the other alcohols are manufacture by far more complex processes, often involving several steps.

 e.g. the synthesis of propan-1-ol can be made in two stages using transition metal based catalysts.

(i) Ethene (from oil cracking) is reacted with carbon monoxide and hydrogen using a rhodium or cobalt compound catalyst to propanal.

H₂C=CH₂  +  CO  +  H₂  ===>  CH₃CH₂CHO 

(ii) The propanal is hydrogenated to make propan-1-ol (1-propanol) using an Ni/Pt catalyst.

CH₃CH₂CHO  +  H₂  ===>  CH₃CH₂CH₂OH

 

Propanol alcohols, as with many other compounds, can be manufactured from basic carbohydrates like sugars in a fermenter. The microbe enzymes do the chemistry - examples of biosynthetic pathways - considered more 'green' chemistry.