Part 4. The chemistry of ALCOHOLS

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

Part 4. 2 Synthesis and Manufacture of Alcohols

Sub-index for this page

4.2.1 Hydrolysis of halogenoalkanes - a laboratory synthesis

4.2.2 Industrial manufacture of alcohols

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4.2.1 The hydrolysis of halogenoalkanes

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 hydroxy 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.

 

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 Reactivity trends of halogenoalkanes - introduction to their nucleophilic substitution reactions

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

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

4.2.2 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)  ===>

(acid catalysed synthesis in the chemical industry)

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

(ii)  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

 

(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.

Doc Brown's Advanced Level Chemistry Revision Notes

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INDEX of notes on ALCOHOLS chemistry

 All Advanced Organic Chemistry Notes

 Index of GCSE/IGCSE Oil - Useful Products Chemistry Revision Notes

 

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