Alcohols
 

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Alcohols RCH₂OH, R₂CHOH, R₃COH.

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Primary alcohol → aldehyde
Reagents: potassium dichromate(VI) solution and dilute sulphuric acid.
Conditions: the dichromate/alcohol mixture is added to hot acid so that the aldehyde distils out of the
                  reaction mixture and further oxidation is minimised:

CH₃CH₂OH + [O] → CH₃CHO + H₂O

The aldehyde may oxidise further to a carboxylic acid

CH₃CHO + [O] → CH₃COOH

Alkanes & alkenes
Arenes
Halogenoalkanes
Alcohols
Aldehydes and ketones
Carboxylic acids
Acyl chlorides
Amines
Nitro compounds
Nitriles
Amides

Secondary alcohol → ketone
Reagents: potassium dichromate(VI) solution and dilute sulphuric acid.
Conditions: heat under reflux

CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O

There is no further oxidation of the ketone under these conditions.

Tertiary alcohols are unaffected by the reactions above.

Alcohol → alkene
Reagents: concentrated sulphuric acid
Conditions: heat to 170^o with excess acid.

CH₃CH(OH)CH₂CH₃ → CH₃CH=CHCH₃ + H₂O

The ease of elimination of water is primary < secondary < tertiary. Tertiary alcohols will sometimes lose water just on heating. If there are two alkenes which can be produced, the major product is that bearing most alkyl groups on the C=C group.

Other reagents can be used which do not cause the charring and oxidation that conc sulphuric acid does. Either (i) syrupy phosphoric(V) acid H₃PO₄ + heat, or (ii) alcohol vapour passed over hot aluminium oxide, are effective.

Alcohol → halogenoalkane. All types of alcohol react similarly.
Reagents: phosphorus(V) chloride
Conditions: at room temperature

CH₃CH₂OH + PCl₅ → CH₃CH₂Cl + POCl₃ + HCl

Alcohol → halogenoalkane. All types of alcohol react similarly.
Reagents: thionyl chloride SOCl₂
Conditions: at room temperature:

CH₃CH₂OH + SOCl₂ → CH₃CH₂Cl + HCl + SO₂

This gives a very clean reaction since the products apart from the halogenoalkane are gaseous.

Alcohol → halogenoalkane.
Reagents: sodium bromide and 50% sulphuric acid. HBr is generated in situ.
Conditions: heat under reflux.

NaBr + H₂SO₄ → HBr + NaHSO₄

CH₃CH₂CH₂CH₂OH + HBr → CH₃CH₂CH₂CH₂Br + H₂O

Primary alcohols react best; others tend to eliminate to give alkenes. This reaction cannot be used for iodides, since KI is oxidised by sulphuric acid. The reaction below is used instead.

Alcohol → bromo- or iodoalkane.
Reagents: moist red phosphorus and bromine or iodine
Conditions: warm.

The phosphorus trihalide is prepared in situ by this reaction.

3CH₃CH₂OH + PBr₃ → 3CH₃CH₂Br + P(OH)₃