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Revision:Alcohols

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TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Chemistry > Alcohols

Alcohols are organic compounds with the -OH group on it. The general formula is CnH2n+1OH. Alcohols have the ending -ol, such as methanol, ethanol. There are 3 types of alcohols:

  • Primary alcohols have 1 group attached to the carbon with the -OH group (Except for methanol) i.e. they have 2 hydrogens on the carbon next to the -OH group.
  • Secondary alcohols have 2 groups attached to the carbon with the -OH group i.e. they have 1 hydrogen on the carbon next to the -OH group.
  • Tertiary alcohols have 3 groups attached to the carbon with the -OH group i.e. they have no hydrogens on the carbon next to the -OH group.

Reactions

  • Combustion:

The combustion of alcohols go as expected \mathrm{2CH_3OH + 3O_2 \longrightarrow 2CO_2 + 4H_2O}

  • Reaction with sodium:

Alcohols react in a similar way to water with sodium \mathrm{2CH_3OH + 2Na \longrightarrow 2CH_3ONa + H_2}

  • Forming haloalkanes:

The -OH group can be replaced by a halide group with phosphorus trihalides - e.g. \mathrm{3CH_3OH + PBr_3 \longrightarrow 3CH_3Br + H_3PO_3}. The phosphorus bromide and iodide can be prepared by adding bromine/iodine to phosphorus. More commonly, however, phosphorus pentachloride is used to for the substitution of chloride into alcohols, as this produces white fumes of hydrogen chloride gas - a test for the -OH group: \mathrm{CH_3OH + PCl_5 \longrightarrow CH_3Cl + POCl_3 + HCl}

  • Oxidation:

Primary and secondary alcohols can be oxidised using acidified (Usually H2SO4) potassium dichromate (K2Cr2O7). In each case, the dichromate ions are reduced to chromate(III) ions. [O] indicates an oxygen atom from an oxidising agent:

Primary alcohols are oxidised to aldehydes: \mathrm{CH_3CH_2OH + [O] \longrightarrow CH_3CHO + H_2O}. The aldehyde produced can then be oxidised further, or distilled off: \mathrm{CH_3CHO + [O] \longrightarrow CH_3COOH}.

Secondary alcohols are oxidised to ketones: \mathrm{(CH_3)_2CHOH + [O] \longrightarrow (CH_3)_2CO + H_2O}

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