Download ALKANOLS (ALCOHOLS)

ALKANOLS (ALCOHOLS)
The alkanols are an homologous series with the hydroxyl group (-OH) as their
functional group. Alkanols with three or more carbons all exist in isomeric form.
Naming:
Straight chain alcohols are named:
 With the suffix –ol
 With the usual prefix to identify the number of carbon atoms e.g. methanol,
ethanol etc
 So that the –OH group is positioned on the lowest possible number
Branched chain alcohols are named so that:
 The parent chain always contains the –OH group
 The position of the hydroxyl group determines the numbering of any alkyl
groups
 The prefixes di and tri are used as required to indicate more than one
hydroxyl or alkyl group
Isomerism and Classification:
Alcohols can exhibit structural isomerism due to:
 The presence and position of alkyl side chains
 The position of the hydroxyl group
Alcohols can also exhibit optical isomerism. This involves the presence of an
asymmetric carbon atom – one bonded to four different atoms or groups of
atoms. A molecule with an asymmetric carbon atom is known as a chiral
molecule. The two forms of the chiral molecule, known as enantiomers or
optical isomers:
 Are mirror images of each other
 Cannot be superimposed on each other
Alcohols are classified as primary, secondary or tertiary, depending upon the
number of alkyl groups attached to the carbon atom to which the OH group is
bonded (or the number of hydrogen atoms attached to the alkanol carbon).
Methanol is considered to be a primary alcohol.
Primary alcohols have a ‘1’ in their name (e.g. propan-1-ol)
Secondary alcohols usually have a ‘2’ or a ‘3’ in their name
Tertiary alcohols have a side chin attached to the carbon that holds the OH group
and so a number is repeated in their name.
(e.g. 2-methylpropan-2-ol).
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Bonding and general properties:
Intermolecular bonding in alcohols results from:
 Strong hydrogen bonding between the polar hydroxyl groups and
neighbouring alcohol molecules.
 Weak dispersion forces and temporary dipole-dipole interactions between
alkyl groups of neighbouring molecules.
In the smaller molecules the hydrogen bonding is more important. As the
molecular mass increases the effect of the polar hydroxyl group decreases and
the non-polar alkyl group determines the properties. Alcohols are non-conductors
and are neutral when liquid or in aqueous solution as the hydroxyl group does
not readily donate or accept protons.
Melting and boiling points are much higher than the corresponding alkanes
because of the hydrogen bonding present in alcohols.
Because hydrogen bonds can form with water and the hydroxyl group of an
alcohol
 Alcohols containing 1 to 3 carbon atoms are very soluble in water
 Alcohols containing 4 carbons are soluble in water
 Alcohols containing 5 o 6 carbons are slightly soluble in water
Alcohols with more than 6 carbons are insoluble in water because of their
relatively large non-polar alkyl regions. Alcohols are useful as commercial solvent
because they can dissolve both polar and non-polar substances.
Reactions:
 Combustion to produce carbon dioxide and water.

Oxidation to form aldehydes and carboxylic acids (primary) or ketones
(secondary). Oxidising agent: acidified dichromate solution.

Chlorination (Lucas Test) – means of distinguishing primary, secondary and
tertiary alcohols. Chlorinating agent: concentrated HCl with ZnCl2 as catalyst.

Substitution reactions: OH group replaced by halogen to produce alkyl halide.
Suitable agents are: SOCl2, HX, PX3, PCl5. (where X = Cl, Br or I). SOCl2 is
most useful with larger alcohols, HX is not used for iodides, but PX3 is largely
used for iodides. (see page 296 of Senior Chem Bk 2)

Dehydration to form alkenes.

Formation of esters. Alcohols react with carboxylic acids to produce esters.
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