Host-Guest Addition Compounds

Certain organic solids such as urea, thiourea, hydroquinone, etc. have crystalline shapes. However, in
some cases, the crystalline shapes of these compounds undergo a change in presence of certain other
compounds. The former compounds whose crystalline structures change are called hosts while the
latter ones in whose presence the crystalline structures of the former change are called guests. 
Depending upon the type of the space available within the crystal lattice of the host molecule the hostguest
addition compounds are divided into following two categories:
1- Inclusion compounds: 
 2-Clatherates:
 Inclusion compounds:
The host-guest addition compounds are known as inclusion compounds
when the space available within the crystal lattice of host molecule is in the form of long channels.
Urea as host:
 The most common host molecule for these compounds is urea. The diameter of the urea channel is about 500 pm and the type of guest molecules will depend upon their shape and size only. Some examples are illustrative.
 Thiourea as host:
Thiourea also forms inclusion compounds with channels of larger diameter.
Consequently, n-alkanes cannot act as guests for thiourea. However, 2-bromooctane, cyclohexane,
chloroform etc., have the right size and shape to be trapped in the channels of thiourea.
 Amylose as host: 
 Starch gives a deep blue colour with iodine, the blue colour is due to the formation of an inclusion complex between amylose and iodine
Uses of inclusion compounds:
Some of the more important uses of inclusion compounds are as
under:
(i) These complexes are quite useful in separating certain isomers that would be otherwise
difficult to separate. For example, n-octane can be separated from its branched chain isomers,
because only n-octane can form inclusion compound with urea.
(ii) Urea inclusion compounds have also been used for resolving racemic mixtures. For example,
(±) racemic mixture of 2-chlorooctane forms two different inclusion compounds
(diastereomeric) which can be separated by fractional crystallization.