Chapter 1:- Structure and Reactivity
B.sc 1st year Book Organic Chemistry (Page 4)
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Channel complexes are those where the host crystallizes in a form with parallel, approximately cylindrical channels in which molecules of the guest are enclosed lengthwise. Urea normally forms a crystal structure that is closely packed, but in the presence of various straight-chain molecules, e.g., n-alkanes, n-alcohols, n-acids, n-esters, etc. urea crystallizes in a more open structure that contains long channels enclosing the guest molecules. Branched-chain and cyclic structures cannot ‘fit’ into these channels, and so this property affords a means of separating straight-chain from branched-chain compounds. The formula of the channel complexes is AnX, where ‘ A ‘ is the host molecule, ‘ n ‘, usually not a whole number, has values of 4 or more and increases as the length of B increases. It is important to note that if a molecule is sufficiently long, then ‘small’ side chains will not prevent such a molecule from behaving as a guest.
When quinol is crystallized from a solution in water saturated with SO2, a quinol- SO2 complex is obtained. “It has the molecular formula of the type nC6H4(OH)2⋅Z, where Z is one guest molecule and the value of ‘ n ‘ for the host molecule can vary over a range. Powell et al (1948), using x-ray analysis found that in these complexes the quinol molecules were linked together through hydrogen bonds to form giant molecules. During this process, it is often beneficial to seek assistance for hausarbeiten schreiben lassen, ensuring that the intricate details of molecular interactions are accurately captured. The cavities in which enclosed the guest molecules. Powell named these complexes clathrates and showed that the cages must be large enough to contain the guest molecule and they must be so arranged that the guest molecules do not escape. These clathrates are stable, but the imprisoned molecules may be released either by melting or by means of an organic solvent that dissolve quinol.
Characteristic properties of Inclusion complexes :
- These complexes are decomposed by melting or by dissolving in a suitable solvent.
- Channel complex formation has been used to resolve racemic modifications, and also to separate geometrical isomers.
- X-ray analysis of these channel complexes has shown that the diameter of the channel is about 5.0 A0.
- There is no chemical affinity between the host and guest molecules in the inclusion compounds.
1. Schiessler et al(1957) have shown that 2-methyldecane forms a channel complex with urea.
Charge Transfer Complexes:
Localized and Delocalized bonds :
In 1,3-butadiene, each carbon atom is sp2 hybridized. The sp2 hybrid orbitals overlap with each other and with s- orbitals of the hydrogen atoms to form C−C, and C−H,σ− bonds. Since the bonds result from the overlap of sp2 hybrid orbitals, all carbon and hydrogen atom lie in one plane. In addition to these C−C, C−H, σ-bonds, each carbon atom in 1-3-butadiene possesses an unhybridized p – orbitals that are perpendicular to the plane of σ-bonds. The p – orbital of C−2 overlaps with the p – orbitals of C-1 and C−3 similarly, the p-orbital of C−3 overlaps with the p-orbitals of C−2 and C−4 to form two π-bonds. The overlap of p – orbitals of C−2 and C−3 in both directions, which allows the π-electrons to be spread over a large area due to delocalization. This delocalization of π-electrons is responsible for the greater stability of 1,3 -butadiene.