The carbon atom in such species is often said to be chiral but this is a misnomer. It is not the carbon atom that is chiral but rather the environment around the carbon atom. The property of chirality, as we have seen, is independent of chemistry and, for that matter, atoms. In the enantiomers 12 and 13 shown below How to manipulate JSmol structures , imagine that the black "ball" is totally invisible against the black background.
The four colored balls form a chiral environment independent of the black ball and the "sticks" that keep the four colored balls spaced apart. A ball achiral in a hand chiral is chiral. In summary, the most common source of chirality in organic chemistry is the "asymmetric carbon", or, better yet, the carbon at a center of asymmetry.
We showed that the carbon only serves the capacity of keeping the four different atoms, or colored balls if you wish, where they belong. But an asymmetric carbon is not the only source of chirality. Conformations of molecules, and molecules themselves, can be chiral without having an asymmetric carbon.
Consider the gauche conformations 16 and 17 of n-butane as an example of a chiral molecular conformation.
These two conformations constitute a racemate whose enantiomers are rapidly interconverting by rotation about the C 2 -C 3 sp 3 hybridized bond. The formation of enantiomers by bond rotation in achiral molecules is called stochastic random chirality [Mislow]. At ambient temperaturethey cannot be separated.
The net racemic gauche conformation and the achiral anti conformation are the major conformations of n-butane, which of course is optically inactive. Gauche butane defines a screw axis, one conformation is left-handed and the other is right-handed. Structures 18 and 19 are the two enantiomers of 1,3-dimethylallene.
Although they define a screw axis, the sp hybridized central carbon prohibits rotation about the central, linear carbon axis C 2 -C 4. There is no asymmetric carbon to be seen in these structures. Diastereomers are stereoisomers that are not enantiomers. How is this possible? Consider two gray tetrahedral "asymmetric carbons" that are bonded to each other and each one has three atoms attached: red , yellow and green.
Structures 20a and 21a represent one staggered conformation of both possible arrangements. Each one is clearly a stereoisomer of the other because they both have the same atom connectivity. Rotation about the C-C bond of staggered conformation 20a , which is a chiral representation, by 60 o gives eclipsed conformation 20b , which clearly is achiral.
A mirror plane can be passed through the C-C bond. Another 60 o rotation of the eclipsed conformation 20b in the same direction produces the en an t iomer mirror image of 20a ,namely, ent - 20a. This is another example of stochastic chirality that was encountered above in the discussion of n-butane. Achiral structure 20 is called a meso compound. It cannot be separated into enantiomers because it is achiral and its mirror image is necessarily superimposable on itself.
On the other hand, the diastereomer of 20 -- namely 21 -- is chiral. In either the staggered conformation 21a or the eclipsed conformation 21b , or in any conformation you wish to make, there is no mirror plane. The toxicity is not due to hexane itself but to one of its metabolites, hexane-2,5-dione. It is believed that this reacts with the amino group of the side chain of lysine residues in proteins, causing cross-linking and a loss of protein function.
The effects of hexane poisoning in humans are uncertain. In , n-hexane was included in the list of chemicals on the Toxic Release Inventory.
In the latter part of the 20th and early part of the 21st centuries, a number of explosions have been attributed to the combustion of hexane gas. In , the U. Environmental Protection Agency issued regulations on the control of emissions of hexane gas due to its potential carcinogenic properties and environmental concerns. Home Hexane C6H C6H14 has 5 isomers in total, they are hexane, 2-methylpentane, 2-methylpentane, 2-methylpentane and 2,3-dimethylbutane.
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