3. Isomerism - Types of isomer PDF

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Types of isomer...

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Organic Chemistry: Isomers Isomer – Different molecules with the same molecular formula. There are a number of different forms of isomerism.

Constitutional Isomers Also known as structural isomers, constitutional isomers have the same molecular formula however the bond orders and thus connections of atoms are different. For example whilst both butane and 2-methylpropane have the molecular formula C 4H10, the connections of the carbon atoms are different, as whilst butane is a straight chain alkane, 2-methylpropane has a methyl side branch on the 2 carbon. These differences have impacts on the physical properties of molecules, such as their boiling points, which can be used to distinguish between constitutional isomers.

Stereoisomers Stereoisomers have the same bond connectivity but different arrangements in space, and can be classified as either enantiomers or diastereomers. Enantiomers Enantiomers are a subclass of stereoisomer in which a molecule cannot be superimposed on its mirror image. This is due to the chirality of the molecule or the plane of substituent groups. A pair of enantiomers have mostly the same physical properties (such as melting/boiling point, solubility etc.), however they differ in their interactions with other chiral molecules, and their optical activity (interaction with plane polarised light).

Enantiomers can be described as chiral. Diastereomers Diastereomers are all other stereoisomers that are not enantiomers, that is they are not mirror images of one another. An example of diastereomers is that of conformational isomers. Diastereomers can be described as achiral. Unlike in enantiomers, where the pair of molecules are mirror images, a pair of diastereomers are not and thus can have completely different physical and chemical properties from one another

Conformational Isomers They have free rotation around a sigma σ bond, and thus an infinite number of conformations are possible via rotation. Each conformation is called a conformer, and each different conformer is an isomer, as they differ by the arrangement of atoms in space. Conformational isomers can be easily visualised by the use of a Newman projection, which takes the viewpoint of looking directly down a carbon-carbon bond, with the attachments on the sides. There are two primary conformations, these being staggered and eclipsed. In a staggered conformation, the attachments are spaced as far apart as possible and these reduced steric interactions lower the molecule’s energy, thus making it more stable. On the other hand, an eclipsed conformation has high energy and is less stable.

Long-chain alkanes have their most stable conformation as an anti-staggered conformation. This is a staggered conformation where the two largest groups are at a planar angle of 180°. This results in the signature zig-zag shape of the long-chain alkane.

Separation of Enantiomers In a racemic mixture (containing both enantiomers of a molecule in equal amounts), it can be very difficult to separate enantiomers as they have the same physical and chemical properties. This is problematic as in many situations only one of the enantiomers is desirable for its interactions with other molecules. One way to conveniently separate the enantiomers is by converting them to diastereomers, as they will then have a different shape and properties, making them easily separable. After separation, they can then be retransformed back into their respective enantiomers....