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Download Organic Isomers - Winston Knoll Collegiate
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Isomers lsomers are different compounds that have the same molecular formula. Being different compounds means that they have different physical properties (melting point, boiling point etc.). They may also have very different chemical properties depending on the type of isomerism present. Isomers may be divided into: Structural isomers, in which the atoms are joined in a different order so that they have different structural formula, and stereoisomers, in which the order in which the atoms are joined is the same, but the molecules have a different spatial arrangement of atoms and hence different 3-D shapes. Structural isomers can be further sub-divided into positional isomers, hydrocarbon chain isomers and functional group isomers; stereoisomers are sub-divided into geometric isomers and enantiomers or optical isomers. The relationship between the different types of isomerism is illustrated below. Examples of structural isomers could be: ! i) 1-propanol and 2-propanol (positional isomers) ! ii) butane and methylpropane (hydrocarbon chain isomers) ! iii) ethanol and dimethyl ether (functional groups isomers) ! iv) 3-hexene and cyclohexane (functional isomers) Positional Isomers have the same hydrocarbon skeleton and the same functional group, it is just that the functional group is joined to a different part of the skeleton. A simple example of this kind of isomerism is 1-propanol and 2-propanol. ! ! ! ! ! ! ! ! ! OH ! ! CH3 - CH2 - CH2 - OH! ! ! ! ! CH3 - CH - CH3 ! 1- propanol! ! ! ! ! ! 2- propanol Hydrocarbon Chain Isomers have different hydrocarbon skeletons that the functional group is attached to. Butane and methylpropane show this kind of isomerism. ! ! ! ! ! ! ! ! ! CH3 - CH2 - CH2 - CH3! butane! ! ! ! ! ! ! ! ! ! ! ! CH3 CH3 - CH - CH3 methylpropane Because of their nature, some functional groups will usually have isomers belonging to a different functional group. For example alcohols, such as ethanol, usually have an ether that is isomeric to them, in this case dimethyl ether. ! ! ! CH3 - CH2 - OH! ethanol! ! ! ! ! ! ! ! ! ! ! ! CH3 - O - CH3 ! dimethyl ether Other common pairs of functional groups that frequently display functional group isomerism with each other are: alkene - cycloalkane ! ! ! C2H5 - CH = CH - C2H5 3-hexene! ! ! ! ! ! cyclohexane aldehyde - ketone ! ! ! ! O CH3 - CH2 - CH! propanal ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! O CH3 - C - CH3 propanone carboxylic acid - ester ! ! ! O! ! CH3 - C - OH!! ethanolic acid! O CH3 - O - CH methyl methanoate ln the case of positional and hydrocarbon chain isomers, the functional group which usually dictates the reactivity of the molecule, is unchanged therefore they have quite similar chemical properties. With functional group isomerism, the change in the functional group can have a profound effect on both the physical and chemical properties of the molecule. In the example above, ethanol is a liquid (at room temperature and pressure) that will react with sodium. Dimethyl ether is a gas (at room temperature and pressure because of no H-bonding) that does not react with sodium. Similarly methyl methanoate is a sweet smelling liquid that forms neutral solutions, while ethanoic acid has a sharp smell (vinegar) and forms acidic solutions. Sterioisomers The most common type of geometric isomers are "cis-trans isomers" where atoms are bonded on the same side "cis" or "trans" meaning opposite. If a carbon atom has four different groups attached to it then there are two different ways in which these groups can be arranged around this carbon atom, which is known as an asymmetric carbon atom or a chiral centre (from the Greek word for “hand”)' The two forms of the molecule, which are known as enantiomers or optical isomers, are mirror images of each other, but cannot be superimposed on each other (like a pair of gloves). This is illustrated below using the amino acid alanine (2-aminopropanoic acid) as an example. The asymmetric carbon atom has a * on it: Mirror images Note that all four groups must be different for this to occur so that glycine (NH2 - CH2 - COOH), in which the methyl group in alanine has been replaced by a second hydrogen atom do not exist as enantiomers. All other 2-amino acids exist as two enantiomeric forms. Because these molecules are so similar, there is very little difference in their physical and chemical properties. In fact, the only difference is that they have differing effects on polarized light, one isomer rotating the plane of polarization clockwise, the other counter clockwise. This can be detected using an instrument known as a polarimeter. Biological systems are much more sensitive to the shape of the molecule and so they tend to form only one of the pair of enantiomers and have different effects.
