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Chemistry 102
Organic Chemistry: Introduction to Isomers Workshop
What are isomers?
Isomers are molecules with the same molecular formula, but different arrangements of atoms. There are different types of isomers, shown by the diagram on the right.
“Structural” isomers are widely called “conformational” isomers. The latter term is preferred in the IUPAC system of nomenclature.
Structural Isomerism
Structural isomerism occurs when two or more organic compounds have the same molecular formulae, but different structures. These differences tend to give the molecules different chemical and physical properties. There are three types of structural isomerism that you need to be aware of: chain isomerism, positional isomerism and functional isomerism. There is a fourth type, known as tautomerism (where there are two isomers are known as the keto and enol isomers) that will not be introduced here.
Chain Isomerism
Chain isomerism occurs when the way carbon atoms are linked together is different from compound to compound. It is also called nuclear isomerism.
There are three chain isomers of C 5 H 12 shown below. Note that these isomers have the same empirical formula as pentane, but different conformations.
pentane 2-methylbutane 2,2-dimethylpropane
Positional Isomerism
Positional isomerism, another type of structural isomerism, occurs when functional groups are in different positions on the same carbon chain. Positional isomers of alcohols, alkenes, and aromatics are common. Below are models of the positional isomers of butanol, butene and methylphenol:
1-butanol 2-butanol
1-butene 2-butene
Note: this is cis -2-butene, which has a geometric isomer called trans -2- butene
2-methylphenol 3-methylphenol 4-methylphenol
- where there is a >C=N bond.
An interesting example of geometric isomerism caused by rings is found in sugars such as glucose, fructose, mannose and galactose.
Below are models of the geometric isomers of 1- butene and 2-butene. 1-Butene does not form geometric isomers, even though it has a C=C bond, because one of the double-bonded carbon atoms has two identical groups on it (hydrogen atoms in this case). 2-Butene does form geometric isomers because each double-bonded carbon atom has two different groups on it. The cis- and trans- prefixes are used to differentiate the positions of the functional groups.
cis -1-butene trans -2-butene Where like groups are on the same side of the double bond, we call it a cis isomer; where they are on opposite sides we call it a trans isomer.
1-butene (^) Although 1-butene contains a C=C bond, it does not form geometrical isomers.
Take care - look for different groups on the double-bonded carbon atoms!
Optical Isomerism
Optical isomerism occurs when substances have the same molecular and structural formulae, but one cannot be superimposed on the other. Put simply, they are mirror images of each other (see the diagram on the right). No matter how hard you try, the molecule on the left will not turn into the molecule on the right – unless you break and make some bonds! Molecules like this are said to be chiral (pronounced ky-ral), and the different forms are called enantiomers.
Optical isomers can occur when there is an asymmetric carbon atom. An asymmetric carbon atom is one which is bonded to four different groups. The groups can be something hideously complex, or something comfortably simple like a hydrogen or chlorine atom. Remember:
- there must be four groups , and
- they must be different.
Optical isomers can rotate the plane of polarization of plane-polarized light:
- one enantiomer rotates the polarized light clockwise (to the right) and is the (+) enantiomer;
- the other rotates the polarized light anticlockwise (to the left) and is called the (–) enantiomer.
A mixture containing equal concentrations of the (+) and (–) enantiomers is not optically active (it will not rotate the plane of polarization). It is called a racemic mixture or racemate.
Below are models of the optical isomers of 2-hydroxypropanoic acid (lactic acid). Lactic acid is a fairly common and simple example of optical isomerism. The (+) enantiomer of lactic acid is found in muscle. Sour milk contains a racemic mixture of the two enantiomers.
(+)-lactic acid (–)-lactic acid
The dashed lines show bonds going into the screen; the wedges show bonds coming out of the screen.
Below is 2,3-dihydroxypropanal (glyceraldehyde). Glyceraldehyde is used as a standard by which other chiral molecules are compared. There are two enantiomers of glyceraldehyde, depending on the position of the –OH (hydroxyl) group on the molecule. These are known as D-glyceraldehyde and L-glyceraldehyde. The little capital letters D and L are deliberate. The positions of the hydroxyl groups on other chiral molecules can be compared with glyceraldehyde to see if they are the D-enantiomer or the L-enantiomer. This is very common in biochemistry. For example, natural sugars are D-enantiomers and amino acids are L- enantiomers. However, knowing whether a molecule is the D or L-enantiomer does not tell us whether it is the (+) or (–) enantiomer – so be careful!
D -glyceraldehyde L-glyceraldehyde
Remember – these are reference molecules; the D and L signs do not tell you if the enantiomer is (+) or (–).
