The two mirror image representations of a carbon atom with four different groups attached to it are depicted in figure 1.6. Imagine aligning the grey carbon atoms and the red and green groups of the two molecules; the blue and white groups are now opposite each other. As expected, the molecule is chiral. Mirror image related molecules are called enantiomers.
A chiral carbon atom and its mirror image.
Activity 1 Enantiomers
Context: The importance of chirality
Previously (ST240, Book 1, page 105, Activity 10), you have seen that organic molecules can change their shape through rotations around single bonds. As no bonds need to be broken in this process, the different arrangements do not create new compounds. By contrast, keeping the same atoms but altering the way they are bonded to each other creates isomers.
n Are enantiomers isomers?
Compounds with chiral carbon atoms fall within a class of isomers called stereoisomers. As with all isomers, a pair of stereoisomers contain the same atoms. Unlike other classes of isomers, the atom connectivities in a pair of stereoisomers is also the same. Stereoisomers only differ in the three dimensional arrangement of the groups bonded to one or more atoms in the molecule.
Activity 2 Drawing three dimensional structures
Objects which can be distinguished from their mirror images are asymmetrical. They lack a plane, axis or centre of symmetry.
Individual carbon atoms which are bonded to four different atoms or groups are asymmetrical. They are said to be chiral.
A chiral compound and its mirror image are called enantiomers.
Enantiomers belong within a class of isomers called stereoisomers.
Stereoisomers can be drawn using filled and dashed wedges to represent groups that lie in front or behind the plane of the paper.
Chiral compounds interact with other chiral compounds in different ways. This is particularly important for molecules interacting within a biological environment, where many of the native molecules are chiral. Chiral molecules rotate plane polarised light by exactly opposite amounts.
A Forward Glance
You will use the concepts presented in this unit in future parts of your course. They will become important when considering the following areas:
Isomerism of alkenes
Substitution reactions of alkyl halides
Addition reaction to alkenes, alkynes and carbonyl compounds
The chemistry of biomolecules such as carbohydrates, amino acids and proteins.
Question 2 Which of the following molecules have a chiral carbon atom
Question 3 Draw the two enantiomers of the amino acid alanine.