1. The Chiral carbon Atom

Learning Objectives
Having completed this unit you should be able to:
1. Distinguish symmetrical and asymmetrical objects
2. Distinguish chiral carbon atoms
3. Interpret and draw three-dimensional representations of organic compounds
4. Discuss the importance of chirality in organic compounds
5. Define the emboldened terms within the text.

Learning Resources
In addition to these web pages, you will require access to your Molymod kit. The web pages contain interactive models that allow molecules to visualised in three dimensions. To view these models, you will require the Chime `plug-in' for your browser. Instructions on how to obtain Chime and an introduction to its functionality are found here. These web pages are an alternative to the paper-based text, they do not contain extra material.

Prior Learning
Before studying this text, you should be familiar with the concepts of bonds, molecules and the basic geometries found in organic compounds. This information was presented in course ST240 Our Chemical Environment. You might find it useful to review section 7.5 of Book 1 (`Models of Matter') of ST240 prior to studying this unit.


1.1 Symmetry and Asymmetry
Chiral carbon atoms are those that do not possess a centre of symmetry. Before progressing to look at asymmetrical carbon atoms, it is worth spending a short time developing an understanding of symmetry and asymmetry in the everyday world.

An object is symmetrical if there is a plane, axis or centre about which there is an exact correspondence of parts of the object. Such objects are said to have a plane, axis or centre of symmetry respectively. For example, if you draw a line running front to back down the middle of a chair, the components of the chair are exactly the same on each side, and equally spaced from the line. A chair thus has a plane of symmetry. Another way of recognising symmetrical objects is that they are indistinguishable from their mirror images. Imagine placing two identical chairs facing each other. They are mirror images, however when placed side by side they are indistinguishable.

Many objects, however, are asymmetrical; they are not the same as their mirror images. Perhaps the most obvious example is a pair of hands.



Figure 1.1
A pair of hands.

The two hands in figure 1.1 are exact mirror images of each other. Unlike the chairs, however, they are not identical: when placed side by side the thumbs, for instance, point in opposite directions. Many objects, and even individual molecules, possess this `hand-like' property. The Greek word for hand is chir, so chemists call asymmetrical molecules chiral.

n Is a plain mug chiral?

n Answer

n Is a mug marked on one side chiral?

n Answer

Why does marking a mug cause it to be chiral when a plain mug is not? Both mugs are based on the symmetrical shape of a cylinder. The plain mug has three unique features - a top, a bottom and a handle and retains a plane of symmetry bisecting the mug top to bottom, and passing through the middle of the handle. The marked mug has a fourth feature, the red X. This makes the two halves of the mug different and so symmetry is lost. The positioning of four unique features around a carbon atom also results in loss of symmetry, and so a carbon atom bonded to four different atoms or groups of atoms will be chiral.

Question 1 Which of the following objects are chiral?

Answers



1.2 Symmetry and Asymmetry at a Carbon Atom
The four carbon building blocks are summarised in figure 1.4. They provide geometries at the carbon atom that are tetrahedral, triangular planar and linear and are typified by the carbon atoms found within methane, ethene, carbon dioxide and ethyne. The methane molecule in figure 1.4 is drawn to give a three-dimensional effect using filled wedges representing bonds coming out of the page towards you and dashed wedges for bonds that are receding away from you. Bonds drawn with a thin line are within the plane of the paper.

Figure 1.4
The structures of methane, ethene, carbon dioxide and ethyne.

The two linear structures (carbon dioxide and ethyne) have an axis of symmetry along the direction of the carbon-carbon bond (figure 1.5a). The triangular planar arrangement contains a plane of symmetry passing through the carbon atom and all three groups attached to it (figure 1.5b). The axis and plane of symmetry in both cases is independent of the groups attached to the carbon atom. With the tetrahedral arrangement, however, there is only a plane of symmetry if two or more of the groups attached to the carbon atom are the same. When this is the case, the plane of symmetry passes through the carbon atom and two of the groups. The remaining two groups - which must be identical - are arranged symmetrically on either side of this plane (figure 1.5c). If all four are different, however, then the two groups either side of the plane are different, just as was the case with the marked mug in the example above, and the molecule is chiral (figure 1.5d).

Figure 1.5a) Linear molecules: Carbon dioxide and ethyne

Figure 1.5b) Triangular planar molecules: Ethene

Figure 1.5c) Symmetrical tetrahedral molecules

Figure 1.5d) Chiral tetrahedral molecules

Figure 1.5
Symmetry at carbon building blocks.

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.

Figure 1.6
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?

n Answer

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

Summary

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

Answer

Question 3 Draw the two enantiomers of the amino acid alanine.

Answer