Topic 15 · page 2 of 28
Drawing enantiomers
A 3D diagram uses plain lines, a wedge and a dashed line around the chiral carbon. The mirror image swaps left and right while the wedge stays a wedge and the dash stays a dash.
Read first: Chiral centres
The three kinds of bond 15.2
Paper is flat, so a 3D diagram uses three bond symbols around the chiral carbon:
| Symbol | Meaning |
|---|---|
| plain line | the bond lies in the plane of the paper |
| solid wedge, wide end at the group | the bond points towards you, out of the paper |
| dashed line | the bond points away from you, behind the paper |
A tetrahedral carbon has four bonds. Draw two in the plane (as a V shape), one wedge and one dash. This is the standard layout and the one a mark scheme expects.
Drawing the mirror image 15.2
To draw the second enantiomer, reflect the first one in a vertical mirror line:
- Draw the first molecule with the chiral carbon, two plain bonds, one wedge and one dash.
- Draw a dashed vertical line to the right as the mirror.
- Redraw each group on the opposite side of the mirror line, so left becomes right and right becomes left.
- Keep each group on the same type of bond. A wedge stays a wedge and a dash stays a dash, because a mirror does not turn the molecule front to back.
MethodCheck that you have drawn the other enantiomer
Compare your two drawings by finding any pair of groups that are in the plane in both. If you now turn one molecule so those two groups match, the wedge group and dash group must be the other way round. If they match as well, you have drawn the same molecule twice.
The wedge and dash groups can be any two of the four. What matters is that the two diagrams are mirror images and not the same.
Check yourself
- Draw 2-chlorobutane as a 3D diagram and then draw its mirror image. Describe your drawing in words: which groups are in the plane and which are on the wedge and dash?
- A student draws two diagrams of CHFClBr. Both have Br and Cl in the plane. In the first, F is on a wedge and H on a dash. In the second, F is on a dash and H on a wedge. Are these the same molecule or enantiomers?
- A student draws the second enantiomer by keeping every group in the same left or right position but turning the wedge into a dash and the dash into a wedge. Is this a valid way to draw the enantiomer? Explain.
- Explain why swapping any two groups on a chiral carbon gives the enantiomer.
Answers cover these until you have tried the questions
- Chiral carbon C2 holds H, Cl, CH₃ and CH₂CH₃. One valid drawing has Cl and H in the plane (a V shape), CH₃ on a wedge and CH₂CH₃ on a dash. The mirror image has Cl and H swapped left to right, and CH₃ and CH₂CH₃ also swapped left to right, with CH₃ still on a wedge and CH₂CH₃ still on a dash. Equally, you may draw the second molecule by swapping any two groups of the first.
- Enantiomers. The two diagrams differ only by swapping two groups (F and H) on the chiral carbon, and CHFClBr has four different groups on carbon, so it is chiral. Swapping two groups gives the mirror image.
- Yes. Swapping the wedge and dash groups exchanges two groups on the chiral carbon, and swapping any two groups gives the mirror image. It is the same as reflecting the molecule in the plane of the paper.
- The four groups occupy four positions on a tetrahedron. Exchanging two of them reverses the handedness of the arrangement, exactly as a mirror does, so the new arrangement is the non-superimposable mirror image.