Chiral centres and geometrical isomers
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The teaching part (2 min) is free to watch. A free account opens the whole lecture: 1 worked example, 2 practice questions, 5 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Left hand, right hand
- Stereoisomerism
- Chiral centres
- Finding the chiral centre
Read the transcript of the teaching part
Last time
Before we start: last time, in Nitrogen, ammonia and oxides of nitrogen, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Left hand, right hand
Your left and right hands are mirror images, but you cannot lay one exactly over the other. Some molecules are just like that: two forms that are mirror images and do not superimpose. In the body, the two forms can have different effects: one flavour, one tasteless; one medicine, one harmful. That is why stereoisomerism matters.
Stereoisomerism
Stereoisomers have the same structural formula, but a different arrangement of the atoms in space. There are two kinds: geometrical isomerism, cis and trans, and optical isomerism. Contrast with structural isomers, which differ in how the atoms are joined.
Chiral centres
A chiral centre is a carbon atom bonded to four different atoms or groups. A molecule with one chiral centre exists as a pair of optical isomers, or enantiomers. They are non-superimposable mirror images. They rotate the plane of plane-polarised light in opposite directions. A mixture with equal amounts of the two is a racemic mixture. It has no effect on polarised light.
Finding the chiral centre
Draw the structure. For each carbon with four single bonds, list the four groups. If all four are different, it is chiral. If two are the same, it is not. In 2-bromooctane, carbon 2 has H, Br, CH three and a C six H thirteen chain. All four are different, so it is chiral. Draw the two enantiomers using wedges and dashes, as mirror images of each other.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Chiral centres and optical isomers.
The whole chapter: An Introduction to AS Level Organic Chemistry, An Introduction to AS Level Organic Chemistry.
Chapter 13: Introduction to organic chemistry
- Functional groups, formulae and structural isomers 6 min
- Chiral centres and geometrical isomers 7 min
Also for A Level Chemistry 9701
Which topics come up most in A Level Chemistry 9701 past papers, counted from every paper since 2004: Paper 2 topic frequency, Paper 4 topic frequency.