Loading...
A common chiral centre is a tetrahedral carbon attached to four different groups. Compare complete groups, not just the first atom: CH3 and CH2CH3 both begin with carbon but are different groups. Enantiomers are non-superimposable mirror images. A wedge-and-dash diagram shows bonds pointing out of and behind the page. Rotating a molecule in space does not create a new enantiomer; exchanging the positions of two groups changes its handedness.
Video by Eliot Rintoul. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Identify a chiral centre
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What is a racemic mixture?
Equal amounts of two enantiomers. Equal opposite rotations cancel.
After watching 2: What must a simple chiral carbon have?
Four different attached groups. Its tetrahedral arrangement can then have a non-superimposable mirror image.
What you'll be able to do
A common chiral centre is a tetrahedral carbon attached to four different groups. Compare complete groups, not just the first atom: CH3 and CH2CH3 both begin with carbon but are different groups. Enantiomers are non-superimposable mirror images. A wedge-and-dash diagram shows bonds pointing out of and behind the page. Rotating a molecule in space does not create a new enantiomer; exchanging the positions of two groups changes its handedness.
Enantiomers rotate plane-polarised light by equal magnitudes in opposite directions under the same conditions. A racemic mixture contains equal amounts of both and has no net rotation because their effects cancel. This does not mean its molecules have ceased to be chiral. The direction of optical rotation cannot be inferred from a drawing merely by deciding which group is on the left. Ordinary boiling points and many reactions in an achiral environment can be the same for enantiomers.
Biological receptors and enzymes are chiral and can interact differently with two enantiomers. One enantiomer may have a different biological effect, so an asymmetric synthesis or a separation can be valuable. Nucleophilic addition to a planar carbonyl can allow attack from either side, producing a racemate when a new chiral centre forms and the environment provides no preference. Evaluate actual structure: a product with two identical groups at the carbonyl-derived carbon is not chiral merely because addition occurred.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Why can reducing propanone not create a chiral centre in propan-2-ol?
The product is not chiral.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.