QUESTION 143 (4 marks)
The neutral structural formula of alanine is shown. Its central carbon is attached to H, NH₂, CH₃ and COOH.
(a) Explain why this carbon is chiral. [2 marks]
(b) Draw the two optical isomers using wedge and dashed bonds, or another clear three-dimensional representation. [2 marks]
Practice marking scheme
Answer
The central carbon is tetrahedral and has four different groups. The two isomers are non-superimposable mirror images.
Working
A tetrahedral carbon bearing H, NH₂, CH₃ and COOH is a stereocentre because all four groups differ. Its two mirror-image arrangements cannot be superimposed. Interchanging two groups at this centre reverses the configuration while preserving every bond connection. No R/S naming is required.
Marking criteria
- Identifies the tetrahedral central carbon. [1 mark]
- Identifies its four different attached groups as the basis of chirality. [1 mark]
- Draws one valid three-dimensional arrangement with the correct four groups. [1 mark]
- Draws its non-superimposable mirror image, preserving connectivity. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.