Chirality and carbonyl evidence
Introduced| English | Meaning |
|---|---|
| enantiomers/eˈnæntɪəməz/ | Stereoisomers that are mirror images of each other and cannot be superimposed. |
| chiral centre/ˈkaɪrəl ˈsentə/ | An atom, commonly carbon bonded to four different groups, around which arrangements can produce non-superimposable mirror images. |
| carbonyl/ˈkɑːbənaɪl/ | The functional group consisting of a carbon atom double-bonded to an oxygen atom. |
| racemate | An equal mixture of the two enantiomers, optically inactive overall. |
What does zero rotation mean?
- An equal mixture of two enantiomers 对映异构体 has no net optical rotation. An achiral substance also has no rotation.
- Use structure as well as optical evidence before deciding which explanation fits.
Zero optical rotation proves that a sample is a racemic mixture.
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An achiral substance is another explanation, so structure or further resolving evidence is needed.
Identify a chiral centre 手性中心
- A tetrahedral carbon with four different groups is chiral in these examples. Butan-2-ol has H, OH, CH₃ and CH₂CH₃ at carbon 2.
- Propan-2-ol has two identical methyl groups and is achiral. A flat formula cannot determine the measured sign of optical rotation.
Which has a chiral carbon?
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Butan-2-ol has four different groups at carbon 2; the other proposed central carbons do not.
Combine carbonyl 羰基 tests
- Both aldehydes and ketones give a 2,4-DNPH precipitate. An aldehyde also gives Tollens’ silver or warm Fehling’s brick-red precipitate.
- A ketone usually resists those reducing tests. LiAlH₄ in dry ether gives a secondary alcohol from a ketone after appropriate work-up.
C₃H₆O with two carbon-13 signals, positive iodoform and no Tollens’ silver fits propanone: two equivalent methyl carbons and one carbonyl environment.
LiAlH₄ reduction of propanone gives propan-2-_____.
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Reduction converts C=O into the secondary alcohol group.
Follow cyanide electron pairs
- CN⁻ attacks the carbonyl carbon through its carbon lone pair, while the C=O π pair moves onto oxygen.
- The O⁻ intermediate takes H⁺ from HCN, regenerating CN⁻. Ethanal gives CH₃CH(OH)CN; either carbonyl face is accessible under achiral conditions.
Which two statements describe CN⁻ addition?
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The nucleophile donates its pair. The oxygen intermediate then gains a proton to form the hydroxynitrile.
Check whether the product is chiral
- Ethanal addition makes a carbon attached to OH, H, CH₃ and CN, producing a racemate 外消旋体 in the ideal achiral model.
- Propanone addition gives two identical methyl groups at that carbon and no chiral centre. The iodoform test identifies a compatible structural feature, not one unique compound.
“Every carbonyl addition product is chiral” is false. Count the four groups on the new tetrahedral carbon.
Use complete test observations, atom connectivity and three-dimensional groups as independent constraints.
Why is the propanone–HCN addition product achiral?
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Two identical groups prevent that carbon from being a chiral centre.