Evidências de quiralidade e carbonila
Introduzido| English | Significado |
|---|---|
| enantiomers/eˈnæntɪəməz/ | enantiômeros |
| chiral centre/ˈkaɪrəl ˈsentə/ | centro quiral |
| carbonyl/ˈkɑːbənaɪl/ | grupo carbonila |
| racemate | racemato |
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.
Rotação óptica nula prova que a amostra é uma mistura racêmica.
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Uma substância aquiral é outra explicação, portanto estrutura ou evidência adicional de resolução é necessária.
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.
Qual possui um carbono quiral?
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O butan-2-ol tem quatro grupos diferentes no carbono 2; os outros propostos não têm.
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.
A redução de LiAlH₄ da propanona produz propan-2-_____.
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A redução converte C=O no grupo álcool secundário.
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.
Quais duas afirmações descrevem a adição de CN⁻?
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O nucleófilo doa seu par. O intermediário de oxigênio então ganha um próton para formar o hidroxinitrilo.
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.
Por que o produto de adição da propanona–HCN é aquiral?
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Dois grupos idênticos impedem que esse carbono seja um centro quiral.