手性和羰基证据。
引入
词汇
| English | 释义 | 拼音 |
|---|---|---|
| enantiomers/eˈnæntɪəməz/ | 对映异构体 | duì yìng yì gòu tǐ |
| chiral centre/ˈkaɪrəl ˈsentə/ | 手性中心 | shǒu xìng zhōng xīn |
| carbonyl/ˈkɑːbənaɪl/ | 羰基 | tāng jī |
| racemate | 外消旋体 | wài xiāo xuán tǐ |
English
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.
练习
测得零旋光度只能证明样品是外消旋混合物。
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非手性物质也是一种可能的解释，因此需要分析结构或进一步的光学分辨证据。
English
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.
练习
哪个分子含有手性碳原子？
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丁烷-2-醇在碳2上连接有四个不同的基团；其他候选中心碳原子则不具备此特征。
English
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.
Example
C₃H₆O with two carbon-13 signals, positive iodoform and no Tollens’ silver fits propanone: two equivalent methyl carbons and one carbonyl environment.
练习
LiAlH₄还原丙酮可得到丙烷-2-_____。
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该还原反应将C=O转化为仲醇官能团。
English
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.
练习
哪两个陈述描述了CN⁻的加成反应？
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亲核试剂提供电子对。随后氧中间体获得质子形成羟基腈。
English
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.
Watch out
“Every carbonyl addition product is chiral” is false. Count the four groups on the new tetrahedral carbon.
Key idea
Use complete test observations, atom connectivity and three-dimensional groups as independent constraints.
练习
为什么丙酮与HCN的加成产物没有手性？
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两个相同的基团阻止该碳原子成为手性中心。