📚 Mastering AQA 3.6 Organic Analysis: Tests, Mass Spectrometry and Infrared Spectroscopy | 掌握 AQA 3.6 有机分析：官能团检验、质谱与红外光谱
Organic analysis is where structure determination comes alive. In AQA 3.6, you must combine simple test-tube reactions with two powerful instrumental techniques: mass spectrometry and infrared spectroscopy. The key skill is using evidence from chemical tests, molecular mass data and bond vibrations to deduce the identity of an unknown organic compound.
有机分析是结构鉴定的核心环节。在 AQA 3.6 中，你需要把试管反应与质谱、红外光谱两种强大的仪器分析方法结合起来。关键技能是利用化学检验、相对分子质量数据和键振动信息，推断未知有机化合物的结构。
1. The Analytical Toolbox: From Wet Tests to Spectra | 分析工具箱：从试管反应到光谱
Organic analysis in AQA 3.6 has three strands. First, functional group tests give rapid qualitative evidence. Second, mass spectrometry provides the relative molecular mass and fragmentation patterns. Third, infrared spectroscopy identifies covalent bonds and functional groups from characteristic absorptions.
AQA 3.6 的有机分析包括三条主线。第一，官能团检验可以快速给出定性证据。第二，质谱提供相对分子质量和碎片信息。第三，红外光谱通过特征吸收识别共价键和官能团。
A complete exam answer usually needs two or more pieces of evidence. For example, a positive Tollens' test tells you an aldehyde is present, but the mass spectrum and IR spectrum are needed to confirm the full structure.
完整的考试答案通常需要两个或更多证据。例如，Tollens 试验呈阳性说明存在醛基，但仍需质谱和红外光谱才能确认完整结构。
2. Testing for Alkenes and Halogenoalkanes | 检验烯烃与卤代烷
dergoes electrophilic addition with Br₂. This test does not require UV light, unlike the substitution reaction of an alkane with bromine.
检验烯烃时，向样品中滴加几滴溴水并振荡。橙色的溴水褪色，因为 C=C 双键与 Br₂ 发生亲电加成反应。该检验不需要紫外光，这与烷烃和溴的取代反应不同。
To test for a halogenoalkane, warm the sample with aqueous sodium hydroxide. This hydrolyses the C-X bond to release halide ions. Acidify with dilute nitric acid, then add aqueous silver nitrate. A white precipitate shows chloride, cream shows bromide, and yellow shows iodide.
检验卤代烷时，将样品与氢氧化钠水溶液共热，使 C-X 键水解生成卤离子。加入稀硝酸酸化后，再加入硝酸银溶液。白色沉淀说明有氯离子，奶油色沉淀说明有溴离子，黄色沉淀说明有碘离子。
- AgCl: white precipitate, soluble in dilute NH₃
- AgBr: cream precipitate, soluble in concentrated NH₃
- AgI: yellow precipitate, insoluble in concentrated NH₃
AgCl 为白色沉淀，溶于稀氨水；AgBr 为奶油色沉淀，溶于浓氨水；AgI 为黄色沉淀，不溶于浓氨水。硝酸酸化的目的是除去多余的 OH⁻，防止生成 Ag₂O 干扰。
3. Identifying Alcohols and Carboxylic Acids | 鉴别醇与羧酸
Primary and secondary alcohols can be oxidised by acidified potassium dichromate, K₂Cr₂O₇, turning the solution from orange to green. Tertiary alcohols are not easily oxidised, so the solution remains orange. This gives quick evidence for the class of alcohol.
伯醇和仲醇可被酸性重铬酸钾 K₂Cr₂O₇ 氧化，溶液由橙色变为绿色。叔醇不易被氧化，因此溶液仍保持橙色。这可以快速判断醇的类型。
Carboxylic acids are easy to identify because they are weak acids. Add sodium carbonate or sodium hydrogencarbonate solution: effervescence of CO₂ is observed. Carboxylic acids also turn blue litmus red and have a pH of about 3-5 in aqueous solution.
羧酸很容易鉴别，因为它们是弱酸。加入碳酸钠或碳酸氢钠溶液，可观察到 CO₂ 气泡。羧酸还能使蓝色石蕊试纸变红，水溶液 pH 约为 3-5。
CH₃CH₂OH + 2[O] → CH₃CHO + H₂O → CH₃COOH
伯醇乙醇先被氧化为乙醛，继续氧化可生成乙酸。仲醇被氧化为酮，叔醇在该条件下不被氧化。
4. Distinguishing Aldehydes from Ketones | 区分醛与酮
Tollens' reagent, [Ag(NH₃)₂]⁺, is the classic test for aldehydes. Warm the sample with Tollens' reagent: an aldehyde is oxidised and a silver mirror forms on the test tube wall. Ketones give no reaction.
Tollens 试剂 [Ag(NH₃)₂]⁺ 是检验醛的经典试剂。将样品与 Tollens 试剂共热：醛被氧化，试管壁上形成银镜。酮不发生反应。
Fehling's or Benedict's solution is another test for aldehydes. The blue Cu²⁺ complex is reduced to a brick-red precipitate of copper(I) oxide, Cu₂O, when warmed with an aldehyde. Ketones again give no reaction.
Fehling 或 Benedict 溶液是另一种检验醛的试剂。与醛共热时，蓝色的 Cu²⁺ 络离子被还原为砖红色的氧化亚铜 Cu₂O 沉淀。酮同样不发生反应。
RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag + 4NH₃ + 2H₂O
醛被氧化为羧酸根离子，同时 Ag⁺ 被还原为银。注意 2,4-DNP 虽然能与醛和酮都生成橙色沉淀，但它只能证明羰基存在，不能区分醛和酮。
5. Mass Spectrometry: Molecular Ions and Fragmentation | 质谱：分子离子与碎片
In a mass spectrometer, a vaporised organic sample is bombarded with high-energy electrons. This knocks out an electron to form a positive molecular ion, M⁺. The m/z value of the molecular ion peak equals the relative molecular mass of the compound.
在质谱仪中，气化的有机样品被高能电子轰击。分子失去一个电子形成正分子离子 M⁺。分子离子峰的 m/z 值等于化合物的相对分子质量。
Molecular ions often fragment because they have excess energy. The molecule breaks at weak bonds to form a positive fragment ion and a neutral radical. The mass spectrum shows a molecular ion peak and several fragment ion peaks at lower m/z values.
分子离子通常因能量过高而发生碎裂。分子在较弱键处断裂，生成正碎片离子和中性自由基。质谱图中会出现分子离子峰和若干 m/z 较低的碎片离子峰。
When interpreting a mass spectrum, look for the highest m/z peak that is not an isotope peak. This is usually the molecular ion M⁺. Common fragment losses include 15 (CH₃), 29 (C₂H₅) and 17 (OH).
解析质谱时，要找到不是同位素峰的最高 m/z 峰，这通常是分子离子 M⁺。常见碎片丢失包括 15（CH₃）、29（C₂H₅）和 17（OH）。
6. High Resolution Mass Spectrometry and Exact Mass | 高分辨率质谱与精确质量
Low resolution mass spectrometry gives integer m/z values, so several compounds can share the same nominal mass. High resolution mass spectrometry measures exact masses to several decimal places, allowing you to determine the molecular formula directly.
低分辨率质谱给出整数 m/z 值，因此不同化合物可能具有相同的名义质量。高分辨率质谱可测量到小数点后多位，从而直接确定分子式。
otope has a characteristic exact mass: C = 12.0000, H = 1.00783, N = 14.0031, O = 15.9949.
例如，CO、N₂ 和 C₂H₄ 的名义质量都是 28，但精确质量不同。这是因为每种同位素都有特征精确质量：C = 12.0000，H = 1.00783，N = 14.0031，O = 15.9949。
| Species | Nominal m/z | Exact mass |
| CO | 28 | 27.9949 |
| N₂ | 28 | 28.0062 |
| C₂H₄ | 28 | 28.0313 |
In exam questions, you may be given the exact m/z of the molecular ion and asked to identify the molecular formula. Always show your working by calculating the exact mass for each possible formula.
在考试题中，可能会给出分子离子的精确 m/z 值，要求确定分子式。一定要写出计算过程，比较每种可能分子式的精确质量。
7. Infrared Spectroscopy: Bond Vibrations and Absorption | 红外光谱：键振动与吸收
Infrared spectroscopy detects the vibrations of covalent bonds. When a bond absorbs infrared radiation, it stretches or bends more energetically. The frequency absorbed depends on the bond strength and the masses of the bonded atoms.
红外光谱检测共价键的振动。当化学键吸收红外辐射时，其伸缩或弯曲振动增强。吸收频率取决于键的强度和成键原子的质量。
An IR spectrum plots transmittance against wavenumber in cm⁻¹. A downward peak means the bond absorbed IR at that frequency. The region below about 1500 cm⁻¹ is called the fingerprint region because it is unique to the whole molecule.
红外光谱图以透光率对波数 cm⁻¹ 作图。向下的吸收峰表示该化学键在此频率吸收了红外辐射。约 1500 cm⁻¹ 以下的区域称为指纹区，因为该区域对整个分子具有唯一性。
t AQA expects you to recognise the main absorptions for O-H, C-H, C=O, C=C, C-O, N-H and C≡N groups.
你不需要记住所有吸收峰，但 AQA 要求识别 O-H、C-H、C=O、C=C、C-O、N-H 和 C≡N 等主要官能团的特征吸收。
8. Key IR Absorption Bands and Spectra Interpretation | 关键红外吸收带与谱图解析
cohol or carboxylic acid. A sharp peak around 1680-1750 cm⁻¹ strongly suggests a C=O bond.
解析红外光谱时可参考下表。2500-3300 cm⁻¹ 附近很宽的峰通常表示醇或羧酸中的 O-H 键。1680-1750 cm⁻¹ 附近的尖锐峰强烈提示 C=O 键。
| Bond | Functional group | Wavenumber range / cm⁻¹ |
| O-H | Alcohol, carboxylic acid | 2500-3300, broad |
| C-H | Alkane, alkene, aromatic | 2850-3100 |
| C=O | Aldehyde, ketone, carboxylic acid, ester | 1680-1750, sharp |
| C=C | Alkene | 1620-1680 |
| C-O | Alcohol, ester, ether | 1000-1300 |
| N-H | Amine, amide | 3300-3500 |
| C≡N | Nitrile | 2220-2260 |
dehyde and a ketone both show C=O, so IR alone cannot always distinguish them; use Tollens' or Fehling's test.
注意：羧酸通常同时显示很宽的 O-H 吸收带和尖锐的 C=O 吸收带。醛和酮都会显示 C=O 吸收，因此仅靠红外光谱不能完全区分它们，需要结合 Tollens 或 Fehling 试验。
9. Using Spectra Together: Solving Unknown Structures | 综合谱图解析未知结构
In AQA exam questions, you are often given a mass spectrum, an IR spectrum and maybe a chemical test result. The best approach is to write down each piece of evidence separately, then combine them into a consistent structure.
在 AQA 考试题中，通常会给出质谱图、红外光谱图，有时还有化学检验结果。最佳做法是分别列出每项证据，再整合为一个一致的分子结构。
d. It turns acidified potassium dichromate from orange to green. After oxidation, the product gives a positive Tollens' test.
例题：某化合物 M⁺ 峰在 m/z 60。红外光谱在约 3350 cm⁻¹ 处有宽 O-H 吸收，但没有 C=O 吸收。它能使酸性重铬酸钾由橙色变为绿色。氧化后的产物与 Tollens 试剂反应呈阳性。
r mass of 60 fits C₃H₈O. Oxidation to an aldehyde means it must be a primary alcohol. Therefore the compound is propan-1-ol, CH₃CH₂CH₂OH.
综合数据：O-H 吸收和无 C=O 吸收说明是醇。相对分子质量 60 符合 C₃H₈O。氧化产物为醛，说明它是伯醇。因此该化合物为丙-1-醇，CH₃CH₂CH₂OH。
10. Required Practical Skills and Common Pitfalls | 必备实验技能与常见误区
AQA required practical 6 covers organic functional group tests. You should be able to describe the reagents, conditions, observations and conclusions for alkenes, alcohols, aldehydes, ketones and carboxylic acids.
AQA 必修实验 6 涵盖有机官能团检验。你应能描述烯烃、醇、醛、酮和羧酸的检验试剂、条件、现象和结论。
Common pitfalls include forgetting to acidify with nitric acid before adding silver nitrate in the halogenoalkane test, using excess bromine water so the colour change is unclear, and not warming Tollens' reagent gently. Always record observations before and after the reaction.
常见误区包括：在卤代烷检验中忘记先用硝酸酸化再加硝酸银；加入过量溴水导致颜色变化不清晰；以及 Tollens 试剂没有温和加热。务必在反应前后都记录观察结果。
Safety matters: acidified potassium dichromate is toxic and corrosive, bromine water is harmful, and silver nitrate stains skin and clothing. Tollens' reagent must be made fresh and disposed of safely because it can form explosive silver compounds.
安全注意事项：酸性重铬酸钾有毒且具有腐蚀性，溴水有害，硝酸银会染色皮肤和衣物。Tollens 试剂必须现配现用并安全处理，因为它可能生成易爆银化合物。
11. Exam Tips and Synoptic Links | 考试技巧与横向联系
AQA questions often ask you to 'identify' or 'deduce' using evidence. When a question gives IR data, quote the wavenumber range and the bond it indicates. When using mass spectrometry, identify the molecular ion peak and any key fragment peaks.
AQA 试题常要求根据证据进行“鉴别”或“推断”。当题目给出红外数据时，要引用波数范围及其对应的化学键。使用质谱数据时，要指出分子离子峰和关键碎片峰。
Synoptic links are important. Organic analysis connects to nomenclature, isomerism, reaction mechanisms and enthalpy changes. For example, recognising a primary alcohol by oxidation links to the aldehyde and carboxylic acid synthesis pathways.
横向联系很重要。有机分析与命名、异构、反应机理和焓变等知识相关。例如，通过氧化反应识别伯醇，与醛和羧酸的合成路径密切相关。
- Always label evidence: 'IR peak at 3350 cm⁻¹ indicates O-H.'
- Show exact mass calculations for high resolution data.
- Check the molecular formula against the M⁺ peak before drawing the structure.
答题时要标注证据，例如“红外 3350 cm⁻¹ 处的吸收表示 O-H”。高分辨率数据要写出精确质量计算。在画出结构前要先核对分子式是否与 M⁺ 峰一致。
12. Summary: AQA 3.6 Checklist | 小结：AQA 3.6 清单
Before the exam, ensure you can describe the test-tube reactions for alkenes, halogenoalkanes, alcohols, aldehydes, ketones and carboxylic acids. You must also be confident reading mass spectra and IR spectra to determine molecular formulas and functional groups.
考试前，确保你能描述烯烃、卤代烷、醇、醛、酮和羧酸的试管检验。你还必须熟练阅读质谱图和红外光谱图，以确定分子式和官能团。
- Understand positive test results and the reagents used.
- Interpret M⁺ and fragment peaks in mass
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