📚 Oxidation of Alcohols: Experimental Procedures from Unit 2 June 2019 Insert | 醇的氧化：Unit 2 2019年6月题库插页实验操作
This article reconstructs and explains the experimental techniques embedded in the AS Chemistry Unit 2 June 2019 Insert, focusing on the controlled oxidation of primary and secondary alcohols. The insert provided detailed equipment lists, step‑by‑step methods, and safety notes for preparing aldehydes and ketones, alongside purification and identification protocols. By breaking down each operation, we connect practical laboratory skills with the underlying theory of redox, distillation, and nucleophilic addition‑elimination reactions commonly tested at this level.
本文还原并解析了AS化学Unit 2 2019年6月题库插页中的实验操作要点，重点介绍了伯醇和仲醇的受控氧化过程。该插页提供了细致的仪器清单、分步操作流程以及安全须知，涉及醛和酮的制备、纯化与鉴定方法。我们将逐一拆解每一项操作，把实践技能与氧化还原、蒸馏和亲核加成‑消除反应的理论联系起来，这些都是该阶段考试常见的核心内容。
1. Introduction to the Insert Task | 插页任务介绍
The June 2019 Unit 2 insert presented a practical scenario: oxidise ethanol (a primary alcohol) to ethanal and simultaneously prepare propanone from propan‑2‑ol, then compare yields and purities. Students were expected to interpret the apparatus diagram, identify the role of anti‑bumping granules, and explain why the oxidising mixture was added dropwise. The insert assessed abilities to plan, monitor, and evaluate an organic synthesis under reflux and distillation conditions.
2019年6月Unit 2插页给出了一个实践情境：将乙醇（伯醇）氧化成乙醛，同时用异丙醇制备丙酮，然后比较产率与纯度。要求考生读懂装置图，指出抗暴沸颗粒的作用，并解释为何氧化剂混合物需要逐滴加入。该插页考查了学生在回流和蒸馏条件下对有机合成进行设计、监控与评价的能力。
2. Safety Precautions and Apparatus Setup | 安全措施与装置搭建
Before handling any chemicals, a risk assessment was required: potassium dichromate(VI) is a strong oxidiser, toxic and harmful to aquatic life; concentrated sulfuric acid is corrosive; ethanol and propanone are highly flammable. The insert stressed wearing eye protection, lab coats, and working in a fume cupboard when heating. Apparatus included a round‑bottom flask, Liebig condenser, still head, thermometer adapter, receiving flask, and anti‑bumping granules to ensure smooth boiling.
在操作任何试剂之前，需要进行风险评估：重铬酸钾(VI)是强氧化剂，有毒且对水生生物有害；浓硫酸有腐蚀性；乙醇和丙酮高度易燃。插页强调必须护目、穿实验服，加热时在通风橱内进行。装置包括圆底烧瓶、李比希冷凝器、蒸馏头、温度计转接头、接收瓶以及确保平稳沸腾的抗暴沸颗粒。
- Acidified dichromate was prepared by dissolving K₂Cr₂O₇ in dilute H₂SO₄ — never add water to acid.
- 用稀硫酸溶解 K₂Cr₂O₇ 配制酸化重铬酸盐——严禁向浓酸中加水。
- Glassware was clamped securely, and joints were lightly greased to prevent vapor leaks.
- 玻璃器皿用铁夹固定牢固，接口稍稍涂抹凡士林以免蒸气泄漏。
3. Preparing the Oxidising Agent | 制备氧化剂
The oxidant for ethanal production was a mixture of potassium dichromate(VI) and dilute sulfuric acid, in the mole ratio specified by the insert: approximately 0.02 mol K₂Cr₂O₇ with 5 cm³ concentrated H₂SO₄ in 25 cm³ water. The orange solution turns green as Cr₂O₇²⁻ ions are reduced to Cr³⁺. This colour change served as a visual indicator for the reaction endpoint — once the mixture remained permanently green, oxidation was deemed complete.
用于制备乙醛的氧化剂是重铬酸钾(VI)和稀硫酸的混合物，插页给出摩尔比约为0.02 mol K₂Cr₂O₇与5 cm³浓硫酸溶于25 cm³水中。随着重铬酸根离子被还原为铬(III)离子，橙色溶液转变为绿色。这一颜色变化可作为反应终点的视觉标志——一旦混合物保持永久绿色，即可认为氧化完成。
Cr₂O₇²⁻ (aq) + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
4. Reaction: Oxidation of Ethanol to Ethanal | 反应：乙醇氧化为乙醛
Approximately 5 cm³ of ethanol was placed in the distillation flask with anti‑bumping granules. The acidified dichromate solution was added dropwise via a tap funnel while the flask was gently heated using a water bath or low‑temperature heating mantle. Immediate distillation was necessary because ethanal (boiling point 21 °C) is highly volatile; without instant removal, it would be further oxidised to ethanoic acid under reflux conditions. The insert emphasised controlling the temperature to stay below 70 °C to avoid over-oxidation.
向蒸馏烧瓶中加入约5 cm³乙醇和几粒抗暴沸颗粒。酸化重铬酸盐溶液通过分液漏斗逐滴加入，同时用水浴或低温加热套温和加热。必须立即进行蒸馏，因为乙醛（沸点21 °C）极易挥发；若不及时分离，将在回流条件下进一步氧化为乙酸。插页强调温度需控制在70 °C以下以防过度氧化。
CH₃CH₂OH + [O] → CH₃CHO + H₂O
- Anti‑bumping granules prevented the formation of large vapor bubbles that could cause violent bumping.
- 抗暴沸颗粒防止形成大气泡而引起剧烈暴沸。
- The tap funnel allowed regulation of the oxidant flow, keeping ethanol in excess locally to favour aldehyde formation.
- 分液漏斗可调控氧化剂流速，保持局部乙醇过量，利于醛的生成。
5. Distillation and Collection of Product | 蒸馏与产物收集
Ethanal vapour passed through the condenser, was cooled back to liquid, and collected in an ice‑cooled receiver to minimise losses from evaporation. The insert’s diagram showed a short fractionating column packed with glass beads, which improved separation by providing surfaces for condensation. The first few drops (fore‑runs) below 20 °C were discarded as they contained dissolved ethanal mixed with water and unreacted ethanol. The main fraction was collected between 20 °C and 24 °C.
乙醛蒸气通过冷凝管被冷却成液体，收集在冰冷却的接收瓶中，以减少蒸发损失。插页的装置图显示一支填充玻璃珠的短分馏柱，通过增加冷凝表面来提高分离效果。低于20 °C的最初几滴（前馏分）被弃去，因为它们含有溶于水的乙醛与未反应乙醇的混合物。主馏分在20 °C至24 °C之间收集。
| Fraction | Temperature (°C) | Main Component |
|---|---|---|
| Fore‑runs | < 20 | Wet ethanal, ethanol |
| Main fraction | 20–24 | Ethanal |
中文解释：馏分、温度、主要成分。
6. Purification: Drying and Redistillation | 纯化：干燥与重蒸馏
The collected ethanal fraction was still wet, containing water that distilled as an azeotrope. The insert instructed transferring the product to a clean conical flask and adding anhydrous sodium sulfate (or anhydrous calcium chloride), swirling until the liquid turned clear — a sign that water had been absorbed. After decanting, the liquid was redistilled, collecting only the fraction at the boiling point of ethanal. This second distillation raised the purity significantly.
收集的乙醛馏分仍然含水，因为水可形成共沸物被带出。插页要求将产物转移至洁净锥形瓶，加入无水硫酸钠（或无水氯化钙），振荡至液体澄清——表明水分已被吸收。倾泌后，对液体进行重蒸馏，仅收集乙醛沸点对应的馏分。二次蒸馏显著提升了纯度。
7. Testing for Aldehyde: Tollens' and Fehling's Tests | 醛基检测：托伦斯试剂与斐林试剂
To confirm the presence of the aldehyde functional group, two qualitative tests were described in the insert footnotes. Tollens' reagent (ammoniacal silver nitrate) produced a silver mirror on the tube wall upon warming with ethanal. Fehling's solution, which contains Cu²⁺ complexed with tartrate in alkaline medium, gave a brick‑red precipitate of Cu₂O. Both reactions rely on the aldehyde being oxidised to a carboxylate anion, while Ag⁺ or Cu²⁺ is reduced.
为确证醛基的存在，插页脚注描述了两种定性检测。托伦斯试剂（氨性硝酸银）与乙醛共热时，在管壁生成银镜。斐林溶液含酒石酸根配位的铜(II)离子，在碱性介质中与醛反应生成砖红色Cu₂O沉淀。两个反应均依赖于醛被氧化为羧酸根离子，同时Ag⁺或Cu²⁺被还原。
- Tollens' test: CH₃CHO + 2[Ag(NH₃)₂]⁺ + 2OH⁻ → CH₃COO⁻ + 2Ag + 4NH₃ + H₂O
- 托伦斯试验：乙醛 + 银氨离子 → 乙酸根离子 + 银 + 氨 + 水
- Fehling's test: CH₃CHO + 2Cu²⁺ + 5OH⁻ → CH₃COO⁻ + Cu₂O + 3H₂O (simplified)
- 斐林试验：乙醛 + 铜(II) + 氢氧根 → 乙酸根 + 氧化亚铜 + 水
8. Determining Boiling Point and Purity | 测定沸点与纯度
A simple distillation apparatus was set up to measure the boiling point of the final ethanal. The insert advised that the thermometer bulb be placed exactly at the junction of the still head and condenser, so it reads the vapour temperature of the condensing liquid. A sharp, constant boiling point of 21 °C indicated high purity; any depression or broad range suggested residual water or ethanol. The measured value was compared with the literature value (21 °C) to calculate the percentage error.
搭建简单蒸馏装置测定最终乙醛的沸点。插页建议温度计水银球必须精确位于蒸馏头与冷凝管的交界处，以读取凝结液体的蒸气温度。锐利恒定的21 °C沸点表明纯度高；若偏低或沸程宽则提示残留水或乙醇。测量值与文献值（21 °C）对比，计算百分误差。
9. Alternative Oxidation to Carboxylic Acid | 替代氧化为羧酸
When the same primary alcohol was heated under reflux with excess acidified dichromate without immediate distillation, full oxidation to ethanoic acid occurred. The insert referenced this as an extension question: students needed to recognise that refluxing forces the aldehyde back into the reaction flask, exposing it to further oxidation. The apparatus was altered by removing the still head and connecting the condenser directly to the round‑bottom flask vertically, allowing vapours to condense and return.
当相同的伯醇与过量酸化重铬酸盐在回流条件下加热且不立即蒸馏时，将发生完全氧化生成乙酸。插页将此作为拓展提问：学生需认识到回流迫使乙醛返回反应烧瓶，使其继续被氧化。装置改装为取下蒸馏头，将冷凝器垂直直连圆底烧瓶，使蒸气冷凝回流。
CH₃CH₂OH + 2[O] → CH₃COOH + H₂O
10. Yield Calculation and Error Analysis | 产率计算与误差分析
culated from the limiting reactant, and the percentage yield was determined as (actual yield / theoretical yield) × 100. Typical values ranged from 60–75%. Sources of loss included evaporation of the low‑boiling product, incomplete reaction, and mechanical hold‑ er tubing. Students were asked to suggest improvements, such as using a more efficient ice‑water trap.
插页给出了数据：5.0 cm³乙醇（密度0.789 g/cm³）得到3.1 cm³纯乙醛（密度0.78 g/cm³）。由限量反应物计算理论产量，再以（实际产量/理论产量）×100%确定产率。典型值在60–75%之间。损失来源包括低沸点产物的蒸发、反应不完全以及冷凝管内壁的机械滞留。要求学生提出改进建议，如使用更高效的冰水收集阱。
11. Preparation of Propanone from Propan‑2‑ol | 由异丙醇制备丙酮
A secondary alcohol oxidation was run in parallel. Propan‑2‑ol (5 cm³) was heated with the same oxidising mixture, but no immediate distillation was needed because the ketone product is resistant to further oxidation. After refluxing for 20 minutes, the mixture was distilled to collect propanone at 56 °C. The insert asked why reflux was now safe: secondary alcohols yield ketones that lack the hydrogen atom attached to the carbonyl carbon, making them stable towards dichromate.
平行进行一个仲醇氧化实验。将异丙醇（5 cm³）用同样的氧化混合物加热，但无需立即蒸馏，因为酮产物不易继续氧化。回流20分钟后，蒸馏收集56 °C的丙酮。插页提问为何此时回流是安全的：仲醇生成的酮缺少与羰基碳相连的氢原子，因此对重铬酸盐稳定。
CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O
12. Common Pitfalls and Exam Tips | 常见易错点与应试提示
Mark schemes frequently highlighted: forgetting to add anti‑bumping granules; misidentifying the purpose of the condenser during reflux vs. distillation; confusing the colour change (orange Cr₂O₇²⁻ to green Cr³⁺) with simple dilution; and failing to justify why aldehyde is distilled off immediately. Students also lost marks for missing key oxidation numbers or for unbalanced redox half‑equations. Remember to cite the redox couple: ethanol is oxidised (oxidation number of α‑carbon increases from –1 to +1 in ethanal).
评分标准经常强调：忘记添加抗暴沸颗粒；混淆回流与蒸馏中冷凝管的作用；将颜色变化（橙色Cr₂O₇²⁻变为绿色Cr³⁺）简单归结为稀释；未能解释为何醛必须立即蒸出。学生还因遗漏关键氧化数或半反应未配平而失分。务必点明氧化还原电对：乙醇被氧化（α‑碳氧化数从–1升为+1，生成乙醛）。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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