📚 Essential Laboratory Techniques for Organic Synthesis | 有机合成基本实验操作技术
In the International A Level Chemistry Unit 5 assessment, mastery of laboratory techniques is crucial. From synthesising an organic liquid to purifying a solid product and determining its purity, each step requires precision and understanding of the underlying principles. This article walks you through the core experimental operations you are likely to encounter, with practical tips for success and safety.
在国际 A Level 化学第五单元的考核中，实验操作的精通至关重要。从合成有机液体到纯化固体产物并测定其纯度，每一步都需要精准以及对原理的理解。本文将带你逐一攻克你可能遇到的核心实验操作，并提供成功与安全的实用建议。
1. Safety and Basic Apparatus | 安全与基本仪器
Always wear safety goggles and a lab coat. Before starting any experiment, familiarise yourself with the location of the eye-wash station, fire extinguisher, and first-aid kit. Quick-fit glassware such as round-bottom flasks, condensers, and still-heads must be checked for cracks and lightly greased on joints to prevent vapour leakage.
始终佩戴护目镜和实验服。开始任何实验前，熟悉洗眼器、灭火器和急救箱的位置。圆底烧瓶、冷凝管、蒸馏头等快装玻璃仪器必须检查有无裂纹，并在接口处涂抹少量凡士林以防蒸气泄漏。
Use anti-bumping granules when heating liquids to ensure smooth boiling. Never heat a sealed apparatus; always leave the system open to the atmosphere at the top of the condenser or through a drying tube to avoid pressure build-up.
加热液体时应使用沸石或防暴沸颗粒以确保平稳沸腾。切勿加热密闭装置；始终在冷凝管顶部或通过干燥管将体系敞开通大气，避免压力积聚。
2. Heating under Reflux | 回流加热
Reflux allows a reaction mixture to be heated at the boiling point of the solvent for an extended period without loss of volatile components. The vapour rises into the vertical condenser, condenses back into liquid, and returns to the flask.
回流使反应混合物能够在溶剂沸点下长时间加热，而不会损失挥发性组分。蒸气上升进入竖直冷凝管，冷凝成液体后流回烧瓶。
Water must enter the condenser jacket at the bottom and exit at the top to ensure efficient cooling and avoid air pockets. The round-bottom flask should be placed in a heating mantle or oil bath, not directly on a hot plate, to prevent uneven heating.
冷却水必须从冷凝管夹套的下口进入、上口流出，以确保高效冷却并避免气穴。圆底烧瓶应置于加热套或油浴中加热，不可直接放在电热板上，以防受热不均。
3. Simple Distillation | 简单蒸馏
Simple distillation is used to separate a liquid from a non-volatile solute or to purify a liquid with a boiling point well below 150°C. The vapour travels from the flask through a still-head, down into a Liebig condenser, where it condenses and is collected as distillate.
简单蒸馏用于分离液体与不挥发性溶质，或纯化沸点远低于150°C的液体。蒸气从烧瓶经蒸馏头进入直形冷凝管，冷凝后以馏出液形式收集。
Place the thermometer bulb exactly at the junction leading to the condenser so it measures the temperature of the vapour entering the condenser. Record the boiling range to assess purity; a pure substance distils over a narrow range of 1–2°C.
温度计水银球应恰好位于通往冷凝管的支管接口处，以测量进入冷凝管的蒸气温度。记录沸程以评估纯度；纯物质的蒸馏温度范围窄至1–2°C。
4. Fractional Distillation | 分馏
Fractional distillation is essential when separating a mixture of miscible liquids with closer boiling points (difference < 25°C). A fractionating column packed with glass beads or Vigreux indentations provides a large surface area for repeated condensation and evaporation, improving separation.
当分离沸点接近（差值<25°C）的互溶液体混合物时，必须使用分馏。填充玻璃珠或刺形分馏柱提供大的表面积，通过反复的冷凝和蒸发提升分离效果。
Collect fractions at steady temperature plateaus. The first fraction typically contains the lower-boiling component, although some overlap may occur. Control the heating rate carefully; too rapid heating results in poor separation.
在温度稳定的平台期收集馏分。第一馏分通常含低沸点组分，但可能会有部分重叠。小心控制加热速率；加热过快会导致分离效果变差。
5. Liquid–Liquid Extraction | 液–液萃取
Use a separating funnel to extract a desired product from an aqueous mixture into an organic solvent. After adding the solvent, stopper the funnel, invert it, and vent by opening the tap to release any built-up pressure from volatile solvents.
使用分液漏斗将目标产物从水溶液混合物中萃取到有机溶剂中。加入溶剂后，塞紧漏斗，倒置并打开活塞放气，释放挥发性溶剂可能积聚的压力。
Shake gently, vent frequently, and then allow the layers to separate. Drain the lower layer (usually aqueous if using dichloromethane, or organic if using diethyl ether) into a clean flask. Repeat the extraction 2–3 times with fresh solvent to maximise yield.
轻轻振摇，不时放气，然后静置分层。将下层（若用二氯甲烷通常为有机层在下，若用乙醚则水层在下）放入干净锥形瓶。用新鲜溶剂重复萃取2–3次，以提高产率。
6. Drying Organic Liquids | 干燥有机液体
After extraction, traces of water must be removed using a drying agent. Add small amounts of anhydrous magnesium sulfate or calcium chloride to the organic layer, swirl, and observe whether the solid clumps. Continue adding until the solid flows freely, indicating that all water has been absorbed.
萃取后残留的水分必须用干燥剂去除。向有机层中少量多次加入无水硫酸镁或氯化钙，摇荡并观察固体是否结块。持续添加直至固体自由流动，表明水分已被完全吸收。
| Drying Agent | Suitable for | Comments |
|---|---|---|
| Anhydrous MgSO₄ | Most organic liquids | Fast, high capacity |
| Anhydrous CaCl₂ | Hydrocarbons, alkyl halides | Not for alcohols/amines |
| Anhydrous Na₂SO₄ | General, gentle drying | Slower but neutral |
After drying, filter or decant the liquid away from the spent drying agent before distillation or solvent removal.
干燥后，在蒸馏或去除溶剂前，应过滤或倾析液体以分离失效的干燥剂。
7. Recrystallisation | 重结晶
Recrystallisation is the primary method for purifying solid organic products. Dissolve the impure solid in the minimum volume of hot solvent. Use a solvent in which the target compound is sparingly soluble when cold but highly soluble when hot.
重结晶是纯化固体有机产物的主要方法。用最小体积的热溶剂溶解不纯固体。选择一种溶剂，使目标化合物在冷时微溶、热时易溶。
If the hot solution is coloured, add a pinch of activated charcoal to adsorb coloured impurities, then filter hot using a fluted filter paper and a heated funnel to prevent premature crystallisation. Allow the filtrate to cool slowly to room temperature and then in an ice bath to maximise crystal yield.
若热溶液带色，可加入一小撮活性炭吸附有色杂质，然后用折叠滤纸和预热漏斗趁热过滤，避免提前结晶。将滤液缓慢冷却至室温，再置于冰浴中，以最大化晶体产率。
Collect the crystals by vacuum filtration using a Buchner funnel, wash with a small amount of ice-cold solvent, and dry on a watch glass or in a desiccator.
用布氏漏斗抽滤收集晶体，用少量冰冷溶剂洗涤，并在表面皿或干燥器中干燥。
8. Melting Point Determination | 熔点测定
The melting point of a solid is a key indicator of purity. A pure substance melts sharply over a narrow range (1–2°C). Impurities lower the melting point and broaden the range, a phenomenon known as melting point depression.
固体的熔点是纯度的重要指标。纯物质在窄温度范围（1–2°C）内敏锐熔融。杂质会降低熔点并拓宽熔程，这一现象称为熔点降低。
Pack a small amount of dry, finely powdered solid into a capillary tube to a depth of 2–3 mm. Use a melting point apparatus (oil bath, Thiele tube, or digital melting point machine) and heat slowly near the expected melting point, about 1–2°C per minute, to obtain an accurate reading.
将少量干燥且研细的固体装入毛细管中，高度2–3mm。使用熔点测定仪（油浴、提勒管或数字熔点仪），在接近预期熔点时缓慢升温，约每分钟1–2°C，以获得准确读数。
9. Thin-Layer Chromatography (TLC) | 薄层色谱法
TLC is used to monitor the progress of a reaction and assess the purity of fractions. Spot a dilute solution of the sample onto a pencil-marked baseline of a silica gel plate. Also spot reference samples or starting materials for comparison.
薄层色谱用于监测反应进程和评估馏分纯度。将样品的稀溶液点在硅胶板铅笔标记的基线上，同时点上对照样品或起始原料以进行比较。
Develop the plate in a sealed jar containing the developing solvent, ensuring the solvent level is below the baseline. When the solvent front has risen to near the top, remove the plate, mark the front, and visualise spots under UV light or using an iodine chamber.
在含有展开剂的密封缸中展开，确保溶剂液面低于基线。当溶剂前沿升至接近顶端时取出板，标记前沿，在紫外灯下或使用碘缸显色。
Calculate the Rf value (retention factor) for each spot using the formula:
Rf = distance travelled by spot ÷ distance travelled by solvent front
计算每个斑点的比移值Rf，公式：
Rf = 斑点移动距离 ÷ 溶剂前沿移动距离
A pure compound shows a single spot; extra spots indicate impurities or incomplete separation.
纯化合物显示单点；额外斑点表明有杂质或分离不完全。
10. Vacuum Filtration | 真空抽滤
Vacuum filtration using a Buchner funnel and side-arm flask is far faster than gravity filtration and useful for isolating crystals. Fit a circle of filter paper that covers all holes in the funnel plate. Wet the paper with solvent, apply gentle suction, and pour the suspension evenly across the paper.
使用布氏漏斗和抽滤瓶进行真空抽滤远比重力过滤快，适用于分离晶体。放入一张覆盖漏斗板所有孔洞的圆形滤纸，用溶剂润湿，打开轻柔的抽气，将悬浮液均匀倾倒在滤纸上。
Wash the solid with a small portion of ice-cold solvent, disconnect the vacuum, and use a spatula to transfer the crystals to a watch glass. Never dispose of organic filtrates down the sink; collect them in an appropriate waste container.
用少量冰冷溶剂洗涤固体，断开抽气，用药匙将晶体转移至表面皿。切勿将有机滤液倒入水槽；应收集于合适的废液容器中。
11. Preparing a Standard Solution | 配制标准溶液
Volumetric techniques are fundamental in quantitative analysis. To prepare a standard solution, accurately weigh a primary standard (e.g., anhydrous sodium carbonate) using a weighing boat and an analytical balance.
容量分析技术是定量分析的基础。配制标准溶液时，用称量舟和分析天平准确称量基准物质（如无水碳酸钠）。
Transfer the solid quantitatively to a beaker, dissolve in distilled water, and pour into a volumetric flask via a funnel. Rinse the beaker and funnel several times with distilled water, transferring all washings into the flask to ensure complete transfer.
将固体定量转移至烧杯，用蒸馏水溶解，再通过漏斗倒入容量瓶。用蒸馏水冲洗烧杯和漏斗数次，所有洗涤液一并转移入容量瓶以确保完全转移。
Fill the flask with distilled water to just below the calibration mark, then use a Pasteur pipette to add water dropwise until the bottom of the meniscus touches the mark. Stopper and invert the flask at least 10 times to homogenise.
加蒸馏水至刻度线稍下方，然后用滴管逐滴加水至弯月面底部与刻度线相切。塞紧瓶塞，翻转摇荡至少10次以混匀。
12. Acid–Base Titration | 酸碱滴定
The titration procedure yields precise concentration data. Rinse a burette with the titrant solution, fill it, and record the initial volume to the nearest 0.05 cm³. Use a pipette and safety filler to transfer a fixed volume of the analyte into a clean conical flask. Add a suitable indicator, e.g., phenolphthalein for a strong base – strong acid titration.
滴定操作可获取精确的浓度数据。用滴定液润洗滴定管，装液并记录初始体积至0.05 cm³。使用移液管和洗耳球量取固定体积的待测液加入洁净锥形瓶中。加入合适指示剂，如强碱–强酸滴定用酚酞。
NaOH + HCl → NaCl + H₂O
Swirl the flask while adding titrant from the burette. Near the endpoint, add titrant dropwise until a persistent pale colour change appears. Record the final burette reading. Repeat until you obtain concordant titres (volumes within 0.10 cm³ of each other).
边摇动锥形瓶边从滴定管滴加滴定液。接近终点时，逐滴加入至出现持久的浅色变化。记录滴定管终读数。重复直至获得一组平行结果（体积相差不超过0.10 cm³）。
Calculate the mean of the concordant titres and use stoichiometric relationships to determine the unknown concentration. Always ensure there are no air bubbles trapped in the burette tip before starting.
计算平行结果的均值，并利用化学计量关系求算未知浓度。务必确保开始前滴定管尖嘴处无气泡残留。
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