回流Reflux

溶剂在沸点处保持恒温，相当于一个简便的恒温器；冷凝管（常用球形冷凝管，冷凝面积大）把蒸气冷凝回流。回流时蒸气上升高度一般不超过冷凝管的 1/3。若反应需防潮，在冷凝管顶部加干燥管；需惰性气氛时接氮气或氩气鼓泡器；需同时滴加试剂时用三口烧瓶加恒压滴液漏斗。

The boiling solvent holds the temperature constant, acting as a simple thermostat, while the condenser, often a bulb (Allihn) condenser with a large cooling surface, returns the vapour. The vapour should rise no more than about a third of the way up the condenser. A drying tube on top excludes moisture, a nitrogen or argon bubbler provides an inert atmosphere, and a three-necked flask with a pressure-equalising dropping funnel allows reagents to be added during reflux.

回流广泛用于需要加热且时间较长的有机反应，如酯化、格氏反应、卤代烃的取代和消除、威廉姆森醚合成等，也用于重结晶时溶解固体和索氏提取。工业精馏塔中“回流”指把部分冷凝液送回塔顶，以提高分离效果。

Reflux is used for long heated organic reactions such as esterification, Grignard reactions, substitutions and eliminations of alkyl halides and Williamson ether synthesis, and for dissolving solids during recrystallisation and in Soxhlet extraction. In industrial columns 'reflux' means returning part of the condensate to the top to improve separation.

加热前先通冷凝水，加沸石或搅拌防暴沸，体系不能密闭（防止压力升高爆裂）。低沸点易燃溶剂（乙醚、丙酮）应使用水浴或加热套而不是明火加热。

Turn on the cooling water before heating, add boiling chips or stir to prevent bumping, and never close the system, which could build pressure and burst. Use a water bath or heating mantle, not a flame, for low-boiling flammable solvents such as ether and acetone.