What are the methods of heating in an organic chemical reaction?
Organic reactions are most commonly heated through the asbestos mesh and cannot be heated directly by fire. Otherwise, the instrument is prone to uneven heat and rupture. If you want to control the heating temperature and increase the heating area so that the reaction substance can be heated evenly and avoid local overheating and decomposition, it is advisable to use an appropriate heating bath.
(1) Water bath: It is suitable for the reaction of heating at a temperature of less than 100 degrees Celsius. If the heating temperature is below 90℃, it can be soaked in water to heat. A boiling water bath or steam bath can be used if the heating temperature is 90-100°C.
(2) Oil bath: When the heating temperature is 100℃-250℃, an oil bath can be used. The advantage of an oil bath is that the temperature can be controlled within a certain range through a temperature control instrument. In addition, the reaction material in the container is heated evenly. The reaction temperature in the container is generally 20°C lower than in the oil bath. Oils used include liquid paraffin, vegetable oil, hardening oil, silicone oil, and vacuum pump oil. The latter two are still more stable at temperatures over 250°C.
(3) Sand bath: When temperatures reach several hundred degrees above, a sand bath is frequently used. Indirectly heating the reaction liquid under the iron plate is achieved by placing the clean and fine sand on the iron plate and burying the reaction vessel in the sand.
(4) Molten salt bath: When the reaction temperature is above a few hundred degrees, it can also be heated by a molten salt bath. Molten salt is stable below 700℃, but care must be taken when using it to prevent splashing on the skin and serious burns.
(5) Electric heating jacket: Electric heating jacket has become a commonly used heating device in laboratories. Especially for the heating and distillation of flammable organic compounds. It is not an open flame, so it has the advantage of being difficult to cause fire and high thermal efficiency. The voltage regulator controls the heating temperature, and the highest temperature is 400°C.
What is the role of zeolite? When do you need zeolite?
Zeolites are small porcelain grains. It has many pores in it, and when the liquid is heated, it creates tiny bubbles, which become the boiling center. This can prevent the liquid from overheating, prevent boiling, and keep the boiling stable. To produce water steam, zeolite is necessary for general heating processes such as reflux, distillation, fractionation, and water steam generators. However, vacuum distillation, steam distillation, and electric stirring reactions do not require it. Zeolite remains effective in continuous distillation. Once halfway stops boiling or distillation, the original zeolite is ineffective When heating distillation begins again, new zeolite should be added. If you forget to add zeolite in advance, never add it when the liquid is heated to boiling. Because this will cause a violent boil and cause the liquid to spill out of the bottle. Fire accidents are also likely to happen. Therefore, it should be cooled for a period before being added.
Anhydrous CaCl2 is a desiccant commonly used in organic synthesis. What substances cannot be desiccated by anhydrous CaCl2?
Since alcohols, phenols, and amines can form complexes with CaCl2, anhydrous CaCl2 cannot be employed to dry these substances. Industrial CaCl2 contains calcium hydroxide and calcium oxide. Therefore, it cannot be used to dry acids either.
When to use reflux condensers, and what are the considerations during operation?
Organic reactions are slow or difficult at room temperature. To expedite the reaction, it is often necessary to keep the reacting substance boiling for a longer time. In this case, it is necessary to use a reflux condensing unit, so that the vapor is continuously condensed in the condensing tube and returned to the reactor to prevent the material in the reaction bottle from escaping and losing. The reflux device generally uses a spherical condensing pipe to pass cooling water from bottom to top. The water flow speed need not be very fast. The amount of material added in general accounts for about 1/3 to 1/2 of the reaction bottle capacity, and no more than 2/3. Before reflux, zeolite should generally be added first. According to the boiling temperature of the liquid in the bottle, you can choose a water bath, oil bath, or electric heating jacket preferably with an appropriate heating bath.
What are the distillation devices composed of? What precautions should be taken during installation and application?
The distillation device includes a distillation flask, a distillation head, a thermometer, a straight condensing tube, a liquid receiving tube, and a receiving bottle. When installing, take the following precautions:
(1) The general installation sequence is bottom-up, from left to right. The heat source height is first determined, and other instruments are installed.
(2) Iron clamps cannot clamp glass instruments directly. Rubber and asbestos should be padded around the clamps to prevent breakage of the instrument.
(3) The atmospheric distillation unit must communicate with the atmosphere.
(4) The amount of material in the distillation bottle should account for 1/3 to 2/3 of the distillation bottle volume.
(5) The standard height of thermometer installation is: the upper end of the mercury bulb should be in a horizontal line with the lower side of the distillation head branch pipe.
(6) Add zeolite properly.
(7) If the product distillation is not dried thoroughly, there will be front-cut fractions that should be received in another bottle. In advance, it is necessary to weigh the bottle that will receive the product.
(8) When distilling substances with boiling points higher than 140℃, air-condensing tubes should be used instead.
What are the precautions when distilling ether?
Ether has a low boiling point, is flammable, volatile, and may contain peroxide. Therefore, when distilling ether, there should be no open flame nearby, never direct heating with a lamp, nor heating with a water bath being heated on a lamp. Instead, use a pre-heated water bath. In order to prevent evaporation and peroxide explosions, general ether cannot be dried.
What should be considered when using a separation funnel for washing and extraction?
When using the separation funnel for washing and extraction, follow these precautions:
(1) When the separation funnel is placed for a long time, to prevent the lid cock from sticking together, it is generally lined with a layer of paper. Before use, remove the backing paper and check the lid and cock for leaks. If it leaks, it should be coated with Vaseline and inspected until it does not. To apply vaseline, apply a thin layer to the cock, plug it in, and rotate for several weeks. However, do not paint around the hole, to avoid blocking it.
(2) Adequate shaking should be done during extraction, and attention should be paid to the correct operation posture and method.
(3) When shaking, gas production often occurs which should be vented in a timely manner.
(4) When separating liquids, the lower liquid should be released from the cock, and the upper liquid should be poured out of the upper mouth.
(5) When separating liquid, you should first open the top lid, or rotate the lid, so that the concave seam or small hole on the lid is aligned with the small hole in the upper neck of the funnel, so as to communicate with the atmosphere.
(6) During extraction and separation, both upper and lower layers of liquid should be retained until the end of the experiment to remedy operational errors.
(7) After the separation funnel is used, it is necessary to wash it, and the cover and cock are lined with paper strips respectively.
What is the difference between a separation funnel and a drip funnel?
The separation funnel is used to separate and extract liquid organic substances. The drip funnel is used to drip reaction material during the reaction. There are two types of drip funnels: spherical drip funnels and constant pressure drip funnels. The spherical drip funnel has a small glass bubble under the cock, through which drip acceleration can be observed and controlled. The constant pressure drip funnel is suitable for dropping volatiles with a low boiling point that cannot come into contact with air.
What is the difference between a straight condensate pipe and an air condensate pipe in use?
The straight condensate pipe is mainly used for distillation; the spherical condensate pipe is mainly used for reflux, and its inner sandwich is spherical, so the cooling area is increased. However, it cannot be used for distillation because of the residual material in the pellets.
When two liquid products have similar boiling points, what method (non-chemical method) is commonly used to separate them?
Fractionation is often used to separate them.
What is the main difference between distillation and fractionation?
Ordinary distillation can only separate liquid and non-volatile substances, or the mixture of components with a boiling point difference between the liquid (generally 30℃). If the boiling points of the components in the mixture are not very different, it is difficult to separate by ordinary distillation. Instead, it is necessary to do so by fractionation. The principle of fractionation is to carry out multiple gasification and condensation in the fractionation column, which is equivalent to carrying out multiple distillations and separating the components. The efficiency of fractionation depends on the type and height of the column. At present, the most sophisticated fractionating equipment can separate mixtures with boiling point differences of 1-2℃.
Can 2-nitro-1, 3-resorcinol be prepared by direct nitration of resorcinol?
No. Because the phenol hydroxyl group is a strong activating group, if directly nitrated, the reaction will be very violent, even explosive. In addition, due to spatial effects, site 2 is less active than sites 4 and 6, so the reaction occurs preferentially at sites 4 and 6. Therefore, sites 4 and 6 should be protected before the nitro group is introduced to site 2.
In the preparation of 2-nitro-1, 3-resorcinol, why should nitration be controlled below 30℃?
Nitric acid is not only a nitrification reagent but also has a strong oxidation ability. If the temperature is higher than 30℃ during nitrification, it will increase the side reaction of oxidation, deepen the product color, and reduce the yield.
What circumstances may be appropriate for steam distillation?
When the reaction mixture is viscous or resinous and cannot be purified by ordinary distillation, crystallization, extraction, etc., steam distillation can be considered for purification.
What conditions must be satisfied for steam distillation?
The substance to be distilled must have the following conditions before steam distillation:
(1) It has a certain vapor pressure at about 100℃ (generally not less than 1.33 kPa).
(2) It can coexist with water for a long time without chemical changes.
(3) Insoluble or almost insoluble in water to facilitate product separation.
What is the basic principle of chromatography in separating mixtures?
The basic principle of chromatographic separation of mixtures is to use the different adsorption capacities of each component in the mixture in the solid phase or the different solubility in the mobile phase (that is, the distribution ratio). The mixture solution flows through the solid phase with repeated adsorption or distribution to separate the components.
When conducting thin-layer chromatography, it is usually necessary to measure the specific shift value of the sample (also known as the Rf value). How to determine the Rf value? What is the significance of measuring Rf values?
The Rf value of the sample is equal to the distance from the center of the highest concentration of the sample to the center of the origin. This is divided by the distance from the front of the solution to the origin center. The Rf value is a physical constant of the sample. It should be the same when measured under the same conditions so that preliminary identification can be made.
Thin layer chromatography in the separation of colorless substances: what are the commonly used color development methods?
(1) Special color development agents: Selection of specific color development agents paired with the substance to be sprayed.
(2) Concentrated sulfuric acid: Certain substances, such as sugars, can be oxidized with concentrated sulfuric acid to produce black spots.
(3) Fluorescence color development: The first method is to separate the fluorescent material with a general chromatography plate under the fluorescent lamp. The measured material will show fluorescent spots. The second method is to separate the non-fluorescent substance using the substance containing fluorescent agent as a thin layer under the fluorescent lamp. The thin plate is bright fluorescence, and the measured substance shows dark spots.
(4) Iodine vapor color development method: Many organic substances react with iodine vapor to produce brown spots.
Organic synthesis may produce a small number of colored substances due to oxidation and other side reactions. How to remove these colored substances?
Add an appropriate amount of activated carbon, heat, and boil for 5-10 minutes, and then heat filtration.
What is the principle of recrystallization?
The solubility of solid organic compounds in solvents is related to temperature. Generally, solubility increases with temperature. If the sample is dissolved in a hot solvent to reach saturation, the solution becomes supersaturated and precipitates crystals due to the decrease of solubility during cooling. By using the different solubility of the solvent to the purified substance and impurities, the purified substance can be precipitated from the supersaturated solution. In addition, all or most of the impurities remain in the solution, to achieve the purpose of purification.
What conditions must be met when applying the sublimation method to purify solid substances? What are the characteristics of sublimation?
Sublimated substances can be purified by sublimation only if they have a fairly high vapor pressure (above 2.67 kPa) below their melting point temperature. Sublimation can remove non-volatile impurities, often resulting in products with higher purity, but its operation time is long, and the loss is also significant, so it is only suitable for the purification of a small number of substances in the laboratory.
Which substance is suitable for vacuum distillation?
Vacuum distillation is particularly suitable for the purification of substances that have been decomposed, oxidized, or polymerized by heat before reaching a boiling point in atmospheric distillation. It’s also suitable for distillations that fail at high boiling points and atmospheric pressure.
Why should atmospheric distillation be used first before vacuum distillation, and then vacuum distillation with a water pump and oil pump?
Distilled materials contain substances with low boiling points. If not removed, they are unable to be condensed down with oil pump vacuum distillation, and then get into the drying system, causing some damage. Later, they will get into the oil pump, reducing the level of vacuum as well. Therefore, before vacuum distillation with an oil pump, atmospheric distillation should be carried out first in order to steam out most of the low boiling point substances. Then, vacuum distillation with a water pump removes residual substances with a low boiling point. The last step can be completed with an oil pump.
What is the proper operating procedure for vacuum distillation?
The correct operating procedure is:
(1) Apply vacuum grease to the grinding edge of the glass instrument, install the distillation instrument and capillary tube, and add the sample.
(2) Switch on the vacuum pump, close the two-way piston on the buffer bottle, open the vacuum gauge, and adjust the spiral clip on the capillary to moderate intake air. Adjust the vacuum degree until the required vacuum degree is reached. Alternatively, check the leakage area and seal it.
(3) Slow heating with appropriate heating methods (preferably water bath or oil bath), steaming speed should not be too fast, to prevent the sample from rushing out of the distillation bottle. Be sure to adjust the vacuum first, then the heating.
(4) At the end of distillation, first remove the heating device, and then slowly rotate the two-way piston on the buffer bottle to ventilate the system, not too fast, so that the mercury in the vacuum gauge breaks through the glass instrument. Finally, turn off the vacuum pump power supply, and disconnect the distillation unit and vacuum system.
There are decompression devices like buffer bottles, cold traps, vacuum gauges, anhydrous CaCl2 drying towers, NaOH drying towers, and solid paraffin drying towers. What is the role of each device?
A buffer bottle buffers and opens to the general atmosphere. Cold traps are collections of low-boiling substances that are difficult to condense. A vacuum gauge measures the vacuum. The anhydrous CaCl2 drying tower absorbs water vapor, and the NaOH drying tower absorbs acidic gases. The solid paraffin drying tower absorbs non-polar or weakly polar organic materials. The cold trap and three drying towers protect the vacuum pump.