Heating of reaction mixtures

Heating of aqueous solutions is most conveniently carried out using a Bunsen burner with the glass vessel suitably supported on a tripod and ceramic-centred gauze it is essential to use a heat resistant bench mat, and under no circumstances should such apparatus be left unattended. It is also imperative that no other worker using flammable solvents is in the vicinity. 

In the case of solutions of flammable liquids having a boiling point below 100 °C, the stainless steel electrically-heated water bath or steam bath provided with a constant-level device must be used. The individual circular type is provided with a series of concentric rings in order to accommodate flasks and beakers of various sizes. A rectangular type, suitable for use in student classes, has several holes each fitted with a series of concentric rings. In both cases the water bath is fitted with an immersion heating element controlled by a suitable regulator. 

High temperatures may be obtained also with the aid of baths of fusible metal alloys, e.g. Woods metal - 4 parts of Bi, 2 parts of Pb, 1 part of Sn and 1 part of Cu - melts at 71 °C Roses metal - 2 of Bi, 1 of Pb and 1 of Sn - has a melting point of 94 °C a eutectic mixture of lead and tin, composed of 37 parts of Pb and 63 parts of Sn, melts at 183 °C. Metal baths should not be used at temperatures much in excess of 350 °C owing to the rapid oxidation of the alloy. They have the advantage that they do not smoke or catch fire they are, however, solid at ordinary temperature and are usually too expensive for general use. It must be remembered that flasks or thermometers immersed in the molten metal must be removed before the metal is allowed to solidify. 

One of the disadvantages of oil and metal baths is that the reaction mixture 

A liquid of the desired boiling point is placed in the flask A which is heated with an electric mantle (see below). The liquid in A is boiled gently so that its vapour jackets the reaction tube BC it is condensed by the reflux condenser at D and returns to the flask through the siphon E. Regular ebullition in the flask is ensured by the bubbler F. The reaction mixture in C may be stirred mechanically. It is convenient to have a number of flasks, each charged with a different liquid changing the temperature inside C is then a simple operation. A useful assembly consists of a 50 ml flask A with a 19/26 joint, a vapour jacket about 15 cm long, a 34/35 joint at B and a 19/26 or 24/29 joint at D. 

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As was mentioned above, every efficient application of microwave energy to perform chemical syntheses requires reliable temperature measurement as well as continuous power feedback control, which enable heating of reaction mixtures to a desired temperature without thermal runaways. Moreover, power feedback control systems that are operated in the most microwave reactors enable a synthesis to be carried out without knowing the dielectric properties or/and conductive properties of all the components of the reaction mixture in detail. On the other hand, temperature control during microwave irradiation is a major problem that one faces during microwave-assisted chemical reactions. In general, temperature in microwave field can be measured by means of ... 

Methanol. Reaction may become vigorous sufficient methanol must be present to dissipate heat of reaction mixtures of powdered Mg with methanol can be detonated.11... 

It is also possible to heat reaction cell by haters isolated from reaction mixture (the so called "indirect heating"). Temperature fields for indirect heating of reaction mixture [1] are shown in Fig.2. Elements isolating heater details from reaction mixture are not shown in Fig.2 for bigger clarity of drawing. 

If we use temperature turn-down equal 90°C (coolant - liquid ethylene), then we shall be able to polymerize about 15 mass % of isobutylene at the expense of adiabatic heating of reaction mixture [60]. If we use liquid ammonia as coolant (Tbou = 243 K), then for the time of reaction mixture heating it is possible to receive not more than 10 mass % of polymer. 

Several highly substituted pyrroles 284 were produced by Kaupp et al. by mechano-chemical one-pot reaction of the enamine ketones 282 with fran -l,2-dibenzoylethene 280 (Scheme 3.75) [50], Depending on the reactivity of substrate, different milling temperatures were applied. In addition, ball milhng of enamine cyclohexenone 281 under same reaction conditions produced tetrahydroindolone 283 in quantitative yield. Quantitative yields of aU products were obtained by heating of reaction mixture after milling for removal of water. In solution, these reactions afforded moderate yields at much higher tanperatuies (Table 3.38). 

Cp = specific heat of reaction mixture m = mass of reactants. 

Methyl esters of carboxylic acids can be used for the formation of oxazolines, but this also requires the heating of reaction mixtures up to 180°C and, hence, seems less convenient in analytical practice. 

Recycling of the catalyst is achieved by either sedimentation of polymer by hexane from aqueous organic solvent, or heating of reaction mixture above room temperature for the reactions in neat water. As the hydrophile-lipophile balance of the polymer varies with temperature, the increase of temperature renders the material less hydrophilic. Catalyst activity is reported not to be degraded after as much as 10 reuses. This is one of the most spectacular durability records set so far in recyclable Pd-catalyzed processes. 

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