Barium hydroxide octahydrate, reaction with

We saw in Section 8.5 that an enthalpy change is often called a heat of reaction because it is a measure of the heat flow into or out of a system at constant pressure. If the products have more enthalpy than the reactants, then heat has flowed into the system from the surroundings and AH has a positive sign. Such reactions are said to be endothermic (endo means "within," so heat flows in). The reaction of 1 mol of barium hydroxide octahydrate with ammonium chloride, for example, absorbs 80.3 kj from the surroundings (AH° = +80.3 kj). The surroundings, having lost heat, become so cold that water freezes around the outside of the container (Figure 8.6). 

FIGURE 8.6 The reaction of barium hydroxide octahydrate with ammonium chloride is so strongly endothermic and draws so much heat from the surroundings that the temperature falls below 0°C and moisture freezes around the container. 

We ve said on several occasions that chemical and physical processes occur spontaneously only if they go "downhill" energetically so that the final state is more stable and has less energy than the initial state. In other words, energy must be released for a process to occur spontaneously. At the same time, though, we ve said that some processes occur perfectly well even though they are endothermic and absorb heat. The reaction of barium hydroxide octahydrate with ammonium chloride shown in Figure 8.6, for example, absorbs 80.3 kj of heat (AH° = +80.3 kj) and leaves the surroundings so cold that moisture freezes around the outside of the container. 

What s going on How can the spontaneous reaction of barium hydroxide octahydrate with ammonium chloride release energy yet absorb heat The answer is that, in the context of a chemical reaction, the words "energy" and "heat" don t refer to exactly the same thing. There is another factor in addition to heat that determines whether energy is released and thus determines whether a reaction takes place spontaneously. We ll take only a brief look at this additional factor right now and return for a more in-depth study in Chapter 17. 

In a typical experiment, benzaldehyde (106 mg, 1 mmol) was added to the finely powdered paraformaldehyde (60 mg, 2 mmol). To this mixture, powdered barium hydroxide octahydrate (631 mg, 2 mmol) was added in a glass test tube and the reaction mixture was placed in an alumina bath (neutral alumina 125 g, mesh 150, Aldrich bath 5.7 cm diameter) inside a household microwave oven and irradiated for the specified time at its full power of 900 W intermittently or heated in an oil bath at 100-110 °C. On completion of the reaction, as indicated by TLC (hexane-EtOAc, 4 1, v/v), the reaction mixture was neutralized with dilute HC1 and the product extracted into ethyl acetate. The combined organic extracts were dried over anhydrous sodium sulfate and the solvent removed under reduced pressure. The pure benzyl alcohol (99 mg, 91%), however, is obtained by extracting the reaction mixture with ethyl acetate prior to neutralization and subsequent removal of the solvent under reduced pressure. 

Silver oxide in DMF was successfully applied for the regioselective reaction of (26) with allyl bromide to give compound (27) in 74% yield [231]. Hydroxyl groups in. positions 2 and 6 in cycloheptaamylose were selectively protected by reaction with allyl bromide, barium oxide, and barium hydroxide octahydrate in a DMF-DMSO mixture [232]. 

There are fire risks associated with the use of sodium hydride and alternative, innocuous bases would be welcome for large scale work One possibility is a mixture of barium oxide and barium hydroxide octahydrate in DMF. In the example shown in Scheme 4 191,355 the reaction is quite slow (7 days) and the yield only 55% but the method deserves further scrutiny. Another metal alkox-ide-based procedure for the regioselective protection of an equatorial alcohol proceeds via the stannylene derivative (see section 4,3.3)356... 

Barium hydroxide octahydrate apparently is dehydrated to the anhydrous hydroxide below 60 °C and to the oxide at 240 °C. Immediate reaction with the residual oxygen in the mass spectrometer occurs to form the known stable peroxide (Fig. 13). 

FIGURE 5.8 Endothermic and exothermic reactions, (a) When ammonium thiocyanate and barium hydroxide octahydrate are mixed at room temperature, the temperature drops, (b) The reaction of powdered aiummum with Fe203 (the thermite reaction) proceeds vigorousiy, reieasing heat and forming Ai203 and moiten iron. 

Or consider the reaction described in the chapter opening, in which crystals of barium hydroxide octahydrate, Ba(OH)2 8H2O, react with crystals of ammonium nitrate, NH4NO3. 

By reaction of Kayaflavone triethyl ether with barium hydroxide octahydrate in refluxing methanol (55%) [3397], Kayaflavone was isolated from dried leaves of Torreya mcifera. 

Barium hydroxide [33, 34] is a strong base which has been employed not only in organic synthesis but also for other purposes. The commercially available octahydrate, l a(OI 1)2 8 11,/) is often used after transformation to the anhydrous form at 200-500 °C. Dehydrated Ba(OH)2 activated at 200°C is denoted C-200. It is known to be a heterogeneous basic catalyst in the Horner-Wadsworth-Emmons (HWE) reaction of triethyl phosphonoacetate (69) with aldehydes 70 to give the 3-substituted ethyl acrylates 71 (Scheme 5.13) [35]. The HWE reaction proceeds at 70 °C in 1,4-dioxane with a small amount of water. The yields of products 71 are usually better than those provided by typical basic catalysts such as NaOH or... 

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