From Other Reagents

A convenient and widely used mettiod for the preparation of late transition metal hydride complexes involves the generation of coordinated alkoxide from base and an alcohol, followed by elimination of an aldehyde or ketone. In one case, the alkoxide intermediate has been observed, while use of the labeled alcohol in Equation 3.120 has confirmed that the a-hydrogen becomes the hydride ligand. This reaction has even been used to produce a Co(III) hydride from a Co(III) aquo complex (Equation 3.121). Only primary and secondary alcohols can be used. 

---

H2S has been very useful in precipitating metal sulfides from their solutions. Controlling the acidity of the solution controls the sulfide ion concentration so that metal ions whose sulfides have sufficiently different solubilities can be separated. The fact that H2S is a gas makes it easy to separate from other reagents. However, H2S is more poisonous than hydrogen cyanide, HCN, and it must be handled with great care. (It is less dangerous than HCN because, unlike HCN, it is so easy to smell that everyone knows that it is present.)... 

Hydroxide ion is different from other reagents with respect to its reactivity toward cobalt(iii) ammine complexes. It reacts very rapidly (as much as 10 times faster than H2O) with cobalt(iii) ammine complexes in a base hydrolysis reaction (33). In this reaction, a first-order dependence on the substituting... 

For experiments in which a compound is synthesized from other reagents, that is, preparative experiments, it is essential to know the main reaction. To perform sfoichiomefric calculations, you should balance the equation for the main reaction. Therefore, before you begin the experiment, your notebook should contain the balanced equation for the pertinent reaction. Using the preparation of isopentyl acetate, or banana oil, as an example, you should write the following ... 

For experiments in which a compound is isolated from a particular source and is not prepared from other reagents, some information described in this section will not be applicable. Such experiments are called isolation experiments. A typical isolation experiment involves isolating a pure compound from a natural source. Examples include isolating caffeine from tea or isolating cinnamaldehyde from cinnamon. Although isolation experiments require somewhat different advance preparation, this advance study may include looking up physical constants for the compound isolated and outlining the isolation procedure. A detailed examination of the separation scheme is important here because it is the heart of such an experiment. 

Catalysts differ from other reagents normally used in a synthesis. They are not used up in the net chemical transformation and may be recovered in their original state at the end of the reaction. The use of homogeneous-phase catalysts in organic synthesis is well-known. 

Girard s reagents Quaternary ammonium salts of the type Me3NCH2CONHNH2 X which form water-soluble compounds with aldehydes and ketones, and are therefore separable from other neutral compounds the aldehyde or ketone may be subsequently regenerated after separation. 

The alkylidene dimethone (dimedone) (I) upon boiling with glacial acetic acid, acetic anhydride, hydrochloric acid and other reagents frequently loses water and passes into a substituted octahydroxanthene or the anhydride (II), which often serves as another derivative. The derivatives (I) are soluble in dilute alkali and the resulting solutions give colourations with ferric chloride solution on the other hand, the anhydrides (II) are insoluble in dilute alkali and hence can easily be distinguished from the alkylidene dimedones (I). 

Hafnium Boride. Hafnium diboride [12007-23-7] HfB2, is a gray crystalline soHd. It is usually prepared by the reaction of hafnium oxide with carbon and either boron oxide or boron carbide, but it can also be prepared from mixtures of hafnium tetrachloride, boron trichloride, and hydrogen above 2000°C, or by direct synthesis from the elements. Hafnium diboride is attacked by hydrofluoric acid but is resistant to nearly all other reagents at room temperature. Hafnium dodecaboride [32342-52-2] has been prepared by direct synthesis from the elements (56). 

Nickel plays a role in the Reppe polymeriza tion of acetylene where nickel salts act as catalysts to form cyclooctatetraene (62) the reduction of nickel haUdes by sodium cyclopentadienide to form nickelocene [1271 -28-9] (63) the synthesis of cyclododecatrienenickel [39330-67-1] (64) and formation from elemental nickel powder and other reagents of nickel(0) complexes that serve as catalysts for oligomerization and hydrocyanation reactions (65). 

Halogenation. Normally, 2-halopropane derivatives are prepared from isopropyl alcohol most economically by reaction with the corresponding acid haUde. However, under appropriate conditions, other reagents, eg, phosphoms haUdes and elemental halogen, also react by replacement of the hydroxyl group to give the haUde (46). 

A " -3-Ketones do not react with Girard reagents under mild conditions and thus can be separated from other reactive ketones. 

The reactivities of the substrate and the nucleophilic reagent change vyhen fluorine atoms are introduced into their structures This perturbation becomes more impor tant when the number of atoms of this element increases A striking example is the reactivity of alkyl halides S l and mechanisms operate when few fluorine atoms are incorporated in the aliphatic chain, but perfluoroalkyl halides are usually resistant to these classical processes However, formal substitution at carbon can arise from other mecharasms For example nucleophilic attack at chlorine, bromine, or iodine (halogenophilic reaction, occurring either by a direct electron-pair transfer or by two successive one-electron transfers) gives carbanions These intermediates can then decompose to carbenes or olefins, which react further (see equations 15 and 47) Single-electron transfer (SET) from the nucleophile to the halide can produce intermediate radicals that react by an SrnI process (see equation 57) When these chain mechanisms can occur, they allow reactions that were previously unknown Perfluoroalkylation, which used to be very rare, can now be accomplished by new methods (see for example equations 48-56, 65-70, 79, 107-108, 110, 113-135, 138-141, and 145-146)... 

Phase-transfer catalysis describes the action of special catalysts that assist the transfer of reactive molecules from a polar ( aqueous ) solvent to a nonpolar ( organic ) solvent. In the absence of the phase-transfer catalyst, one of the reagents is confined to one solvent, and the other reagent is confined to the other solvent, so no reaction occurs. Addition of a small amount of catalyst, however, enables one of the reagents to pass into the other solvent thereby initiating a reaction. 

Formamide is distilled under vacuum before use. /-Butyl alcohol may be dried by distillation from sodium or calcium hydride (3 g/100 ml). All other reagents should be dry. 

Trimethylhyctdzinium iodide is a novel, highly reactive reagent for at via VNS, as shown in Eq 9 49, in which the regioselecdviry is different from that obtained v/ith other reagents... 

Paraffins are relatively inactive compared to olefins, diolefins, and aromatics. Few chemicals could be obtained from the direct reaction of paraffins with other reagents. However, these compounds are the precursors for olefins through cracking processes. The C -Cg paraffins and cycloparaffms are especially important for the production of aromatics through reforming. This section reviews some of the physical and chemical properties of C1-C4 paraffins. Long-chain paraffins normally present as mixtures with other hydrocarbon types in different petroleum fractions are discussed later in this chapter. 

A major use of propane recovered from natural gas is the production of light olefins by steam cracking processes. However, more chemicals can be obtained directly from propane by reaction with other reagents than from ethane. This may be attributed to the relatively higher reactivity of propane than ethane due to presence of two secondary hydrogens, which are easily substituted. 

---