Synthesis of Alkanes and Cycloalkanes

The term paraffin, however, was probably not an appropriate one. We all know that alkanes react vigorously with oxygen when an appropriate mixture is ignited. This combustion occurs, for example, in the cylinders of automobiles, in furnaces, and, more gently, with paraffin candles. When heated, alkanes also react with chlorine and bromine, and they react explosively with fluorine. We shall study these reactions in Chapter 10. 

A chemical synthesis may require, at some point, the conversion of a carbon-carbon double or triple bond to a single bond. Synthesis of the following compound, used as an ingredient in some perfumes, is an example. 

This conversion is easily accomplished by a reaction called hydrogenation. There are several reaction conditions that can be used to carry out hydrogenation, but among the common ways is use of hydrogen gas and a solid metal catalyst such as platinum, palladium, or nickel. Equations in the following section represent general examples for the hydrogenation of alkenes and alkynes. 

Alkenes and alkynes react with hydrogen in the presence of metal catalysts such as nickel, palladium, and platinum to produce alkanes. The general reaction is one in which the atoms 

The reaction is usually carried out by dissolving the alkene or alkyne in a solvent such as ethyl alcohol (C2H5OH), adding the metal catalyst, and then exposing the mixture to hydrogen gas under pressure in a special apparatus. One molar equivalent of hydrogen is required to reduce an alkene to an alkane. Two molar equivalents are required to reduce an alkyne. (We shall discuss the mechanism of this reaction in Chapter 7.) 

---

Synthesis of Alkanes and Cycloalkanes 4.18A Hydrogenation of Alkenes and Alkynes... 

Alkanes and cycloalkanes. Obviously a variety of acyclic and cyclic hydrocarbon structures can be synthesized from the appropriate alkyl- or aryl-thiophenes (Scheme 46). The reaction is especially useful for the construction of macrocycles (Scheme 47). One of the most interesting applications of this reaction is in the synthesis of the chiral hydrocarbon butylethylmethylpropylmethane (210) (80JOC2754). The chiral acid (209) was the precursor, in which the thiophene was the potential n-butyl group. Raney nickel desulfurization, followed by standard manipulations to convert the acetic acid unit into an ethyl group, gave the hydrocarbon (210) (Scheme 48) this had [a]578 = -0.198°. It was established that... 

We have shown that perfluorination of the phthalocyanine ligand enhances the stability and catalytic activity of RuFiePc. Encapsulation of this complex in zeolite NaX by the synthesis method dramatically improves the activity and selectivity of RuFiePc. These results suggest that RuFi Pc-NaX is one of the best alkane oxidation catalysts of its kind. Although cycloalkanes are readily oxidized, the complete range of possible substrates is uncertain at this point. The oxidation of other alkanes and olefins will be the subject of continuing studies. 

The synthesis of cycloalkane-1,3-diones starts from bis(trimethylsilyloxy) bicyclo[n.l.O]alkanes and FeCl3, followed by treatment with Na02CCH3 in MeOH (Equation (lO)) 60... 

A high degree of deuterium incorporation into benzene and cycloalkanes has been achieved. However, the reactions are reversible, which implies that for synthesis of deuterated alkanes at even higher conversion a larger excess of the deuterated substrate is needed. High conversion is obtained (up to approximately 80%) because the reactions were repeated under high pressure conditions up to eight times. 

There is some increase in selectivity with functionally substituted carbenes, but the selectivity is still not high enough to prevent formation of mixtures. Phenylchlorocarbene gives a relative reactivity ratio of 2.1 1 0.09 in insertion reactions with isopropylbenzene, ethylbenzene, and toluene.132 For cycloalkanes, tertiary positions are about 15 times more reactive than secondary positions toward phenylchlorocarbene.133 Carbethoxycarbene inserts at tertiary C—H bonds about three times as fast as at primary C—H bonds in simple alkanes.134 Because of low selectivity, intermolecular insertion reactions are seldom useful in synthesis. Intramolecular insertion reaction are of considerably more use. Intramolecular insertion reactions usually occur at the C—H bond that is closest to the carbene, and good yields can frequently be obtained. Intramolecular insertion reactions can provide routes to highly strained structures that would be difficult to obtain in other ways. 

---