Substitution of Alkanes

Halogenation. Fluorination, chlorination, and bromination of alkanes catalyzed by superacids have been reported.1,2 Reactions may be carried out in the liquid phase, or in the gas phase over solid superacids or supported noble metal catalysts. High selectivity and relatively mild reaction conditions are the main features of these transformations. 

Chloroform plays a crucial role in the reaction. Among others its somewhat electrophilic hydrogen serves as an acceptor for the fluoride ion. 

Electrophilic fluorination of methane itself was effected with good selectivity to methyl fluoride (accompanied by minor amounts of methylene fluoride) by reacting it with NFj SbFg or N2F+AsFg in liquid HF or pyridinium poly(hydrogen fluoride) solution.6 

Electrophilic chlorination and bromination of methane over supported noble metals9-11 (Pt on A1203, Pd on BaSC ) and solid superacid catalysts9-13 (e.g., TaOF3 on alumina, Nafion-H, zeolites, SbF5-graphite, sulfated zirconia) have been studied (see Section 3.4.2). Monosubstitution with selectivities better than 

90% are usually achieved. Insertion of electrophilic halogen species into the 

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These halogenations are like free-radical substitutions of alkanes (see Section 4.4). The order of reactivity of H-abstraction is... 

Shorter routes, in some cases via alternate feedstocks, e.g. substitution of alkanes for alkenes and aromatics... 

Substitution reactions involve the replacement of one atom, or a group of atoms, by another. For example, the free-radical substitution of alkanes by chlorine in sunlight... 

The rest of this chapter describes a completely different method for preparing alkyl halides one that uses alkanes as reactants It involves substitution of a halogen atom for one of the alkane s hydrogens... 

Methane ethane and cyclobutane share the common feature that each one can give only a single monochloro derivative All the hydrogens of cyclobutane for example are equivalent and substitution of any one gives the same product as substitution of any other Chlorination of alkanes m which the hydrogens are not all equivalent is more com plicated m that a mixture of every possible monochloro derivative is formed as the chlo rmation of butane illustrates... 

Bromine reacts with alkanes by a free radical chain mechanism analogous to that of chlorine There is an important difference between chlorination and brommation how ever Brommation is highly selective for substitution of tertiary hydrogens The spread m reactivity among pnmary secondary and tertiary hydrogens is greater than 10 ... 

Substitutive lUPAC nomenclature names epoxides as epoxy derivatives of alkanes According to this system ethylene oxide becomes epoxyethane and propylene oxide becomes 1 2 epoxypropane The prefix epoxy always immediately precedes the alkane ending it is not listed m alphabetical order like other substituents... 

Ethers are named m substitutive lUPAC nomenclature as alkoxy derivatives of alkanes Functional class lUPAC names of ethers are derived by listing the two alkyl groups m the general structure ROR m alphabetical order as separate words and then adding the word ether at the end When both alkyl groups are the same the prefix di precedes the name of the alkyl group... 

Recall from Section 6 18 that epoxides may be named as epoxy derivatives of alkanes in substitutive lUPAC nomenclature... 

Section 16 1 Ethers are compounds that contain a C—O—C linkage In substitutive lUPAC nomenclature they are named as al/coxy derivatives of alkanes In functional class lUPAC nomenclature we name each alkyl group as a separate word (m alphabetical order) followed by the word ether... 

Halogenation (Sections 4 14 and 12 5) Replacement of a hy drogen by a halogen The most frequently encountered ex amples are the free radical halogenation of alkanes and the halogenation of arenes by electrophilic aromatic substitution... 

Trifluoromethyl-substituted diazonium betaines [176]. Synthetic routes to trifluoromethyl-substituted diazo alkanes, such as 2,2,2-trifluorodiazoethane [ 177, 7 78, 179] and alkyl 3,3,3-trifluoro-2-diazopropionates [24], have been developed Rhodium-catalyzed decomposition of 3,3,3-tnfluoro-2-diazopropionates offers a simple preparative route to highly reactive carbene complexes, which have an enormous synthetic potential [24] [3-1-2] Cycloaddition reactions were observed on reaction with nitnles to give 5-alkoxy-4-tnfluoromethyloxazoles [750] (equation 41)... 

Epoxides are normally nmned as epoxy derivatives of alkanes or as substituted oxiranes. 

Sulfides are sulfur analogs of ethers they contain the C—S—C functional group. They are named as alkylthio derivatives of alkanes in substitutive lUPAC nomenclature. The functional class lUPAC nmnes of sulfides are derived in the same manner as those of ethers, but the concluding word is sulfide. 

Depending on the relative amounts of the two reactants and on the time allowed, a sequential substitution of the alkane hydrogen atoms by chlorine occurs, leading to a mixture of chlorinated products. Methane, for instance, reacts with CI2 to yield a mixture of CH3CI, CH2CI2, CHCI3, and CCI4. We ll look at this reaction in more detail in Section 5.3. 

Problem 5.2 Radical chlorination of alkanes is not generally useful because mixtures of products often result when more than one kind of C-H bond is present in the substrate. Draw and name all monochloro substitution products CgM 13CI you might obtain by reaction of 2-methylpentane with C)2. 

The hydrophobicity index is also suitable for correlating the cM values of various substituted sodium alkane 1-sulfonates [68]. The perfluoroalkyl substituent, e.g., 8 17 has a pronounced hydrophobic effect (/ = 1.66 at 75°C, sodium salt), whereas the hydrophilic disulfonates have values distinctly below 1 (for a-disulfonates, / = 0.75 was derived [70]). Further, it was somewhat surprisingly shown that substituents like 1-hydroxymethyl, 3-hydroxyethoxy, or even the hydroxyethoxyethoxy groups have hydrophobic effects. 

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