Pentane radical chlorination

Radical chlorination of pentane is a poor way to prepare 1-chloropentane, but radical chlorination of neopentane, (CI- C, is a good way to prepare neopentyl chloride, (CHj CO Cl. Explain. 

Assume that you have carried out a radical chlorination reaction on (P)-2-chloro-pentane and have isolated (in low yield) 2,4-dichIoropentane. How many stereoisomers of the product are formed and in what ratio Are any of the isomers optically active (See Problem 10.24.)... 

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 C6H13CI you might obtain by reaction of 2-methyl pentane with CI2. 

Let us examine a reaction that illustrates this principle, the radical chlorination of pentane ... 

Each isomer has its individual set of physical and chemical properties however, these properties are similar (Table 6). The fundamental chemical reactions for pentanes are sulfonation to form sulfonic acids, chlorination to form chlorides, nitration to form nitropentanes, oxidation to form various compounds, and cracking to form free radicals. Many of these reactions are used to produce intermediates for the manufacture of industrial chemicals. Generally the reactivity increases from a primary to a secondary to a tertiary hydrogen (37). Other properties available but not Hsted are given in equations for heat capacity and viscosity (34), and saturated Hquid density (36). 

The ratio of 2,2-dimethyl butane to 2-methyl pentane produced by these reactions will be k5a/k5h. In general, chlorine atom is less selective in hydrogen abstraction reactions than are hydrocarbon free radicals and hence fc2a/ 2b > > k5a/k5h. Consequently, one would expect that the first increment of HCl would decrease the ratio of 2,2-dimethyl butane to 2-methyl pentane in the C6 alkylation product. 

Although chlorine atoms attack bicyclo[l.l.l]pentane to give about equal parts of the bridgehead radical and the 2°-radical from attack at one of the three methylene groups, EPR experiments at — 33°C indicate that t-BuO attack yields only the bridgehead radical . Furthermore, the bridgehead radical is stable to 37 °C and the rearranged radical 20 is not observed by EPR spectroscopy the rate constant for the conversion of 19 to 20 must be less than 10 s . ... 

The seminal work of Hassner has shown that halogen azides react with alkenes both in ionic and radical pathways leading to opposite regiochemistry (Scheme 8.13). The nature of the reagent and the polarity of the media play a fundamental role. Iodine azide reacts via an ionic pathway whereas the two mechamsms may operate for bromine and chlorine azides. Apolar solvents such as pentane and irradiation of the reaction mixture favor the radical process. 

Similar considerations apply to the products of a particular radical reaction. Whenever their structure or stoichiometry is not exactly defined (or obvious) they are given as products or in terms of an overall stoichiometry, e.g. (CjHj,) would denote the mixture of radicals which results from hydrogen atom abstraction from pentane. An undefined product radical is also given as substrate minus the abstracted atom, e.g. ethanol (—H ) or trichloroethylene (—Cl ) would refer to the radicals left after hydrogen atom abstraction from ethanol and chlorine atom abstraction from trichloroethylene, respectively. Generally, if the unpaired electron can be assigned to a particular atom X it is indicated as X (radical dot on top of atom). 

Though inert towards chlorine at room temperature in the dark, perfluoropenta-1,2-diene is rapidly converted into the saturated tetrachloride, 1,2,2,3-tetrachloro-octafluom-n-pentane (95 %), and a small amount of mixed dimers, (CjFjIj (4%), when photolysed in the presence of an excess of chlorine. Photochemical hydrobromination yields mainly 3/f-2-bromo-octafluoropent-1-ene (34) and 1 W-2-bromo-octafluoropent-2-ene (35), a result which indicates that bromine atom preferentially attacks the central carbon of the allenic system to give an intermediate radical (36) which exists long enough to permit 90° rotation of the orbital containing the unpaired electron, so that it is transformed into an allylic radical (37) (see Scheme 14). 

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