1- Chloro-4-bromobutane

With n dissimilar chiral atoms the number of stereoisomers is 2" and the number of racemic forms is 2" as illustrated below for 2-chloro-3-bromobutan.e (n = 2). The R,S configuration is shown next to... 

Problem 6.53 Outline the steps needed for the following syntheses in reasonable yield. Inorganic reagents and solvents may also be used, (a) IXhloropentane to 1,2-dichloropentane. (b) IXhloropentane to 2-chloro-pentane. (c) IXhloropentane to 1-bromopentane. (d) l-Bromobutane to 1.2-dihydroxybutane. (e) Isobutyl chloride to... 

In an earlier paper Barton and Onyon considered the unimolecular mechanism of dehydrochlorination to be of more universal application than the radical chain mechanism and postulated that a chloro-compound will decompose by a radical chain mechanism only so long as neither the compound itself nor the reaction products will be inhibitors for the chains . On the basis of this postulate the authors correctly predicted the mechanism of decomposition of a number of chlorine compounds. The postulate does not hold well for bromine compounds which show a greater tendency to decompose via radical chain mechanisms. However, from their early studies on 2-bromopropane 2-bromobutane, t-butyl bromide, and bromo-cyclohexane, Maccoll et a/.234,235,397,410,412 concluded that these compounds also decompose unimolecularly via a four-centre transition state similar to that proposed by Barton and Head. 

Allyl halides have been reduced with electrogenerated tris(bipyridine)cobalt(I) to afford 1,5-hexadiene [369,370]. Some of the earliest work with cobalt(I) salen involved its use for the catalytic reduction of bromoethane [371], bromobenzene [371], and /er/-butyl bromide and chloride [372]. More recently. Fry and coworkers examined the cobalt(I) salen-cata-lyzed reductions of benzal chloride [373-375] and of benzotrichloride [376], and the catalytic reductions of 1-bromobutane [377,378], 1-iodobutane [378], 1,2-dibromobutane [378], benzyl and 4-(trifluoromethyl)benzyl chlorides [379], iodoethane [380], diphenyl disulfide [381], 1,8-diiodooctane [382], and 3-chloro-2,4-pentanedione [383] have been investigated. 

The versatility of TOT in clathrate formation is further illustrated by the discovery of several other clathrate types belonging to various space groups PT (/raw-stilbene, cw-stilbene a-bromobutyric acid benzene ) Pl(PT) (methyl tran -cinnamate, methyl cw-cinnamate) P2 (weso-2,3-butanediol carbonate) P2j/c (weTO-2,3-di-bromobutane) C2jc (3-bromooctane ethyl a-bromobuty-rate ° ) Pbca (l,l,l-trifluoro-2-chloro-2-bromoethane) Pbcn (dl-2,3-dibromobu-tane) hexagonal R (a-chlorotetrahydropyran) The clathrates for which X-ray crystal structure determinations have been completed are indicated in Table 1. 

The qualitative interpretation of chiral discrimination through stereochemical data rests on the premise that, within restrictions imposed by molecular shape and size, coincidence of guest symmetry with cavity symmetry should promote higher e.e. values. Since the cage-type clathrates contain dissymmetric cavities they are expected to discriminate more efficiently those guests displaying a twofold axis. This is indeed the case for the thiirane and oxirane derivatives in Table 3. On the other hand, the high enantiomeric purity of 2-chloro- and 2-bromobutane is rather unexpected on symmetry consideration alone. In the type of enantiomeric... 

A familiar example of a stereoselective reaction would be the formation of a higher yield of f rans-2-butene than cis-2-butene in an E2 reaction, no matter whether the starting material is (R)- or (S)-2-bromobutane. The addition of bromine to trans-2-butene to produce mcso-2,3-dibromobutane or the addition of bromine to the cis isomer to produce an equimolar mixture of the two enantiomers of 2,3-dibromobutane is a stereospecific reaction. Note that a reaction that gives only one of a pair of eirantiomers is not necessarily stereospecific. A yeast-mediated reduction of 3-chloropropiophenone gives (S)-3-chloro-l henylpropan-l-ol, with no evidence for formation of the R enantiomer. Because the reactant cannot exist as stereoisomers, it is not possible for stereoisomerically different reactants to give stereoisomerically different products, and the reaction can only be considered stereoselective, not stereospecific. 

Label a series of ten clean and dry test tubes (10 X 75 mm test tubes may be used) from 1 to 10. In each test tube, place 2 mL of a 15% Nal-in-acetone solution. Now add 4 drops of one of the following halides to the appropriate test tube (1) 2-chlorobutane, (2) 2-bromobutane, (3) 1-chlorobutane, (4) 1-bromobutane, (5) 2-chloro-2-methylpropane (f-butyl chloride), (6) crotyl chloride CH3CH = CHCH2CI (see Special Instructions), (7) benzyl chloride (a-chlorotoluene) (see Special Instructions), (8) bromobenzene, (9) bromocyclohexane, and (10) bro-mocyclopentane. Make certain you return the dropper to the proper container to avoid cross-contaminating these halides. 

When 2-methyl-2-propanol tert-hvXyl alcohol, 65) is treated with concentrated HCl, 2-chloro-2-methylpropane (2-chloro-2-methylpropane ter -butyl chloride, 93) is isolated in 90% yield. Similarly, when 1-butanol (94) is treated with 48% HBr in the presence of sulfuric acid, a 95% yield of 1-bromobutane (96) is obtained. In both reactions, the oxygen of the alcohol reacts as a Br0nsted-Lowry base in the presence of the protonic acids, HCl, or sulfuric acid. The fact that alkyl halides are produced clearly indicates that these are substitution reactions. In previous sections, tertiary halides gave substitution reactions when a nucleophilic halide ion reacted by an Sfjl mechanism that involved ionization to a carbocation prior to reaction with the halide. Primary halides react with a nucleophilic halide ion by an 8 2 mechanism. It is reasonable to assume that tertiary alcohols and primary alcohols will react similarly, i/the OH unit is converted to a leaving group. 

One of the most frequently used reagents is n-butyllithium, which can be prepared from chloro- or bromobutane in hexane, benzene or ether. Solutions in hexane (usually 3.6moldm BuLi) are commercially available. Methyl chloride, bromide and iodide all react satisfactorily with lithium in diethyl ether. Normally alkyl iodides are unsuitable as they react too quickly with the organolithium compound RI + RLi -> R—R + Lil. A similar problem arises with benzyl chloride, which gives 1,2-diphenylethane. Aryl bromides and iodides, however, often give good yields, although the chlorides are often insufficiently reactive. 

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