Methylbutane, 2-, chlorination

Chloro-3-methylbutane 2-Chloro-2-methylbutane (Chlorination at C) (Chlorination at D)... 

Primary alkyl chlorides are fairly stable to fluorine displacement. When fluorinated, 1-chloropropane is converted to 1-chloroheptafluoropropane and 1-chloto-2-methylbutane produces 39% l-chlorononafluoro-2-methylbutane and 19% perfluoro-2-methylbutane. Secondary and tertiary alkyl chlorides can undergo 1,2-chlorine shifts to afford perfluonnated primary alkyl chlorides 2-Chloro-2-methylpropane gives l-chlorononafluoro-2-methylpropane, and three products are obtained by the fluorination of 3-chloropentane [7] (equation 1). Aerosol fluorina-tion of dichloromethane produces dichlorodifluoromethane which is isolated in 98% purity [4 (equation 2). If the molecule contains only carbon and halogens, the picture is different. Molecular beam analysis has shown that the reaction of fluorine with carbon tetrachlonde, lodotrichloromethane, or bromotrichloromethane proceeds first by abstraction of halogen to form a trichloromethyl radical [5]... 

Bromination of an optically active form of the corresponding chloro compound (l-chloro-2-methylbutane) also results in an optically active product, and retention of configuration. It may be that an actual bridged radical is formed, but a somewhat less concrete interaction seems more likely, as halogenation with the more reactive chlorine is found to lead wholly to racemisation. 

It was shown in 1940 by Brown, Kharasch, and Chao that, during free radical chlorination of optically active 2-methyl-l-chlorobutane chloride, the product l,2-dichloro-2-methylbutane was racemic, implying that the intermediate... 

Exercise 4-15 Use the data given above for the percentages of the monochlorides formed in the vapor-phase chlorination of 2-methylbutane at 300° and take into account the statistical factors for the different numbers and kinds of hydrogens in answering the following ... 

The rates at which the various C-H bonds of 2-methylbutane are broken by attack of chlorine atoms approach 1 1 1 as the temperature is raised above 300°. At higher temperatures both chlorine atoms and hydrocarbons become more reactive because of increases in their thermal energies. Ultimately, temperatures are attained where a chlorine atom essentially removes the first hydrogen with which it collides regardless of position on the hydrocarbon chain. In such circumstances, the composition of monochlorination products will correspond to that expected from simple statistics. 

In a multistep synthesis, the overall percent yield is the product of the fractional yields in each step times 100 and decreases rapidly with the number of steps. For this reason, a low-yield step along the way can mean practical failure for the overall sequence. Usually, the best sequence will be the one with the fewest steps. Exceptions arise when the desired product is obtained as a component of a mixture that is difficult to separate. For example, one could prepare 2-chloro-2-methylbutane in one step by direct chlorination of 2-methyl-butane (Section 4-5A). But because the desired product is very difficult to separate from the other, isomeric monochlorinated products, it is desirable to use a longer sequence that may give a lower yield but avoids the separation problem. Similar separation problems would be encountered in a synthesis that gives a mixture of stereoisomers when only one isomer is desired. Again, the optimal synthesis may involve a longer sequence that would be stereospecific for the desired isomer. 

PROBLEM 23.12 Draw all the monochloro substitution products (C5HnCl) you would expect to obtain from chlorination of 2-methylbutane. 

This example demonstrates that the rule of thumb is an estimate at best. Most importantly, it is a reminder that the tertiary product will not necessarily be the primary product. Bromine is more selective than chlorine and substituting bromine for chlorine in the same reaction will result in predominately 2-bromo-2-methylbutane. Fluorine, on the other hand, is so reactive that the primary7 product would pre-dominate. 

Allylic Chloride vs. tert-Chloride Reactivity. There is some question in the literature as to whether the allylic chloride moiety or ferf-chloride group is more responsible for the thermal instability of poly (vinyl chloride) (I, 2). To shed some light on this problem we compared the relative reactivities at 100 °C. in chlorobenzene of 4-chloro-2-pentene and 2-chloro-2-methylbutane with dibutyltin -mercaptopropionate. Data are summarized in Table I. The half-time for the reaction of the allylic chloride with the stabilizer mercaptide group was less than 15 minutes, whereas the half-time for the tert-chloride was nearly 20 times longer. The greater reactivity of the allyl chloride suggests that it is the more important functionality in polymer degradation. However, these results on rates of chlorine substitution are not necessarily an exact measure of thermal instability. 

Free radical chlorination (Cl2 and light) of (R)-1 -chloro-2-methylbutane gives the two products shown, in addition to other products. These two products are always formed in exactly equal amounts. Explain this observation. 

Predict the ratios of products that result from chlorination of isopentane (2-methylbutane). 

Draw and name the monochlorination products you imgfil obtain by radical chlorination of 2-methylbutane. Which of the products are chiral Are any of the products optically active ... 

In the example the product is shown to be 1-chlorobutane, but in fact the reaction produces a mixture of 1-chlorobutane and 2-chlorobutane. If the reaction were to occur purely by chance, we would expect the ratio of products to be 6 4 because there are six primary hydrogens and four secondary hydrogens on butane. But because a secondary C—H bond is weaker than a primary C—H bond (95 vs 98 kcal mole" ), we might expect more 2-chlorobutane than chance would dictate. In the chlorination of 2-methylbutane ... 

In l,2-dichloro-3-methylbutane the Cl, H, H of CH2CI takes priority over the C, C, H of isopropyl. Chlorine has a higher atomic number than carbon, and the fact that there are /wo C s and only one Cl does not matter. (One higher number is worth more than two—or three—of a lower number.)... 

Problem 7.10 Altogether, the free-radical chlorination of (S)-(+)-l-chloro-2-methylbutane gave six fractions of formula CsHioCl2. Four fractions were found to be optically active, and two fractions optically inactive. Draw structural formulas for the compounds making up each fraction. Account in detail for optical activity or inactivity in each case. 

In the chlorosulfonation of 4-chloro-2-methylbutane (52), the electrophilic chlorine radical abstracts a hydrogen atom from the most electron-rich site to... 

Isoprene epoxidized on the vinyl group (l,2-epoxy-3-methyl-3-butene, 14) would also be an attractive synthon for monoterpenoids if it were more readily accessible. Among the possible preparative methods (Scheme 2), acid-catalyzed isomerization of 2,3-epoxy-l-halo-3-methylbutanes (epoxidized prenyl halides) and dehydrohalogenation of the resulting isomers seemed the best route until chlorination of prenyl acetate (15, R = OAc) with hypochlorous acid was published. Dehydration of the mixture of products gave a maximum of the chloroacetate 16, which yielded the epoxide 14 with sodium hydroxide in methanol. ... 

In one case the product stream was analysed for chlorinated compounds (ZSM-5, 400°C, WHSV=24) as shown in Table 1. The total selectivity to chlorinated reaction products (chloroisobutane, 2-chloro-2-methylbutane) is 4% and stays... 

When (5)-(+)-l-chloro-2-methylbutane reacts with chlorine, one of the products formed is (—)-l,4-dichloro-2-methylbutane. Does this product have the R or the S configuration ... 

Dr. A1 Cahall wanted to determine experimentally the relative ease of removal of a hydrogen atom from a tertiary, a secondary, and a primary carbon by a chlorine radical. He allowed 2-methylbutane to undergo chlorination at 300 °C and obtained as products 36% l-chloro-2-methylbutane, 18% 2-chloro-2-methylbutane, 28% 2-chloro-3-methylbutane, and 18% l-chloro-3-methylbutane. What values did he obtain for the relative ease of removal of tertiary, secondary, and primary hydrogen atoms by a chlorine radical under the conditions of his experiment ... 

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. Calculate approximate AH values for the possible monochlorination reactions of 2-methylbutane. Use the bond dissociation energies measured for OHjOH2-H, H-CH(CH3>2, and H-CCCH as representative of typical primary, secondary, and tertiary C-H bonds. 

Since there are three pentanes, eight possible amyl chlorides, and therefore eight arrangements of the alcohol CsHuOH, the process is not so simple as indicated in the equations above. In the commercial synthesis, normal pentane and isopentane (2-methylbutane) are submitted to chlorination and subsequent hydrolysis. 

Aldehydes from amino acid chlorination, such as 2-methylpropanal (derived from valine), 2- and 3-methylbutanal (derived from leucine and isoleucine, respectively), and phenylacetaldehyde (derived from phenylalanine), have been found to be potent contributors to taste-and-odor problems in the drinking water supply of Edmonton, Alberta. The incidents occur in the spring following snowmelt and concomitant increases in raw water color and TOC (Hrudey et al., 1988). Interestingly, the aldehydes were also formed during monochloramine disinfection. 

The enantioselective a-chlorination of -keto esters was achieved with up to 88% ee using NCS with a commercially available TADDOL ligand. The chiral bisoxazoline copper(II) complexes have also been reported to induce the asymmetric a-chlorination of -keto esters when reacted with NCS. The asymmetric a-chlorination of aldehydes has been achieved using NCS and (2/ ,5/ )-diphenylpyrrolidine as a chiral catalyst. For example, the enantioselective chlorination of 3-methylbutanal with NCS proceeds in 95% yield and 94% ee (eq 17). ... 

There is experimental evidence that alkyl radicals are planar, or pyramidal, structures that can become planar through inversion (like ammonia). For example, chlorination of (-l-)-l-chloro-2-methylbutane (10) produced racemic ( )-l,2-dichloro-2-methylbutane (12). The results are most consistent with an intermediate radical 11, which is planar or which has only a small barrier to inversion so that the reaction of 11 can occur equally well from either face of the radical center. The loss of optical activity shows that radical is probably planar (Scheme 4.5). 

For example, the chlorination of iso-butane in the gas phase at lOC C gives comparable quantities of iso-butyl chloride and tert-butyl chloride, and at 300 °C 2-methylbutane gives a mixture of products (Scheme 4.18). 

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