Chloropentane

Let us now examine what happens when a chiral molecule (containing one chirality center) reacts so as to yield a product with a second chirality center. As an example consider what happens when (5)-2-chloropentane undergoes chlorination at C3 (other products are formed, of course, by chlorination at other carbon atoms). The results of chlorination at C3 are shown in the box below. 

The products of the reactions are (25,35)-2,3-dichloropentane and (2S,3R)-2,3-dichloropentane. These two compounds are diastereomers. (They are stereoisomers but they are not mirror images of each other.) The two diastereomers are not produced in equal amounts. Because the intermediate radical itself is chiral, reactions at the two faces are not equally likely. The radical reacts with chlorine to a greater extent at one face than the other (although we cannot easily predict which). That is, the presence of a chirality center in the radical (at C2) influences the reaction that introduces the new chirality center (at C3). 

Both of the 2,3-dichloropentane diastereomers are chiral and, therefore, each exhibits optical activity. Moreover, because the two compounds are diastereomers, they have different physical properties (e.g., different melting points and boiling points) and are separable by conventional means (by gas chromatography or by careful fractional distillation). 

Abstraction of a hydrogen atom from C3 of (S)-2-chloropentane produces a radical that is chiral (it contains a chirality center at C2).This chiral radical can then react with chlorine at one face [path (a)] to produce (2S, 3S)-2,3-dichloropen-tane and the other face [path (b)] to yield (2S, 3R) -2,3-dichloropentane. These two compounds are diastereomers, and they are not produced in equal amounts. Each product is chiral, and each alone would be optically active. 

Consider the chlorination of (5)-2-chloropentane at C4. (a) Write structural formulas for the products, showing three dimensions at all chirality centers. Give each its proper R,S) designation, (b) What is the stereoisomeric relationship between these products (c) Are both products chiral (d) Are both optically active (e) Could the products be separated by conventional means (f) What other dichloropentanes would be obtained by chlorination of (5)-2-chloropentane (g) Which of these are optically active  

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Ghloropentane. Use the quantities given in the previous preparation, but substitute 22 g. (27 ml.) of methyl n-propyl carbinol (b.p. 118-5°) for iso-amyl alcohol. Collect the 2-chloropentane at 96-98°. 

Cyclization of the two pendant alkyl side chains on barbiturates to form a spiran is consistent with sedative-hypnotic activity. The synthesis of this most complex barbiturate starts by alkylation of ethyl acetoacetate with 2-chloropentan-3-one to give 152. Hydrolysis and decarboxylation under acidic conditions gives the diketone, 153. This intermediate is then reduced to the diol (154), and that is converted to the dibromide (155) by means of hydrogen bromide. Double Internal alkylation of ethyl... 

Look carefully at the reactions shown in the previous section. In each case, an unsymmetrically substituted alkene has given a single addition product, rather than the mixture that might have been expected. As another example, 1-pentene might react with HC1 to give both 1-chloropentane and 2-chloropentane, but it doesn t. Instead, the reaction gives only 2-chloropentane as the sole product. We say that such reactions are regiospecific (ree-jee-oh-specific) when only one of two possible orientations of addition occurs. 

The 50.31 MHz 13C NMR spectra of the chlorinated alkanes were recorded on a Varian XL-200 NMR spectrometer. The temperature for all measurements was 50 ° C. It was necessary to record 10 scans at each sampling point as the reduction proceeded. A delay of 30 s was employed between each scan. In order to verify the quantitative nature of the NMR data, carbon-13 Tj data were recorded for all materials using the standard 1800 - r -90 ° inversion-recovery sequence. Relaxation data were obtained on (n-Bu)3SnH, (n-Bu)3SnCl, DCP, TCH, pentane, and heptane under the same solvent and temperature conditions used in the reduction experiments. In addition, relaxation measurements were carried out on partially reduced (70%) samples of DCP and TCH in order to obtain T data on 2-chloropentane, 2,4-dichloroheptane, 2,6-dichloroheptane, 4-chloroheptane, and 2-chloroheptane. The results of these measurements are presented in Table II. In the NMR analysis of the chloroalkane reductions, we measured the intensity of carbon nuclei with T values such that a delay time of 30 s represents at least 3 Tj. The only exception to this is heptane where the shortest T[ is 12.3 s (delay = 2.5 ). However, the error generated would be less than 10%, and, in addition, heptane concentration can also be obtained by product difference measurements in the TCH reduction. Measurements of the nuclear Overhauser enhancement (NOE) for carbon nuclei in the model compounds indicate uniform and full enhancements for those nuclei used in the quantitative measurements. Table II also contains the chemical... 

DCP REDUCTION. As illustrated below DCP (D) is sequentially transformed into 2-chloropentane (M) and then to pentane (P) during its reduction with (n-Bu)3SnH. 

Figure 3. Distribution of reactants (D, M) and products (M, P) observed in the (n-Bu)3SnH reduction of DCP. D = DCP, M = 2-chloropentane and P = pentane (see Figure 2).

Our 13C NMR analysis (2) of the E-V copolymers obtained via the (n-Bu)3SnH reduction of PVC led to k /kr=1.31 0.1 in excellent agreement with the kinetics observed for the removal of chlorines from m- and r-DCP. We also found no W diads in those E-V copolymers made by removing more than 80% of the chlorines from PVC. This observation is confirmed in the (n-Bu)3SnH reduction of DCP where the chlorines in this PVC diad model compound were found to be 4 times easier to remove than the isolated chlorines in 2-chloropentane, 2-, and 4-chlorooctane. 

Neither 1-chloropentane nor 3-chloropentane contains a stereocenter, but 2-chloropentane does, and it is obtained as a racemic form. 

Abstraction of a hydrogen atom from C2 produces a trigonal planar radical that is achiral. This radical is achiral then reacts with chlorine at either face [by path (a) or path (b)]. Because the radical is achiral the probability of reaction by either path is the same therefore, the two enantiomers are produced in equal amounts, and a racemic form of 2-chloropentane results. 

A small quantity of 2,4-dichloropentan-3-one (5%) is obtained with the a-monochloropentanone 2 during the distillation. The NMR 8uialysis for 2-chloropentan-3-one 2 is described by Wyman and Kaufman. ... 

Dimethylpropane. b) CHjCHjCHjCH CH, gives 2-chloropentane and 3-chloropentane. 2-Methyl-butane gives 2-chloro-3-methylbutane. (c) 2-Methylbutane gives 2-chloro-2-methylbutane. 

Problem 7.53 Give structures of all alkenes formed and underline the major product expected from E2 elimination of (a) 1-chloropentane, (fc) 2-chloropentane. 

S-methyIbutyraldehyde, 3-methylbutanal (c) a-chlorovaleraldehyde, 2-chloropentanal (d) methyl isopropyl ketone, 3-methyl-2-butanone (e) ethyl phenyl ketone, 1-phenyl-1-propanone (propiophenone) (/) methyl vinyl ketone, 3-buten-2-one. The C=0 group has numbering priority over the C=C group. 

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