Cl bond distances in some individual compounds

Crystal structures of some highly chlorinated compounds were selected from the CSD according to the following criteria (a) number of chlorine atoms 5, (b) compound contains only H, N and/or O beside C and Cl, (c) R factor 0.05, (d) standard deviation for C—C bonds 0.5 pm, (e) publication between 1983 and 1991. With these restrictions 135 C(sp3)—Cl bonds were listed with a mean value of 176.8 pm (o - 1.8 pm). The individual values range from 173.2 to 184.4 pm. The above criteria result in 81 C(sp2)—Cl bonds with a mean value of 172.0 pm (cr = 1.6 pm), and individual bond lengths ranging from 168.7 to 178.0 pm. Some examples of highly chlorinated compounds of relatively small size are presented in Table 30 [C(sp3)—Cl distances] and Table 31 [C(sp2)—Cl distances]. 

The typical C(sp2)—Cl bond distances in Table 29 are split into bonds to terminal carbon (substructures VII and IX) and into bonds to aromatic rings (substructures IX and X). Since the values are nearly equal in both cases (173.4 and 173.9 pm) and shorter by about 

Chlorinated dibenzo-p-dioxines are of great interest because of their toxicity which depends strongly on the degree and on the position of chlorination. C—Cl bond lengths of 

TABLE 32. C(sp2)—Cl bond distances (in pm) in some chlorinated dibenzo-p-dioxines 

Two crystal structures with highly unusual C—Cl bond distances have been reported very recently. An extremely long C(sp2)—Cl bond of 185.5(7) pm occurs in the ethylene part of 1 -chloro-2,2-bis(4-chlorophenyl)-1 -lithioethylene (43), more than 10 pm longer than the typical value for vinyl C—Cl bonds (173.4 pm)292. A very short C—Cl bond of 166.8 pm has been determined for 44, which is the first crystal structure analysis for a chlorinated carbocation293. The shortening of this bond is rationalized by chlorine back-donation and partial double-bond character of the C—Cl bond. The aromatic C—Cl bond has a normal length of 172.6(9) pm. 

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