Chlorine bonding

Here the electrophile is tert butyl cation formed by a hydride migration that accompa nies ionization of the carbon-chlorine bond... 

Unlike the case of benzene in which ionization involves loss of a tt electron from the ring electron impact induced ionization of chlorobenzene involves loss of an elec tron from an unshared pair of chlorine The molecular ion then fragments by carbon-chlorine bond cleavage... 

Carbon-Hydrogen and Carbon-Chlorine Bond Dissociation Energies of Selected Compounds... 

The carbon-bromine bond is longer than the carbon-chlorine bond therefore although the charge e in the dipole moment expression p, = e d k smaller for the bromine than for the chlo nne compound the distance d is greater... 

The aromaticity of the nng is retained when chlorine bonds to the benzyhc carbon aro maticity is lost when chlorine bonds to one of the ring carbons... 

The fluoride ion is the least polarizable anion. It is small, having a diameter of 0.136 nm, 0.045 nm smaller than the chloride ion. The isoelectronic E and ions are the only anions of comparable size to many cations. These anions are about the same size as K" and Ba " and smaller than Rb" and Cs". The small size of E allows for high coordination numbers and leads to different crystal forms and solubiUties, and higher bond energies than are evidenced by the other haUdes. Bonds between fluorine and other elements are strong whereas the fluorine—fluorine bond is much weaker, 158.8 kj/mol (37.95 kcal/mol), than the chlorine—chlorine bond which is 242.58 kJ/mol (57.98 kcal/mol). This bond weakness relative to the second-row elements is also seen ia 0-0 and N—N single bonds and results from electronic repulsion. 

Organic fluorine compounds were first prepared in the latter part of the nineteenth century. Pioneer work by the Belgian chemist, F. Swarts, led to observations that antimony(Ill) fluoride reacts with organic compounds having activated carbon—chlorine bonds to form the corresponding carbon—fluorine bonds. Preparation of fluorinated compounds was faciUtated by fluorinations with antimony(Ill) fluoride containing antimony(V) haUdes as a reaction catalyst. 

Most chlorofluorocarbons are hydrolytically stable, CCI2F2 being considerably more stable than either CCl F or CHCI2F. Chlorofluoromethanes and ethanes disproportionate in the presence of aluminum chloride. For example, CCl F and CCI2F2 give CCIF and CCl CHCIF2 disproportionates to CHF and CHCl. The carbon—chlorine bond in most chlorofluorocarbons can be homolyticaHy cleaved under photolytic conditions (185—225 nm) to give chlorine radicals. This photochemical decomposition is the basis of the prediction that chlorofluorocarbons that reach the upper atmosphere deplete the earth s ozone shield. 

Replacement of Labile Chlorines. When PVC is manufactured, competing reactions to the normal head-to-tail free-radical polymerization can sometimes take place. These side reactions are few ia number yet their presence ia the finished resin can be devastating. These abnormal stmctures have weakened carbon—chlorine bonds and are more susceptible to certain displacement reactions than are the normal PVC carbon—chlorine bonds. Carboxylate and mercaptide salts of certain metals, particularly organotin, zinc, cadmium, and antimony, attack these labile chlorine sites and replace them with a more thermally stable C—O or C—S bound ligand. These electrophilic metal centers can readily coordinate with the electronegative polarized chlorine atoms found at sites similar to stmctures (3—6). 

Stabilization Mechanism. Zinc and cadmium salts react with defect sites on PVC to displace the labHe chloride atoms (32). This reaction ultimately leads to the formation of the respective chloride salts which can be very damaging to the polymer. The role of the calcium and/or barium carboxylate is to react with the newly formed zinc—chlorine or cadmium—chlorine bonds by exchanging ligands (33). In effect, this regenerates the active zinc or cadmium stabilizer and delays the formation of significant concentrations of strong Lewis acids. 

Substitution at the Carbon—Chlorine Bond. Vinyl chloride is generally considered inert to nucleophilic replacement compared to other alkyl halides. However, the chlorine atom can be exchanged under nucleophilic conditions in the presence of palladium [7440-05-3] Pd, and certain other metal chlorides and salts. Vinyl alcoholates, esters, and ethers can be readily produced from these reactions. 

Chlorosulfuric acid is a strong acid containing a relatively weak sulfur—chlorine bond. Many salts and esters of chlorosulfuric acid are known, most of them are relatively unstable or hydrolyze readily in moist air. 

The properties of chlorine azide resemble those of bromine azide. Pon-sold has taken advantage of the stronger carbon-chlorine bond, i.e., the resistance to elimination, in the chloro azide adducts and thus synthesized several steroidal aziridines. 5a-Chloro-6 -azidocholestan-3 -ol (101) can be converted into 5, 6 -iminocholestan-3l -ol (102) in almost quantitative yield with lithium aluminum hydride. It is noteworthy that this aziridine cannot be synthesized by the more general mesyloxyazide route. Addition of chlorine azide to testosterone followed by acetylation gives both a cis- and a trans-2iddMct from which 4/S-chloro-17/S-hydroxy-5a-azidoandrostan-3-one acetate (104) is obtained by fractional crystallization. In this case, sodium borohydride is used for the stereoselective reduction of the 3-ketone... 

Reductive cleavages of carbon-chlorine bonds by active metals and with photochemical activation figure in recent studies aimed at HFCs and HCFCs Sodium amalgam [3J] (equation 25), zinc powder [34] (equation 26), and alumi-mun/tin chloride [35] (equation 26) are all used in conjunction with protic solvents in reactions giving high yields and conversions... 

Copyrolysis of 1,1-diehloroperfluoroindane and chlorodifluoromethane or tetrafluoroethylene gives 1-perfluoromethyleneindane as the major product and three minor products [3] (equation 2) Insertion of difluorocatbene into the benzylic carbon-chlorine bond and subsequent loss of a chlonne molecule is observed in the copyrolysis of chlorodifluoromethane and pentafluorobenzotnchlonde to give a-chloroperfluorostyrene as the major product. Aromatic carbon-chlorine bonds are unreactive to the difluorocarbene in this reaction [4] (equation 3). 

Next, examine the structure of 1-phenyl-1-ethyl cation-chloride anion, an ion pair that is initially generated. What evidence is there for cai bon-chlorine bond cleavage Examine the electrostatic potential map for the ion pair. Which face of the cation is more available for attack How could the other enantiomer form ... 

The halogen migration is completely suppressed by halogen-metal exchange when the chloroethynyl group is in position 5 of the pyrazole ring. The concentrations of 3-pyrazolyl and 4-pyrazolyl anions are probably small, and they cannot compete with NH2 anions for chlorine bonded to the acetylenic carbon. 

The mechanism for the transformation of 5 to 4 was not addressed. However, it seems plausible that samarium diiodide accomplishes a reduction of the carbon-chlorine bond to give a transient, resonance-stabilized carbon radical which then adds to a Smni-activated ketone carbonyl or combines with a ketyl radical. Although some intramolecular samarium(n)-promoted Barbier reactions do appear to proceed through the intermediacy of an organo-samarium intermediate (i.e. a Smm carbanion),10 ibis probable that a -elimination pathway would lead to a rapid destruction of intermediate 5 if such a species were formed in this reaction. Nevertheless, the facile transformation of intermediate 5 to 4, attended by the formation of the strained four-membered ring of paeoniflorigenin, constitutes a very elegant example of an intramolecular samarium-mediated Barbier reaction. 

Resonance of a Carbon-Chlorine Bond and an Adjacent Double Bond.—We expect the phosgene molecule to resonate among the structures... 

---