1-Chloroethene, bonding

Double bonds, representing the sharing of two pairs of electrons, are inferred by writing a double line. Vinyl chloride (systematically chloroethene) is shown as two different representations according to the conventions we have just seen for propanol. Note that it is customary always to show the reactive double bond, so that CH2CHCI would not be encountered as an abbreviation for vinyl chloride. 

By contrast, vinyl halides such as chloroethene, CHj CHCl, and halogenobenzenes are very unreactive towards nucleophiles. This stems from the fact that the halogen atom is now bonded to an sp hybridised carbon, with the result that the electron pair of the C—Cl bond is drawn closer to carbon than in the bond to an sp hybridised carbon. The C—Q is found to be stronger, and thus less easily broken, than in, for example, CH3CH2CI, and the C—Q dipole is smaller there is thus less tendency to ionisation (8 1) and a less positive carbon for OH to attack Sf l) the n electrons of the double bond also inhibit the close approach of an attacking nucleophile. The double bond would not help to stabilise either the 8 y2 transition state or the carbocation involved in the 8 1 pathway. Very much the same considerations apply to halogenobenzenes, with their sp hybridised carbons and the tt orbital system of the benzene nucleus their reactions, which though often bimolecular are not in fact simply 8 2 in nature, are discussed further below (p. 170). 

In contrast, compounds such as chlorobenzene and chloroethene, in which the halogen is attached directly to a multiply bonded carbon atom, do not exhibit SNl-type reactions. Evidently then, unsaturated carbon cations such as phenyl or ethenyl are appreciably less stable (more difficult to form) than tert-alkyl cations ... 

The reactivity order also appears to correlate with the C-X bond energy, inasmuch as the tertiary alkyl halides both are more reactive and have weaker carbon-halogen bonds than either primary or secondary halides (see Table 4-6). In fact, elimination of HX from haloalkenes or haloarenes with relatively strong C-X bonds, such as chloroethene or chlorobenzene, is much less facile than for haloalkanes. Nonetheless, elimination does occur under the right conditions and constitutes one of the most useful general methods for the synthesis of alkynes. For example,... 

The cyclopropyl halides are exceptional in that their behavior is much more like alkenyl halides than like secondary alkyl halides. Thus cyclopropyl chloride undergoes SN1 and SN2 reactions much less rapidly than isopropyl or cyclohexyl chlorides. A relationship between the reactivity of cyclopropyl chloride and chloroethene is not surprising in view of the general similarity between cyclopropane rings and double bonds (Section 12-5). This similarity extends to cyclopropylmethyl derivatives as well. Cyclopropylmethyl chloride is reactive in both SN-1 and SN2 reactions in much the same way as 3-chloropropene ... 

The haloalkenes, for example, chloroethene [ethylene chloride, C1(H)C=CH2], prepared in this and related ways and discussed in more detail with the other organic halides (Chapter 7) serve as starting material for a host of other types of compounds. However, additional substitution reactions (also apparently occurring through addition-elimination pathways) might be effected. Typical of these is the apparent arylation of an alkene in which an aryl group is substituted for a hydrogen attached to the double bond. The process is known as the Heck reaction and is shown in Scheme 6.54. ... 

B3LYP/6-31 lG-l-l-(d,p) calculations on the reactions of (1-chlorovinyl)- and (1-chlorocyclopropyl)magnesium chlorides solvated with two molecules of dimethyl ether, with a chloride ion, methyl anion, vinyl anion, or an acetylide anion showed that the reactions with magnesium compounds have lower AG values than either chloroethene or chlorocyclopropane. This is attributed to the longer, more reactive, C-Cl bond in the magnesium compounds. The potential energy surfaces, AG values, and the transition state structures for the reactions are given. 

Several technical processes have been developed in which reagents presence of a catalyst add to the triple bond. For example, the catalyzed addition of hydrogen chloride gives chloroethene (vinyl chloride), and addition of hydrogen cyanide produces propenenitrile (acrylonitrile). 

The C—Cl bond energy in chloroethane is 341 kj mole for chloroethene (CHj=CHCl), it is 368 kj mole . Why is the bond in chloroethene stronger The C—Cl bond length in chloroethane is 178 pm. Explain whether you expect the C—Cl bond length in vinyl chloride to be longer or shorter than the C—Cl bond in chloroethane. 

Most alkenes are weakly polar. For example, propene has a dipole moment of 0.3 D. The dipole moments of alkenes containing substituents with bond moments of known direction can be used to establish the bond moment for an sp —sp carbon—carbon bond. The dipole moment of chlo-roethene is 1.4 D. Because chlorine is more electronegative than carbon, the chlorine atom has a partial negative charge. The net dipole moment of rn r-l-chloropropene is 1.7 D. It results from the cumulative effect of the carbon—carbon single bond and the carbon—chlorine bond. Because dipole moment of tntwr-l-chloropropene is larger than that of 1-chloroethene, the two contributing bond moments in tntwr-l-chloropropene reinforce each other. 

Answer The carbon atom in chloroethene is sp hybridi2ed and forms shorter and stronger bonds than the sp -hybridized carbon atom of chloroethane. The C—Cl bond length should be about 2% shorter, or 175 pm. 

The -f-H2C—CHCl section of the polymer chain is the repeat unit of poly(chloroethene) (Figure 15.18). In poly(alkenes) made of one type of monomer, the repeat unit is the same as the monomer except that the C=C double bond is changed to a C—C single bond. Notice that, as in any other addition reaction of the alkenes, addition polymerisation yields only one product. 

Rememberfrom Chapter 15 that addition polymerisation occurs when an unsaturated monomer, such as an alkene, bonds to itself in an addition reaction. Poly(ethene), poly(chloroethene) and poly(phenylethene) are all addition polymers. 

The electron-withdrawing inductive effect of the chlorine of chloroethene makes its double bond less electron rich than that of ethene. This causes the rate of reaction of chloroethene with an electrophile (i.e., a proton) to be slower than the corresponding reaction of ethene. 

Although it may seem counterintuitive, there are ground-state effects that give evidence for this phenomenon. In chlorobenzene, the contribution of resonance forms such as 12.8, in which there is a positive charge on chlorine, results in a shortened carbon-chlorine bond (1.69 A vs. 1.77 A in chloroethene and 1.72-1.75 A in haloalkanes) and a lowered dipole moment. 

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