2-Phenylethyl chloride

An example with the characteristics of the coupled displacement is the reaction of azide ion with substituted 1-phenylethyl chlorides. Although the reaction exhibits second-order kinetics, it has a substantially negative p value, indicative of an electron deficiency at the transition state. The physical description of this type of activated complex is the exploded S 2 transition state. 

Stabilization of a carbocation intermediate by benzylic conjugation, as in the 1-phenylethyl system shown in entry 8, leads to substitution with diminished stereosped-ficity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. The system has been analyzed in terms of the fate of the intimate ion-pair and solvent-separated ion-pair intermediates. From this analysis, it has been estimated that for every 100 molecules of 1-phenylethyl chloride that undergo ionization to an intimate ion pair (in trifluoroethanol), 80 return to starting material of retained configuration, 7 return to inverted starting material, and 13 go on to the solvent-separated ion pair. 

Studies of the solvolysis of 1-phenylethyl chloride and its p-substituted derivatives in aqueous trifluorethanol containing azide anion as a potential nucleophile provide details relative to the mechanism of nucleophilic substitution in this system. 

C KIE in the base-promoted elimination reactions (equation 257) of 2,2-diaryl-1,1,1-trichloroethanes (388), substituted 2-phenylethyltrimethylammonium bromides (389), substituted 2-phenylethyl chlorides (390) and substituted 1-phenylethyl chlorides (391), labelled successively at the a and the p carbons, have been measured 70 and it was concluded that these reactions proceed according to the E2 mechanism with transition states having varying degrees of Elcb or El character. 

The effect of methyl groups in the aromatic ring of 1-phenylethyl chloride is shown in Table 3. The small but significant rate increase of the o-CH3 group was reasonably ascribed to steric acceleration. 

Fig. 35 The Y-T plots for the reactions of ring substituted 1-phenylethyl chlorides in 20% acetonitrile in water at ionic strength 0.8 (NaC104) at 25°C r = 1.15. Reproduced with permission from Richard et al. (1984b). Copyright 1984 American Chemical...

In Fig. 36, log for the unimolecular reaction and log k2 for the bimolecular one are plotted against Y-T a with r = 1.15. The dependence of the SnI rate on the aryl substituents is clearly far greater than that of the Sn2 rate. Although the unimolecular SnI rate constants could be obtained only for substituents more ED than H, the substituent effect on the unimolecular process can be described by the Y-T equation (2) with a p value of —5.0 and an r value of 1.15. On the other hand, the plot of logA 2 vs. 5(r = 1.15) for the bimolecular (AnDn) process gives a smooth curve. The correlation for the former unimolecular process is identical with that for the corresponding solvolyses of 1-phenylethyl chloride in solvolysing solvents (Tsuno et al., 1975 Fujio et al., 1984). 

Fig. 37 More O Ferrall-Jencks diagram for the Menschutkin reactions of 1-phenylethy] and benzyl chlorides with pyridine. The structures of transition states were optimized by ab initio MO calculation (RHF/b-Sf G ). O, substituted 1-phenylethyl chlorides with pyridine , benzyl chlorides with pyrindine , with 4-nitropyridine O, methyl and A, ethyl chlorides with pyridine (Fujio et al, unpublished).

The overall consumption rate of the covalent precursor (ks) is determined by HPLC and/or titration measurements this correlates with monomer consumption in propagation. The rate of racemization of optically active 1-phenylethyl chloride (ka) is determined by polarimetric measurements, Racemization is usually faster than solvolysis, confirming that activation is reversible and that internal return may occur before the carbenium ion reacts with an external nucleophile, Racemization requires not only that the C—Cl bond of the covalent precursor is broken, but that the lifetime of the ion pair is long enough for the flat carbenium ion to rotate, such that both sides of the carbenium ion are completely equivalent as shown in Eq. 18. 

Phenylethyl chloride ionizes and therefore racemizes in nitrometh-ane both spontaneously and in the presence of various acids (e.g., HC1) [47], HC1 elimination (dehydrochlorination) also occurs. However, unimo-lecular racemization is four times faster than this side reaction at 99° C ... 

Spontaneous racemization is very slow in nonpolar solvents at ambient temperature, but is greatly accelerated by protonic and Lewis acids. Racemization is first order in both 1-phenylethyl chloride and acid. Racemization catalyzed by SnCL, in CC14 at 25° C proceeds with a rate constant ka = 1.5 x 10-2 mol, -L-sec [48]. Because styrene and 1-phenylethyl chloride consumption is 90 times slower than racemization, the rate of racemization is not affected by adding styrene to the system. That is, the efficiency of ion capture by styrene is low, whereas the ion pair collapse must be very fast. Racemization of 1-phenylethyl chloride with SnCl4 is nearly 100 times faster in benzene than in CCL, ka = 1.3 mol-, L sec- at 25° C [49], with activation parameters AHt = 35 kJ-rnol- and A St = 120/mor K-. ... 

Boron trichloride and tribromide exchange ligands when used with alkyl halides and acetates initiators. For example, as shown in Eq. (35), 1-phenylethyl bromide and BBr2Cl are formed rapidly and quantitatively when 1-phenylethyl chloride is mixed with an equimolar amount of boron tribromide [59,67]. 

Figure 23 MWDs of polystyrene obtained with the l-phenylethyl chloride/SnCl4 initiating system in CH2Cl2 solvent at - 15° C [styrene]0 = 1.0 M [1-phenylethyl chloride]o = 20 mA/ [SnCl4]o = 100 mil [salt]0 = 40 mM conversion >90%. Additives (a) none (salt-free) (b) nBmNCl (c) nBu4NBr. (From Ref. 25.)...

This author however did not search for 1-phenylethyl chloride among the products. 

One can conclude that the 1-phenylethyl halide is by far the preferred product of these interactions, but that suitable conditions of polarity and temperature can create the conditions required for some polymerisation to take place. Pocket et al. have demonstrated that 1-phenylethyl chloride exchanges chlorine atoms with HCl in nitromethane, i.e., that carbenium ion pairs can be generated in tiiis system. It seems likely that the low yields of polymers observed in specific conditions as described above resulted from the solvation of the halide by excess acid, a contingency which restricted active species to a short period. From the few scattered results published it is nevertheless difficult to derive a sound interpretation of such apparent anomalies as the fact that the strongest of all hydrogen halides, HI, failed to induce polymerisation. 

Note that the polarised complex formed in tiris interaction is not active for initiation, but (-)-1-phenylethyl chloride is racemised by SnCl4 in CCl4 ... 

While these processes occur, the removal of SnCl4 2 H2O to form the products described allows some of the precipitate to reenter the solution, until there is sufficient catalyst not bound to 1-phenylethyl chloride for polymerisation to start. 

That the induction period is linked specifically to 1-phenylethyl chloride is not only proved by the fact that addition of this compound to polymerising mixtures with styrene stops the process but also because no induction period is observed in the dimerisation of 1,1-diphenylethylene by SnCl4-H20 in benzene while if styrene is pol3mierised by the same system induction periods are noticed. We believe that initiation with the latter monomer needs one molecule of free SnCl4 and one of dihydrate. 

Our tentative explanation of the waves in the conversion-time curves observed in the polymerisation of styrene in CCI4 or toluene is also based on the presence of a reservoir of undissolved initiator. We assume that the precipitated SnCl4 2 H2O reenters the solution at a slow constant rate. The presence in the system of 1-phenylethyl chloride and/or chain ends possessing a similar stmcture, and of HCl, does not allow the entering initiator to generate a sufficient number of active species and one observes... 

We wish to stress that the above conjectures are not thoroughly backed by experimental evidence, but based on rather circumstantial information. They should therefore be considered as working hypotheses to be verified by future investigations, particularly concerning the role fo 1-phenylethyl chloride. 

This section deals with the difficult problem of establishing the most plausible initiation mechanism for polymerisation systems in which cocatalysis is ascribed to alkyl or aralkyl halides. This type of cocatalysis is by no means a general phenomenon. We have already mentioned in preceding sections dealing with other types of cocatalysis that, for example, r-butyl chloride is not a cocatalyst in the polymerisation of isobutene induced by titanium tetrachloride and that 1-phenylethyl chloride is not a cocatalyst in conjunction vdth stannic chloride - Indeed, the reaction... 

A recent study by Bos and Treloar on the interaction of mercuric chloride with trityl chloride in ethylene chloride showed that the trityl trichloromercurate is formed as a mixture of free ions and ion pairs, roughly in equal amounts when die concentrations of the precursors were similar to those used in the polymerisation studies reported above. This conclusion makes the interpretation of Sambhi s results even more difficult, because the extremely low polymerisation rates he observed are incompatible wdfli initiation by free trityl ions. In fact, the work of Johnson and Pearce discussed below, clearly showed that initiation is fast with styrene when one uses a trityl salt whichis appreciably dissociated into free ions. Tliis paradox raises a new question, namely that concerning the role of the counterion in these processes. It is conceivable that with HgCli transfer of a chloride ion is so favoured that appreciable quantities of 1-phenylethyl chloride orhcnnologous oligomers could be formed and not be detected as polymer . Sambhi s few polymerisation curves do not allow to assess this point. 

Further studies on the pyrolysis of chlorinated and brominated hydrocarbons have been reported by Maccoll et al. for 3-bromopentane , menthyl and neo-menthyl chlorides , r-alkyl chlorides , dimethylallyl chlorides , a-chloro-o-xylene , and substituted 1-phenylethyl chlorides . Other workers have reported on the thermal reactions of ethyl chloride , monochloropentanes , 1-... 

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