MECHANISMS Electrophilic Addition1 In this mechanism, a positive species approaches the double or triple bond and in the first step forms a bond by donation of the p pair of electrons2 to the electrophilic species to form a s pair
!he I"#AC I"#AC desi$nation for this mechanism is
%
As in electrophilic substitution &p% '()*, + need not actually be a positive ion but can be the positive end of a dipole dip ole or an induced dipole, dipo le, ith the ne$ative part brea-in$ b rea-in$ off either durin$ the first step or shortly after% !he second step is a combination of 1 ith a species carryin$ an electron pair and often bearin$ a ne$ative char$e% !his step is the same as the second step of the SN1 mechanism% Not all electrophilic additions follo the simple mechanism $iven above% In many brominations it is fairly certain that 1, if formed at all, very rapidly cycli.es to a bromonium ion &2*
!his intermediate is similar to those encountered in the nei$hborin$/$roup mechanism of nucleophilic substitution &see p% 00'*% !he attac- of on an intermediate li-e 2 is an SN2 step% hether the intermediate is 1 or 2, the mechanism is called AdE2 &electrophilic
addition, bimolecular*% In investi$atin$ the mechanism of addition to a double bond, perhaps the most useful type of information is the stereochemistry of the reaction% !he to carbons of the double bond and the four atoms immediately attached to them are all in a plane &p% 3*4 there are thus three possibilities% 5oth + and may enter from the same side of the plane, in hich case the addition is stereospecific and syn4 they may enter from opposite sides for stereospecific anti addition4 or the reaction may be nonstereospecific% In order to determine hich of these possibilities is occurrin$ in a $iven reaction, the folloin$ type of e6periment is often done +is added to the cis and trans isomers of an al-ene of the form A5C7C5A% e may use the cis al-ene as an e6ample% If the addition is syn, the product ill be the erythro dl pair, because each carbon has a (89 chance of bein$ attac-ed by +
:n the other hand, if the addition is anti, the threo dl pair ill be formed
:f course, the trans isomer ill $ive the opposite results the threo pair if the addition is syn and the erythro pair if it is anti% !he threo and erythro isomers have different physical
properties% In the special case, here + ; &as in the addition of 5r2*, the <or e6ample,the positive char$e mi$ht be stabili.ed by an attraction for + that does not involve a full bond &see *%
!he second $roup ould then come in anti% A circumstance that ould favor syn addition ould be the formation of an ion pair after the addition of +0
Since is already on the same side of the plane as +, collapse of the ion pair leads to syn addition% Another possibility is that anti addition mi$ht, at least in some cases, be caused by the operation of a mechanism in hich attac- by and +are essentially simultaneous but from opposite sides
!his mechanism, called the AdE mechanism &termolecular addition, I"#AC ANAE*,( has the disadvanta$e that three molecules must come to$ether in the transition state% Hoever, it is the reverse of the E2 mechanism for elimination, for hich the transition state is -non to possess this $eometry &p% 10?)*% !here is much evidence that hen the attac- is on 5r@ &or a carrier of it*, the bromonium ion 2 is often an in termediate and the addition is anti% As lon$ a$o as 1311, Mcen.ie and >ischer independently shoed that treatment of maleic acid ith bromine $ave the dl pair of 2,/dibromosuccinic acid, hile fumaric acid &the trans isomer* $ave the meso compound%' Many similar e6periments have been performed since ith similar results% >or triple bonds, stereoselective anti addition as shon even earlier% 5romination of dicarbo6yacetylene $ave ?89 of the trans isomer%?
!here is other evidence for mechanisms involvin$ 2% e have already mentioned &p% 003* that bromonium ions have been isolated in stable solutions in nucleophilic substitution reactions involvin$ bromine as a nei$hborin$ $roup% Such ions have also been isolated in reactions involvin$ addition of a 5r@ species to a double bond%) !he folloin$ is further evidence% If the to bromines approach the double bond from opposite sides, it is very unli-ely that they could come from the same bromine molecule% !his means that if the reaction is performed in the presence of nucleophiles, some of these ill compete in the second step ith the bromide liberated from the bromine% It has been found, indeed, that treatment of ethylene ith bromine in the presence of chloride ions $ives some 1/chloro/2/ bromoethane alon$ ith the dibromoethane%3 Similar results are found hen the reaction is carried out in the presence of ater &1(/08* or of other nucleophiles%18 Ab initio molecular orbital studies sho that 2 is more stable than its open isomer 1 &+ 7 5r*%11 !here is evidence that formation of 2 is reversible%12 Hoever, a number of e6amples have been found here addition of bromine is not stereospecifically anti% >or e6ample, the addition of 5r2 to cis/ and trans/1/ phenylpropenes in CCl0 as nonstereospecific%1 >urthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent% In solvents of lo dielectric constant, the addition as 38B1889 anti, but ith an increase in dielectric constant, the
reaction became less stereospecific, until, at a dielectric constant of (, the addition as completely nonstereospecific%10 Di-eise in the case of triple bonds, stereoselective anti addition as found in bromi nation of /he6yne, but both cis and trans products ere obtained in bromination of phenylacetylene%1( !hese results indicate that a bromonium ion is not formed here the open cation can be stabili.ed in other ays &e%$%, addition of 5r@ to 1/phenylpropene $ives the ion #hCHCH5rCH, hich is a relatively stable ben.ylic cation* and that there is probably a spectrum of mechanisms beteen complete bromonium ion &2, no rotation* formation and completely open/cation &1, free rotation* formation, ith partially brid$ed bromonium ions &, restricted rotation* in beteen%1' e have previously seen cases &e%$%, p% 0'1* here cations reuire more stabili.ation from outside sources as they become intrinsically less stable themselves%1? >urther evidence for the open cationmechanism here aryl stabili.ation is present as reported in an isotope effect study of addition of 5r2 to ArCH7CHCHAr= &Ar 7 p/nitrophenyl, Ar=7 p/tolyl*% !he 10C isotope effect for one of the double/bond carbons &the one closer to the N:2 $roup* as considerably lar$er than for the other one%1) hen the F/bond of an al-ene attac-s Cl@,13 I@,28 and GS@,21 the result is similar to that hen the electrophile is 5r@4 there is a spectrum of mechanisms beteen cyclic intermediates and open cations% As mi$ht be e6pected from our discussion in Chapter 18 &p% 00'*, iodonium ions compete ith open carbocations more effectively than bromonium ions, hile chloronium ions compete less effectively% !here is -inetic and spectral evidence that at least in some cases, for e6ample, in the addition of 5r2 or ICl, the electrophile forms a F comple6 ith the al-ene before a covalent bond is formed%22 hen the electrophile is a proton,2 a cyclic intermediate is not possible, and the mechanism is the simple AH @ AN process shon before
!his is an A/SE2 mechanism &p% (2(*% !here is a $reat deal of evidence20 for it, includin$ 1% !he reaction is $eneral/acid, not specific/acid/cataly.ed, implyin$ ratedeterminin$ proton transfer from the acid to the double bond%2(
2% !he e6istence of open carbocation intermediates is supported by the contrast in the pattern of al-yl substituent effects2' ith that found in brominations, here cyclic intermediates are involved% In the latter case, substitution of al-yl $roups on H2C7CH2 causes a cumulative rate acceleration
until all four hydro$ens have been replaced by al-yl $roups, because each $roup helps to stabili.e the positive char$e%2? In addition of H, the effect is not cumulative% Geplacement of the to hydro$ens on one carbon causes $reat rate increases &primary secondary tertiary carbocation*, but additional substitution on the other carbon produces little or no acceleration%2) !his is evidence for open cations hen a proton is the electrophile%23 % :pen carbocations are prone to rearran$e &Chapter 1)*% Many rearran$ements have been found to accompany additions of H and H2:%8 It may also be recalled that vinylic ethers react ith proton donors in a similar manner &see 18/'*% !he stereochemistry of H addition is varied% E6amples are -non of predominant syn, anti, and nonstereoselective addition% It as found that treatment of 1,2 dimethylcyclohe6ene &0* ith H5r $ave predominant anti addition, 1 hile addition of ater to 0 $ave eual amounts of the cis and trans alcohols2
:n the other hand, addition of J5r to acenaphthylene &(* and to indene and 1/phenylpropene $ave predominant syn addition%
In fact, it has been shon that the stereoselectivity of HCl addition can be controlled by chan$in$ the reaction conditions% Addition of HCl to 0 in CH2Cl2 at /3)KC $ave predominantly syn addition, hile in ethyl ether at 8C, the addition as mostly anti%0 Addition of H to triple bonds has the same mechanism, althou$h the intermediate in this case is a vinylic cation, '%(
In all these cases &e6cept for the AdE mechanism*, e assumed that formation of the intermediate &1, 2, or * is the slo step and attac- by the nucleophile on the intermediate is rapid, and this is probably true in most cases% Hoever, some additions have been found in hich the second step is rate determinin$%'
