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Properties of the Tert-Butyl Halide Solvolysis Transition States In PCCP View Article Online PAPER View Journal | View Issue Properties of the tert-butyl halide solvolysis transition states† Cite this: Phys. Chem. Chem. Phys., a b bc 2021, 23, 3311 Michael H. Abraham, * Filomena Martins, * Ruben Elvas-Leita˜o and bd Luı´s Moreira We have obtained properties (or descriptors) of the transition states in the solvolysis of tert-butyl chloride, bromide and iodide. We show that all three transition states, in both protic and in aprotic solvents, are highly dipolar and are strong hydrogen bond acids and strong hydrogen bond bases, except for the Received 27th September 2020, tert-butyl iodide transition state in aprotic solvents, which has a rather low hydrogen bond acidity. Thus, Accepted 25th January 2021 the transition states are stabilized by solvents that are hydrogen bond bases (nucleophiles) and are DOI: 10.1039/d0cp05099g hydrogen bond acids (electrophiles). We show also that the partition of the transition states between water and solvents is aided by both nucleophilic and electrophilic solvents and conclude that the rate of rsc.li/pccp solvolysis of the three halides is increased by both nucleophilic and electrophilic solvents. Creative Commons Attribution-NonCommercial 3.0 Unported Licence. 1 Introduction et al.14 This is contrary to a long-established position15–18 that the rate of solvolysis of the tert-butyl halides is increased by nucleo- Although the mechanism of solvolysis of tert-butyl chloride has philic solvent participation, or by ‘‘nucleophilic assistance/cation been studied for the past 85 years,1 questions about the solvation’’. mechanism still remain unsettled, especially whether or not The tert-butyl halide transition states are generally regarded to there is nucleophilic assistance by solvents. Gajewski2 has be somewhere in between the relatively non-polar tert-butyl referred to this solvolysis as ‘‘...the most misunderstood reac- halides and the polar tert-butyl halide ion pairs, with the leaving This article is licensed under a tion in organic chemistry’’ and has put forward arguments halide atom in the transition state CÁÁÁX bond carrying a partial based on his multiple parameter equation3 that there is no negative charge. Then electrophilic solvents (hydrogen bond acids) positive nucleophilic solvent participation in the solvolysis. can solvate the leaving halide, leading to a reduction in the energy 4–7 Open Access Article. Published on 25 January 2021. Downloaded 9/26/2021 3:47:46 AM. Dvorko et al. used the multi-parameter Koppel-Palm equa- of the transition state and to an increase in the rate constant. tion to reach a similar conclusion for the solvolysis of tert-butyl There will also be a corresponding partial positive charge on the chloride, as did Ponomarev et al.8 for the solvolysis of tert-butyl carbon atom of the CÁÁÁXbond–asshowninScheme1for bromide and tert-butyl iodide.9 Indeed, these workers4–8 suggest hydroxylic solvents. Then nucleophilic solvents (hydrogen bond that there is a small negative effect of nucleophilic solvation in bases) would be expected to solvate this area of the transition the solvolysis of the three halides in protic solvents. However, it state, leading again to an increase in the rate constant. But should be mentioned that Serebryakov10 regards the various according to various workers,2–9,11–14 there is no rate enhancement correlation analyses4–8 to be unsound. Application of the multi- due to nucleophilic solvents, and hence there must be no nucleo- parameter Kamlet–Taft equation to rate constants for the tert-butyl philic solvation of the transition state. Dvorko et al.6 rationalized chloride solvolysis also indicated11–13 that there was no, or little, the lack of nucleophilic assistance as being due to an intermediate positive nucleophilic assistance, as also suggested by Farcasiu ion pair that they described as RÀ[O]XÀ butthisdoesnotseemto make the cause of the lack of nucleophilic assistance any clearer. We have set out a multi-parameter equation,19–22 that we have a Department of Chemistry, University College London, 20 Gordon St, London used to obtain information on the properties of a large number WC1H 0AJ, UK. E-mail: [email protected] b Centro de Quı´mica Estrutural (CQE), Faculdade de Cieˆncias, Universidade de Lisboa, Campo Grande, Edifı´cio C8, 1749-016, Lisboa, Portugal. E-mail: [email protected] c A´rea Departamental de Engenharia Quı´mica, Instituto Superior de Engenharia de Lisboa, IPL, R. Conselheiro Emı´dio Navarro, 1959-007 Lisboa, Portugal d Instituto Superior de Educaça˜oeCieˆncias (ISEC Lisboa), Alameda das Linhas de Torres, 174, 1750-142 Lisboa, Portugal † In memoriam of Professor Michael H. Abraham, who passed away on January 19th, 2021, during the revision of this manuscript. Scheme 1 This journal is the Owner Societies 2021 Phys. Chem. Chem. Phys.,2021,23, 3311À3320 | 3311 View Article Online Paper PCCP of neutral compounds from the effects of solvents on compound A is the overall solute hydrogen bond acidity, B is the overall solute solubilities or on compound water–solvent partitions. More hydrogen bond basicity, V is the McGowan’s characteristic mole- recently, we have extended the equations to ionic compounds cular volume in cm3 molÀ1/100 and L is the logarithm of the gas to such as permanent ions and ion-pairs.23,24 Very relevant to the hexadecane partition coefficient at 298 K. present study is the application to betaine25 and to amino- In the extension of eqn (1)23,24 to include permanent ions 26 + À + À acids. In these cases, we found that the ionic form of the such as K and Cl , ion pairs such as Me4N Cl , deprotonated À equation had to be used, in accordance with complete charge carboxylic acids, RCO2 , and protonated amines, the independent separation in these compounds. So application of our equations variables E, S, A, B and V werethesameineqn(1)and(3)sothat to the tert-butyl halide transition states may lead to information only two extra variables were needed to incorporate the ionic as to the extent of charge separation in the transition states, to solutes, J+ for cationic solutes and JÀ for anionic solutes. Then the hydrogen-bond acidity (electrophilicity) and hydrogen-bond eqn (1) becomes eqn (3), where J+ =0foranions,JÀ =0forcations basicity (nucleophilicity) of the transition states, and hence to and J+ = JÀ = 0 for neutral molecules. the role of solvent as a nucleophile (hydrogen bond base) and an Log Ps = c + eE + sS + aA + bB + vV + j+J+ + jÀJÀ electrophile (hydrogen bond acid). (3) 2 Materials and Methods The coefficients in eqn (1) and (3) for various solvents are given in Table 1.19–24 2.1 Materials For experiments carried out in this work, tert-butyl chloride and tert-butyl bromide were purchased from BDH (purity 4 99%) 3 Results and discussion and tert-butyl iodide was purchased from Aldrich (purity 4 95%). Partition coefficients of the tert-butyl halide transition states, tert-Butyl iodide was regularly purified by column chromatogra- Tr, from water to solvents, as log Ps, were obtained from eqn (4), Creative Commons Attribution-NonCommercial 3.0 Unported Licence. phy (Silica gel 60). Solvents were obtained commercially and as described before29,30 used without further purification from Aldrich, Merck and Koch- Light (499% + purity) and their water content was always Log Ps (Tr) = log Ps (tert-BuX) + log k(S) À log k(W) o2% v/v. Solvent mixtures were prepared by volume using pure (4) solvents and freshly collected double deionized water, obtained with a Milli-Q system from Millipore (Bedford, MA, USA) with a Here, Ps (Tr) is the water (W) to solvent (S) partition coefficient resistivity of 18.2 MO. of a tert-butyl halide transition state, log Ps (tert-BuX) is the water to solvent partition coefficient of the corresponding 2.2 Procedures halide, and k(S) and k(W) are the tert-butyl halide rate constants This article is licensed under a Kinetic curves were followed by conductimetry using an auto- in the solvent S and in water. 29–31 mated Wayne Kerr B905 conductance bridge. Temperature Thermodynamic data on the initial states were available, control was always better than Æ0.01 K. Substrate concen- and these in turn were used to derive the descriptors for the tert- À3 butyl halides in eqn (1) and in eqn (2), as set out in detail Open Access Article. Published on 25 January 2021. Downloaded 9/26/2021 3:47:46 AM. tration was 0.01 mol dm and, whenever deemed necessary, 21,22,30 0.02 mol dmÀ3 of 2,6-lutidine (a weak base) was added to the elsewhere. The obtained values are in Table 2. In Table 3, solvent to capture the produced acid and prevent its further Kw is the gas to water partition coefficient. There is nothing reactivity. In some cases, calibration curves were performed. exceptional about the descriptors for the tert-butyl halides. In Reactions were followed up to 90% of the apparent plateau. particular, they have zero electrophilicity (hydrogen-bond acidity, A) Mean k values result from at least 3 different runs and showed a and very little nucleophilicity (hydrogen-bond basicity, B). standard deviation better than 5%. All k values were determined 3.1 tert-Butyl chloride using a Microsoft Excel spreadsheet specifically designed by us for this purpose.27,28 Several years ago we studied the structure of the tert-butyl chloride transition state using reaction field theory36 Our 2.3 Theory results indicated that the transition state had decidedly different Our method is based on eqn (1) and (2), set up for correlation of structures in protic and in aprotic solvents, and so it seems processes involving a given neutral solute in a variety of appropriate to apply our equations separately to data in protic solvents.19–22 The dependent variable is log Ps, where Ps is a and aprotic solvents.
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