The alpha-effect in gas-phase SN2 reactions: existence and the origin of the effect

The origin of enhanced reactivity of alpha-nucleophiles in SN2 reactions was examined on the basis of computational results at the high level G2(+) method for 22 gas-phase reactions: Nu- + RCl --> RNu + Cl- [R = Et and i-Pr; Nu- = HO-, CH3O-, HS-, Cl-, Br-, NH2O-, HOO-, FO-, HSO-, ClO-, and BrO-]. The results clearly indicate the existence of the alpha-effect, whose size varies depending on the R group and the identity of the alpha-atom. The alpha-effect is larger for i-PrCl than EtCl, and for an alpha-nucleophile with a harder alpha-atom. Analyses of the present results, together with previously reported ones for MeF and MeCl reactions, reveal that several rationales so far presented to explain the alpha-effect, such as thermodynamic product stability, transition state (TS) tightness, electrostatic interaction, ET rationale, and polarizability, cannot explain the observed size of the alpha-effect. The importance of deformation energy on going from the reactant to the TS is presented.     

The alpha-effect in gas-phase SN2 reactions revisited

This paper re-examines gas-phase S(N)2 reactions at saturated carbon for model reactions Nu(-) + CH(3)Cl --> CH(3)Nu + Cl(-) (Nu(-) = HO(-), MeO(-), NH(2)(-), HS(-), Cl(-), Br(-), I(-), HOO(-), MeOO(-), HSS(-), and NH(2)NH(-)) using the G2(+) theory. The calculated results show that the alpha-effect does exist in the gas-phase S(N)2 reaction at the sp(3) carbon, contrary to the currently accepted notion of the absence of the alpha-effect in the gas phase.     

Does α‐effect exist in E2 reactions? A G2(+) investigation

The gas-phase base-induced bimolecular elimination (E2) reactions at saturated carbon with 13 bases, B(-) + CH3CH2Cl --> BH + CH2=CH2 + Cl(-) (B = HO, CH3O, CH3CH2O, FCH2CH2O, ClCH2CH2O, Cl, Br, FO, ClO, BrO, HOO, HSO, and H2NO), were investigated with the high-level G2(+) theory. It was found that all alpha-bases with adjacent lone pair electrons examined exhibited downward deviations from the correlation line between the overall barriers and proton affinities for the normal bases without adjacent lone pair electrons, indicating the existence of the alpha-effect in the gas phase E2 reactions. The sizes of the alpha-effect for the E2 reaction, DeltaH(alpha)(E2), span a smaller range if the alpha-atoms are on the same column in the periodic table, in contrast to the corresponding S(N)2 reactions, where the DeltaH(alpha)(S(N)2) values significantly decrease from an upper to a lower column. The origin of the alpha-effects in E2 reactions can be interpreted by the favorable orbital interaction between the lone-pair electrons and positively charged anti-bonding orbital. It is worth noticing that the neighboring electron-rich pi lobe instead of lone pair electrons could also cause the alpha-effect in E2 reaction.     

Theoretical investigation of enolborane addition to alpha-heteroatom-substituted aldehydes. Relevance of the Cornforth and polar Felkin-Anh models for asymmetric induction

The addition of enolborane nucleophiles to chiral alpha-heteroatom-substituted aldehydes (CH(3)CH(X)CHO, X = F, Cl, OMe, SMe, NMe(2), and PMe(2)) was investigated using density functional theory by means of B3LYP/6-31G(d) calculations, with particular emphasis on determining the relevance of the polar Felkin-Anh and Cornforth models for asymmetric induction in these reactions. The relative energy of the polar Felkin-Anh and Cornforth transition-state structures is found to depend on the nature of the alpha-heteroatom substituent, with electronegative substituents (F, OMe, Cl) favoring Cornforth structures, while less electronegative substituents (PMe(2), SMe, NMe(2)) favor polar Felkin-Anh structures. These transition-state preferences are correlated with the relative energy of the corresponding rotamer of the uncomplexed reactant aldehyde, indicating that the transition states are particularly sensitive to the conformation of the aldehyde. The proposed Nu --> sigma*(C-X) interaction that forms the basis of the polar Felkin-Anh model appears to be insignificant in reactions with enolborane nucleophiles. The calculated transition-state structures for the addition of E- and Z-enolborane nucleophiles to 2-methoxypropanal predict a diastereofacial selectivity that is in good agreement with the experimentally determined values.     

Origin of the SN2 benzylic effect

The S N2 identity exchange reactions of the fluoride ion with benzyl fluoride and 10 para-substituted derivatives (RC6H 4CH 2F, R = CH3, OH, OCH 3, NH2, F, Cl, CCH, CN, COF, and NO2) have been investigated by both rigorous ab initio methods and carefully calibrated density functional theory. Groundbreaking focal-point computations were executed for the C6H5CH 2F + F (-) and C 6H 5CH2Cl + Cl (-) SN2 reactions at the highest possible levels of electronic structure theory, employing complete basis set (CBS) extrapolations of aug-cc-pV XZ (X = 2-5) Hartree-Fock and MP2 energies, and including higher-order electron correlation via CCSD/aug-cc-pVQZ and CCSD(T)/aug-cc-pVTZ coupled cluster wave functions. Strong linear dependences are found between the computed electrostatic potential at the reaction-center carbon atom and the effective SN2 activation energies within the series of para-substituted benzyl fluorides. An activation strain energy decomposition indicates that the SN2 reactivity of these benzylic compounds is governed by the intrinsic electrostatic interaction between the reacting fragments. The delocalization of nucleophilic charge into the aromatic ring in the SN2 transition states is quite limited and should not be considered the origin of benzylic acceleration of SN2 reactions. Our rigorous focal-point computations validate the benzylic effect by establishing SN2 barriers for (F (-), Cl (-)) identity exchange in (C6H5CH2F, C6H 5CH2Cl) that are lower than those of (CH3F, CH3Cl) by (3.8, 1.6) kcal mol (-1), in order.     

CH_2X(X=H,FCI)与臭氧反应机理的最子化学研究

用量化学UMP2方法，在6-311++G**基组水平上研究了CH_2X(X=H,FCI)与臭氧反 应机理，全参数优化了反应过程中反应物、中间体、过渡态和产物的内何构型，在 UQCISD（T）/6-311++G**水平上计算了它们的能量，并对它们进行了振动分析，以 确定中间体和过渡态的直实性。从CH_2X(X=H,FCI)与O_3的反应机理的研究结果看 ，它们与O_3反应的活性都比较强，相对而言，活性大小顺序为CH_2F＞CH_3＞ CH_2CI，也就是说，CH_2F自由基与臭氧间的反应活性最强，对大气臭氧的损耗将 是最大的。同时研究还发现CH_2X(X=H,FCI)系列自由基与O_3的反应都是强放热反 应。     

Comprehensive theoretical studies on the gas phase SN2 reactions of anionic nucleophiles toward chloroamine and N-chlorodimethylamine with inversion and retention mechanisms

The anionic S(N)2 reactions at neutral nitrogen, Nu(-) + NR(2)Cl → NR(2)Nu + Cl(-) (R = H, Me; Nu = F, Cl, Br, OH, SH, SeH, NH(2), PH(2), AsH(2)) have been systematically studied computationally at the modified G2(+) level. Two reaction mechanisms, inversion and retention of configuration, have been investigated. The main purposes of this work are to explore the reactivity trend of anions toward NR(2)Cl (R = H, Me), the steric effect on the potential energy surfaces, and the leaving ability of the anion in S(N)2@N reactions. Our calculations indicate that the complexation energies are determined by the gas basicity (GB) of the nucleophile and the electronegativity (EN) of the attacking atom, and the overall reaction barrier in the inversion pathway is basically controlled by the GB value of the nucleophile. The retention pathway in the reactions of NR(2)Cl with Nu(-) (Nu = F, Cl, Br, OH, SH, SeH) is energetically unfavorable due to the barriers being larger than those in the inversion pathway by more than 120 kJ mol(-1). Activation strain model analyses show that a higher deformation energy and a weaker interaction between deformed reactants lead to higher overall barriers in the reactions of NMe(2)Cl than those in the reactions of NH(2)Cl. Our studies on the reverse process of the title reactions suggest that the leaving ability of the anion in the gas phase anionic S(N)2@N reactions is mainly determined by the strength of the N-LG bond, which is related to the negative hyperconjugation inherent in NR(2)Nu (R = H, Me; Nu = HO, HS, HSe, NH(2), PH(2), AsH(2)).     

Definitive ab initio studies of model SN2 reactions CH(3)X+F- (X=F,Cl, CN,OH, SH,NH(2), PH(2))

The energetics of the stationary points of the gas-phase reactions CH(3)X+F(-)-->CH(3)F+X(-) (X=F, Cl, CN, OH, SH, NH(2) and PH(2)) have been definitively computed using focal point analyses. These analyses entailed extrapolation to the one-particle limit for the Hartree-Fock and MP2 energies using basis sets of up to aug-cc-pV5Z quality, inclusion of higher-order electron correlation [CCSD and CCSD(T)] with basis sets of aug-cc-pVTZ quality, and addition of auxiliary terms for core correlation and scalar relativistic effects. The final net activation barriers for the forward reactions are: E (b/F,F)=-0.8, E (b/F, Cl)=-12.2, E (b/F,OH)=+13.6, E b/F,OH=+16.1, E b/F,SH=+2.8, Eb/F, NH=+32.8, and E b/F,PH =+19.7 kcal x mol(-1). For the reverse reactions E b/F,F= -0.8, Eb/Cl,F =+18.3, E b/CN,F=+12.2, E b/OH,F =-1.8, E b/SH,F =+13.2, E b/NH(2),=-1.5, and E b/PH(2) =+9.6 kcal x mol(-1). The change in energetics between the CCSD(T)/aug-cc-pVTZ reference prediction and the final extrapolated focal point value is generally 0.5-1.0 kcal mol(-1). The inclusion of a tight d function in the basis sets for second-row atoms, that is, utilizing the aug-cc-pV(X+d)Z series, appears to change the relative energies by only 0.2 kcal x mol(-1). Additionally, several decomposition schemes have been utilized to partition the ion-molecule complexation energies, namely the Morokuma-Kitaura (MK), reduced variational space (RVS), and symmetry adapted perturbation theory (SAPT) techniques. The reactant complexes fall into two groups, mostly electrostatic complexes (FCH(3).F(-) and ClCH(3).F(-)), and those with substantial covalent character (NCCH(3).F(-), CH(3)OH.F(-), CH(3)SH.F(-), CH(3)NH(2).F(-) and CH(3)PH(2).F(-)). All of the product complexes are of the form FCH(3).X(-) and are primarily electrostatic.     

Gaseous ion activation dynamics: the role of the bulk gas in the racemization of chiral oxonium ions.

The kinetics of the inversion of configuration of a family of chiral oxonium ions, that is, O-protonated 1-aryl-1-methoxyethanes [YMe+], were investigated in two different gaseous media (in CH3X with X=F and X=Cl) at 720 torr of pressure and in the temperature range: 25-140 degrees C. The activation parameters of the [YMe+] inversion reaction were found to obey two different isokinetic relationships (IKR), depending on the nature and the position of the substituents in the oxonium ions and on the nature of the bulk gas employed. The observation of two IKR for the same family of reactions was related to a switchover in the resonant vibrational energy exchange between the reactants' critical mode, active in the transition state (omega), and the discrete vibrational levels v of the bulk gas. In CH3F, this vibrational-vibrational coupling switchover concerns the out-of-plane C-F...H-O bending) the phi family) and the H3C-F stretching (the gamma family) modes in the proton-bound [CH3F.YMe+] complex. In CH3Cl, the coupling switchover concerns the out-of-plane C-Cl...H-O bending (the phi family) and the H3C-Cl methyl group rocking (the gamma family) modes in the proton-bound [CH3Cl.YMe+] complex. The [YMe+] activation dynamics also determine the inversion dynamics. The [YMe+]ret<==>[YMe+]inv isomerization for the phi family involves the same "thermodynamically most favorable" transition state in both the CH3F and the CH3Cl media, whereas the same process for the gamma family proceeds through different, dynamically favored transition states.     

卡宾与醚C——H键插入反应的理论研究(Ⅳ)——CX2(X=H,F,Cl)与甲基苄基醚C-H键的插入反应

The C--Hbond insertion reactions between benzyl methyl ether and CX₂(X=H, F, Cl) have been studied by using density functional theory at B₃LYP/631G^*level.The potential barriers for the C--Hbond insertions in methyl group of benzyl methyl ether are123.3 kJ/mol(X=Cl) and240.4 kJ/mol(X=F), and those in benzyl group are37.5 kJ/mol(X=Cl) and112.2 kJ/mol(X=F) respectively.No potential barriers are present in both the insertion reactions with methylene groupThe C--Hbond insertion reactions between benzyl methyl ether and CX₂(X=H,F,Cl) take place primarily at α carbon of the benzyl group and the phenyl group promotes the C-Hbond insertion by carbene at its neighboring α-carbon more easily     

Stereoelectronic and inductive effects on 1H and 13C NMR chemical shifts of some cis-1,3-disubstituted cyclohexanes

This work presents the substituent effects on the 1H and 13C NMR chemical shifts in the cis-isomer of 3-Y-cyclohexanols (Y = Cl, Br, I, CH3, N(CH3)2 and OCH3) and 3-Y-1-methoxycyclohexanes (Y = F, Cl, Br, I, CH3, N(CH3)2 and OCH3). It was observed that the H-3 chemical shift, due to the substituent alpha-effect, increases with the increase of substituent electronegativity when Y is from the second row of the periodic table of elements, (CH3 *sigma(C3--H3a) interaction energy. This interaction energy, for the halogenated compounds, decreases with an increase in size of the halogen, and this is a possible reason for the largest measured chemical shift for H-3 of the iodo-derivatives. The beta-effect of the analyzed compounds showed that the chemical shift of hydrogens at C-2 and C-4 increases with the decrease of n(Y) --> *sigma(C2-C3) and n(Y) --> *sigma(C3-C4) interaction energies, respectively, showing a behavior similar to H-3. The alpha-effect on 13C chemical shifts correlates well with substituent electronegativity, while the beta-effect is inversely related to electronegativity in halogenated compounds. NBO analysis indicated that the substituent inductive effect is the predominant effect on 13C NMR chemical shift changes for the alpha-carbon. It was also observed that C-2 and C-4 chemical shifts for compounds with N(CH3)2, OCH3 and F are more shielded in comparison to the compounds having a halogen, most probably because of the larger interaction of the lone pair of more electronegative atoms (n(N) > n(O) > n(F)) with *sigma(C2-C3), *sigma(C3-C4) and *sigma(C3-H3a) in comparison with the same type of interaction with the lone pair of the other halogens.     

The rates of S(N)2 reactions and their relation to molecular and solvent properties

The energy barriers of symmetrical methyl exchanges in the gas phase have been calculated with the reaction path of the intersecting/interacting-state model (ISM). Reactive bond lengths increase down a column of the Periodic Table and compensate for the decrease in the force constants, which explains the near constancy of the intrinsic barriers in the following series of nucleophiles: F(-) approximately Cl(-) approximately Br(-) approximately I(-). This compensation is absent along the rows of the Periodic Table and the trend in the reactivity is dominated by the increase in the electrophilicity index of the nucleophile in the series C相似文献   

Modified Gaussian-2 level investigation of the identity ion-pair SN2 reactions of lithium halide and methyl halide with inversion and retention mechanisms

Identity ion-pair S(N)2 reactions LiX + CH(3)X --> XCH(3) + LiX (X = F, Cl, Br, and I) have been investigated in the gas phase and in solution at the level of the modified Gaussian-2 theory. Two possible reaction mechanisms, inversion and retention, are discussed. The reaction barriers relative to the complexes for the inversion mechanism [DeltaH(cent) ( not equal )(inv)] are found to be much higher than the corresponding values for the gas phase anionic S(N)2 reactions, decreasing in the following order: F (263.6 kJ mol(-1)) > Cl (203.3 kJ mol(-1)) > Br (174.7 kJ mol(-1)) > I (150.7 kJ mol(-1)). The barrier gaps between the two mechanisms [DeltaH(cent) ( not equal ) (ret) - DeltaH(cent) ( not equal ) (inv)] increase in the order F (-62.7 kJ mol(-1)) < Cl (4.4 kJ mol(-1)) < Br (24.9 kJ mol(-1)) < I (45.1 kJ mol(-1)). Thus, the retention mechanism is energetically favorable for fluorine and the inversion mechanism is favored for other halogens, in contrast to the anionic S(N)2 reactions at carbon where the inversion reaction channel is much more favorable for all of the halogens. The stabilization energies for the dipole-dipole complexes CH(3)X. LiX (DeltaH(comp)) are found to be similar for the entire set of systems with X = F, Cl, Br, and I, ranging from 53.4 kJ mol(-1) for I up to 58.9 kJ mol(-1) for F. The polarizable continuum model (PCM) has been used to evaluate the direct solvent effects on the energetics of the anionic and ion-pair S(N)2 reactions. The energetic profiles are found to be still double-well shaped for most of the ion-pair S(N)2 reactions in the solution, but the potential profile for reaction LiI + CH(3)I is predicted to be unimodal in the protic solvent. Good correlations between central barriers [DeltaH(cent) ( not equal ) (inv)] with the geometric looseness of the inversion transition state %C-X( not equal ), the dissociation energies of the C-X bond (D(C-X)) and Li-X bond (D(Li-X)) are observed, respectively.     

The periodic table and the intrinsic barrier in s(n)2 reactions

The identity S(N)2 reactions on nitrogen (see eq 3) with nucleophiles having the general structure H(n)()X(-) where X belongs to the group of nonmetallic elements which do not border the line separating them from the metallic elements (X = F, Cl, Br, I, O, S, Se, N, P, and C) were studied at the G2+ level. The results show that, similarly to the previously observed phenomenon for S(N)2 reaction on carbon (J. Am. Chem. Soc. 1999, 121, 7724), the Periodic Table, through the valence of the element X, controls the intrinsic barrier for the reaction. The average intrinsic barriers obtained for nitrogen substrates were 20, 27, 39, and 57 kcal/mol for the mono-, di-, tri-, and tetravalent X's, respectively. It is also concluded that the intrinsic barriers are similar for N- and C-based substrates and dimethyl substitution on both raises the intrinsic barrier by ca. 10 kcal/mol.     

硫原子上亲核取代反应的密度泛函理论研究

在B3LYP/6-311 G(2df,p)的水平上,对反应X- CH3SCl(X=F,Cl,Br,I)进行了理论研究.计算结果表明:X-(X=Cl,Br,I)与CH3SCl作用时,实际发生的是在硫原子上而不是在碳原子上的亲核取代反应,而且属于加成-消去机理.但是F-与CH3SCl作用则容易发生脱质子反应.     

Theoretical dynamic studies on the reactions of CH3C(O)CH3-nCl(n) (n = 0-3) with the chlorine atom

The theoretical investigations were performed on the reaction mechanisms for the title reactions CH(3)C(O)CH(3) + Cl --> products (R1), CH(3)C(O)CH(2)Cl + Cl --> products (R2), CH(3)C(O)CHCl(2) + Cl --> products (R3), and CH(3)C(O)CCl(3) + Cl --> products (R4) by ab initio direct dynamics approach. Two different reaction channels have been found: abstract of the H atom from methyl (--CH(3)) group or chloromethyl (--CH(3-n)Cl(n)) group of chloroacetone and addition of a Cl atom to the carbon atom of the carbonyl group of chloroacetone followed by methyl or chloromethyl eliminations. Because of the higher potential energy barrier, the contribution of addition-elimination reaction pathway to the total rate constants is very small and thus this pathway is insignificant in atmospheric conditions. The rate constants for the H-abstraction reaction channels are evaluated by using canonical variational transition state theory incorporating with the small-curvature tunneling correction. Theoretical overall rate constants are in good agreement with the available experimental values and decrease in the order of k(1) > k(2) > k(3) > k(4). The results indicate that for halogenated acetones the substitution of halogen atom (F or Cl) leads to the decrease in the C--H bond reactivity and more decrease of reactivity is caused by F-substitution.     

Theoretical study on the mechanism of the CH2F + NO2 reaction

Despite the importance of the Fluoromethyl radicals in combustion chemistry, very little experimental information on their reactions toward stable molecules is available in the literature. Motivated by recent laboratory characterization about the reaction kinetics of Chloromethyl radicals with NO2, we carried out a detailed potential energy survey on the CH2F + NO2 reaction at the B3LYP/6-311G(d,p) and MC-QCISD (single-point) levels as an attempt toward understanding the CH2F + NO2 reaction mechanism. It is shown that the CH2F radical can react with NO2 to barrierlessly generate adduct a (H2FCNO2), followed by isomerization to b1 (H2FCONO-trans) which can easily interconvert to b2 (H2FCONO-cis). Subsequently, Starting from b (b1, b2), the most feasible pathway is the C--F and N--O1 bonds cleavage along with N--F bond formation of b (b1, b2) leading to P1 (CH2O + FNO), or the direct N--O1 weak-bond fission of b (b1, b2) to give P2 (CH2FO + NO), or the 1,3-H-shift associated with N--O1 bond rupture of b1 to form P3 (CHFO + HNO), all of which may have comparable contribution to the reaction CH2F + NO2. Much less competitively, b2 either take the 1,4-H-shift and O1--N bond cleavage to form product P4 (CHFO + HON) or undergo a concerted H-shift to isomer c2 (HFCONOH), followed by dissociation to P4. Because the rate-determining transition state (TSab1) in the most competitive channels is only 0.3 kcal/mol higher than the reactants in energy, the CH2F + NO2 reaction is expected to be rapid, and may thus be expected to significantly contribute to elimination of nitrogen dioxide pollutants. The similarities and discrepancies among the CH2X + NO2 (X = H, F, and Cl) reactions are discussed in terms of the electronegativity of halogen atom. The present article may assist in future experimental identification of the product distributions for the title reaction, and may be helpful for understanding the halogenated methyl chemistry.     

Reactivity of niobium and tantalum pentahalides with cyclic ethers and the isolation and characterization of intermediates in the polymerization of tetrahydrofuran

The complexes MX5(THF) (M = Nb, X = Cl, 2a; M = Ta, X = F, 2c, X = Cl, 2d) and [MX4(THF){O(CH2)4O(CH2)3CH2)}][MX6] (M = Nb, X = Cl, 3a; M = Ta, X = Cl, 3d, X = Br, 3e, X = I, 3f) result from reactions of MX5 with 0.5 and 1.5 equiv of THF, respectively. Compounds 3 contain the unprecedented 4-(tetrahydrofuran-1-ium)-butan-1-oxo ligand and are likely to play a role in the course of THF polymerization catalyzed by MX5. The addition of L (L = 2,5-dimethyltetrahydrofuran, tetrahydropyran, 1,4-dioxane) to MX5 results in the formation of the hexacoordinated complexes MX5(L). The molecular structures of 2d, 3d, and NbCl5(dioxane), 6a, have been ascertained by X-ray diffraction studies.     

Probing the reactivity of microhydrated α‐nucleophile in the anionic gas‐phase SN2 reaction

To probe the kinetic performance of microsolvated α‐nucleophile, the G2(+)_M calculations were carried out for the gas‐phase S_N2 reactions of monohydrated and dihydrated α‐oxy‐nucleophiles XO^?(H₂O)_{n = 1,2} (X = HO, CH₃O, F, Cl, Br), and α‐sulfur‐nucleophile, HSS^?(H₂O)_{n = 1,2}, toward CH₃Cl. We compared the reactivities of hydrated α‐nucleophiles to those of hydrated normal nucleophiles. Our calculations show that the α‐effect of monohydrated and dihydrated α‐oxy‐nucleophiles will become weaker than those of unhydrated ones if we apply a plot of activation barrier as a function of anion basicity. Whereas the enhanced reactivity of monohydrated and dihydrated ROO^? (R = H, Me) could be observed if compared them with the specific normal nucleophiles, RO^? (R = H, Me). This phenomena can not be seen in the comparisons of XO^?(H₂O)_{n = 1,2} (X = F, Cl, Br) with ClC₂H₄O^?(H₂O)_{n = 1,2}, a normal nucleophile with similar gas basicity to XO^?(H₂O)_{n = 1,2}. These results have been carefully analyzed by natural bond orbital theory and activation strain model. Meanwhile, the relationships between activation barriers with reaction energies and the ionization energies of α‐nucleophile are also discussed. © 2015 Wiley Periodicals, Inc.     

Gas phase reactions of NH2Cl with anionic nucleophiles: Nucleophilic substitution at neutral nitrogen

Reactions of chloramine, NH2Cl, with HO-, RO- (R = CH3, CH3CH2, CH3CH2CH2, C6H5CH2, CF3CH2), F- , HS- , and Cl- have been studied in the gas phase using the selected ion flow tube technique. Nucleophilic substitution (S(N)2) at nitrogen to form Cl- has been observed for all the nucleophiles. The reactions are faster than the corresponding S(N)2 reactions of methyl chloride; the chloramine reactions take place at nearly every collision when the reaction is exothermic. The thermoneutral identity S(N)2 reaction of NH2Cl with Cl-, which occurs approximately once in every 100 collisions, is more than two orders of magnitude faster than the analogous reaction of CH3Cl. The significantly enhanced S(N)2 reactivity of NH2Cl is consistent with a previous theoretical prediction that the barrier height for the S(N)2 identity reaction at nitrogen is negative relative to the energy of the reactants, whereas this barrier height for reaction at carbon is positive. Competitive proton abstraction to form NHCl- has also been observed with more highly basic anions (HO-, CH3O-, and CH3CH2O-), and this is the major reaction channel for HO- and CH3O-. Acidity bracketing determines the heat of deprotonation of NH2Cl as 374.4 +/- 3.0 kcal mol(-1).