Electronic Properties of Chlorine, Methyl, and Chloromethyl as Substituents to the Ethylene Group—Viewed from the Core of Carbon

Abstract

"Substituent effects" is an important and useful concept in organic chemistry. Although there are many approaches to parametrizing the electronic and steric effects of substituents, the physical basis for the parameters is often unclear. The purpose of the present work is to explore the properties of chemical shifts in carbon 1s energies as a well-defined basis for characterizing substituents to an ethylene C═C moiety. To this end, high-resolution carbon 1s photoelectron spectra of six chloro-substituted ethenes and seven chloro-substituted propenes have been measured in the gas phase. Site-specific adiabatic ionization energies have been determined from the spectra using theoretical ab initio calculations to predict the vibrational structures. For two molecules, 3-chloropropene and 2,3-dichloropropene, the spectral analyses give quantitative results for the conformer populations. The observed shifts have been analyzed in terms of initial-state (potential) and relaxation effects, and charge relaxation has also been analyzed by means of natural resonance theory. On the basis of core-level spectroscopy and models, chlorine, methyl, and chloromethyl have been characterized in terms of their effect on the carbon to which they are attached (α site) as well as the neighboring sp(2) carbon (β site). The derived spectroscopic substituent parameters are characterized by both inductive (electronegativity) effects and the ability of each substituent to engage in electron delocalization via the π system. Moreover, the adopted approach is extended to include substituent-substituent interaction parameters.

Carbocation stability as predictor for electrophilic addition of HCl to chlorinated ethenes and propenes in the gas phase

“…Since this attempt has been met with only limited success for the present choice of nucleophiles, chlorinated ethenes and propenes, it prompts the issue whether shifts in carbon 1s energies would correlate better with our computed activation energies for addition of HCl. This issue is explored in Figure , where C1s ionization energies from the studies of Zahl et al and Sæthre et al are paired with activation energies for electrophilic attack at the same sp 2 carbon. The data points show significant scatter but arguably fall into three subsets characterized by the number of chlorine atoms directly attached to the ionized carbon; N C l = 0,1,2, with mean activation and ionization energies increasing with N C l .…”

“…The further analysis is simplified by drawing on substituent parameters, α and β , as done above for PA and activation energy. The relevant substituent parameters for the core‐ionization process are obtained from Zahl et al For CH 2 Cl, the parameters refer to the gauche conformer. Figure shows that except for the α C l point in the upper right corner, the remaining points fall approximately on a single line passing through the origin.…”

“…To predict gas‐phase activation energies (E a , g a s ), geometries for HCl, the neutral ground‐state alkenes, and the corresponding transition states for addition of HCl were optimized using B3LYP and basis sets of triple‐ ζ quality ( tzp ) . In each case, it was verified that the potential energy surface has the appropriate local topology (zero or one imaginary vibrational frequency, respectively).…”

“…This allows us to investigate how the presence of chlorine, chloromethyl, and methyl substituents around a C–C double bond affects electrophilic addition of HCl as well as the formation of a carbocation by protonation, and to address the title issue in terms of their mutual variation with substitution of the alkene. Finally, the reactivity data are compared with recently published carbon‐1s ionization energies, as a possible alternative to the PA for predicting activity in electrophilic addition reactions.…”

How do electronic substituent effects work? – Additional contrasting cases for a differentiated inquiry illustrated by the example of alkaline ester hydrolysis

“…Der + M-Effekt trittb esondersi nm echanistischen Kontexten zutage,indenen in reaktivenZwischenstufenElektronenmangelzentren entstehen, died urch diesen Effekt stabilisiert werden.I mV ergleich zurv on unsg ewähltenM odellreaktionw irdd eutlich, wiei n unterschiedlichenR eaktionenv erschiedeneE ffekte einesS ubstituenteni nd en Vordergrund treten kçnnen[36]:W ährendd er + M-Effekt vonBrom-Substituentenfürdie alkalische Esterhydrolyse von nachrangiger Bedeutungi st,k ommt ihmb ei S E Ar-Reaktionena n monosubstituierten Benzolen hingegen eine entscheidendeR olle zu,indem er denSchlüssel zurErklärung deren"chamäleonartiger" Wirkungauf dieReaktivitätund Regioselektivitätder Reaktion bietet[5].…”