Alkyl Inductive Effects on Molecular Ionization Potentials. XVI. Esters

Abstract

Correlation of molecular ionization potentials, E,, with Taft's alkyl inductive substituent constants, cr,(,,, has now been extended to include the esters. It is found that three series of esters, the forrnates, acetates, and methyl alkanoates, fall on three separate straight lines in accord with the general equation (,,, where Eo is the intercept and a, the slope. The average standard error for the three lines is 0.024 eV with an average correlation coefficient of 0.992, showing that the alkyl substituent exerts an inductive influence on the electron density at the site suffering the electron loss: a lone pair on either the carbonyl or alkoxy oxygen atom. Evidence is deduced for the site of electron expulsion and a prediction is made concerning the site of vapor phase protonation of esters and carboxylic acids. E, values presently not available experimentally are predicted for several esters. [Traduit par le journal I Can. J. Chem., 51.3963 (1973) We demonstrate in this paper that the ionization potentials of various series of carboxylate esters are linearly correlated with Taft's alkyl inductive substituent constants, o, (1).A huge body of data relating to many physical properties of organic molecules has been correlated with different sets of substituent constants, such as o,, o* (2), and Hammett's original (3) 0 and its offshoots. The ionization energies of the alcohols (4a), ethers (4b), thiols (4c), thioethers (4d), alkyl halides (4e), ketones (4f), carboxylic acids (4g), copper acetylacetonates (4h), amines (44, nitriles (5), nitroalkanes (5), n-alkanes (6), alkenes (7), alkynes (8), and disulfides (9) have recently been shown to be linear functions of both the inductive and polar substituent constants.Assuming the validity of Koopmans' theorem (10) which identifies the first adiabatic ionization energy with the energy of the highest occupied molecular orbital, the electron must be lost from a non-bonding lone pair on one of the oxygens of the carboxyl group (I I). This may be represented by the following equationEvidence will be given later in this paper pertaining to wlzich oxygen is the probable site of electron loss. It is quite apparent that electronreleasing alkyl groups should increase the electron density at the relatively electronegative carboxyl group and consequently facilitate ionization of the molecule. A similar effect should manifest itself in a greater basicity, the greater the electron density at the carboxyl group, as in the case of alcohols (12).The ol(,, values and the experimentally determined (1 3) photoionization potentials of three series of esters, HCOOR, MeCOOR, and RCOOMe are given in Table 1

The Alkyl Inductive Effect, II Theoretical Calculation of Inductive Parameters

“…We have also found good results for aldehydes [5], Cu and Cr acetylacetonates [10,13], and benzene Cr tricarbonyls [13], as well as for benzene [14], pyridine and thiophene derivatives [15]. It is interesting to note that quantum mechanical calculations using simple Hiickel molecular orbital theory for obtaining values of highest occupied molecular orbitals (HOMO) have resulted, for alkenes [1G], disulfides [17], alkylbenzenes [14] and for pyridine and thiophene derivatives [15] constants [5].…”

“…(2) were found to be in excellent agreement [3,5] with Taft's original <xi (R) values, and <7I(R) values were calculated for 23 groups for which no previous estimates were available [3].…”

“…[3] and [5] should put to rest the ideas held in some quarters, that the alkyl inductive effect is nonexistent and that CTI(R) = 0 for all alkyl groups [18]; or that the effect does, indeed, exist but is a minor one, and the GI values can be known only to one significant figure [4].…”

Cyano phosphate: an efficient intermediate for the chemoselective conversion of carbonyl compounds to nitriles

“…Synth. 1977,56, adamantane-2-carbonitrile (9) 4 100 5 cyclohexadec-5-enone (10) cyclohexadec-5-enecarbonitrile (11) 3 97 6 phenylpropanal (12) 4-phenylbutyronitrile (13) is possible by a combination of cyanophosphorylation and Sml2 reduction followed by recyclization of the oxirane ring opening product21 with triethylamine, to give the epoxy nitrile 47 in 54% yield (Scheme IV).…”

Kinetics of the peroxydisulfate oxidation of sulfoxides in buffered solution. 2. The effect of initial sulfoxide concentration with diethyl and diphenyl sulfoxides

“…Mechanism e, somewhat modified since the time it was first proposed,3 is the mechanism that we currently favor, and involves the heterolytic autoionization of S2082" into sulfate and the zwitterionic sulfur tetroxide. The very first step probably gives an intimate ion pair, followed perhaps by subsequent equilibria involving solvated ionic species, but the subsequent equilibria, for the sake of simplicity, will not be shown: ~03-S-^0^-0 S03~S042-+ +0-SO3"" (slow) (12) and the rate -d[S20821/dz = fc2[R2SO] [SO40] = fci[S2082"] [R2S0]/([R2S0] 4 k^SOf']/^} (13) But since the rate in any given run is fc[S2082'], it follows that the observed rate constant is given by k = fc,[R2SO]/([R2SO] 4 k.x[S0421 /k2) (14) identical with eq 2 if b = fc_i[S042_]//c2.…”