Ionization of Organic Compounds: Iii. Basicities of Propionic Acid and Propionamide

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The ionization in aqueous sulphuric acid of thiolactams consisting of five-, six-, and sevenmembered rings has been studied. With increasing concentration of acid the ionization ratio increases more rapidly than ho. A possible explanation involving the hydration of the thiolactam is given. Changes in the basicity of thiolactams with ring size parallel changes previously found for lactams.By analogy with the resonance structures of simple amides (I), the structure of lactams (I; X = 0 ) would be expected to be made up by a 60y0 contribution from structure Ia (X = 0 ) and 40% contribution from structure Ib (X = 0), although the relative weights of the contributions could be altered somewhat by ring strain (2). Thiolactams (I; X = S) have higher dipole moments (3), and so should be represented to a preponderant extent by the dipolar form I b (X = S). The high polarity of the thiolactams (I; X = S, n = 3 , 4 , and 5) may in part account for the fact that they are solids a t room temperature while the corresponding lactams (I; X = 0, n = 3, 4, and 5 ) are liquid, and that (as postulated later in this paper) they appear to be more extensively hydrated in a-queous solution than are the lactams.It is probable that lactams and thiolactams in strong acid are protonated on the oxygen or sulphur atom to give the conjugate acids (11; X = 0 or S) (4). Consequently i t became of interest to compare the basicities of the two classes of compounds. I t seemedpossible that in spite of the greater intrinsic basicity of oxygen as compared with sulphur (cf. OH-and SH-), the greater negative charge on the sulphur of thiolactams would cause them to be more basic than lactams. We have accordingly determined the protonation constants of the thiolactams (I; X = S, n = 3, 4, and 5) for comparison with the protonation constants reported for the lactams (I; X = 0 , n = 3, 4, and 5) (2).

Section: Resultsmentioning

confidence: 99%

Ionization of Organic Compounds: V. Thiolactams in Sulphuric Acid

Edward¹,

Stollar²

1963

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“…However the vast majority of workers in this field have concluded that the site of protonation is the carbonyl oxygen atom and a large amount of evidence now exists in favour of oxygen protonation (8)(9)(10)(11)(12)(13)(14)(15)(16). Their studies also include (i) Hammett's acidity scale cannot be used to calculate the pK, value for this prototropic reaction (8-1 1) ; thus a new scale, HA, was developed to calculate the pK, value in the real thermodynamic sense (10); (ii) the absorption spectra of the monocations of the amides depend on the environment and a regular red shift in the absorption spectrum is observed with an increase in acidity I ( l l c , d ) .…”

Section: Introductionmentioning

confidence: 99%

Prototropic reactions for 1-, 2-, and 4-fluorenamides in the ground and excited singlet states

Manoharan¹,

Dogra²

1988

RAMASAMY MANOHARAN and SNEH K. DOGRA. Can. J. Chem. 66, 2375Chem. 66, (1988. Absorption and fluorescence spectra of 1-, 2-and 4-fluorenamides (1-FAm, 2-FAm, 4-FAm) were studied in different solvents and at various acid concentrations. This study indicates that the long wavelength absorption band is long-axis polarised in 2-FAm whereas in 1-FAm and 4-FAm, it is composed of long-and short-axis polarised transitions. The -CONH2 group in 1-FAm and 4-FAm is coplanar with the fluorene moiety, whereas the same group in 2-FAm is non-coplanar with the fluorene ring in non-polar solvents, both in So and S, states, but attains coplanarity in the polar and hydrogen bonding solvents in the SI state. Hammett's acidity scale is not valid for the protonation reaction of amides and a medium effect is observed on the absorption spectra of the monocations in strong sulphuric acid. The prototropic equilibrium for the protonation of ring carbon atom is not established in the S1 state, due to the slower rate of protonation reaction. Proton-induced fluorescence quenching of the monocations is followed before the monocations are being protonated.RAMASAMY MANOHARAN et SNEH K. DOGRA. Can. J. Chem. 66,2375Chem. 66, (1988. Utilisant diffkrents solvents et des concentrations diffkrentes d'acide, on a Ctudit les spectres de fluorescence des fluorhamides-1, -2 et -4 (FAm-1, FAm-2, FAm-4). Ces Ctudes indiquent que la grande longueur d'onde d'absorption est polariste selon le grand axe dans le composC FAm-2 tandis que dans les composCs FAm-I et FAm-2 cette longueur d'onde est composCe de transitions polarisCes selon les deux axes. Le groupe -CONH2 du compC FAm-I et I'unitt fluorkne sont coplanaires, tandis que dans le composC FAm-2, cette coplanCaritC n'existe pas dans les solvants non-polaires dans les ttats So et S I , mais on l'observe dans 1'Ctat SI dans les solvants polaires et dans les solvants contenant des liaisons hydrogknes. L'Cchelle d'aciditk de Hammett n'est pas valable pour la reaction de protonation des amides et on observe un effet du milieu sur les spectres d'absorption des monocations dans l'acide sulfurique fort. On n'a pas pu dCterminer 1'Cquilibre prototropique de la protonation I de l'atome de carbone du cycle dans 1'Ctat S I , cause de la vitesse plus lente de la rCaction de protonation. On observe l'arrkt I de fluorescence du monocation, induit par le proton, avant la protonation des monocations.

“…We have studied the acid strength The study of the pK,, values of protonated carboxylic acids has attracted Inany workers lately. Electronic, steric and solvent effects were the most widely studied aspects (1)(2)(3)(4)(5). Sorensen (6) considered structure 1 for the protonated crotonic acid on the basis of the symn~etrical structure proposed by earlier workers (3, 7) for the protonated carboxyl group.…”

Section: Introductionmentioning

confidence: 99%

“…of protonated a,P-unsaturated cyclic carboxylic acids 2 (17 = 3, 4,5).' Also, the study of the influence of alkyl substitution at the 2 position on the acid strength of protonated crotonic acid was thought to be important since it serves as a model compound for the a,P-unsaturated cyclic carboxylic acids.…”

Section: Introductionmentioning

confidence: 99%

pK_a values of protonated α,β-unsaturated cyclic carboxylic acids. Effect of ring size on acidities

Bhat¹,

Rao²,

Ranganayakulu³

1978

S. N. BI-IAT, RAMA RAO, and K. RANCANAYAKULU. Can. J. Cheni. 56,2003Cheni. 56, (1978. The pK, values of protonated a,p-unsaturated acids, naniely tiglic, cyclopentene carboxylic, cyclohexene carboxylic, and cycloheptene carboxylic acids have been determined spectrophotonietrically and the values are -4.08, -4.05, -3.88, and -3.84, respectively. The conjugate acid of tiglic acid is a stronger acid than that of crotonic acid (pKa -3.94) and the diference i n acid strength is explained on the basis of steric inhibition of resonance by the a-methyl group. All the protonated cyclic acids are shown to be somewhat weaker than the corresponding acyclic ones. Several factors have been considered to explain the acid strength of these cyclic a,p-unsaturated acids. It has been shown that the ease of placing the double bond eso to the ring system is responsible for the changes in acidity of the above cyclic systems. S. N. BHAT, RAMA RAO et K. RANGANAYAKULU. Can. J. Cheni. 56.2003Cheni. 56. (1978 On a determine spectrophotonietriquen~ent les valeurs de pK, des acides a,p non saturis protones suivants: I'acide tiglique et les acides cyclopentene, cyclohexene et cycloheptenecarboxyliques; les valeurs des pK, sent respectivement de -4.08, -4.05, -3.88 et -3.84. L'acide conjugue de l'acide tiglique est un acide plus fort que I'acide crotonique (pK, -3.94) e t on expliclue la difference dans la force de I'acide en se basant sur une inhibition sterique a l a resonance provenant de la presence du groupe methyle a. On a niontreque tous les acides cycliques protones sont un peu plus faibles que les acides acycliques correspondants. On a considert plusieurs facteurs pour expliquer la force de I'acide de ces acides a,B non satures cycliques. On a montre que la facilite de placer la double liaison eso dans un systeme cyclique est responsable dcs changements d'acidite dans les systkmes cycliques nientionnts ci-dessus.[ T r a d~~i t par le journal] Introduction carboxvlic acids. We have studied the acid strength The study of the pK,, values of protonated carboxylic acids has attracted Inany workers lately. Electronic, steric and solvent effects were the most widely studied aspects (1-5). Sorensen (6) considered structure 1 for the protonated crotonic acid on the basis of the symn~etrical structure proposed by earlier workers (3, 7) for the protonated carboxyl group. The fact that alkyl substituents at the 3 position remarkably influence the acid strength supports the allylic cation structure 1. Since resonance