Maryvonne Helbert

Stéréochimie de la liaison hydrogène sur le groupe carbonyle

Abstract The asymmetric shape or splitting of the ν(XH) or ν(XD) bands of OH(OD), NH, SH and CH proton donors hydrogen bonded to the carbonyl group of ketones, aldehydes, esters, amides, ureas and carbamates have been explained by the existence of two stereoisomeric complexes: a linear complex (along the axis of the carbonyl bond) and an angular complex (in the direction of a lone pair). Bulky substituents on the carbonyl group or near the XH bond destabilize the angular arrangement. Inductive electron-withdrawing substituents on the carbonyl group favour the linear arrangement.

Hydrogen-bond basicity of secondary and tertiary amides, carbamates, ureas and lactams

The hydrogen-bond basicity scale pKHB(logarithm of the formation constant of 4-fluorophenol–base complexes in CCl4) has been measured for tertiary and secondary amides, carbonates, ureas and lactams. The hydrogen-bonding fixation site is the carbonyl group, even for the very hindered amide ButCON(C6H11)2. In the amides R1CONR2R3 the hydrogen-bond basicity is decreased more by bulky R1 substituents on the carbonyl carbon than by bulky R2 and R3 substituents on nitrogen. The field effect of X substituents operates more effectively on hydrogen-bond basicity than the resonance effect in the XCONMe2 series. The hydrogen-bond basicity is increased by six-membered cyclisation.

Polarity and basicity of solvents. Part 2. Solvatochromic hydrogen-bonding shifts as basicity parameters

The solvatochromic hydrogen-bonding shifts of p-nitrophenol and of p-nitroaniline have been measured by the thermosolvatochromic comparison method for an extended sample of oxygen, nitrogen, carbon, halogen, and sulphur bases. Their significance as a hydrogen-bonding parameter has been tested by their correlation with formation constants, n.m.r. shifts, vibrational shifts, and enthalpies for hydrogen-bonding formation. Family-dependent correlations are generally found between the above properties. The correlation of a hydrogen-bonding property for an OH donor versus the same property for an NH donor is family (polar oxygen bases, ethers, pyridines, and tertiary aliphatic amines) dependent. The only significant family-independent correlation is for the solvatochromic shift of p-nitrophenol versus the enthalpy of hydrogen-bond formation of p-fluorophenol. It is shown that the β scale is mainly a scale of NH hydrogen-bond acceptor basicity. The averaging process used to define β is criticized and it is recommended that correlation analysis of basicity is undertaken with clearly defined models.

Super-basic nitriles

Alkyl substitution on the amino nitrogen, vinylogy (but not benzology), iminology, and alkyl substitution on the functional carbon of the amidine skeleton in cyanamide iminologues increase the hydrogen-bonding basicity of cyanamide and produce, on the pKHB scale, super-basic nitriles more basic than tertiary amines. On the gas-phase basicity scale iminology also increases the basicity of cyanamide, but sp-nitrogen bases remain less basic than sp2- or sp3-nitrogen bases.

The influence of solvent on the inductive order of substituents from infrared measurements on 4-substituted camphors: a new model of inductive effects

4-Substituted camphors (1)–(24) have been chosen as purely inductive models of substituent effects in order to study the effect of solvent on the inductive order of substituents. The probe was the carbonyl stretching vibration, the frequency of which was measured in (i) the gas phase, (ii) non-polar solvents (heptane and carbon tetrachloride), (iii) a hydrogen-bonding acceptor solvent (pyridine), (iv) a hydrogen-bonding donor solvent (hexafluoropropan-2-ol), and (v) an amphoteric solvent (methanol). Correlation analysis shows the consistency of data for gas-phase and apolar solvents, the deviation of hydrogen-bonding donor substituents (OH, NHCO2Et, NH2, CO2H, CONH2) in pyridine, and the deviation of hydrogen-bonding acceptor substituents (all except H, alkyls, C6H5, CHCH2, halogens) in hexafluoropropan-2-ol. In methanol the two kinds of deviation are so attenuated that most substituents behave inductively, in the same way as in apolar solvents. This explains the apparent, but fallacious independence of the inductive scale of substituents (defined mainly in alcoholic or aqueous media) on the solvent.