One for all? The nucleophilicity parameters N, which have been derived from the rate constants k of reactions of nucleophiles with carbocations, also hold for SN2‐type reactions (see scheme). A general equation is suggested which includes established correlations (Swain–Scott, Ritchie) as special cases.

The reactions of substituted benzhydryl bromides Ar(2)CHBr with primary and secondary amines in DMSO yield benzhydryl amines Ar(2)CHNRR', benzophenones Ar(2)C=O, and benzhydrols Ar(2)CHOH. Kinetic investigations at 20 degrees C revealed the rate law -d[Ar(2)CHBr]/dt = (k(1) + k(2)[HNRR'])[Ar(2)CHBr], where the amine independent term k(1) gave rise to the formation of Ar(2)C=O and Ar(2)CHOH and the amine-dependent term k(2)[HNRR'] was responsible for the formation of Ar(2)CHNRR'. Clear evidence for concomitant S(N)1 and S(N)2 processes was obtained. While the rate constants of the S(N)1 reactions correlate with Hammett's sigma(+) constants (rho = -3.22), the second-order rate constants k(2) for the S(N)2 reactions are not correlated with the electron releasing abilities of the substituents, indicating that the transition states of the S(N)2 reactions do not merge with the transition states of the S(N)1 reactions. The correlation equation log k(20 degrees C) = s(E + N), where nucleophiles are characterized by N and s and electrophiles are characterized by E (J. Am. Chem. Soc. 2001, 123, 9500-9512), was used to calculate the lifetimes of benzhydrylium ions in the presence of amines and DMSO. The change from S(N)1 to S(N)2 mechanism occurred close to the point where the calculated rate constant for the collapse of the benzhydrylium ions with the amines just reaches the vibrational limit; that is, the concerted S(N)2 mechanism was only followed when it was enforced by the lifetime of the intermediate. The nucleophile-specific parameters N and s needed for this analysis were determined by studying the kinetics of the reactions of a variety of amines with amino-substituted benzhydrylium tetrafluoroborates (Ar(2)CH(+)BF(4)(-)) of known electrophilicity E in DMSO. Analogously, the rates of the reactions of laser flash photolytically generated benzhydrylium ions Ar(2)CH(+) with DMSO in acetonitrile were employed to determine the nucleophile-specific parameters N and s of DMSO, and it is reported that DMSO is a significantly stronger O-nucleophile than water and ordinary alcohols.

The kinetics of the reactions of benzhydrylium ions with some alcohols and alkoxides dissolved in the corresponding alcohols were photometrically investigated. Using the correlation equation log k (20°C) = s(E + N), the N and s parameters of methoxide, ethoxide, n-propoxide, and isopropoxide in alcohol-acetonitrile (91:9, v/v) were determined. The cosolvent acetonitrile has only a little influence on the rate constants of the reactions of alcohols and alkoxides. The order of N values (OH -<< MeO -< EtO -< n-PrO -< i-PrO -) shows that alkoxides differ only moderately in reactivity but are considerably more nucleophilic than hydroxide. As a consequence, the nucleophilic reactivity of a 0.5 mmol/L aqueous hydroxide solution increases by a factor of 13 when 10% (v/v) methanol is added. In 1-10 mmol/L alkoxide solutions in alcohols, weak electrophiles react considerably faster with alkoxides than with the corresponding alcohols. With increasing electrophilicity, the preference for alkoxides decreases, and electrophiles of -3 < E < 3 react with alkoxides (1-10 mmol/L) and alcohols with comparable rates. Stronger electrophiles will react with alcohols exclusively when alkoxides are present in concentrations ≤10 mmol/L. Résumé :Faisant appel à la photométrie, on a étudié la cinétique des réactions d'ions benzhydrylium avec quelques alcools et alcoolates dissous dans les alcools correspondants. Utilisant l'équation de corrélation log k (20°C) = s(E + N) on a déterminé les paramètres N et s du méthylate, de l'éthylate, du propylate et de l'isopropylate dans un mélange 91 : 9 (v/v) d'alcool et d'acétonitrile. L'acétonitrile utilisé comme cosolvant n'a que peu d'effet sur les constantes de vitesse des réactions des alcools et des alcoolates. L'ordre des valeurs de N (OH -<< MeO -< EtO -< n-PrO -< i-PrO -) montre que les réactivités des alcoolates ne diffèrent que légèrement, mais qu'ils sont beaucoup plus nucléophiles que l'hydroxyde. En conséquence, la réactivité nucléophile d'une solution d'hydroxyde aqueux à une concentration de 0,5 mmol/L augmente par un facteur de 13 quand on lui ajoute 10 % (v/v) de méthanol. Dans des solutions d'alcoolates à des concentrations allant de 1 à 10 mmol/L, les électrophiles faibles réagissent beaucoup plus rapidement avec les alcoolates qu'avec les alcools correspondants. Avec une augmentation du caractère électrophile, la préférence des alcoolates diminue et les électrophiles de -3 < E < 3 réagissent avec les alcools et les alcoolates (à des concentrations allant de 1 à 10 mmol/L) avec des vitesses comparables. Les électrophiles plus forts ne réagissent qu'avec les alcools lorsque les alcoolates sont présents à des concentrations inférieures à 10 mmol/L.

Professor Dieter Enders zum 60. Geburtstag gewidmet Seit mittlerweile mehr als fünfzig Jahren versuchen Chemiker, quantitative Nucleophilie-Skalen zu entwickeln.[1] Dem vielversprechenden Anfang in den 1950ern, aus dem die Swain-Scott-Gleichung [Gl. (1)] [2] und die Edwards-Gleichung [3] hervorgingen, folgte in den 1960ern eine Phase der Ernüchterung, da immer mehr Faktoren gefunden wurden, die bei einer quantitativen Beschreibung der Nucleophilie berücksichtigt werden müssen. [4] lg ðk=k H2O Þ ¼ s Hintergrundinformationen zu diesem Beitrag sind im WWW unter

The kinetics of the reactions of the azide ion with benzhydrylium ions were investigated photometrically in different alcohol/acetonitrile mixtures and in DMSO at 20 8C. Since the competing reactions of the benzhydrylium ions with the solvents are generally slower, the second-order rate constants of the reactions of the benzhydrylium ions with the azide ion can unambiguously be determined to yield nucleophilicity parameters N and s for the azide ion in different alcohols. Several reactions of the benzhydrylium ions with the azide ion are reversible, which allowed the determination of the equilibrium constants and the rate constants of the reverse reactions. It is found that benzhydrylium ions with annelated six-membered rings are better electrophiles and better electrofuges than the five-membered ring analogs due to the lower intrinsic barriers of the former reactions.

The kinetics of the reactions between eleven carbanions 2a-k (nitronates and malonic acid derivatives) and benzhydrylium ions 1a-g were investigated photometrically in methanol/acetonitrile (91:9, v/v) at 20°C. Since the competing reactions of the benzhydrylium ions with methanol and methoxide ions are generally slower, the second-order rate constants of the reactions between the benzhydrylium ions 1a-g and the carbanions 2a-k can be determined with high precision. The second-order rate constants (log k) correlate linearly with the electrophilicity parameters E of the benzhy-

No abstract