Hammett's Law - Hammett's Law

This is an empirical rule proposed by LP Hammett in 1935 to quantitatively discuss the effect of substituents on the reaction or equilibrium of organic compounds. For example, the ionization equilibrium constant of benzoic acid increases when a nitro group is added to the benzene nucleus, and decreases when an amino group is added. Thus, in meta- or para-substituted aromatic compounds, the substituents affect the reaction, and the following simple equation holds true:

Here, k ₀ and k are the rate constants or equilibrium constants for a compound without a substituent and a compound with a substituent, respectively. ρ is the Hammett reaction constant (or ρ value) that is determined by the type and conditions of the reaction (temperature, dielectric constant of the solvent, etc.), and σ is called the substituent constant (or σ value) that is determined by the type and position of the substituent. Hammett set ρ = 1 for the dissociation reaction of benzoic acid derivatives in an aqueous solution at 25 °C, and calculated the σ values ​​of various substituents. In the case of no substitution, σ = 0, and there is a tendency for σ to be < 0 for electron-donating groups and σ to be > 0 for electron-withdrawing groups. To date, the validity of this rule has been recognized for more than 3,000 reactions. This rule was later extended to polysubstituted and heterocyclic compounds, as well as aliphatic compounds by RW Taft and others, and the polar effects, steric effects, and resonance effects of the substituents were also separated, leading to the development of ortho-substituted compounds.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

有機化合物の反応または平衡に及ぼす置換基の影響を定量的に論じるために，1935年にL.P. Hammettによって提唱された経験則．たとえば，安息香酸の電離平衡定数はベンゼン核にニトロ基を入れると増大し，アミノ基を入れると減少する．このように，芳香族化合物のメタまたはパラ置換体では置換基が反応に影響を及ぼし，その間に次の簡単な式が成立する．

ここで，k₀，kは置換基をもたない化合物と，置換基をもった化合物のそれぞれの速度定数または平衡定数である．ρはその反応の種類と条件(温度，溶媒の誘電率など)によって決まるハメットの反応定数(またはρ値)であり，σは置換基の種類と位置によって決まる置換基定数(またはσ値)とよばれるものである．ハメットは安息香酸誘導体の25 ℃ の水溶液中における解離反応についてρ ＝ 1とし，種々の置換基のσ値を求めた．無置換の場合σ ＝ 0であって，電子供与性基ではσ ＜ 0，電子求引性基ではσ ＞ 0となる傾向がみられる．現在まで，3000以上の反応について，この規則の妥当性が認められている．この規則は，のちに多置換体や複素環式化合物などをはじめ，R.W. Taftらによって脂肪族化合物にまで拡張され，置換基の極性効果，立体効果，共鳴効果などの分離も行われてオルト置換体にまで発展された．

出典　森北出版「化学辞典（第2版）」化学辞典 第2版について　情報