to inductive effects, with resonance accounting for the remaining difference. Hydrogen bonding In carbon tetrachloride and alkane solvents phenol hydrogen bonds with a wide range of Lewis bases such as pyridine, diethyl ether, and diethyl sulfide. The enthalpies of adduct formation and the IR frequency shifts accompanying adduct formation have been studied. Phenol is classified as a hard acid which is compatible with the C/E ratio of the ECW model with EA = 2.27 and CA = 1.07. The relative acceptor strength of phenol toward a series of bases, versus other Lewis acids, can be illustrated by C-B plots. Phenoxide anion The phenoxide anion is a strong nucleophile with a nucleophilicity comparable to the one of carbanions or tertiary amines. It can react at both its oxygen or carbon sites as an ambident nucleophile (see HSAB theory). Generally, oxygen attack of phenoxide anions is kinetically favored, while carbon-attack is thermodynamically preferred (see Thermodynamic versus kinetic reaction control). Mixed oxygen/carbon attack and by this a loss of selectivity is usually observed if the reaction rate reaches diffusion control. Tautomerism Phenol exhibits keto-enol tautomerism with its unstable keto tautomer cyclohexadienone, but only a tiny fraction of phenol exists as the keto form. The equilibrium constant for enolisation is approximately 10−13, which means only one in every ten trillion molecules is in the keto form at any moment. The small amount of stabilisation gained by exchanging a C=C bond for a C=O bond is more than offset by the large destabilisation resulting from the loss of aromaticity. Phenol therefore exists essentially entirely in the enol form. 4, 4' Substituted cyclohexadienone can undergo a dienone–phenol rearrangement in acid conditions and form stable 3,4‐disubstituted phenol. Phenoxides are enolates stabilised by aromaticity. Under normal circumstances, phenoxide is more reactive at the oxygen position, but the oxygen position is a "hard" nucleophile whereas the alpha-carbon positions tend to be "soft". Reactions Phenol is highly reactive toward electrophilic aromatic substitution. The enhance nucleophilicity is attributed to donation pi electron density from O into the ring. Many groups can be attached to the ring, via halogenation, acylation, sulfonation, and related processes. Phenol's ring is so strongly activated that bromination and chlorination lead readily to polysubstitution. Phenol reacts with dilute nitric acid at room temperature to give a mixture of 2-nitrophenol and 4-nitrophenol while with concentrated nitric acid, additional nitro groups are introduced, e.g. to give 2,4,6-trinitrophenol. Aqueous solutions of