A study on equilibrium and kinetics of carbocation-to-carbinol conversion for di- and tri- arylmethane dye cations in aqueous solutions: Relative stabilities of dye carbocations and mechanism of dye carbinol formation

Abstract

<span style="font-size:14.0pt;line-height: 115%;font-family:" times="" new="" roman";mso-fareast-font-family:fd1596850-identity-h;="" color:black;mso-ansi-language:en-in;mso-fareast-language:en-in;mso-bidi-language:="" hi"="" lang="EN-IN">Arylmethane dye cations form a structurally interesting set of stable carbocations. A detailed study on rate-equilibria of carbinol formation from two diarylmethane and nine triarylmethane dye carbocations in aqueous solutions has been carried out using spectrophotometric measurements. The conclusions reached are : (i) The stability order found (auramine O>crystal violet <span style="font-size:14.0pt;line-height:115%;font-family:" times="" new="" roman";="" mso-fareast-font-family:fd1612762-identity-h;color:black;mso-ansi-language:="" en-in;mso-fareast-language:en-in;mso-bidi-language:hi"="" lang="EN-IN">= <span style="font-size:14.0pt;line-height:115%;font-family:" times="" new="" roman";="" mso-fareast-font-family:fd1596850-identity-h;color:black;mso-ansi-language:="" en-in;mso-fareast-language:en-in;mso-bidi-language:hi"="" lang="EN-IN">methyl violet > victoria blue R > victoria pure blue BO <span style="font-size:14.0pt;line-height:115%;font-family:" times="" new="" roman";="" mso-fareast-font-family:fd1612762-identity-h;color:black;mso-ansi-language:="" en-in;mso-fareast-language:en-in;mso-bidi-language:hi"="" lang="EN-IN">= <span style="font-size:14.0pt;line-height:115%;font-family:" times="" new="" roman";="" mso-fareast-font-family:fd1596850-identity-h;color:black;mso-ansi-language:="" en-in;mso-fareast-language:en-in;mso-bidi-language:hi"="" lang="EN-IN">ethyl violet > pararosaniline > brilliant green > malachite green> carbocation form of Michler's hydrol > methyl green), seems to be determined by an interplay of dye carbocation / carbinol conformation and stereoelectronic effects of substituents; and (ii) carbinol formation is general base catalysed and occurs by the rate determining attack of a H₂O molecule on the dye carbocation centre <i>via</i> two kinetic pathways one mediated by another H₂O molecule and the other by a OH^- ion.</span></span></span></span></span>