Isothiouronium

In organic chemistry, isothiouronium is a functional group with the formula [RSC(NH₂)₂]⁺ (R = alkyl, aryl) and is the acid salt of isothiourea. The H centres can also be replaced by alkyl and aryl. Structurally, these cations resemble guanidinium cations. The CN₂S core is planar and the C–N bonds are short.^[1]

S-Ethylisothiouronium diethylphosphate: an example of an isothiuronium compound

Synthesis

Salts comprising these cations are typically prepared by alkylation of thiourea:

SC(NH₂)₂ + RX → [RSC(NH₂)₂]⁺X⁻

Reactions

Hydrolysis of isothiouronium salts gives thiols.^[2]

[RSC(NH₂)₂]⁺X⁻ + NaOH → RSH + OC(NH₂)₂ + NaX

Isothiouronium salts in which the sulfur has been alkylated, such as S-methylisothiourea hemisulfate (CAS number: 867-44-7), will convert amines into guanidinium groups. This approach is sometimes called the Rathke synthesis^[3] after Bernhard Rathke^[4] who first reported it in 1881.^[5]

RNH₂ + [CH₃SC(NH₂)₂]⁺X⁻ → [CH₃N(H)C(NH₂)₂]⁺X⁻ + CH₃SH

Chelating resins with isothiouronium groups are used to recover mercury and other noble metals including platinum from solutions.^[6]