How is iron chloride stain made and why is it selective to phenols when devlopin
ID: 1026632 • Letter: H
Question
How is iron chloride stain made and why is it selective to phenols when devloping TLC plates?
Question 6 10 pts How is iron chloride stain made and why is it selective to phenols when developing TLC plates? 50mL of water is combined with 50mL of methanol and 1 gram of ferric) chloride; Iron Chloride acts as a Lewis acid and phenols, which are acidic with a pka of-10, act as the Lewis base. The color is a result of the interaction of up to three phenol molecules donating their non-bonding electrons to a single iron acting as a Lewis acid and accepting the non-bonding electrons. 50mL of water is combined with 50mL of methanol and 1 gram of ferric (lI) chloride; Phenols are acidic (with a pka of -10) and donate protons to the chloride atoms from the iron making hydrochloric acid. The phenol anion then reacts with the methanol to methyoxybenzene also known as anisole. The anisole decomposes under heat leading to the color seen on the TLC plate O 1% FeCl2 is combined with 50% aqueous methanol, the reaction on the TLC plate is the result of the decomposition of the product containing an ionic bond between the negatively charged phenolate and the positively charged chlorine as this is the product when phenol reacts with FeCl2 in the presence of silica and methanol No answer text provided.Explanation / Answer
OPTION 1. Iron (III) chloride stain for spotting phenols on TLC plates is prepared as a 1% solution of ferric chloride with 50% aqueous methanol. The usually Prussian blue colour of phenols is due to the complexation of three or more phenol molecules with Fe3+ through the lone pairs on the phenolic oxygens forming thus a Lewis acid-base complex. This complex's colour however, can vary from blue, green or even red depending upon the nature of the phenol present in the TLC plate.
Due to the electronic nature of ferric ion being a "hard acid" similar to oxygen atom which is a "hard base" as per HSAB theory, ferrous chloride does not form the usual intense coloured complex expected of an iron chloride stain and works only if ferric chloride is used. Finally, there occurs no chemical reaction on the phenols due to this staining process as the complexed chloride ions have a very high bond dissociation enthalpy making it rather difficult to displace them to give HCl and anisole.