In: Chemistry
4. Omit the questions on p. 10-3 of the 262 lab
manual. Answer the following questions instead.
a) A research paper describes that a photochemical reaction takes
place in a quartz vessel but not in a Pyrex vessel. Rationalize
this observation.
b) Why does the benzopinacol precipitate from the reaction mixture
even though it is a diol?
c) What is the purpose of washing with 95% ethanol?
d) The acid-catalyzed rearrangement of benzopinacol to
benzopinacolone is irreversible. Explain.
a ) The photochemical light having shorter wawelenght ( < 200 cm-1) have a high energy sufficient to breck covalent bonds.If we use pyrex vessel it cannot withstand to such aa energy & will be brocken down.For the stability of pyrex vessel the waveleght required is above 200 cm-1 .The quartz vessel is stable to such photochemicals energy & therefore used in photochemical reaction.
b ) the light is absorbed & passed through pinacolon mixture without initiating the reaction & hence the measurable conversion is less & therefore precipitated out.
c ) The most of the impurities in the precipitates are dissolved in 95 % ethanol & we get pure precipitates.
d) The protonation of hydroxyl group by acid makes leaving group & expel oit as water. The one of the phenyl group is the transfered to electron deficient carbon atom makinf C-O double bond.& hence reaction is irreversible.