A method of preparing a very pure alkane is to react the corresponding alkyl mag

Question

A method of preparing a very pure alkane is to react the corresponding alkyl magnesium halid (grignard reagent) with water and collect the alkane product. Often a chemist has the problem of determining structural characteristics of an unknown pure substance. In an analytical procedure known as the Zerewittenoff process the number of moles of active hydrogen in an unknown compound is determined by reatment of the unknown compound with methyl magneium bromide and measurement of the volume of  methane evolved. An active hydroge is defined as one that will react with the Grignard reagent. Usually it is a hydrogen bonded to oxygen, nitrogen, sulfur, phosphorous and only occasionaly to carbon.

A sample of 1.83mg of an unknown compound of molecular weight about 92 amu gave 1.34mL of methane gas (corrected to STP) when reacted with an excess of methyl magnesium bromide. How many active (that is acidic) hydrogens are there per molecule? Assuming all the active hydrogens to be -OH groups, suggest a structure for the unknown compound.

Explanation / Answer

Step1 Moles of compound =     1.83x10^-3   /92 =1.99 x10^-5

Step2 Moles of methane = 1.34/22400 = 5.99x10^-5

Step3 Moles of OH group =     5.99x10^-5 /1.99x10^-5 = 3; Active hydrogen groups =3

Step4   Formula is CH2OH CHOH CH2OH ; 1,2,3 Propane triol or glycerol

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