A Grignard reagent, methylmagnesium iodide, reacts with a compound that contains an O-H bond and…
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A Grignard reagent, methylmagnesium iodide, reacts with a compound that contains an O-H bond and releases methane. For each active hydrogen atom, one molecule of methane is released, and the rest of the reagent stays in the flask. This is how the active hydrogen of a compound can be counted. A sample holds of ethane-1,2-diol, which has two hydroxyl groups. Another holds of propane-1,2,3-triol, which has three. Take as the volume of one mole of gas at standard temperature and pressure. Find the volume of methane from each. A third sample of of an unknown compound gives . How many active hydrogen atoms does each of its molecules have?
Need a reminder? The key ideaShow
A halogen deactivates the ring but directs to the ortho and para positions. There are two ortho positions, two meta positions and one para position, so the reactivity of one position is its share divided by the number of positions of that kind. The Wurtz-Fittig and Fittig reactions couple halides at random with sodium. A Grignard reagent takes a proton from any O-H or N-H bond and gives an alkane.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Plan. The amount of methane equals the amount of active hydrogen atoms.
Step 1. Diol. of methane. .
Step 2. Triol. of methane. .
Step 3. Unknown. of methane. Per molecule: active hydrogen atom.
Answer: , and 1 active hydrogen atom.
Check. The volume is proportional to the number of hydroxyl groups: , and for 1, 2 and 3 groups.
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