Four compounds are each treated first with dilute and then with concentrated sodium hydroxide…
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Four compounds are each treated first with dilute and then with concentrated sodium hydroxide: methanal, 2,2-dimethylpropanal, butanal and cyclohexanone. How many of them undergo the Cannizzaro reaction in concentrated base? Butanal () gives an aldol in dilute base, and the aldol loses water on heating to give 2-ethylhex-2-enal (): . Find the mass of 2-ethylhex-2-enal from of butanal, in a yield.
Need a reminder? The key ideaShow
An alpha hydrogen is needed for the aldol reaction (dilute base). With no alpha hydrogen, concentrated base gives the Cannizzaro reaction. A crossed aldol of two enolisable compounds gives four products. The carbonyl group is meta-directing. For the masses, use moles = mass / molar mass and the equation ratio.
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.
Step 1. Cannizzaro. It needs an aldehyde with no alpha hydrogen. Methanal and 2,2-dimethylpropanal have none. Butanal and cyclohexanone have alpha hydrogens, and they do aldol reactions. So 2 compounds.
Step 2. Moles of butanal. .
Step 3. Product. Two moles of butanal give one mole of the product, so .
Step 4. Mass. .
Answer: 2 compounds, and .
Check. The product mass is less than the mass of butanal because water is lost: , with of water.
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