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Which alkene on heating with alkaline $\ce{KMnO4}$ solution gives acetone and a gas that turns lime water milky ?

  1. 2-methyl-2-butene
  2. isobutylene
  3. 1-butene
  4. 2-butene

I know that $\ce{CO2}$ turns lime water milky but, I'm not able to find the alkene in the question. Also, I want the mechanism involved.

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    $\begingroup$ Permanganate chops the double bond, leaving oxygens on both ends, and then finishes off any aldehyde groups it can find. See where this gets you. $\endgroup$ – Ivan Neretin May 23 '18 at 10:27
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    $\begingroup$ What diagrams? Draw a structural formula of any alkene on a sheet of paper. Cut it in half with scissors across the double bond. Glue each half on a separate blank sheet. Attach $\ce{=O}$ to the severed bonds. That would be it. $\endgroup$ – Ivan Neretin May 23 '18 at 11:11
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    $\begingroup$ @Oscar Yeah, that's what I said in the first comment. $\endgroup$ – Ivan Neretin May 23 '18 at 14:08
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The best approach, and good practice, is to draw out the reaction products for each case. You should know that alkaline potassium permanganate replaces a carbon-carbon double bond with two carbon-oxygen bonds, one to each of the original double-bonded carbons. For instance, with 2-butene we have:

$\ce{CH3-CH=CH-CH3 -> CH3-CH=O + CH3-CH=O}$

Here the product molecules are the same because the alkene is, of course, symmetric.

But wait, there's more. The carbonyl compounds pictured in the above example are stable in the alkaline permanganate medium if they are ketones, with only carbon atoms attached to the carbonyl function. If there are aldehydes, the carbon-hydrogen bond(s) to the carbonyl group will be oxidized further. Thus the "acetaldehyde" identified above for 2-butene is further oxidized to break the carbon-hydrigen bond at the carbonyl group:

$\ce{CH3-CH=CH-CH3 -> 2 CH3-CH=O -> 2 CH3-C(O)-OH}$

where in the alkaline medium the acetic acid will appear as acetate ion.

As you work through your choices you find that there is one aldehyde where the carbonyl carbon is bonded only to hydrogen atoms allowing that function to be oxidized all the way to carbon dioxide (or, again accounting for the alkali, carbonate or bicarbonate ion). Find it, then see which choice gives that along with the other given product acetone. Bonus question: Would the acetone hold up against further oxidation? How would you tell?

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