In the following pyrolysis reaction, we have to find how many $\ce{CO2}$ is formed.

According to me it should be 5 as there are five carboxylic acid.

But the answer is 4, how?

4-oxocyclohexane-1,1,3,3,5-pentacarboxylic acid

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    $\begingroup$ Tip: the presence of a carbonyl group in beta position to a carboxyl facilitates decarboxylation. Try to remove all carboxyl groups (one at a time) which fulfill that condition. $\endgroup$ – vapid Jul 22 '16 at 14:26
  • $\begingroup$ @vapid so there would be three carboxyl group $\endgroup$ – Koolman Jul 22 '16 at 14:28
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    $\begingroup$ A carboxyl group also includes carbonyl. $\endgroup$ – vapid Jul 22 '16 at 14:30
  • $\begingroup$ @vapid yeah i know that $\endgroup$ – Koolman Jul 22 '16 at 14:31
  • $\begingroup$ @vapid Please elaborate your answer . Am I coorect that there are 3 carboxyl group for decarboxylation $\endgroup$ – Koolman Jul 22 '16 at 14:32

While it would be possible to decarboxylate almost any carboxylic acid with appropriate conditions, simple aliphatic acids are not readily decarboxylated. Carbonyl group in beta position facilitate decarboxylation due to formation of a cyclic transition state:enter image description here
Now, your example has five carboxyl group, and each has a carbonyl in beta position (carboxyl also counts as a carbonyl compound). Let's look at the picture: enter image description here

First we can remove the three top carboxyls marked with arrows (A). There is a carbonyl in the ring that will aid in the decarboxylation, so we will arrive to a compound at B) (I'm not saying that the reaction will proceed that way, it is just an explanation). As I said before, one carboxyl group can facilitate the removal of the other carboxyl. But once we remove one group, we get compound C). That compound does not have any groups that would help to get rid of the last carboxyl. So there is your answer.

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