Counting number of acidic-H in a molecule

In a recent exam, following question was asked:



> 
> Number of acidic hydrogen present in X are:
> 
> 
> [![enter image description here](https://i.stack.imgur.com/bgbYD.png)](https://i.stack.imgur.com/bgbYD.png)
> 
> 
> 


I've found 8 of them as shown:


[![enter image description here](https://i.stack.imgur.com/ZWmoj.png)](https://i.stack.imgur.com/ZWmoj.png)


7 of them (marked with numerals) were correct according to solution, but 8th one (pointing with red arrow) was not. Is it because of the fact that the carbon was bridge-headed or something else?




An aldehyde or a ketone with an alpha hydrogen forms a carbanion that resonates to enolate form. This leads to two canonical structures that are in resonance. The example below is a polyphenol compound extracted from turmeric, called curcumin.


[![enter image description here](https://i.stack.imgur.com/iDQFu.png)](https://i.stack.imgur.com/iDQFu.png)


source: Biomaterials. 2010 May;31(14):4179-85. doi: 10.1016/j.biomaterials.2010.01.142. Epub 2010 Feb 23. (<https://www.ncbi.nlm.nih.gov/pubmed/20181392>)


The hydrogens between two keto groups are most acidic. This is due to formation of two equivalent enol structures stabilized by hydrogen bonding. The structural requirement would be **planarity** of atoms involved in hydrogen bonding. 


Below are 3-D structures of the given compound in the question above.


* Structure without hydrogens


Bridgehead carbon is 4. Keto groups are at 2 , 5 and 8 (shaded in grey). Keto groups on 5 and 8 are not in the same plane . Hence Hydrogen on 4 , 22 (see figure with hydrogens) does not form stable hydrogen bonded enol structures as discussed in the above example.


Therefore, hydrogen at **8th one (pointing with red arrow)** may not be acidic.


Note: 3-D structures have been drawn on Chem3Dpro


[![enter image description here](https://i.stack.imgur.com/mwkmG.jpg)](https://i.stack.imgur.com/mwkmG.jpg)


* Structure with hydrogens


[![enter image description here](https://i.stack.imgur.com/xMpt1.jpg)](https://i.stack.imgur.com/xMpt1.jpg)


* Structure with hydrogens (2nd view)


[![enter image description here](https://i.stack.imgur.com/pu3zX.jpg)](https://i.stack.imgur.com/pu3zX.jpg)


 3

It is because of the fact that a double bond is not usually stable at the bridge head position in smaller fused rings.
Therefore the hydrogen cannot be considered as acidic.


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