[![enter image description here][1]][1]


  [1]: http://i.stack.imgur.com/lKgRK.png

Given this compound in an acidic medium, what would get protonated first?
Would not the double bond get protonated preferentially, since oxygen is more electronegative than carbon and less willing to donate an electron pair for protonation?

I checked the ***pka*** value for the conjugate acid of  aldehyde, and it was around ***-10***, which is pretty low, and hence suggests that aldehyde would have a very ***low basicity***

Also alkene (ethene) had a pka value of around 43.

Going by this information does it not make sense for a double bond to get protonated first?

*ON the other hand, the **oxonium ion, in this case is resonance stabilised**, which would indicate that it is protonated in preference to the double bonds.*

***Which of these reasoning is correct?
And more importantly, in these situations how are we supposed to predict which factor dominates?***