Vinylic Protons Are Acidic Because

We know that a proton alpha to a carbonyl group is pulled downfield.

Vinylic protons are acidic because.

Is this the proper resonance structure for the red proton s conjugate base. If so then this explains why the red proton is more acidic because it has a delocalized pair of electrons while the blue proton does not have a delocalized pair of electrons. The species formed is the conjugate acid of that base. The allylic position is the one on carbon next to double bond.

Notice that the proton closest to the carbonyl group is at a higher chemical shift than the proton in cyclohexene 6 05 ppm for cyclohexenone vs. A better question would be what s an allylic position because allylic proton is nothing more than proton hydrogen in allylic position. H2s also has a proton attached to an s and has a pka of 7 0 which is fairly acidic. The o h and n h protons are exchangeable and this is handy feature because when in doubt you can add a drop of deuterated water d 2 o and make the signal disappear since deuterium does not resonate in the region where protons do.

Protons c is bonded to an sp2 carbon which makes it a poor acid. Typical proton proton coupling constants. This is not surprising since the proton is not only vinylic but it is also alpha to a carbonyl group. The complementary process when a proton is added transferred to a brønsted lowry base is protonation or hydronation.

Other groups that give broad and sometimes deuterium exchangeable signals are the amines amides and thiols. For vinylic protons in a trans configuration we see coupling constants in the range of 3j 11 18 hz while cis protons couple in the 3j 5 10 hz range. Deprotonation or dehydronation is the removal transfer of a proton or hydron or hydrogen cation h from a brønsted lowry acid in an acid base reaction the species formed is the conjugate base of that acid. Meldrum s acid and barbituric acid historically named acids are in fact a lactone and a lactam respectively but their acidic carbon protons make them acidic.

Proton b is bonded to a more electronegative atom s. It is not one of the functional groups in the list above but there is a similar proton is table of acids that we learned. The book indicates the red proton is the more acidic proton.