Vinylic Protons Are Acidic Because

Typical proton proton coupling constants.

Vinylic protons are acidic because.

A better question would be what s an allylic position because allylic proton is nothing more than proton hydrogen in allylic position. 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. The α protons of carbonyl groups are acidic because the negative charge in the enolate can be partially distributed in the oxygen atom. The 2 bond coupling between protons on the same alkene carbon referred to as geminal protons is very fine generally 5 hz or lower.

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. Proton b is bonded to a more electronegative atom s. This is not surprising since the proton is not only vinylic but it is also alpha to a carbonyl group. Other groups that give broad and sometimes deuterium exchangeable signals are the amines amides and thiols.

The allylic position is the one on carbon next to double bond. The book indicates the red proton is the more acidic proton. The species formed is the conjugate acid of that base. 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.

Is this the proper resonance structure for the red proton s conjugate base. We know that a proton alpha to a carbonyl group is pulled downfield. 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. 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.

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. Allyl is a name for this radical. The complementary process when a proton is added transferred to a brønsted lowry base is protonation or hydronation.