In the following acid–base reactions, 1 . determine which species are...

In the following acid–base reactions,

1 . determine which species are acting as electrophiles (acids) and which are acting as nucleophiles

(bases).

2 . use the curved-arrow formalism to show the movement of electron pairs in these reactions, as well as the imaginary movement in the resonance hybrids of the products.

3 . indicate which reactions are best termed Brønsted–Lowry acid–base reactions.

This reaction is a proton transfer from HCl to the group of acetaldehyde. Therefore, it is a Brønsted–Lowry acid–base reaction, with HCl acting as the acid (proton donor) and acetaldehyde acting as the base (proton acceptor). Before drawing any curved arrows, remember that arrows must show the movement of electrons from the electron-pair donor (the base) to the electron-pair acceptor (the acid). An arrow must go from the electrons on acetaldehyde that form the bond to the hydrogen atom, and the bond to chlorine must

break, with the chloride ion taking these electrons. Drawing these arrows is easier once we

draw valid Lewis structures for all the reactants and products.

The resonance forms of the product show that a pair of electrons can be moved between

the oxygen atom and the pi bond. The positive charge is delocalized over the carbon

and oxygen atoms, with most of the positive charge on oxygen because all octets are satisfied

in that resonance structure.

In this case, no proton has been transferred, so this is not a Brønsted–Lowry acid–base reaction.

Instead, a bond has formed between the

group. Drawing the Lewis structures helps to show that the group (the nucleophile in this reaction) donates the electrons to form the new bond to acetaldehyde (the electrophile). This result agrees with our intuition that a negatively charged ion is likely to be electron-rich and therefore an electron donor.

Notice that acetaldehyde acts as the nucleophile (Lewis base) in part (a) and as the electrophile (Lewis acid) in part (b). Like most organic compounds, acetaldehyde is both acidic and basic. It acts as a base if we add a strong enough acid to make it donate electrons or accept a proton. It acts as an acid if the base we add is strong enough to donate an electron pair or abstract a proton.