Answer
The electrophile and the nucleophile, and the curved arrow mechanism are shown in the picture.
This reaction is Brønsted–Lowry acid–base reaction.
Work Step by Step
1) HCl is an electrophile because it accepts the lone pair of electrons and $CH_3COH$ is a nucleophile because it donates the pair of electron.
2) The curved arrow originated from the $CH_3COH$ lone pair of oxygen and goes towards the H in HCl. Then the electrons involved in the bond between H and Cl, go to Cl.
3) In this reaction there is a proton transfer from HCl to the C=O 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).
