The Morita-Baylis-Hillman (MBH) reaction of methyl acrylate (MA) with p-nitrobenzaldehyde (pNBA) catalysed by tertiary amines in methanol is chosen as the model reaction. We focus on the mechanism as outlined in Figure which is supported by experimental and computational studies. The rate-limiting step of the reaction is the proton transfer between intermediate 2 and 3 which is aided by a methanol molecule, the corresponding transition state (TS3) is shown in Figure highlighted in grey.