Could someone please help with a mechansim question.

From the way I understand this, in C=O the C is delta positive and the O is delta negative due to electronegativity. So, when the C is attacked by the NH3 lone pair, the electrons in C=O move to oxygen to allow the new electrons to form a bond.

The O- pair wants to join the C again to reform the C=O as this will allow further stability of charge. To do this, a pair of electrons will need to be removed from the C to allow this new pair. The Cl will leave because it is most able to accept the pair of electrons (very electronegative), it can also accept a proton (forming HCl) from the NH3+. Because of those 2 things Cl will be the most stable group to leave, Cl- is a weak base, it can accept the proton but can most readily accept the electron pair to be able to leave.

Hope this makes sense

this is a very good answer, however tagsmags if you were unable to predict this then you haven't learn the mechanisms. all acyl chlorides mechanisms occur like this with nucleophiles. its nucleophilic addition elimination

Thanks for the response, but I don't think any acyl chloride mechanisms are covered in the Edexcel spec. What spec are you using?

this is a very good answer, however tagsmags if you were unable to predict this then you haven't learn the mechanisms. all acyl chlorides mechanisms occur like this with nucleophiles. its nucleophilic addition elimination