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Could someone please help with a mechansim question.

Hello I have found a bit of an unusal question from Edexcel 2017 Past Paper 3 Question 8 (b).

"Predict the mechanism for the reaction of CH3CH2COCl with ammonia Include curly arrows and relevant lone pairs"

The answer is attached, however I don't understand how you were meant to predict that there would be heterolytic fission of the pi bond in C=O and that the C-Cl bond would break in the second step?

As far as I am aware the closest reaction you learn on the spec is the addition of cyanide ions to carbonyl compounds, however this still would not provide information on the fission of the C=O and C-Cl bond. Any help would be appreciated.
(edited 1 year ago)
There's always heterolytic fission in the C=O bond because homolytic fission will give C-O bond with unpaired electrons on both atoms which just doesn't happen.
I'm not sure how you were meant to predict that C-Cl bond breaks after tho.

Hope that helps.. somewhat...
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
Original post by jjeeeeeea
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
Reply 4
Original post by B7861
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?
Original post by Tagsmags
Thanks for the response, but I don't think any acyl chloride mechanisms are covered in the Edexcel spec. What spec are you using?

oh I apologise, I do AQA and they are covered in the spec....
Original post by B7861
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


yeah - that answer went into way more detail than you need for aqa a level chem. in fact you don't need to know any of the background - just memorise the mechanism and that's just how it is with acyl chlorides + nucleophiles

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