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Why is C-Cl weaker than C-OH

I'm trying to figure out why Cl is a better leaving group which makes acyl chlorides more reactive compared to carboxylic acids but I can't figure out why the C-Cl bond is weaker than C-OH

Thank you
Reply 2

ohh god i was expecting a simple answer haha, wow

i think i understand now, thank you :smile:
Original post by LeaX
ohh god i was expecting a simple answer haha, wow

i think i understand now, thank you :smile:


A simple explanation I learnt at school which may not be 100% correct is that Cl- is the conjugate base of a strong acid (HCl), and so is a pretty weak base, so the equilibrium is well off to the right and OH- is the conjugate base of a weak acid (Water) so is not as weak as the CL- group, pushing the equilibrium more to the left, thus less reaction happening. Take this with a grain of salt because even though it sounds right it may not be a good reason :P
(edited 9 years ago)
Reply 4
Original post by monkeyvonban
A simple explanation I learnt at school which may not be 100% correct is that Cl- is the conjugate base of a strong acid (HCl), and so is a pretty weak base, so the equilibrium is well off to the right and OH- is the conjugate base of a weak acid (Water) so is not as weak as the CL- group, pushing the equilibrium more to the left, thus less reaction happening. Take this with a grain of salt because even though it sounds right it may not be a good reason :P

ohhh i see, thank you :smile: that makes so much sense
Reply 5
Original post by LeaX
ohh god i was expecting a simple answer haha, wow

i think i understand now, thank you :smile:


The answer is simple; it's just a case of peeling apart that post to get to the juicy explanation at the centre! The key point is that the chloride anion is much bigger than hydroxide (181 pm radius versus 110 pm) so the negative charge is spread out over a larger volume. This means chloride is more stable (less electron-electron repulsion), so it's a better leaving group. The acid strengths that monkeyvonban mentioned are further evidence of chloride being more stable, not an explanation.

For what it's worth, the bond energy is not very different (360 kJ/mol for C-O versus 330 kJ/mol for C-Cl) so you should focus an explanation of the acyl chloride's better reactivity on the above stability argument, not a bond strength argument.
Reply 6
Original post by BJack
The answer is simple; it's just a case of peeling apart that post to get to the juicy explanation at the centre! The key point is that the chloride anion is much bigger than hydroxide (181 pm radius versus 110 pm) so the negative charge is spread out over a larger volume. This means chloride is more stable (less electron-electron repulsion), so it's a better leaving group. The acid strengths that monkeyvonban mentioned are further evidence of chloride being more stable, not an explanation.

For what it's worth, the bond energy is not very different (360 kJ/mol for C-O versus 330 kJ/mol for C-Cl) so you should focus an explanation of the acyl chloride's better reactivity on the above stability argument, not a bond strength argument.


ahh that seems much more simple now, thank you for clarifying :smile:

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