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Topic: Anhydrides, acid chlorides, why so reactive?  (Read 11704 times)

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Offline Nescafe

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Anhydrides, acid chlorides, why so reactive?
« on: May 05, 2012, 07:07:02 PM »
Hi,

I know acid chlorides and any anhydride you form are usually very reactive, but why, what makes these species so reactive?

Cheers,

Nescafé.

Offline Organic_lover*

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Re: Anhydrides, acid chlorides, why so reactive?
« Reply #1 on: May 05, 2012, 07:49:25 PM »
Hey Nescafe!

The reactivity of acid chlorides is due the presence of Cl atom on it's structure, besides the carbonyl group (that have a good polarity). The acid chlorides can participate on SN2 reactions and any other reactions where the Cl atom can "go out" from the acid chloride due to a nucleophilic attack on the carbonyl carbon (if you remember, the chloride is a good leaving group).



The anhydride, on the other hand, has it's reactivity due the two carbonyl groups on it's structure by the "same" reason: the difference of polarity of the carbon and oxigen atoms of carbonyl groups allow the attack of some nucleophile.



Hope this examples help you to understand. :)

See ya \o
"Nothing in life is to be feared, only understood. Now is the time to understand more to fear less." {Marie Curie}

Offline Nescafe

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Re: Anhydrides, acid chlorides, why so reactive?
« Reply #2 on: May 05, 2012, 11:34:27 PM »
Maybe you can help me with this. I read that chloride is a better leaving group than an anhydride (when adjacent to a carbonyl, hence the anhydride).

I do not understand why this is so.

Cheers,

Nescafe.

Offline zs3889

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Re: Anhydrides, acid chlorides, why so reactive?
« Reply #3 on: May 06, 2012, 12:25:05 AM »
I guess chloride is more electronegative than oxygen?

Offline Nescafe

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Re: Anhydrides, acid chlorides, why so reactive?
« Reply #4 on: May 06, 2012, 08:58:26 PM »
I guess chloride is more electronegative than oxygen?

Yea, spot on.

Offline Babcock_Hall

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Re: Anhydrides, acid chlorides, why so reactive?
« Reply #5 on: May 07, 2012, 08:57:11 AM »
For compounds that are closely related, leaving group ability correlates better with basicity, not with electronegativity.  For example, in terms of leaving group ability Cl- < Br- < I-.  This is anticorrelated with electronegativity.  However I- is the weakest base of the three, which is another way of saying that HI is the strongest acid of the three.

With respect to acetyl chloride versus acid acetic anhydride reacting with an amine, you can still use basicity to understand reactivity.  Which is the stronger base, Cl- or acetate ion?  The stronger base is usually the poorer leaving group (there are some exceptions when you compare compounds that are very different in structure).  I am not 100% certain that this is what you questions was about, so please let me know if it needs to be clarifed.

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