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Author Topic: Nucleophilic substitutions  (Read 974 times)

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mycotheologist

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Nucleophilic substitutions
« on: August 04, 2012, 08:37:51 AM »

I'm trying to get a good understanding of why one nucleophile, replaces another nucleophile during a nuclephilic substitution. For instance, halide ions are strong nucleophiles so why are they so easy to replace, even by weak nucleophiles like OH-? I read that a way to estimate the strength of a leaving group, you look at the its pKa when attached to a hydrogen atom. The lower the pKa, the better the leaving group. With the exception of HF, all the hydrohalic acids are very strong which explains why they make good leaving groups.  I read that tosylate groups are as good as halide groups for leaving so I'm guessing tosylic acid has a pretty low pKa. So is it mainly the strength of the leaving group that underlies nucleophilic substitutions?

Do nitro groups participate in nucleophilic substitution? Their corresponding acid is nitrous acid which is pretty strong.
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AWK

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kirushanth

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Re: Nucleophilic substitutions
« Reply #2 on: August 04, 2012, 06:24:21 PM »

Just one more doubt. HCl is strong acid. So, Cl(-) is a weak base. So, it is supposed to be a good leaving group. What property of being a weak base does Cl(-) have, that makes it a good leaving group?

For example, RCl+OH(-) gives ROH+Cl(-)

Why does Cl(-) have to leave? Does it stabilize the system?
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mycotheologist

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Re: Nucleophilic substitutions
« Reply #3 on: August 08, 2012, 01:12:22 PM »

Are you sure its not weak acids that have weak conjugate bases? For example, sodium acetate is acetic acids conjugate base. Its a weak base because it bonds to protons forming acetic acid, which has a dissociation constant that determines how many protons will remain bonded to it. HCl is a strong acid so its conjugate base Cl- doesn't accept protons at all. Its therefore not a weak base.
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Re: Nucleophilic substitutions
« Reply #4 on: August 08, 2012, 10:24:51 PM »

Bronsted theory requires conjugated acid and base. then Cl- is extremely weak base.
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fledarmus

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Re: Nucleophilic substitutions
« Reply #5 on: August 09, 2012, 02:42:53 AM »

Are you sure its not weak acids that have weak conjugate bases? For example, sodium acetate is acetic acids conjugate base. Its a weak base because it bonds to protons forming acetic acid, which has a dissociation constant that determines how many protons will remain bonded to it. HCl is a strong acid so its conjugate base Cl- doesn't accept protons at all. Its therefore not a weak base.

Consider the following two reactions in light of your description:

CH3COO- + H+  :rarrow: CH3COOH

Cl- +  H+  :rarrow: HCl

Which one is more likely to occur, according to your description?

The stronger base between CH3COO- and Cl- would be the one for which the reaction above was the most likely to occur.
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