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Topic: Question on nucleophilic substitution  (Read 2095 times)

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Offline wizrak

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Question on nucleophilic substitution
« on: October 18, 2011, 03:27:57 PM »
We generally say that Chlroine is a good leaving group due to:
its long bond length = weaker bond
and also forms a stable anion.

I fail to understand how the chlroide anion is more stable than lets say hydroxide anions.

Also, when a nucleophile attacks the polar carbon with a C=O bond breaks (as the pi bond is weakest)
but does the C=O bond get regenerated due to the electronegativity of the oxygen and cause the nucleophile to breakoff again

Offline Honclbrif

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Re: Question on nucleophilic substitution
« Reply #1 on: October 18, 2011, 05:16:50 PM »
For the first question: it has more to do with the polarizability of the carbon-halogen bond.

For the second question: yes. If a species which is a good nucleophile and a good leaving group attacks a C=O bond, the tetrahedral intermediate can eject the incoming nucleophile and regenerate itself.
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Offline wizrak

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Re: Question on nucleophilic substitution
« Reply #2 on: October 18, 2011, 06:24:48 PM »
thanks for a fast answer

When u mention the polarzability of the C-Cl bond, do you mean that chlorines electronegativity causes the carbon to be delta positive = vulnerable for nucleophilic attack?

BUT WHY is it that if a molecule has both OH- and Cl- group
Cl is substitued instead of OH- group, despite the oxygen being the more electronegative group the chlorine?

Offline Jeremy

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Re: Question on nucleophilic substitution
« Reply #3 on: October 18, 2011, 06:39:35 PM »
thanks for a fast answer

When u mention the polarzability of the C-Cl bond, do you mean that chlorines electronegativity causes the carbon to be delta positive = vulnerable for nucleophilic attack?

BUT WHY is it that if a molecule has both OH- and Cl- group
Cl is substitued instead of OH- group, despite the oxygen being the more electronegative group the chlorine?
I think it's because the C-O bond is stronger than those introduced by most nucleophiles.

Offline fledarmus

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Re: Question on nucleophilic substitution
« Reply #4 on: October 18, 2011, 07:02:55 PM »
Strong bases are poor leaving groups, and the hydroxide ion is a terrible leaving group. Leaving group activity correlates fairly well with the pKa of the conjugate acid of the leaving group - HCl has a very low pKa and Cl- is a pretty good leaving group.

You can do nucleophilic substitutions on alcohols, but you have to do them in acidic conditions so the alcohol is protonated. That makes water the leaving group, rather than OH-

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