October 29, 2020, 03:43:03 PM
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Topic: Organic synthesis  (Read 254 times)

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Offline Rosalind Franklin

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Organic synthesis
« on: October 09, 2020, 10:47:49 AM »
We were asked to try out the mechanism for this organic conversion by using Ph2S:

I have two questions when it comes to the starting step which is Ph2S: attacking 1-chloro-3-iodo propane:

1) If I use Ph2S: why does it act as a nucleophile and attack CH2 ? Why can't it act as a Lewis base and abstract a proton from CH2?
2) I understand that Iodine is a better leaving group than Chlorine, but if we compare electrophilicity, the CH2 attached to Cl is more electrophilic(Cl being more electronegative than I), then why does the nucleophile(provided my first question is answered) attack CH2 next to Iodine? Is leaving group ability given more preference over the electrophilicity of CH2? 


Offline Babcock_Hall

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Re: Organic synthesis
« Reply #1 on: October 09, 2020, 04:55:22 PM »
How strong is Ph2S as a base?

Offline Rosalind Franklin

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Re: Organic synthesis
« Reply #2 on: October 27, 2020, 12:02:35 PM »
The lone pair on Sulfur is in resonance with the Ph group which makes it a poor base? But that means the lone pair isn't available for donation, which will make it a poor nucleophile as well? Also, there are two Ph groups, so we should consider steric effects too right? I don't understand how it can be a good nucleophile when it is not a good base either. There are just too many factors at play here and it is a bit confusing to decide which predominates.

Offline Babcock_Hall

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Re: Organic synthesis
« Reply #3 on: October 27, 2020, 02:59:38 PM »
I would think along the lines that Ph2SH1+, being positively charged, is a relatively strong acid; therefore, its conjugate base must be weak.  This can be verified by looking up the pKa value of the acid.

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