Strong nucleophile but weak base

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electrax

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7+ Year Member
Is it possible for a weak base to be a strong nucleophile? I'm reading about it in organic odyssey and it seems to me that the properties that would make a strong nucleophile would tend to make a strong base as well?

Thanks!
 
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Yes; for example, CN-, N3-, Cl-, Br-, I-, SH-, and SR- are strong nucleophiles but weak bases. An example of a strong base but a weak nucleophile is the t-butoxide ion, while examples of strong bases as well as strong nucleophiles are OH- and OR-.
 
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electrax said:
Is it possible for a weak base to be a strong nucleophile? I'm reading about it in organic odyssey and it seems to me that the properties that would make a strong nucleophile would tend to make a strong base as well?

Thanks!
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Nucleophilicity often parallels basicity, as my professor once told me.......BUT not always. Consider I- CH3S-, CN- . These are very powerful nucleophiles but weak bases. Often the reason lies with the size of these reagents, but not always. Sometimes these bases prefer to do SN2 as in the case of CN- and RS-....while LDA, and t-butoxide do E2. The explanation has to do with a reaction being controlled by charge or by molecular orbitals. For the DAT.....simple recognize the nucleophiles stated above and their mechanisms, and you should be fine.

Hope this helps.

Dr. Jim Romano
 
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DAT Destroyer said:
Nucleophilicity often parallels basicity, as my professor once told me.......BUT not always. Consider LDA, DBU, CH3S-, CN- and t-butoxide. These are very powerful nucleophiles but weak bases. Often the reason lies with the size of these reagents, but not always. Sometimes these bases prefer to do SN2 as in the case of CN- and RS-....while LDA, and t-butoxide do E2. The explanation has to do with a reaction being controlled by charge or by molecular orbitals. For the DAT.....simple recognize the nucleophiles states above and their mechanisms, and you should be fine.

Hope this helps.

Dr. Jim Romano
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I thought t-butoxide and LDA are strong bases but weak nucleophiles?
 
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