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07-17 - Nucleophilic Substitution.docx

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McMaster University
Jeff Landry

Nucleophilic Substitution of Haloalkanes  Nucleophile is something that is electron-rich (and thus is attracted to the nucleus) o Lewis Base, attacks something that is bare of electrons o Nucleophile will attack another nucleus to kick out a leaving group  The C-X bond is polarized (more electron density around the halogen X than around the carbon)  Can’t kick off a hydrogen from CH b4cause it would result in a single hydrogen with 2 electrons (hydride), which is EXTREMELY high energy, so that would never happen o Leaving group will be a halogen  C-X (X = halogen) bond strength increases down the periodic table (as you go down, halide gets bigger, so bond length gets bigger, so bond strength gets weaker) o Weaker bond → easier to break off (and be leaving group)  Fluoride is very electronegative, so makes very strong bonds, so it is rarely a leaving group (hard to break it off)  As you make a longer chain, bond strength increases (higher boiling point)  When H is attacked, we call the attacker a Base, but when C is attacked, we call the attacker a Nucleophile o Nucleophiles are usually negatively charged (electron-rich)  Alkyl Halides are good starting materials to make Alcohols, Ethers, and Nitriles  By changing concentrations and measuring the resulting rates, you can see what elements are involved in the slow step of the reaction  How to study a mechanism: o Kinetics (modify concentrations to see what slow step is) o Stereochemistry o Modify subs
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