uken_z 發問於 科學及數學化學 · 10 年前

[Organic Chem] SN2 and SN1

1) Increase the polarity of solvent can facilitate SN1 or SN2 reaction , and why?

2) Why does water rapidly hydrolyses tertiary haloalkane to alcohol but primary and secondary haloalkanes undergo alkaline hydrolysis ? I need the mechanisms.

3) Why do alcohol solvents tend to raise the basicity of hydroxide ion and therefore facilicate the elimination reaction ?

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    1. Increase in polarity of solvent can facilitate SN1 or SN2 reaction, and why? As SN2 reaction is a nucleophilic substitution reaction, it involves a negatively charged or neutral nucleophile to yield neutral products or positively charged product. So, increase in polarity of solvent can facilitate SN2 reaction. This is because polar solvents can dissolve metal cations rather than nucleophilic anion.So, the unsolvated anions have a greater nucleophilicity, the ground state energy of nucleophile can be increased.Recall that the rate of chemical reaction is determined by activation energy, ΔG‡ , it is the energy difference between reactant ground state and transition state.As groundstate energy of nucleophile can be increased, the ΔG‡ can be decreased. With a smaller ΔG‡, SN2 reaction will take place at a correspondingly faster rates. Although SN1 reaction is also a substitution reaction, in contrast to SN2 reaction, the reaction rate of SN1 reaction depends only on concentration of haloalkane but is independent on concentration of nucleophile(we call it first-order process).So, the rate of SN1 reaction will not increase with the increase in polarity. 2) Why does water rapidly hydrolyses tertiary haloalkane to alcohol but primary and secondary haloalkanes undergo alkaline hydrolysis ? I need the mechanisms. Because tertiary haloalkane is much more reactive than primary and secondary haloalkanes (about 1,200,000 times). The mechanism of hydrolyses of tertiary haloalkane:

    圖片參考:http://imgcld.yimg.com/8/n/HA00656648/o/7011080200...

    3) Why do alcohol solvents tend to raise the basicity of hydroxide ion and therefore facilicate the elimination reaction? I’m not sure about this.

    (I deduce it from alcohol + water ---> alkoxide ion + hydronium ion) Maybe it is because of the formation of alkoxide ion, which is a strong base.

    The mechanism is as below.

    圖片參考:http://imgcld.yimg.com/8/n/HA00656648/o/7011080200...

    Hope I can help you!~

    資料來源: 黑白灰_唯物主義者_魑魅魍魎
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