股市巨象 發問時間: 科學化學 · 1 0 年前

有機化學問題kinetic control

the reaction of 1,3-butadiene with HBr gives two products ,one is major

product at low reaction temperature , contrarily another product predominates at high temperature

(a)what are these two products?

(b)which one is kinetic control product?

(c)use a free energy versus reaction coordinate diagram to explain this reaction

中譯和詳解

3 個解答

評分
  • ike
    Lv 5
    1 0 年前
    最佳解答

    http://www.cem.msu.edu/~reusch/VirtualText/addene2...

    The expected addition product from reactions of this kind is the result of 1,2-addition, i.e. bonding to the adjacent carbons of a double bond. The unexpected product comes from 1,4-addition, i.e. bonding at the terminal carbon atoms of a conjugated diene with a shift of the remaining double bond to the 2,3-location. These numbers refer to the four carbons of the conjugated diene and are not IUPAC nomenclature numbers. Product compositions are often temperature dependent, as the addition of HBr to 1,3-butadiene demonstrates.

    CH2=CH-CH=CH2 + HBr .........

    reaction temperature

    CH3CHBr-CH=CH2 +

    1,2 addition yield

    CH3CH=CHCH2Br

    1,4 addition yield

    0 ºC

    40 ºC

    70%

    15%

    30%

    85%

    Bonding of an electrophilic atom or group to one of the end carbon atoms of a conjugated diene (#1) generates an allyl cation intermediate. Such cations are stabilized by charge delocalization, and it is this delocalization that accounts for the 1,4-addition product produced in such addition reactions. As shown in the diagram, the positive charge is distributed over carbons #2 and #4 so it is at these sites that the nucleophilic component bonds. Note that resonance stabilization of the allyl cation is greater than comparable stabilization of 1,3-butadiene, because charge is delocalized in the former, but created and separated in the latter.

    An explanation for the temperature influence is shown in the following energy diagram for the addition of HBr to 1,3-butadiene. The initial step in which a proton bonds to carbon #1 is the rate determining step, as indicated by the large activation energy (light gray arrow). The second faster step is the product determining step, and there are two reaction paths (colored blue for 1,2-addition and magenta for 1,4-addition).The 1,2-addition has a smaller activation energy than 1,4-addition, but the 1,4-product is more stable than the 1,2-product. At low temperatures, the products are formed irreversibly and..

    請照網址就可以看到圖了!!

  • Leonis
    Lv 5
    1 0 年前

    這題也是有機化學常見考題,答案"issac"大大已經說明的很詳細了,我補充說明的是動力學控制的要素是活化能較低,因此在低溫時是主產物,而熱力學控制的要素是產物相對來說較穩定,因此在較高溫時為主產物

  • ?
    Lv 5
    1 0 年前

    根據古老的記憶....

    (A)順反CH3CH2BrCH2BrCH3,反=major

    (B)順式

    (C)...

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