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
- ikeLv 51 0 年前最佳解答
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 .........
1,2 addition yield
1,4 addition yield
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..
- LeonisLv 51 0 年前
- ?Lv 51 0 年前