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The chemical reduction of 1,3-butadiene with sodium in alcohol gives mainly the 1,4-addition product that is but-2-ene. Under these condition the isolated double bond are not reduced. This suggest that dienes are much more reactive than simple alkenes.

Why is diene more reactive than alkene even though the conjugated double bond of diene (in the example of excerpts given above) is stabilized with resonance? How is 1,4-addition product the major product?

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It is not only the diene itself which is stabilised by resonance, but on electrophilic addition the carbocation formed is also stabilised by resonance if the diene is conjugated. Thus, forming the intermediate for a conjugated diene is easier than forming the intermediate for the alkene, even though the starting compound itself is more stable in case of conjugated dienes and therefore conjugated dienes react faster.

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  • $\begingroup$ Also can you tell how 1,4-addition product is the major product? $\endgroup$ – pcforgeek Dec 24 '15 at 3:11
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    $\begingroup$ The 1,4 addition product is the major product at high temperature, as the actual stability of the product comes into picture rather than the kinetics, which dominates at lower temperature.The 1,4 adduct is thermodynamically stabilised by hyperconjugation. $\endgroup$ – Aditya Anand Dec 24 '15 at 3:19
  • $\begingroup$ Can you elaborate it comparing how it is major product than 1,2 addition product? $\endgroup$ – pcforgeek Dec 24 '15 at 3:23
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    $\begingroup$ The formed carbocation tells you only about kinetics.Thermodynamic formation of 1,4 adduct is easier.Draw the structures of the 1,2 and 1,4 addition products and you will see that there are 2 alpha C-H bonds for the 1,2 product and there are 6 for 1,4 addit ion product.See chemwiki.ucdavis.edu/Organic_Chemistry/Conjugation/… for more details $\endgroup$ – Aditya Anand Dec 24 '15 at 3:33
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There is a long and elaborate answer by Philipp which will tell you a lot about why — possibly more then you need to know. It boils down to reactiviy meaning easier accessable orbitals meaning a lower LUMO or a higher HOMO in energy terms. Well, going from ethene to butadiene, you go from two π to four and thus you can imagine two additional orbitals to be sandwiched in the middle between the (all)bonding and (all)antibonding ones. This is also shown including an energy scale in Philipp’s answer:

energy levels of ethene versus butadiene

If you liked this answer, please also go and upvote Philipp’s.

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Conjugated dienes have enhanced stability due to resonance. So they are less reactive when compared to alkenes in general. But many reactions proceed through high-energy cation or free radical intermediates; in these cases the resonance stabilization of the intermediate allyl species makes conjugated dienes much more reactive than alkenes.

enter image description here

As to your other question, a Michael addition is a 1-4 addition, where a nucleophile attacks the β carbon, and produces the thermodynamically favored product, which happens at high temperatures. On the other hand, a 1-2 reaction (on the carbonyl) gives the kinetic product, and is obtained at low temperatures. At higher temperatures, it is not only important that the product forms, but it is also important as to whether the formed product will be stable under the conditions.

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