Where is the more reactive double bond? [closed]








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I was thinking that the addition reaction would occur more easily in the double bond (marked with an arrow) where the carbons are less substituted, because they are more unstable.


[![image 2](https://i.stack.imgur.com/0OgFqm.png)](https://i.stack.imgur.com/0OgFqm.png) 




I would like to agree with you somewhat.


The stability of substituted alkenes is due to two factors:


1. Stabilizing interaction between the C=C (pi bond) (**hyperconjugation**)
2. Adjacent C-H (sigma bonds) on substituents.


The more substituents that are present in the double bond, the more hyperconjugation and the stable the alkene.


Consider the heats of of hydrogenation of these alkenes:


[![a](https://i.stack.imgur.com/ZjTZJ.png)](https://i.stack.imgur.com/ZjTZJ.png)


Heat of hydrogenation of alkenes is a measure of the stability of carbon-carbon double bonds.


Under similar conditions, the smaller the numerical value of heat of hydrogenation of an alkene, the **more stable** the double bond therein


To sum up,it is much is easier to create a new bond where the bond is weak/less stable,which is the reason why alkenes (esp polysub.) easily undergo addition reactions.


**Please note that, alkenes behave as nucleophiles (Lewis bases) in polar reactions, donating a pair
of electrons from their electron-rich C=C bond to an electrophile (Lewis acid),So in this case they need stabilisation (tert. carbocation more stable)**


**Update** 


Electrophilic addition of alkenes


As previosly noted alkenes behave as Lewis basis in polar reactions.


One classic example is reaction with HBr


* The reaction occurs in two steps, protonation and bromide addition,
and involves a carbocation intermediate.
* A hydrogen atom on the electrophile HBr is attacked by electrons from the nucleophilic double bond, forming a new C–H bond. This leaves the other carbon atom with a + charge and a vacant p orbital. Simultaneously, two electrons from the H–Br bond move onto bromine, giving bromide anion.
* The carbocation intermediate that results is itself an electrophile, which can accept an electron pair
from nucleophilic Br- ion to form a C-Br bond and yield a neutral addition
product.


**Markovnikov’s rule**


* In the addition of HX to an alkene, the H attaches to the carbon with
fewer alkyl substituents and the X attaches to the carbon with more
alkyl substituents.
* In the addition of HX to an alkene, the more highly substituted
carbocation is formed as the intermediate rather than the less highly
substituted one.


References


Image and some content from McMurry Organic chemistry


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