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Grade: 12
Which is most stable?
(a) ethene
(b) 1-butene
(c) cis-but-2-ene
(d) trans-but-2-ene
(e) 2,3-dimethyl but-2-ene
Please help
5 months ago

Answers : (2)

Mahima Kanawat
991 Points
Dear student 
Stability of alkene is decided by Hyperconjugation  
2,3 dimethyl but - 2-en is most stable beacuse it exhibits Hyperconjugation and has maximum no. Of alpha H i. e., 12.
5 months ago
227 Points
Basically, the following features help in determining stability order:
=> Inductive Effect
=> Electromeric effect
=> Resonance
=> Hyperconjugation (No-bond resonance)
Now, what is an alpha carbon and alpha hydrogen?
If you say with respect to a double bond, then Alpha Carbon is the Carbon adjacent to the Carbon having the double bond, and the hydrogens substituted in the adjacent (alpha) carbon are called alpha hydrogen.
Hyperconjugation (no bond resonance) takes place when the alpha carbon takes up the electron pair from the alpha hydrogen (making the hydrogen positive and turning itself negative). More is the number of alpha hydrogens, more will be the hyperconjugative structures – more will be the stability of the substance which have many hyperconjugative structures.
Let’s Check out the structure of Ethene: It has two carbons attached with double bonds. In each carbon having the double bond, there is no more attached carbon, (alpha carbon), and so, there are no hyperconjugative structures of ethene, so there is no stability factor.
When you consider 1-butene, the structure is CH2=CH-CH2-CH3. The first and second carbons are attached with the double bond. In the first carbon, no adjacent carbon is attached. However, in the second carbon, one adjacent carbon with two alpha hydrogens is present. So, there are two hyperconjugative structures.
When you consider Cis-2-butene and Trans-2-butene, they are geometrical isomers. For Cis-2-butene and trans-2-butene both, each carbon (of the two) attached with double bond has one adjacent alpha carbon with three alpha hydrogens. So, both have 6 hyperconjugative structures.
Lastly, when you consider 2,3-dimethyl But-2-ene, the 2nd and the 3rd carbon are attached with double bonds. When you draw the structure, it is clear that the second carbon has two adjacent alpha carbons with three hydrogens each (6 hydrogens for one carbon having a double bond). And, the 3rd carbon is also attached with two adjacent alpha carbons, having 3 Hydrogen atoms each (6 alpha hydrogens).
This make a total of 12 alpha hydrogens, so we get 12 hyperconjugative structures.
Hence we conclude that 2,3-dimethyl But-2-ene is the most stable, followed by the geometrical isomers, 1-butene and lastly, ethene.
5 months ago
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