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Why do we say that an alpha hydrogen is necessary for hyperconjugation? What exactly is an alpha hydrogen? From what I read online, it is the hydrogen attached to a functional group. Why is that required?

For example, in $\ce{CH3-CH=CH2}$, hyperconjugation takes place, but I don't see clearly any functional group that would facilitate it.

Alternatively, in $\ce{CH3-COOH}$ it does not take place.

Why is this?

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To remember:

Hyperconjugation is the stabilising interaction that results from the interaction of the electrons in a σ-bond (usually C-H or C-C) with an adjacent empty or partially filled p-orbital or a π-orbital to give an extended molecular orbital that increases the stability of the system. Further information.

The proposed systems to take into account.

Looking at the resonance structures (which is a synonym of mesomeric effect) of the propene

Resonance structures of propene
(source: tutorvista.com)

and acetic acid

Resonance structures of acetic acid
(source: nonsibihighschool.org)

Therefore, in hyperconjugation analysis you need to look at the position of the electron density. Not the vicinal hydrogen.

Moreover and with the aim to give a complete answer, the hyperconjugation of the acetic acid can be very well understood at here.

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    $\begingroup$ Your pic is for mesomeric stabilisation of acetate not hyperconiugation of acetic acid. $\endgroup$ – Mithoron Dec 30 '15 at 22:51
  • $\begingroup$ But why does hyperconjugation not take place in acetic acid? Besides, why can only alpha hydrogen participate? Can you explain more about the electron density part? $\endgroup$ – Shodai Dec 31 '15 at 4:29
  • $\begingroup$ @Shodai, I invite you to take a look at: chemistry.stackexchange.com/questions/27799/… $\endgroup$ – Another.Chemist Dec 31 '15 at 4:35
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    $\begingroup$ @Shodai, also, the relevance of the alpha hydrogen to a functional group with a carbonil is when you are doing a synthesis through an electrophilic substitution at this hydrogen. If this is what you want to know. I recommend you this site: www2.chemistry.msu.edu/faculty/reusch/virttxtjml/crbacid3.htm $\endgroup$ – Another.Chemist Dec 31 '15 at 4:43

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