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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Winga on November 10, 2004, 08:12:32 AM

Title: About the stability of alkene!
Post by: Winga on November 10, 2004, 08:12:32 AM

By Saytzeff's rule, the greater no. of alkyl groups attached to the double bonded carbons, more stable the alkenes.

Can I say that the double bonded carbons are said to be e- deficient?
So, the e- donating groups, alkyl groups can stabilize C=C carbons. By hyperconjugation?

I want to know that the bond pair e- in C-H bond (sigma orbital) donates to which orbital of C=C so that C=C carbons can be stabilized.


One more factor to explain more alkyl groups attached to C=C carbons, more stable the alkenes is the bond strain, right?

Title: Re:About the stability of alkene!
Post by: Mitch on November 10, 2004, 04:38:46 PM

What does stable really mean? I hate these types of words. If stability was defined by which one has a lower (delta)H_formation then the lone ethylene would have the lowest and thus be most "stable".

Title: Re:About the stability of alkene!
Post by: dexangeles on November 10, 2004, 10:54:19 PM

yes Zaitsev's rule states that, meaning

Stability:

monosubstituted < disubstituted < trisubstituted < tetra substituted
(less stable)                                                       (more stable)

then you have to factor out cis vs trans (steric hindrace/strain)
trans is usually more stable than cis except for cycloalkenes
trans is said to only appear in cylcooctene and above, but is still less stable than cis
around 12C ring, their approximately equal, then over 12 trans is more stable

and yes, hyperconjugation affects it's stability more than induction effect

it would be better to say that double bonds are electron attracting than e- deficient

their stability can be shown through their potential energy map
the lower the potential energy, the more stable

bond strains in double bonds are prominents in cycloalkenes

e.g.
the Carbon is sp2 hybridized, so the angles between bonds are said to be around 120 degrees
in cyclobutene (in example) the bond in the cycloalkene is at 90 degrees
this is a cause of angle strain and makes it not stable

Title: Re:About the stability of alkene!
Post by: Winga on November 11, 2004, 08:00:14 AM

Do alkyl groups donate e- by hypercojugation + inductive effect or just hyperconjugation?

Title: Re:About the stability of alkene!
Post by: dexangeles on November 11, 2004, 12:12:14 PM

inductive effect is the effect (pull) that more electronegative atoms have on other electrons from other sites of the molecule, so i'd say hyperconjugation