Alkene Comparison

[Sportfreunde]

I have an assignment where I have to compare 10 alkenes. 8 of them are decatrienes with different conformations like (1 E, 3E, 5E)-1,3,5-decatriene. The other two are phynalbutane and (3,5-cyclohexadienyl)-butene. I’ve used Argus lab to come up with the 3-D structure and have also been given 3D structures from which I have the heat of formation for each molecule (ex: 13.68 kcal/mol for (2E, 5Z, 8E)-2,5,8-decatriene. I have to use this information to basically comment on the molecules and explain which ones are stable, note why the discrepencies exist in the heats of formation. So I need to relate the stability to the structure and also heat of formation. My question is, what should I comment on here and what things should I be looking for. Also what significance would the LUMO and HOMO have for each molecule...this is the part I’m most unsure of because I don’t understand LUMO/HOMO.

[kyle1990]

-Conjugated alkenes are more stable than isolated C=C double bonds (this is where LUMO/HOMO come into play)
-E isomers are more stable than Z isomers due to steric reasons
-More substitued double bonds = more stable alkene
-Aromatic rings are unusually stable due to its overlapping pi network

These are just a few general guidelines I could think of. I'll post some more if I think of them.

[chrisf]

E isomers are more stable than Z isomers due to steric reasons

This is not always (but usually) the case and it's not just an issue of sterics, to my knowledge. One isomer is favored over another because of an increased net-dipole moment that arises from the arrangement of the molecule. E isomers usually have some kind of hydrogen IMF which makes them more favorable. But you're right, steric hindrance is one reason E is favored.

HOMO/LUMO helps to explain the shape/hybridization of the electron orbital cloud and the reactivity of the orbitals.

HOMO/LUMO help explain why conjugated alkenes are so stable. Actually, it's for the exact same reason you mentioned for the stability of aromatic rings - the overlapping network (adjacent) pi bonds. The adjacent pi bonds contribute to an overall lower energy which increases the stability of the molecule.

[kyle1990]

This is not always (but usually) the case and it's not just an issue of sterics, to my knowledge. One isomer is favored over another because of an increased net-dipole moment that arises from the arrangement of the molecule. E isomers usually have some kind of hydrogen IMF which makes them more favorable. But you're right, steric hindrance is one reason E is favored.

Glad you pointed that out chris because I didn't know that =)