McMurry (Kap.7,5)

Question 1

Although the cis-trans isomer conversion does not happen naturally, it can be brought about by treating an alkene with a strong acid catalyst.

If we interconvert the two, and have them reach equilibrium, they dont have the same stability/energy.

Which one is more stable/less energy?

Answer 1

The trans isomer is more stable and have less energy then the cis isomer of the disubstituted carbon.

more stable by 2,8 kJ/mol (0,66 kcal/mol) at room temparature.

Question 2

Why are trans isomers more stable then cis isomers?

Answer 2

Because of the steric strain between the large substituents, on the same side of the double bond (up or down).

Question 3

The interconversion from trans to cis is used to establish a more stable equilibrium, a more generel methid is taken advantage of.

Which one?

Answer 3

Alkenes undego a (hydrogenation reaction), to yield the corresponding alkane (by using palladium or platinum and H2 gas).

You can see the energy diagram for the hydrogenation of cis and trans but-2-ene on page 192 (the cis release more energy because it is more unstable, and have a higher (delta G) value) because of that).

Question 4

On page 193 you can see the heats of hydrogenation value for some alkenes.

The cis has a more negative heat of hydrogenation value then the corresponding trans isomer (negative mean more heat).

The difference of heat of hydrogenation measured between the two corresponds well with the 2,8kJ/mol energy difference calculated from the equilibrium.

There are some differences though, why?

Answer 4

1) it is hard to measure heat of hydrogenation exactly.

2) The free energy changes (delta-g) and (delta-H) heat of hydrogenation dont measure the exact same thing, therefore a small difference is expected.

Question 5

The more substituted an alkene is, the more stable it is?

Why? (page 193)

Answer 5

1) Hyperconjugation (a stabilizing ineraction between the C=C (pi) bond and the adjacent C-H (sigma) bonds. The more substituents, the greater the stabilization of the alkene. (the R groups increase - more hyperconjucation - more stabilization).

2) Bond strenght. A bond between SP2 - SP3 is stronger then SP3 - SP3.

More substituted alkenes have a higher ratio of SP2-SP3 bonds then the less substituted alkenes, and therefore they are more stable (see page 194).