[me1967]

Hello,

I am trying to re-learn Org Chemistry from 15 years ago and ran into the following problem that I hope can be explained to me.

The heat of hydrogenation of 1,4-pentadiene is 254.4 kJ/mol.  The heat of hydrogenation of 1-pentyne is approximately 292.9 kJ/mol.

1.  Which structure is more stable, 1,4-pentadiene or 1-pentyne?

2. Consider an unknown with the molecular formula C4H6.  How many degrees of unsaturation are present?

Great site, I have learned a lot by reading each post and am sure I will be back with more questions.

Thanks in advance for any and help.

[english]

One is releasing more heat than the other.  What does this tell you?


One degree of unsaturation is the lost of 2 hydrogens.

[UWSteve]

One degree of unsaturation is the lost of 2 hydrogens.

Also, if you have to answer any other questions regarding degrees of saturation, this way will not give you the correct answer if there are O's, N's, or halogens in the formula.

[lavoisier]

Hello,

I am trying to re-learn Org Chemistry from 15 years ago and ran into the following problem that I hope can be explained to me.

The heat of hydrogenation of 1,4-pentadiene is 254.4 kJ/mol.  The heat of hydrogenation of 1-pentyne is approximately 292.9 kJ/mol.

1.  Which structure is more stable, 1,4-pentadiene or 1-pentyne?

2. Consider an unknown with the molecular formula C4H6.  How many degrees of unsaturation are present?

Great site, I have learned a lot by reading each post and am sure I will be back with more questions.

Thanks in advance for any and help.

In addition to the (correct) answers you got from previous posts, I think you should take notice of a couple of things that might not be obvious for everybody.

Concerning question 1, you are asked about the stability of two substances based on their heat of hydrogenation. But note that you're allowed to do so because the final product is the same for both reactions (n-pentane), and (I think) because the number of moles of hydrogen added in the two reactions are the same. So you can compare the energy of the starting molecules thanks to the fact that the final state is the same.

For question 2, the D.U. is the number of double bonds plus the number of cycles (i.e. each cycle counts as one degree of unsaturation). Benzene has a D.U. of 4 because it has 3 double bonds and is a cycle. The formula to calculate D.U. (in a molecule containing C,H,O,N) is:

D.U. = C - H/2 + N/2 + 1

or more in general:

D.U. = 1 + Sum_i ((valence_i - 2) * n_i/2)