[THC]

Cyclopropane or propene? Which one would you pick as a fuel?


This is a test without books, notes etc.
I guess I have to pick the one with the lowest delta H. Since their chemical formula is C3H6, the products are the same (CO2 and H2O).
My guess is that propene is the best fuel since there's more energy in double bonds compared to single bonds.

[macman104]

Is this a question on a test that you will be handing in?  If so, then...you probably shouldn't be posting for help...

[THC]

Is this a question on a test that you will be handing in?  If so, then...you probably shouldn't be posting for help...

No. But that would be a very naive teacher who assigns a test for the students to solve outside of class and hope they don't cheat Also, I would be very stupid to mention it if it, in fact, was a test. Well, since you're asking - I'm practising for a test by reviewing the tests from last year. Can't figure this one out, though.

[Arkcon]

So which gives more energy, the triple bond, or breaking the strained ring?  Take a guess, then see what books will tell you.  How will they change the question for this year's exam?  Cyclobutane?

[sjb]

So which gives more energy, the triple bond, or breaking the strained ring? 

breaking the *double* bond, surely

S

[Arkcon]

Ah, oops, I misread the question, I though it said propyne.  Heh. Srry.

[THC]

Well, when you put it like that: Cyclopropane?

But I still don't understand why.

[sjb]

Just noticed we haven't really resolved this.

Here's my take on it (as this is closed book).

All other things being equal, in cyclopropane we have 6 C-H bonds, and 3 C-C sigma bonds. In propene there are still 6 C-H bonds, but 2 C-C sigma bonds and the pi bond of the alkene.

2 single bonds are usually stronger than a sigma and a pi, so I'd perhaps suggest that the cycloalkane would release more energy.

What did your tutor/teacher/whoever say?

[adam]

Also an important point is the availabilty of the twoo molecules...which is cheaper and easy to obtain...??

[THC]

Just noticed we haven't really resolved this.

Here's my take on it (as this is closed book).

All other things being equal, in cyclopropane we have 6 C-H bonds, and 3 C-C sigma bonds. In propene there are still 6 C-H bonds, but 2 C-C sigma bonds and the pi bond of the alkene.

2 single bonds are usually stronger than a sigma and a pi, so I'd perhaps suggest that the cycloalkane would release more energy.

What did your tutor/teacher/whoever say?

The answer sheet says cyclopropane but doesn't offer an explanation. But I guess you offered one. Anyway, this type of question didn't appear on the test, so I'm happy