ochem question

[keibee82]

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Which of the following has the lowest heat of hydrogenation
per mole of H2 absorbed?

a. 1,3-pentadiene
b. 1,4-pentadiene
c. 1,2-pentadiene
d. 1,3,5-hexatriene
e. 1,2,4-hexatriene

 

[Notoriousjae]

Which of the following has the lowest heat of hydrogenation
per mole of H2 absorbed?

a. 1,3-pentadiene
b. 1,4-pentadiene
c. 1,2-pentadiene
d. 1,3,5-hexatriene
e. 1,2,4-hexatriene

the lowest heat of hydrogenation would be given out by the most stable alkene. In terms of stability, conjugated diene > isolated diene > cumulated diene.
so the answer would be a. 1,3-pentadiene.

 

[keibee82]

the lowest heat of hydrogenation would be given out by the most stable alkene. In terms of stability, conjugated diene > isolated diene > cumulated diene.
so the answer would be a. 1,3-pentadiene.

thats what i was thinking too, but the answer is D somehow.

 

[lintydent]

the lowest heat of hydrogenation would be given out by the most stable alkene. In terms of stability, conjugated diene > isolated diene > cumulated diene.
so the answer would be a. 1,3-pentadiene.

Wouldn't the most conjugate be more stable ie. wouldn't C=C-C=C-C=C 1,3,5-hexatriene be more conjugated than C=C-C=C-C 1,3-pentadiene?

 

[Notoriousjae]

thats what i was thinking too, but the answer is D somehow.

after i posted the answer, i realized that stability increases if the alkene is more substituted or has a greater number of double bonds while being a conjugated alkene. i looked it up in my orgo textbook and it happens to be true so yes d. would be the right answer. In essence, the triene would have lower energy than the diene because it is more stable and would thus have a lower catalytic hydrogenation value.
btw i'm just curious but where did you get this question from? it seems rather detailed.

 

[keibee82]

after i posted the answer, i realized that stability increases if the alkene is more substituted or has a greater number of double bonds while being a conjugated alkene. i looked it up in my orgo textbook and it happens to be true so yes d. would be the right answer. In essence, the triene would have lower energy than the diene because it is more stable and would thus have a lower catalytic hydrogenation value.
btw i'm just curious but where did you get this question from? it seems rather detailed.

i got this question from ochem practice exam in kaplan #1.
hey so more conjugated alkene means more double bond??
wouldn't choice d and e both have three double bonds?

 

[lintydent]

i got this question from ochem practice exam in kaplan #1.
hey so more conjugated alkene means more double bond??
wouldn't choice d and e both have three double bonds?

More conjugated alkene means more conjugated double bonds.
BUT ...Conjugated = multiple bonds that alternate with single bonds.
1,2,4 is not every other....not conjugated. 1,3,5 is every other...conjugated.

I'm sure there's a definition in your ochem book if you need detailed explaining.
You'll need to understand conjugation for aromatics.

 

[Notoriousjae]

i got this question from ochem practice exam in kaplan #1.
hey so more conjugated alkene means more double bond??
wouldn't choice d and e both have three double bonds?

conjugated alkene means the double bonds would have to be separated by a single bond. If you look at e. it would not be a conjugated triene because the double bonds would be adjacent to each other on the first and second carbon.
btw good explanation lintydent. you know your shiz 👍 😀