akalin24

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What has the LOWEST Delta Hf..

cyclopropane
cyclobutane
cyclopentane
cyclohexane

I think its cyclohexane.. least amount of ring strain..

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pmpndaflip

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Its cyclopropane...LOWEST delta H meaning most negative enthalpy or most exothermic...since G= H - TS the lower the delta H means lowest delta G...so which is most exothermic upon reaction and the one with the greatest ring straing is most exothermic. :thumbup:
 

ASDIC

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no...its cyclohexane!!!

check out this website from the Yale University Chemistry Dept:

http://classes.its.yale.edu/chem220a/studyaids/thermo/heats-formation.html#Cycloalkanes

cyclopropane ==> delta Hf is +12.7 kcal/mol
cyclobutane ===> delta Hf is +6.6
cyclopentane ==> -18.3
cyclohexane ===> -29.5

clearly, -29.5 is lower than 12.7. So the answer is cyclohexane due to decreased ring strain favoring a very negative exothermic reaction.
 
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liverotcod

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I remember asking this same question of my organic instructor when we were doing that topic, and made the same argument about negative numbers being smaller than positive numbers. But really, it's a trick question either way. The "lowest" in this case, according to him, is defined as the one that gives off the least heat, so cyclopropane. It really could go either way, depending on the exact phrasing of the question.
 

akalin24

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I found this..

Delta Hf values in (kJ/mol)

2 ethylene 52.2625
3 cyclopropane 53.0987
4 cyclobutane 28.4308
5 cyclopentane -76.9304
6 cyclohexane -123.3395
7 cycloheptane -117.9042
8 cyclooctane -124.1757
9 cyclononane -132.5377
10 cyclodecane -154.2789
 

liverotcod

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If the question was really the LOWEST delta Hf, then cyclohexane is correct. Aside from all of the previous discussion, it has more bonds than any of the others. It's just that the wording of the question is really important for this type of problem. A question that asked lowest delta Hf per carbon atom might have a difference answer, for example.
 
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