heat of hydrogenation

[Tina324]

how the heck is hydrognation exothermic?
you go from a double bond to a single bond.
isn't a double bond more stable than a single?..therefore producing an endothermic reaction?

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[Danny289]

how the heck is hydrognation exothermic?
you go from a double bond to a single bond.
isn't a double bond more stable than a single?..therefore producing an endothermic reaction?

you are breaking Pi bond then creating sigma bond.

 

[Chesnok]

I know it's confusing when you look at these aspects from gen.chem and orgo. chem. But for sure if orgo says it's exothermic ,it means that when you break higher-energy bond to make it single, energy is released
In gen chem it says that you need energy to break the double bond which is also true so it's endothermic . lol . It depends at what area you are asked for. IMO

 

[Tina324]

haha...well im thinking about it in terms of hybridization. an alkene has sp2 and an alkane has sp3..the alkene has more s character and therefore is more stable!...right? lol

 

[Contach]

you are breaking Pi bond then creating sigma bond.

No you aren't...

A double bond is a sigma and a pi
a single bond is a sigma only.
to hydrogenate something, you are only breaking the pi bond.

 

[Contach]

haha...well im thinking about it in terms of hybridization. an alkene has sp2 and an alkane has sp3..the alkene has more s character and therefore is more stable!...right? lol

you are getting your apples and oranges mixed up i think.

you are correct in that the bond of an alkene is stronger than the bond of an alkane. because an alkene is a pi and a sigma and an alkane is a sigma only. thus the bond order of an alkene is 2 and stronger, and of shorter length.

however. when we talk about the stability of the molecule, it goes beyond bond strength and involves molecule stability. Alkanes are more stable than alkenes. Therefore alkenes are higher on the PE diagram than alkanes, and when you break a bond, you are going from a high PE, to a low PE... therefore exothermic. but you are correct once again in that it will take energy to break that bond.. as illustrated by the activation of the reaction.

I wish this was MSN and I could draw a PE diagram.. but you will have to work it out yourself. (remember that things lower on the PE diagram are more stable).

This point is pretty important in understanding. Another application of this is comparing the relative energies of a benzene ring and a cyclohexane.
You may be tempted to say that the benzene ring is more stable than the cyclohexane, because the benzene bonds are stronger than the single bonds of cyclohexane. But this is infact wrong!!! Cyclohexane is more stable than Benzene.

Why cyclohexane is more stable than benzene, and why an alkane is more stable than an alkene is a question someone else can hopefully answer... Because I cannot begin to fathom what level chemistry is involved in deciphering entire-molecule stability.

Hope this helps!
ez,
contach.

 

[Tina324]

thanks everyone! still confused tho =)
i understand all that PE stuff, BUT what I don't understand is HOW an alkane is more stable than an alkene (according to hybridization and alkene is more stable. Is an alkane more stable because alkene's readily react and do elimination reactions?

also..what??? i was always taught that benzene is the MOST stable! dude. it's aromatic, how much more stable can u get?

 

[alanan84]

thanks everyone! still confused tho =)
i understand all that PE stuff, BUT what I don't understand is HOW an alkane is more stable than an alkene (according to hybridization and alkene is more stable. Is an alkane more stable because alkene's readily react and do elimination reactions?

also..what??? i was always taught that benzene is the MOST stable! dude. it's aromatic, how much more stable can u get?

Just think of what sort of reactions alkanes go through. You don't see alot of reactions with alkanes because they are relatively unreactive. The double bond in alkenes makes them able to go through more reactions. In fact, most of the reactions that alkenes go through, turns them into alkanes!

Also, be careful with benzene. It is unusually stable considering it has three double bonds but that doesn't make it the most stable. Any sort of alkane will be more stable.

 

[sony102]

No you aren't...

A double bond is a sigma and a pi
a single bond is a sigma only.
to hydrogenate something, you are only breaking the pi bond.

true, but aren't you also adding Hydrogen afterwards, (dafinition of Hydrogentation is adding Hydrogen) and making bonds is exothermic

http://www.cem.msu.edu/~reusch/VirtualText/addene2.htm

 

[Tina324]

OooOo I think i have it..(this might answer ur q contach)

Ok so "stability" can be looked through the lens of chemical reactivity and thermodynamics.

In regards to chemical reactivity, benzene is more stable than cyclohexane because of aromaticity (conjugation, huckel, planar, cyclic..all these things make benzene less reactive). BUTTT, thermodynamically benzene has more potential energy than cyclohexane because..hmm i dunno lol (because it has a double bond?)

Now talking abt heat of hydrogenation.. the more stable a compound is, the LESS exothermic it's heat of hydrogentation....so WHY IS BENZENE highly exothermic? I just did a q from kap that said "1,3,5 hexatriene has 3 double bonds in complete conjugation and therefore is not as exothermic as alkanes for instance, or non conjugated alkenes"

anyone have a clue?

 

[alanan84]

OooOo I think i have it..(this might answer ur q contach)

Ok so "stability" can be looked through the lens of chemical reactivity and thermodynamics.

In regards to chemical reactivity, benzene is more stable than cyclohexane because of aromaticity (conjugation, huckel, planar, cyclic..all these things make benzene less reactive). BUTTT, thermodynamically benzene has more potential energy than cyclohexane because..hmm i dunno lol (because it has a double bond?)

Now talking abt heat of hydrogenation.. the more stable a compound is, the LESS exothermic it's heat of hydrogentation....so WHY IS BENZENE highly exothermic? I just did a q from kap that said "1,3,5 hexatriene has 3 double bonds in complete conjugation and therefore is not as exothermic as alkanes for instance, or non conjugated alkenes"

anyone have a clue?

I don't think you can use heats of hydrogenation to compare two different molecules. It is only effective when comparing isomeric alkenes. So comparing benzene with cyclohexane is ineffective because one has 3 double bonds and the other has none.