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El_Cadejo's avatar

Question regarding the difference in vaporization of two like compounds.

Asked by El_Cadejo (34031 points ) September 24th, 2010

Not really sure how to phrase that question without it being a mile long sooo on to the details and explanation.

Ok so I have two compounds of a very similar nature but slightly different.

When compound A is vaporized it burns off clean leaving no residue at all.

When compound B is vaporized it burns off but leaves a small black residue behind.

What could one say about compound B that would be causing these residual resins? Would they be impurities in the compound or could it just be something like the molecular make-up of said compound that disables it from being broken down completely or maybe not enough heat to break all the bonds? I have a somewhat rudimentary understanding of chemistry so my suggestions could be way out in left field for all I know.

If it helps any, here are pictures of the molecular structures of compound A and compound B. From what I can gather , compound A has an extra benzene ring attached, while compound B lacks said ring but has an extra oxygen and hydrogen molecule on the end of it. The thing that I find confusing about this is that compound A is the one burnin off clean, if I had to guess I would have thought the other way around due to the extra carbon associated with it.

hmm this turned out to be a lot longer of a question than I previously anticipated lol

So yeaaaa anyone got any idea what about the molecular structure of compound B that would lead to this residue?

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5 Answers

FireMadeFlesh's avatar

If you had asked me which one would leave a residue, I would’ve intuitively gone with your answer and said A.

Given the results you have found though, the answer must lie in their ability to disperse in air. Considering compounds A and B have 24 and 20 carbons respectively, the extra oxygen B has would have little impact. It must be a result of how well each compound vaporises, and how well it can disperse with atmospheric oxygen prior to combustion. The extra benzene ring in compound A would give it a higher temperature of combustion, which would give it a little more time to disperse in the air and get greater access to atmospheric oxygen. Compound B would burn earlier, without being well dispersed, so combustion would be incomplete.

I hope that helps. Its been a long time since I studied chemistry.

El_Cadejo's avatar

Ah so it appears im not crazy in my guessing lol

so would a lower heat circumvent this with compound B ? Or is this just something that is inherent to its molecular make up?

I did also notice compound B vaporized much much faster compound A so you appear to be correct in that hypothesis.

also, what exactly would effect its ability to disperse in the air? Like how can one see this from looking at its structure? Or is this somethin your just guessing at?

FireMadeFlesh's avatar

I would expect every similar compound to have a curve that describes the relationship between ambient heat and the completeness of combustion. If you cool the ambient temperature, compound B should burn more completely and therefore leave less residue, but there would also be a minimum temperature at which it can be vaporised. It would depend on whether this minimum temperature is greater or less than the temperature at which it would undergo complete combustion. Maybe you could circumvent the problem by changing your vaporisation apparatus to make a finer mist, or you could raise the atmospheric oxygen levels (although that could be dangerous).

Zyx's avatar

I actually would have guessed B was the freaky one, but I might be chemistry-impaired. Is it at all relevant that A seems more symmetrical at the carbonyl bond?

El_Cadejo's avatar

@Zyx hell if i know. I really wanna ask my chem professor but i feel like he will be asking me then why i want to know about these compounds that are easily recognizable as indoles

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