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Topic: How to identify volatile molecules  (Read 375 times)

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Offline SeigiroSpica

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How to identify volatile molecules
« on: February 05, 2020, 09:39:50 PM »

(image: 52- This are pheromones of different insects.
a- Write it's name accordingly to the IUPAC
b- Pheromones are volatile compounds that enable communications between insects. What is the nature that make them volatile?)

In general, I have a problem to identfy volatile compounds.

I don't understand wich are the characteristics that make them more or less volatile.
I understand that volatile is the capacity that a sustance have to go to vapor. As it is more volatile it's easier to go to vapor with less energy.

¿What do I see in common in mor than 3 molecules?

All of them have a functional group ester.
All of them have double bond.
All of them have oxygen atoms with two pair of electron unpaired each.

I think that it can be volatile by nature of the double bond.
The Carbon that present double bond, generally are hybrized SP2. Meaning that they have a P orbital without hybridization, and form a bond π where the elecontr's pair that's there isn't limited sterically, and is easy to attack.

Meaning that isn't require much energy to break the bond.

But, I don't understand why they aren't triple bond instead of double, followin the logic that a triple bond would make a more volatile molecule.

My question is, what are the characteristics that make a compound more or less volatile and how can I identify it?

Sorry for my band english, I did my best, if you can't understand something I'll do my best to clarify it.

Offline rolnor

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Re: How to identify volatile molecules
« Reply #1 on: February 06, 2020, 05:38:34 AM »
What about size of the molecule, is that important? If you compare water and diethyl ether, why has diethyl ether lower boiling point?

Offline hollytara

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Re: How to identify volatile molecules
« Reply #2 on: February 06, 2020, 10:24:54 AM »
To be volatile, molecules of a compound have to be able to easily move from a condensed phase (liquid or solid) to the vapor phase.  What properties do they need to have - or not have?

Offline Babcock_Hall

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Re: How to identify volatile molecules
« Reply #3 on: February 06, 2020, 10:50:19 AM »
"Meaning that isn't require much energy to break the bond."  It is a common misconception that covalent bonds break when a molecule leaves the liquid phase and enters the gas phase.  It is the intermolecular interactions that are lost.

Offline Enthalpy

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Re: How to identify volatile molecules
« Reply #4 on: February 10, 2020, 11:40:33 AM »
¡Bienvenido, SeigiroSpica!

So the question boils down, if I dare to write, to:

1) How strong bonds are between two molecules, and
2) How many such bonds two molecules can make.

While nearly any two atoms can stick a little bit to an other, some subgroups of atoms, or "functions", make much stronger intermolecular forces, while just HC skeletons don't. Have you already learned some of these intermolecular forces, like for instance hydrogen bonds?

Because some bonds are much stronger than others, the kind of possible bonds by a given molecule matters more than the number of strong bonds, which matters more than the total number or mass of the atoms.

So based on the functions present in the pheromones, what kind of intermolecular bonds do you expect? Are these bonds strong?

Sidenote: this reasoning works pretty well for evaporation. There are even quantitative rules that, helped if needed by simple software, predict boiling points to typically 15K accuracy, as well as vapour pressure. But for melting points, which depend completely on packing lots of molecules tightly, every geometrical detail counts, and similar quantitative rules fail horribly, easily by >60K. Software exists for that too, you shall not trust its predicted melting points.

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