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How an O-H increases boiling point.

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Hi, I'm curious to know why adding an O-H to the end of a molecule can, or will, increase the boiling point of the molecule. How can this happen? What goes on when an alcohol group is added?

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Kind of, it was in a presentation from my college. They provided a 'rough' definition of the reason why it increases, which I can't remember for the sakes of me, so I figured to ask the people here. So its not necessarily homework, but it's out of curiosity.

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Intermolecular forces is a force between many molecules, it's much weaker than intramolecular. Electrostatic interaction is caused by atoms/molecules that have are attracted to opposite charges. I know that electrostatic interaction is the cause of ions interacting with each other since they can have the same or opposite charge.

Please correct me if I'm wrong anywhere.

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19 hours ago, Questionasker said:

Electrostatic interaction is caused by atoms/molecules that have are attracted to opposite charges. I know that electrostatic interaction is the cause of ions interacting with each other since they can have the same or opposite charge.

 

They don't necessarily need to be charged, they can be partially electron deficient / electron rich. Do you know what effect the strength and extent of these interactions might have on things like boiling point? 

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I think I have a broad understanding, so if atoms are electrostatically pulled together then it will mean that it would require more energy to break their bonds. This could apply to molecules too.

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Yes, more or less. So with an alcohol, can you identify what sort of intermolecular interactions might exist? Hydrogen bonding, London dispersion, dipole, Van der Waals, etc.? 

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Posted (edited)

With an alcohol, i believe, intermolecular forces will exist since O-H has both a slightly negative and positive charge. The electrons of the O-H are unevenly distributed since Oxygen has a higher electronegativity than hydrogen, therefore pulling the electron towards it. This will then cause a Dipole dipole effect when there are different charges at opposite end of the alcohol. However, this happens for a very short amount of time since electrons move incredibly fast. So due to the Van der waals effect, the alcohol will have a temporary dipole effect which means that it can bond to other molecules, but I can't imagine that it'll be strong. So going by what I mentioned, If it's correct, the hydrogen should be oscillating between neutral and having a slighy positive charge. This must mean that it can covalently bond the hydrocarbon, such as ethane, to another ethane molecule.

 

 

Edited by Questionasker
Needed correction

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3 hours ago, Questionasker said:

With an alcohol, i believe, intermolecular forces will exist since O-H has both a slightly negative and positive charge. The electrons of the O-H are unevenly distributed since Oxygen has a higher electronegativity than hydrogen, therefore pulling the electron towards it. This will then cause a Dipole dipole effect when there are different charges at opposite end of the alcohol. However, this happens for a very short amount of time since electrons move incredibly fast. So due to the Van der waals effect, the alcohol will have a temporary dipole effect which means that it can bond to other molecules, but I can't imagine that it'll be strong. So going by what I mentioned, If it's correct, the hydrogen should be oscillating between neutral and having a slighy positive charge. This must mean that it can covalently bond the hydrocarbon, such as ethane, to another ethane molecule.

 

 

 

On average, the OH bond will be polarised. In fact, it can participate in a particular type of dipole interaction called hydrogen bonding. As with all electrostatic interactions, it is not overly strong when you compare it to covalent bond strengths, but hydrogen bonds are considered quite a bit stronger than other intermolecular attractive forces. I have bolded a sentence that is not quite right. You won't covalently bond two ethane molecules together. You will get Van der Waals (also London dispersion forces) between molecules of ethane, but that's about it. They are very weak interactions. You will also get Van der Waals interactions with alcohol molecules. When you compare the two, you have (for example) ethanol, which has hydrogen bonding and Van der Waals interactions, and ethane, which only contains Van der Waals interactions. As you noted above, the stronger the interactions and the more of them that there are, the more energy it requires to break intermolecular bonds and pull molecules apart, which translates to a higher boiling point. As such, you would reasonably expect ethanol to have a higher boiling point to ethane. Another interesting comparison is between diethyl ether, acetic acid, ethanol and ethane. Can you predict the order of boiling points between those molecules from highest to lowest? 

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Hi, sorry for the late reply.

Just by looking at what they are, keep in mind that i'm only going to predict the boiling points. I actually have never heard of 'Diethyl ether' and 'Acetic acid' I predict it will go:

* Diethyl ether

* Acetic acid

*Ethanol

* Ethane

 

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16 hours ago, Questionasker said:

Hi, sorry for the late reply.

Just by looking at what they are, keep in mind that i'm only going to predict the boiling points. I actually have never heard of 'Diethyl ether' and 'Acetic acid' I predict it will go:

* Diethyl ether

* Acetic acid

*Ethanol

* Ethane

 

 

Not quite. Three of these molecules can undergo hydrogen bonding. One cannot. Since they are close in terms of molecular size, you can rank them roughly by looking at the number of potential hydrogen bonds they can form to other molecules. Acetic acid has two oxygens that can both hydrogen bond, and a polar hydrogen that can form hydrogen bonds. Ethanol has one oxygen and a polar hydrogen, and diethyl ether only has an oxygen. Ethane has nothing that can hydrogen bond. Hence, you might predict that acetic acid would have the highest boiling point, since it has the highest number of potential hydrogen bond sites, followed by ethanol, diethyl ether, and then ethane. 

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Thanks for the correction, but how can we know that different molecules can form hydrogen bonds with themselves?

 

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Hydrogen bonding requires a polar atom with a lone pair of electrons, such as nitrogen and oxygen, and an electropositive hydrogen in a polar bond, such as -OH hydrogens and -NH hydrogens. So, the ether can't actually H-bond to itself because it doesn't meet the second requirement, but the alcohol and carboxylic acids both can. 

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