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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: confusedstud on February 24, 2013, 10:59:39 PM

Title: Are diols polar?
Post by: confusedstud on February 24, 2013, 10:59:39 PM: Are diols such as ethylene glycol polar? Because from diagrams the bond dipoles of each hydroxyl group seem to cancel each other out. Furthermore, shouldn't the carbon chains be able move around? So in longer diols how are we going to tell what are their net dipole moment?
Title: Re: Are diols polar?
Post by: argulor on February 24, 2013, 11:19:46 PM: Hydrogen bonding is the dominant force here. Also, remember that these are sp³ hybridized carbons and that there's constant rotation around the C-C bond. Unlike sp² hybridized carbons:
$C(=C/Cl)\Cl$ and $C(=C\Cl)\Cl$ .
Title: Re: Are diols polar?
Post by: confusedstud on February 25, 2013, 01:48:39 AM: Hydrogen bonding is the main dominant force, but what is the link between this and net dipole moment? Also, due to them being able to rotate shouldn't the net dipole moment always fluctuate?
Title: Re: Are diols polar?
Post by: argulor on February 25, 2013, 10:50:37 AM: Hydrogen bond donors and acceptors will almost always be active (at STP) regardless of symmetry.

Quote from: confusedstud on February 25, 2013, 01:48:39 AM
Hydrogen bonding is the main dominant force, but what is the link between this and net dipole moment? Also, due to them being able to rotate shouldn't the net dipole moment always fluctuate?

Yes, and essentially we measure the average dipole moment of this rotation.
Title: Re: Are diols polar?
Post by: confusedstud on February 25, 2013, 11:07:15 AM: Quote from: argulor on February 25, 2013, 10:50:37 AM
Hydrogen bond donors and acceptors will almost always be active (at STP) regardless of symmetry.

Quote from: confusedstud on February 25, 2013, 01:48:39 AM
Hydrogen bonding is the main dominant force, but what is the link between this and net dipole moment? Also, due to them being able to rotate shouldn't the net dipole moment always fluctuate?

Yes, and essentially we measure the average dipole moment of this rotation.

Oh I understand this better now. But what do you mean by "hydrogen bond donors and acceptors will almost always be active"? Also, since they are constantly fluctuating how is that we can measure them to be considered polar? Thanks for the help :)
Title: Re: Are diols polar?
Post by: opsomath on February 25, 2013, 03:53:12 PM: In any molecule with hydrogen bonding ability, forces due to the overall dipole moment can be almost completely ignored. What matters much more is local dipoles, ie, polarized bonds.
Title: Re: Are diols polar?
Post by: Babcock_Hall on February 25, 2013, 03:53:34 PM: Speaking only for myself, I don't use the word polarity to mean exactly the same thing as the size of the dipole moment. Polarity is a more multifaceted and complex concept. It is related to relative solubility in different solvents and mobility in silica chromatography, among other things. In my experience, the presence or absence of hydrogen bond donor groups on a molecule is a major contributor to its overall polarity.
Title: Re: Are diols polar?
Post by: confusedstud on February 25, 2013, 10:57:54 PM: Quote from: opsomath on February 25, 2013, 03:53:12 PM
In any molecule with hydrogen bonding ability, forces due to the overall dipole moment can be almost completely ignored. What matters much more is local dipoles, ie, polarized bonds.

does this include diketones too? Because if I had a polar solvent, say HCl and I were to dissolve my diol in this. So how do we explain that the diol is polar too so it can dissolve in a polar solvent like HCl?
Title: Re: Are diols polar?
Post by: Babcock_Hall on February 26, 2013, 09:37:40 AM: @OP, How are the intermolecular forces in a diketone like or unlike a diol?
Title: Re: Are diols polar?
Post by: confusedstud on February 26, 2013, 11:22:24 AM: The diketone is only capable of accepting hydrogen bonds while the diol is capable of donating and accepting hydrogen bonds.

Thanks for the help :)
Title: Re: Are diols polar?
Post by: Needaask on March 07, 2013, 04:01:47 AM: Quote from: Babcock_Hall on February 26, 2013, 09:37:40 AM
@OP, How are the intermolecular forces in a diketone like or unlike a diol?

hi i was going through the forum posts and this is also one of the chapters that i don't quite understand.

but in the solution since the maximum number of H bonds doesn't change so does it mean that diketone is infinitely soluble in HCl? I'm thinking since the number of H bonds doesn't change so the intermolecular interactions are the same so they should be infinitely soluble in each other?