Polarity: Alcohol and Ketone
Started by coolchix321
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No
OH is more polar than Ketone
it was an question/answer on AAMC 7
OH is more polar than Ketone
it was an question/answer on AAMC 7
Alcohols are more polar than ketones. Ketones don't have O-H bonding. Increasing the O-H bonds will increase not only the acidity but also the boiling points of alcohols compared to ketones. Remember the difference in the electronegativity of the O and H is greater than the difference between C and O of the carbonyl.
this is correct
also remember that more polar=higher boiling point but not necessarily higher melting point as well..
if you look at a trans alkene vs. cis alkene. The cis will have the higher boiling point because it is polar and the trans will have a higher melting point because it can stack better than the kinks that will result from cis alkenes.
for example, butter is solid at room temperature because its an unsaturated trans fat while oil is liquid at room temperature because its a cis unsaturated fat. at least i think thats correct...
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Obviously alcohol is more polar. Have you watched a droplet of acetone evaporate vs ethanol? The acetone evaporates much faster. Why? Less intermolecular forces.
after a carbonyl is reduced with a reducing agen (Nabh4 or LiAlh4), it is converted to an alcohol and the alcohol is more polar?
okay, fair enough with the H-bond argument, but I thought the carbonyl was more polar because it has "2" bonds to O whereas the Alcohol only has "1". Doesn't this, by definition, make the carbonyl more polar because the electrons that are in that bond are pulled more towards the oxygen and increase the polarity of the carbonyl in comparison to the OH.
okay, fair enough with the H-bond argument, but I thought the carbonyl was more polar because it has "2" bonds to O whereas the Alcohol only has "1". Doesn't this, by definition, make the carbonyl more polar because the electrons that are in that bond are pulled more towards the oxygen and increase the polarity of the carbonyl in comparison to the OH.
I've looked up dipole moments in the past, and the carbonyl ketone bond has a larger dipole moment than the alcohol O-R bond.
I think it might be an issue of a poorly phrased question and a virtue of how dipole moments are calculated. I think dipole moments are calculated using one molecule (disregarding any intermolecular interactions, which alcohols have big time). They didn't specify which case. They also didn't
specify along what bond they want you to consider the dipole, but it only makes sense to compare c-o to c-o.
To answer your question, memorize and go with aamc.
specify along what bond they want you to consider the dipole, but it only makes sense to compare c-o to c-o.
To answer your question, memorize and go with aamc.
For molecules with larger numbers of carbon atoms, alcohols being protic are more polar than ketones that are aprotic.
The complexity occurs when you have compounds with a smaller number of carbon atoms where shape is more influential.
For example...
iso-propanol (1.66 D)
acetone (2.69 D)
n-propanol (3.09 D)
All of the compounds have 3 carbons and one oxygen and one alcohol is more polar and one is less polar than the ketone.
With larger compounds, this effect is less pronounced and usually the protic substance is more polar than the aprotic substance.
That's logical, thanks for the explanation. Would you agree then that it's a poor question?
It is a poor question, if the question indeed asked "what is more polar, alcohol or ketone?" (or some derivation thereof)
For one, there is no one standard method of measuring polarity. People often use dipole moments, electronegativity differences, dielectric constants, etc. but none are perfect. For example, water is considered the standard polar solvent, but acetone has a higher dipole moment. Ethanol is generally considered more polar than acetone, but has dipole moment comparable to water and lower than acetone. Not to mention that dipole moments are measured in gas phase, whereas polarity really matters most for liquid phase.
For one, there is no one standard method of measuring polarity. People often use dipole moments, electronegativity differences, dielectric constants, etc. but none are perfect. For example, water is considered the standard polar solvent, but acetone has a higher dipole moment. Ethanol is generally considered more polar than acetone, but has dipole moment comparable to water and lower than acetone. Not to mention that dipole moments are measured in gas phase, whereas polarity really matters most for liquid phase.
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The dipole in the carbonyl group (C=O) is stronger than the dipole in the hydroxyl group (C-O-H) because in hydroxyl groups, the C-O opposes the dipole in the O-H. In organic reactions, the carbonyl carbon often acts as an electrophile (meaning it is fairly positive and attracts negative charge), while the oxygen becomes negative (sometimes acting as a nucleophile). It's a little less common to see reactions of alcohols where the oxygen acts as a nucleophile and the carbon acts as an electrophile. Also, remember that hydrogen bonding does not determine a single molecule's dipole moment.
