Polarities of alcohol and carbonyl groups

[Tokspor]

---

Members do not see this ad.

Why would an alcohol group placed on a hydrocarbon be of greater polarity than a carbonyl group on the same position? I thought the polarity of the carbonyl was what made it so susceptible to nucleophilic attacks.

 

[eric51]

I think you might be misreading the question. They are probably talking about the polarity of the bond that is at the end of the hydrocarbon. In that case, the alcohol would have the terminal hydrocarbon carbon bonded to an oxygen (a quite polar bond) while the carbonyl would have it connected to another carbon (not very polar). So while the actual carbonyl group is polar, they are not asking about the group polarity, but rather of the polarity of the bond *to* the group.

I could be wrong of course. What exactly is the question you are having trouble with?

 

[Tokspor]

I think you might be misreading the question. They are probably talking about the polarity of the bond that is at the end of the hydrocarbon. In that case, the alcohol would have the terminal hydrocarbon carbon bonded to an oxygen (a quite polar bond) while the carbonyl would have it connected to another carbon (not very polar). So while the actual carbonyl group is polar, they are not asking about the group polarity, but rather of the polarity of the bond *to* the group.

I could be wrong of course. What exactly is the question you are having trouble with?

The question is from AAMC 7. There's a figure of two similar molecules which are different at just one position--whereas one of them has an alcohol group, the other has a carbonyl. The question is as follows:

A student uses thin layer chromatography on silica plates to monitor the progress of the reaction below. Does the product have a higher or lower Rf than the starting material?

A) Higher, because the product is more polar than the starting material
B) Higher, because the product is less polar than the starting material
C) Lower, because the product is less polar than the starting material
D) Lower, because the product is more polar than the starting material

The answer is D. Why does the -OH contribute more to the polarity of the molecule than an O double-bonded to a carbon would?

 

[Isoprop]

The alcohol can H-bond to the silica gel better. It has both a hydrogen bond donor and a hydrogen bond acceptor. The carbonyl has only a hydrogen bond receiver.

You should memorize this order of polarities:

acids > alcohols > carbonyls> amines > ethers > alkanes

 

[medfromeng]

The alcohol can H-bond to the silica gel better. It has both a hydrogen bond donor and a hydrogen bond acceptor. The carbonyl has only a hydrogen bond receiver.

You should memorize this order of polarities:

acids > alcohols > carbonyls> amines > ethers > alkanes

An old thread, but it ranks highly in search so I'll correct this.

The OP's question referred to Rf / TLC - _not_ polarity specifically. H-bonding is very significant in interaction with the TLC's stationary phase. Polarity also affects this. Alcohols will have a lower Rf because they have both polarity AND Hydrogen bond donor AND acceptor.

alcohols in general are NOT more polar than carbonyls/ketones. Consider isopropyl alcohol (dipole~=1.6) versus the similarly structured acetone (dipole~=2.8) - the ketone is unambiguously MORE polar than the alcohol.

 

[jhmmd]

Why would an alcohol group placed on a hydrocarbon be of greater polarity than a carbonyl group on the same position? I thought the polarity of the carbonyl was what made it so susceptible to nucleophilic attacks.

That's not key here. It's the greater tendency of the negative charge. Does that make sense?