AAMC chem qpack #19

[poochimaster]

So the answer is D, that reducing =O into -OH will increase its polarity.

The answer explanation was rather lackluster, so can someone explain to why this is the case?

I read somewhere that the dipole moment depends on charge and distance. Is it because single bonds are longer than double bonds, and thus the OH would have a stronger dipole moment than the =O, is this why?

thanks!

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[theonlytycrane]

One time on an ochem lab test, I got an answer wrong because I ended up having to choose between whether a ketone was more polar than an alcohol and I choose ketone. It really annoyed me and I may have never let that go...

Okay I admit this doesn't help answer the question. But maybe you'll remember that an alcohol is considered more polar because that random poster (me) told a silly story about his ochem lab test.

Okay I'm done

 

[aldol16]

So the answer is D, that reducing =O into -OH will increase its polarity.

The answer explanation was rather lackluster, so can someone explain to why this is the case?

I read somewhere that the dipole moment depends on charge and distance. Is it because single bonds are longer than double bonds, and thus the OH would have a stronger dipole moment than the =O, is this why?

thanks!

The answer explanation isn't that great. Whether a ketone or alcohol has a larger dipole moment depends on the situation and ketones are quite electrophilic - hence the vast array of carbonyl chemistries that are found in nature.

A better explanation is this. Think about how TLC works. It's basically a column but on a thin piece of cardboard. The stationary phase is silica and the mobile phase is your solvent. If something sticks well to the silica, it'll have a smaller Rf because it won't want to move with the mobile phase. So ask yourself: what sticks best to silica?

If you can answer that question correctly, then it becomes obvious why the answer is so bad. It's not merely because the compound is polar that it sticks to silica - it's that it hydrogen bonds. Anything that can hydrogen bond will love silica. So the alcohol will stick more tightly than the ketone. This will narrow your choices down to C and D. From there, you'll have to pick the "best" one (not necessarily the correct answer choice in absolute terms but the one that is closest), which is D.

 

[keepSwimmingNemo]

why is the answer A

 

[poochimaster]

The answer explanation isn't that great. Whether a ketone or alcohol has a larger dipole moment depends on the situation and ketones are quite electrophilic - hence the vast array of carbonyl chemistries that are found in nature.

A better explanation is this. Think about how TLC works. It's basically a column but on a thin piece of cardboard. The stationary phase is silica and the mobile phase is your solvent. If something sticks well to the silica, it'll have a smaller Rf because it won't want to move with the mobile phase. So ask yourself: what sticks best to silica?

If you can answer that question correctly, then it becomes obvious why the answer is so bad. It's not merely because the compound is polar that it sticks to silica - it's that it hydrogen bonds. Anything that can hydrogen bond will love silica. So the alcohol will stick more tightly than the ketone. This will narrow your choices down to C and D. From there, you'll have to pick the "best" one (not necessarily the correct answer choice in absolute terms but the one that is closest), which is D.

Ah, thank you for the answer! I hadn't thought about hydrogen bonding playing a role in allowing the alcohol to stick better to the silica.