Question about Official Guide Questions (Spoiler)

[artist27]

Question 12 from the C/P section.
This is probably a pretty simple question, but I am confused about the classification of polar vs nonpolar substances. Are amides and ethers always considered polar?

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[theonlytycrane]

They are actually non-polar in this example.

The crown ether (big circular R-O-R) structure is non-polar. The Y group (amide) is non-polar because of the long carbon chain attached to it.

The X group is negatively charged when deprotonated and is the polar group. This is what helps to transiently associate with the cations for transport.

 

[artist27]

They are actually non-polar in this example.

The crown ether (big circular R-O-R) structure is non-polar. The Y group (amide) is non-polar because of the long carbon chain attached to it.

The X group is negatively charged when deprotonated and is the polar group. This is what helps to transiently associate with the cations for transport.

Whoops sorry my question was supposed to be "are they always considered nonpolar." I thought crown ethers were polar because of the electronegativity difference between the O and C. But I guess if the structure is big enough, that difference becomes negligible. Is that accurate?

 

[theonlytycrane]

There is an electronegativity difference, but ethers are usually non-polar because the dipoles cancel one another out in the R-O-R structure.

For polar groups, look for things like COOH that can lose a proton.

Also, in this context, the crown-ether is helping cations cross a membrane. The membrane is non-polar and that's where the crown ether is, so it must be non-polar. But it's helping cations move so part of it must be polar as well.