Help me answer my burning questions!

[dhb10]

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Hi,

I'm preparing to take my mcat in a few weeks and I have had some orgo/gen chem questions that I would like to get answered before my test day (I haven't had anyone to ask).

Please help me by answering one or more of these questions:

1. For sugars and amino acids, does the D configuration always mean that the compound rotates light clockwise? (I know D doesn't always signify the R stereochemistry but I'm just confused about the correlation of +/- rotation and D/L configuration)

2. For compounds with aromatic rings attached (ex: Diphenylmethanol - methanol with two benzene rings attached) - in this case, if the OH were protonated and acted as a leaving group resulting in a carbocation, will the rings make the C+ more stable? I'm confused because in compounds like phenol (benzene with OH directly attached), the aromatic ring draws electron density from the OH making the bond weaker and more acidic. Based on that logic, I would think that the rings in diphenylmethanol would draw more electron density away from the C+ making it MORE positive and unstable but my books says it will stabilize it.

3. For atomic orbitals, are electrons at a higher orbital at a higher energy or lower energy? In the Berkley review, it states that the energy of an orbital is proportional to 1/n (n=orbital number). This would make higher orbitals at a lower energy. But I always assumed higher orbitals were more unstable and more reactive. For example, in biology class we learn that photosynthesis pushes electrons to a higher energy level (failing to fill a lower energy level first like Hund's rule predicts).

4. Can meso compounds have unever chiral centers? A problem in the Berkley review orgo set seemed to suggest a 5 carbon sugar that was the configuration R, R, S and R,S,S for carbons 2,3, and 4 respectively were meso regardless of the configuration of the middle carbon (carbon 3). I always thought I learned that meso compounds have to have an even number of chiral centers.

I appreciate any response even if it doesn't answer all the questions!

 

[monkeyvokes]

2. For compounds with aromatic rings attached (ex: Diphenylmethanol - methanol with two benzene rings attached) - in this case, if the OH were protonated and acted as a leaving group resulting in a carbocation, will the rings make the C+ more stable? I'm confused because in compounds like phenol (benzene with OH directly attached), the aromatic ring draws electron density from the OH making the bond weaker and more acidic. Based on that logic, I would think that the rings in diphenylmethanol would draw more electron density away from the C+ making it MORE positive and unstable but my books says it will stabilize it.

for phenol, one of the lone pairs on oxygen can resonate into the ring. Thus, electron density is taken away from oxygen and the positive proton is less likely to stick around.
--Removing electron density from an acidic proton increases its acidity.

for the phenylic carbocation, you have a source of electrons to stabilize the formation of a positive charge. If electron density is delocalized around a positive charge, then the positive charge isn't so unfavorable.
--Adding electron density to a carbocation stabilizes it (same principle for why tertiary is favorable over primary)

 

[monkeyvokes]

1. For sugars and amino acids, does the D configuration always mean that the compound rotates light clockwise? (I know D doesn't always signify the R stereochemistry but I'm just confused about the correlation of +/- rotation and D/L configuration)

Correct me if I'm wrong, but I don't believe there is any optic correlation to S, R, D, or L

 

[monkeyvokes]

4. Can meso compounds have unever chiral centers? A problem in the Berkley review orgo set seemed to suggest a 5 carbon sugar that was the configuration R, R, S and R,S,S for carbons 2,3, and 4 respectively were meso regardless of the configuration of the middle carbon (carbon 3). I always thought I learned that meso compounds have to have an even number of chiral centers.

Wasn't sure about #4 so I googled it and found this: http://www.physicsforums.com/showthread.php?t=536103

tldr: yes 2n+1 chiral centers, but meso compounds require 2n stereocenters. e.g.: (1R, 2S,4r)-1,2,4-cyclopentanetriol

great questions btw

 

[dhb10]

Wasn't sure about #4 so I googled it and found this: http://www.physicsforums.com/showthread.php?t=536103

tldr: yes 2n+1 chiral centers, but meso compounds require 2n stereocenters. e.g.: (1R, 2S,4r)-1,2,4-cyclopentanetriol

great questions btw

Hmm alright, I think I understand now. I actually think you meant to say that all meso compounds have 2n CHIRAL centers (Rs and Ss) but can have 2n + 1 STEREOCENTERS. In your example of 1,2,4-cyclopentanetriol, only carbons 1 and 2 are chiral centers (have four DIFFERENT groups attached - because carbon 4 has two symmetrical groups). THUS, it is named 1R and 2S but carbon 4 does NOT have R/S configuration because it is NOT a chiral center. HOWEVER, Carbon 4 is a stereocenter because if you swap the OH group and the H (making the OH go into or out of the page), you get a different compound. THUS, the molecule has 2 chiral centers (aka 2 R or S carbons) and 1 extra stereocenter.

Also, I found this online about D/L configurations: http://chemistry.umeche.maine.edu/CHY251/dlwrong.html

It seems like D molecule BY DEFINITION rotate plane polarized light + (clockwise) and L vise versa. However, D/L has no correlation with R/S stereocenters (even though more often that not, D sugars have R stereochemistry at the last chiral center in a sugar for example).

Thanks a lot for your help on these questions. If anyone else can help me specifically with question #3, I would greatly appreciate it.

 

[monkeyvokes]

Hmm alright, I think I understand now. I actually think you meant to say that all meso compounds have 2n CHIRAL centers (Rs and Ss) but can have 2n + 1 STEREOCENTERS. In your example of 1,2,4-cyclopentanetriol, only carbons 1 and 2 are chiral centers (have four DIFFERENT groups attached - because carbon 4 has two symmetrical groups). THUS, it is named 1R and 2S but carbon 4 does NOT have R/S configuration because it is NOT a chiral center. HOWEVER, Carbon 4 is a stereocenter because if you swap the OH group and the H (making the OH go into or out of the page), you get a different compound. THUS, the molecule has 2 chiral centers (aka 2 R or S carbons) and 1 extra stereocenter.

Also, I found this online about D/L configurations: http://chemistry.umeche.maine.edu/CHY251/dlwrong.html

It seems like D molecule BY DEFINITION rotate plane polarized light + (clockwise) and L vise versa. However, D/L has no correlation with R/S stereocenters (even though more often that not, D sugars have R stereochemistry at the last chiral center in a sugar for example).

Thanks a lot for your help on these questions. If anyone else can help me specifically with question #3, I would greatly appreciate it.

"you have to be careful with how you through around the terms "stereocenter" and "chiral center." If your instructor said "A meso compound cannot have 2n+1 number of CHIRAL centers," then they are correct. It would be incorrect to say "a meso compound that contains STEREOcenters must have 2n stereocenters." In other words, these terms do not have the same meaning. All chiral centers are stereocenters, and not all stereocenters are chiral centers. the term "stereocenter" or "stereogenic center" means that swapping two groups on that center would give a different stereoisomer, whereas "chiral center" means that the atom doesn't occupy a molecular plane of symmetry (4 different substituents bonded to the atom). "

 

[nOchemallday]

"you have to be careful with how you through around the terms "stereocenter" and "chiral center." If your instructor said "A meso compound cannot have 2n+1 number of CHIRAL centers," then they are correct. It would be incorrect to say "a meso compound that contains STEREOcenters must have 2n stereocenters." In other words, these terms do not have the same meaning. All chiral centers are stereocenters, and not all stereocenters are chiral centers. the term "stereocenter" or "stereogenic center" means that swapping two groups on that center would give a different stereoisomer, whereas "chiral center" means that the atom doesn't occupy a molecular plane of symmetry (4 different substituents bonded to the atom). "

IDK why you plagarized a quote from another forums, but OP clearly understands the difference between stereocenters and chiral centers. It must have 2n chiral centers; but frankly may have up to 2 (potentially even more) stereocenters in addition to the 2n chiral centers.
[Meso compound containing 2chiral AND 2 stereo centers: (1R,3S) -1,2,3,4-tetrahydroxyl-2-methylcyclobutane]

In any case, you are correct in saying that all meso compounds must have an even number of CHIRAL centers. And as for TBR suggesting that the stereocenter can be R or S, that is also true. (but to my understanding it is denoted lower case 'r' or 's' indicating that it is not a true chiral center) As long as the 2 chiral centers have opposite (1R and 1S) carbons, the stereocenter can be facing either direction.

3. For atomic orbitals, are electrons at a higher orbital at a higher energy or lower energy? In the Berkley review, it states that the energy of an orbital is proportional to 1/n (n=orbital number). This would make higher orbitals at a lower energy. But I always assumed higher orbitals were more unstable and more reactive. For example, in biology class we learn that photosynthesis pushes electrons to a higher energy level (failing to fill a lower energy level first like Hund's rule predicts).

Electrons in a higher orbital are at a lower energy.
This is easily identified by the trend in ionization energy by periods. If we look at the noble gases of the first 3 periods (first 3 rows), the ionization energy, or energy required to remove an electron from the valence shell, decreases. For instance, Helium has a higher ionization energy than Neon, which has a higher ionization energy than Argon. This is because the valence shell of Helium is n=1 (1s^2), Neon n=2 (2p^6), Argon n=3 (3p^6). So we can see that the energy "holding" the electron is lower in each orbital, because it requires less energy to remove it.

We can also readily see this with the radius of each orbital being further from the nucleus. Further away means less energy to hold those electrons in orbit.

When you reference photosynthesis, we are talking not about different orbitals, but excited vs ground state. We say that an atom that absorbs a photon enters an "excited state" of higher energy than the "ground state". This is like going from the 2s^2 orbital to the 2p^1 orbital. But we are still in the same n=2 orbital.

Hope this helps.

 

[monkeyvokes]

IDK why you plagarized a quote from another forums, .

quote was from the website i linked in my previous reply.
and yeah i see that i had accidentally typed the information backwards, my mistake

 

[dhb10]

1

 

[dhb10]

Hey, thanks a lot for your help. This makes tons of sense!