Just a small tip for y'all!!! if you already have a good background on these chapters like from your premed classes, watching these practice problems before the chapter helps a lot because you know where your gaps are. Also, Professor Eman has a link in the description about the notes written in Google docs format, so you practically don't have to retake notes. You can just use her notes to save you a lot of time and focus your time on the things you are struggling with more. Work smart, not harder!!!
Hi Eman!! thank you so much for these videos as they are saving me a month before my mcat. Quick question, for the last question, why is it eclipsed if it is 120 degrees apart? I understand why the answer is c based on process of elimination. Thank you again
Hi! I should’ve been more clear with my language so allow me to try to explain. The “partially eclipsed” conformations are the eclipsed conformations where the two large groups are eclipsing the small groups. They are eclipsed, but not with respect to each other. The dihedral angle between the two groups is 120°. The correct answer to the question about the methyl groups of butane being 120 degrees apart is that this molecule is in its middle-energy eclipsed form. When the methyl groups are 120 degrees apart in a Newman projection, they are in a partially eclipsed conformation. In this conformation, the groups are not directly aligned (0 degrees) but are still causing torsional strain due to partial alignment. This makes the energy higher than the anti conformation (180 degrees), where the groups are farthest apart, but lower than the fully eclipsed conformation. I hope this clarifies it! Feel free to ask if you have more questions!
Hello! Do you post the question slides without the correct answer highlighted? I would love to be able to work through the practice problems, and then watch the video explaining the correct answers. Thanks!
Around 10:10 when you discuss the diaxial methyl groups on the chair conformation of cyclohexane, why are they closer to the ring's hydrogens and thus confer less stability than if the methyl groups were on the equatorial position?
In the chair conformation of cyclohexane, the axial positions are those that are perpendicular to the plane of the ring, alternating up and down around the ring. Equatorial positions, on the other hand, are roughly in the plane of the ring, extending out to the sides. When you have a methyl group in the axial position on cyclohexane, it is closer to the hydrogen atoms on the adjacent carbons (especially those on carbons 1, 3, and 5 when looking at carbon 2, for example). This spatial proximity leads to steric hindrance, which is a type of strain due to repulsive forces between electron clouds of the axial methyl group and the axial hydrogens on the adjacent carbons. This repulsion is termed 1,3-diaxial interactions. In contrast, when methyl groups are in the equatorial position, they are farther away from the hydrogens on adjacent carbons, resulting in less steric hindrance because the groups are splayed out to the sides, away from the ring. This position allows for more space around the substituents, leading to a lower energy and more stable conformation. Therefore, cyclohexane with methyl groups in the equatorial positions is generally more stable than when the methyl groups are in the axial positions due to reduced steric hindrance and lower overall energy of the molecule.
quick question: in question 6 does relative configuration mean the configuration in the reactants side, and the absolute configuration is the configuration for the product?
Great question! Let’s break it down: 1. Relative configuration in this context refers to the comparison of the chiral centers before and after the reaction. If the stereochemistry at the chiral center relative to a reference group (often hydrogen or another chiral center in the molecule) remains unchanged, we say the relative configuration is retained. 2. Absolute configuration pertains to the actual R or S assignment of the chiral center before and after the reaction. If the assignment changes (from R to S , or vice versa), the absolute configuration has changed. In summary, in the context of question 6, relative configuration deals with how the stereocenter’s arrangement compares before and after the reaction, while absolute configuration deals with the specific R or S stereochemistry.
Meso compounds are achiral because they have a plane of symmetry and this will lead to a mirror image which is superimposable to the original molecule. The fastest method of determining if a compound is meso is to use these checkboxes: 1. Two (or more) actual stereocenters 2. But achiral due to a plane of symmetry 3. and "opposite" stereocenters
Hi! In organic chemistry, the symbols (+) and (-) indeed refer to a compound's optical activity. Specifically: (+): Indicates that a compound rotates plane-polarized light in a clockwise direction, or to the right. This is also known as dextrorotatory and can be represented by the prefix d- or D-. (-): Indicates that a compound rotates plane-polarized light in a counterclockwise direction, or to the left. This is also known as levorotatory and can be represented by the prefix l- or L-.
My check off list that I tell students to help them determine Meso compounds with two chiral centers is the following: 1. Two chiral centers 2. Reflectional symmetry 3. One has R/S configuration at chiral center 1 and 2 respectively and the other has S/R configuration at chiral center 1 and 2 respectively. If you were given enough information in a written problem that checks off this list, you might be able to get away with not drawing anything. I recommend drawing out structures though!
Enantiomers are pairs of non-superimposable mirror images, meaning they are distinct chiral molecules that rotate plane-polarized light in opposite directions. However, a meso compound, due to its internal symmetry, is superimposable on its mirror image. This superimposability means that meso compounds do not exist as pairs of enantiomers; instead, they are a single achiral molecule. Therefore, while meso compounds may have stereocenters, they are not enantiomers because they do not have non-superimposable mirror images.
Meso compounds criteria are : 1. 2 chiral centers 2. Reflectional symmetry and 3. One molecules having r/s and s/r. I go over this in more details if you watch my MCAT tutoring session I just uploaded as well
@@professoreman2289 Thank you for explaining. I am just a bit confused. I thought there was reflectional symmetry in these compounds, chiral centers and opposite R/S at each chiral center.
Hi, one has R/R at chiral center 1 and 2. The other molecule S/S at chiral center 1 and 2. For the trick I explained earlier, one would have to have R/S at chiral center 1 and 2. And the other molecule would have to be S/R. I do a similar problem to this more in depth in a recent video I put up (tutoring video) if you want more information. But I hope that helps a little more:)
@@professoreman2289 Hi professor Eman, could you please point me to which video explains this in more detail? And where I can find it? I am still struggling a little with grasping why these are not meso compounds. Thank you!
The way you explain concepts and reiterate them in such a digestible way is amazing! Thank you for all your hard work Professor Eman!
Wow, thank you! Your comment means a lot to me:)
Just a small tip for y'all!!! if you already have a good background on these chapters like from your premed classes, watching these practice problems before the chapter helps a lot because you know where your gaps are. Also, Professor Eman has a link in the description about the notes written in Google docs format, so you practically don't have to retake notes. You can just use her notes to save you a lot of time and focus your time on the things you are struggling with more.
Work smart, not harder!!!
Absolutely great tip!!!:)
Hi Eman!! thank you so much for these videos as they are saving me a month before my mcat. Quick question, for the last question, why is it eclipsed if it is 120 degrees apart? I understand why the answer is c based on process of elimination. Thank you again
Hi! I should’ve been more clear with my language so allow me to try to explain.
The “partially eclipsed” conformations are the eclipsed conformations where the two large groups are eclipsing the small groups. They are eclipsed, but not with respect to each other. The dihedral angle between the two groups is 120°.
The correct answer to the question about the methyl groups of butane being 120 degrees apart is that this molecule is in its middle-energy eclipsed form. When the methyl groups are 120 degrees apart in a Newman projection, they are in a partially eclipsed conformation. In this conformation, the groups are not directly aligned (0 degrees) but are still causing torsional strain due to partial alignment. This makes the energy higher than the anti conformation (180 degrees), where the groups are farthest apart, but lower than the fully eclipsed conformation.
I hope this clarifies it!
Feel free to ask if you have more questions!
As always! Thank you so much for the video!
You are very welcome! Hope studying is going well!
Hello! Do you post the question slides without the correct answer highlighted? I would love to be able to work through the practice problems, and then watch the video explaining the correct answers. Thanks!
Hey, I’ve done that for the most recent MCAT content and plan to do it for this subject soon as I’m remaking it completely.
Around 10:10 when you discuss the diaxial methyl groups on the chair conformation of cyclohexane, why are they closer to the ring's hydrogens and thus confer less stability than if the methyl groups were on the equatorial position?
In the chair conformation of cyclohexane, the axial positions are those that are perpendicular to the plane of the ring, alternating up and down around the ring. Equatorial positions, on the other hand, are roughly in the plane of the ring, extending out to the sides.
When you have a methyl group in the axial position on cyclohexane, it is closer to the hydrogen atoms on the adjacent carbons (especially those on carbons 1, 3, and 5 when looking at carbon 2, for example). This spatial proximity leads to steric hindrance, which is a type of strain due to repulsive forces between electron clouds of the axial methyl group and the axial hydrogens on the adjacent carbons. This repulsion is termed 1,3-diaxial interactions.
In contrast, when methyl groups are in the equatorial position, they are farther away from the hydrogens on adjacent carbons, resulting in less steric hindrance because the groups are splayed out to the sides, away from the ring. This position allows for more space around the substituents, leading to a lower energy and more stable conformation.
Therefore, cyclohexane with methyl groups in the equatorial positions is generally more stable than when the methyl groups are in the axial positions due to reduced steric hindrance and lower overall energy of the molecule.
quick question: in question 6 does relative configuration mean the configuration in the reactants side, and the absolute configuration is the configuration for the product?
Great question! Let’s break it down:
1. Relative configuration in this context refers to the comparison of the chiral centers before and after the reaction. If the stereochemistry at the chiral center relative to a reference group (often hydrogen or another chiral center in the molecule) remains unchanged, we say the relative configuration is retained.
2. Absolute configuration pertains to the actual R or S assignment of the chiral center before and after the reaction. If the assignment changes (from R to S , or vice versa), the absolute configuration has changed.
In summary, in the context of question 6, relative configuration deals with how the stereocenter’s arrangement compares before and after the reaction, while absolute configuration deals with the specific R or S stereochemistry.
@@professoreman2289 thank you so much!
You’re so welcome! Lmk if you have any more questions :)
Thank you so much for your videos! Are meso compunds always identical/ the same molecule?
Meso compounds are achiral because they have a plane of symmetry and this will lead to a mirror image which is superimposable to the original molecule.
The fastest method of determining if a compound is meso is to use these checkboxes:
1. Two (or more) actual stereocenters
2. But achiral due to a plane of symmetry 3. and "opposite" stereocenters
for question 8 at 18:41, does the (+) / (-) just refer to its optical activity?
Hi! In organic chemistry, the symbols (+) and (-) indeed refer to a compound's optical activity.
Specifically:
(+): Indicates that a compound rotates plane-polarized light in a clockwise direction, or to the right. This is also known as dextrorotatory and can be represented by the prefix d- or D-.
(-): Indicates that a compound rotates plane-polarized light in a counterclockwise direction, or to the left. This is also known as levorotatory and can be represented by the prefix l- or L-.
I thought diastereomers have to have 2 chiral centers. But cis-2-butene and trans-2-butenes are not chiral (they have double bonds)
Cis and trans isomers can be considered a type of diastereomers under certain contexts.
Question 11 @ 22:30 ish. Is it possible to determine if a compound is meso without drawing it?
My check off list that I tell students to help them determine Meso compounds with two chiral centers is the following:
1. Two chiral centers
2. Reflectional symmetry
3. One has R/S configuration at chiral center 1 and 2 respectively and the other has S/R configuration at chiral center 1 and 2 respectively.
If you were given enough information in a written problem that checks off this list, you might be able to get away with not drawing anything.
I recommend drawing out structures though!
for number 14: I was under the impression that if the configurations are opposite they would be enantiomers, how come that is not an answer?
Enantiomers are pairs of non-superimposable mirror images, meaning they are distinct chiral molecules that rotate plane-polarized light in opposite directions. However, a meso compound, due to its internal symmetry, is superimposable on its mirror image. This superimposability means that meso compounds do not exist as pairs of enantiomers; instead, they are a single achiral molecule. Therefore, while meso compounds may have stereocenters, they are not enantiomers because they do not have non-superimposable mirror images.
17:53 why are the molecules in question 7 not meso compounds
Meso compounds criteria are : 1. 2 chiral centers 2. Reflectional symmetry and 3. One molecules having r/s and s/r. I go over this in more details if you watch my MCAT tutoring session I just uploaded as well
@@professoreman2289 Thank you for explaining. I am just a bit confused. I thought there was reflectional symmetry in these compounds, chiral centers and opposite R/S at each chiral center.
Hi, one has R/R at chiral center 1 and 2. The other molecule S/S at chiral center 1 and 2. For the trick I explained earlier, one would have to have R/S at chiral center 1 and 2. And the other molecule would have to be S/R. I do a similar problem to this more in depth in a recent video I put up (tutoring video) if you want more information. But I hope that helps a little more:)
@@professoreman2289 Hi professor Eman, could you please point me to which video explains this in more detail? And where I can find it? I am still struggling a little with grasping why these are not meso compounds. Thank you!