Chemistry Newman projections of meso-2,3-butanediol

[Physics lover]

Homework Statement

Which of the following are Newman projections of meso-2,3-butanediol

Relevant Equations

Newman projection
Fischer projection

The question and options are-:

Ok so what I did was that i converted all of them to their fischer projection.But only P gave me meso-2,3-butanediol.Others were not meso.Please help.

 

[TeethWhitener]

Homework Statement:: Which of the following are Newman projections of meso-2,3-butanediol
Relevant Equations:: Newman projection
Fischer projection

Ok so what I did was that i converted all of them to their fischer projection.But only P gave me meso-2,3-butanediol.Others were not meso.Please help.

This is incorrect, but we'd need to see your work to determine why.

Start by explaining in your own words 1) what a meso compound is, and 2) how to determine whether a compound is meso.

 

[Physics lover]

This is incorrect, but we'd need to see your work to determine why.

Start by explaining in your own words 1) what a meso compound is, and 2) how to determine whether a compound is meso.

ok
As per my knowledge a meso compound is one that has a chiral centre as well as a plane of symmetry or axis of symmetry.
So if a molecule has any kind of symmetry like Plane,axis or centre,it is regarded as meso.

 

[TeethWhitener]

ok
As per my knowledge a meso compound is one that has a chiral centre as well as a plane of symmetry or axis of symmetry.
So if a molecule has any kind of symmetry like Plane,axis or centre,it is regarded as meso.

This isn’t quite correct. A meso compound has at least two chiral centers. Your issue in the OP seems to be that you haven’t tried rotating the two centers about their bond. Try drawing out the different configurations of each compound (P, Q, R, S) in a Newman projection.

 

[sysprog]

ok
As per my knowledge a meso compound is one that has a chiral centre as well as a plane of symmetry or axis of symmetry.
So if a molecule has any kind of symmetry like Plane,axis or centre,it is regarded as meso.

How do you decide whether a chiral molecule has also a non-chiral symmetry? A chiralic enantiomer has a resemblance that may loosely be called a symmetry, but it's not really immediately symmetrical -- hint: your hands 'mirror' each other, and if you put them palm to palm and finger to finger, there's a line of symmetry between them $\dots$

 

[Physics lover]

This isn’t quite correct. A meso compound has at least two chiral centers. Your issue in the OP seems to be that you haven’t tried rotating the two centers about their bond. Try drawing out the different configurations of each compound (P, Q, R, S) in a Newman projection.

I actually don't know much about Newman projections.I think If I rotate the molecule by 60° or 120° or any other multiple of 60° the configuration will remain the same.Am i right?

 

[sysprog]

I actually don't know much about Newman projections.I think If I rotate the molecule by 60° or 120° or any other multiple of 60° the configuration will remain the same.Am i right?

Another hint: the word 'surgery' is a contractional reformation of 'chirurgery' -- sometimes which dimension you rotate something in is important -- not by itself as important always is by how many degrees -- I understand people just wanting the answer -- please understand that here on PF part of the mission is that the maestros want the learning -- how else will you become one of the new maestros?

 

[TeethWhitener]

I actually don't know much about Newman projections.I think If I rotate the molecule by 60° or 120° or any other multiple of 60° the configuration will remain the same.Am i right?

Newman projections are drawings of a molecule from the perspective of looking straight down a bond axis. Google provides a number of decent pages with primers on how to draw Newman projections.

I noticed I used “configuration” above when I should have used “conformation.” The conformation is just the arrangement of groups in space relative to one another, without necessarily breaking any bonds. So in a Newman projection, you can rotate either of the axis atoms’ groups by however much you want and you won’t change the compound. With that in mind, try to draw out trans, gauche, staggered, and eclipsed conformations for each compound in OP.

 

[chemisttree]

Rotate the front group and try to line up the methyls or the hydroxyls. You will see...

 

[Physics lover]

Rotate the front group and try to line up the methyls or the hydroxyls. You will see...

I just wanted to know that can i rotate about a single carbon at a time or i can rotate both of them at a time.I think it's about single at a time.

 

[chemisttree]

Think of it as a propeller.

 

[TeethWhitener]

I just wanted to know that can i rotate about a single carbon at a time or i can rotate both of them at a time.I think it's about single at a time.

You can do either.

 

[Physics lover]

You can do either.

ok thanks i got my answer as P and R.I drew the structure of P from Fischer and then rotated it to get R.

 

[TeethWhitener]

Good work. Incidentally, when you only have two stereocenters, there's another quick and dirty way to solve this problem, provided that you know how to assign R and S absolute configurations: A meso compound will always be (R,S) or (S,R), whereas the chiral stereoisomers will be (R,R) or (S,S).

 

[Physics lover]

Good work. Incidentally, when you only have two stereocenters, there's another quick and dirty way to solve this problem, provided that you know how to assign R and S absolute configurations: A meso compound will always be (R,S) or (S,R), whereas the chiral stereoisomers will be (R,R) or (S,S).

ok But for that i would have to convert the Newman to any other projection as I cannot assign R and S to a Newman projection,Can I?

 

[chemisttree]

Of course you can. You need to visualize the chiral carbon center in question and manipulate it in your mind. Molecular models are very helpful support for this.