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Meso compound question
Started by virtualmaster999
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I'm sure either feralis or dr. Romano will come in and answer this better than me. So I will let you look at this link in case you can't wait for them to come.
http://organicchemistrysolutions.co...ule-and-a-tricky-term-called-the-stereo-unit/
http://organicchemistrysolutions.co...ule-and-a-tricky-term-called-the-stereo-unit/
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In order to be a meso compound you need a chiral center. Answer choices A and C do not have that so they are out. Choice D is not a meso compound because the Bromine's are attached differently (one on a wedge and one on a dash) hence it has no plane of symmetry. Answer choice B has free rotation about the single bond, so both C-F bonds are wedges or dashes depending on how you turn the molecule in your head and the centers are chiral. Hence you can draw a plane of symmetry right down the middle and it's a meso compound!!
Also, I attached an example of a little trick you can use to rotate the molecule between the two different representations.
It's also important to know why you can do this. At any time with a planar substituent on an sp3 carbon, 2 of the bonds attached to the carbon will be "in the plane," 1 will be Wedge and the other will be Dash, so you are not actually changing the molecule by doing this, you're just making it easier to compare.
It's also important to know why you can do this. At any time with a planar substituent on an sp3 carbon, 2 of the bonds attached to the carbon will be "in the plane," 1 will be Wedge and the other will be Dash, so you are not actually changing the molecule by doing this, you're just making it easier to compare.
Attachments
It's a single bond, so there's free rotation, and as the explanations above have made clear there can certainly be an internal plane of symmetry.
A nice trick to use to check for meso when they give you a straight chain with two chiral centers: check if both chiral carbons are attached to the same substituents. If they are, evaluate each chiral center for R/S. If one is R and the other is S (or vice versa), it will be meso.
A nice trick to use to check for meso when they give you a straight chain with two chiral centers: check if both chiral carbons are attached to the same substituents. If they are, evaluate each chiral center for R/S. If one is R and the other is S (or vice versa), it will be meso.
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The symmetry is hidden in the notation.......Simply do the R and S......one carbon that is chiral is R...the other is S.....and the molecule has connectivity that allows for symmetry.
Dr. Romano
Continuing off of mess compounds....Dr. Romano, are there ways/things to look out for when they give you a big molecule and say " how many stereoisomers are there." I know the 2^n formula, and could solve it out for simpler molecules but how do we know they have meso compounds? Are there characteristics/properties to look out for on the original compound, or do we simply have to draw all the stereoisomers out?
Great question......I try to really bang this into my students heads !!! The 2n rule gives the Maximum number of stereoisomers....... For example....look at the molecule cholesterol. We have 8 chiral carbons. Try this as an exercise ! Clearly this molecule has NO SYMMETRY.....thus you are fine. If a molecule has possible symmetry ....then it will be less than what this rule gives. For example.....1, 3 -dimethylcyclopentane has 2 chiral carbons.....thus we expect 4 stereoisomers. HOWEVER..... brief inspection of the molecule should alert you to possible symmetry, Thus we have only 3. The cis 1, 3 molecule is meso....and will not have an enantiomer. Most likely you will not see this on the DAT.....If you can ID all the chiral carbons....and predict the number of stereoisomers,,,you will be fine.
Hope this helps
Dr. Romano
