Simple ORGO question

[BobbyBeavis]

What makes you determine which group obtains the wedge or dash? For instance, on the answer (meso), what if I labeled the methyl on the left side with a dash and the ethyl a wedge, while giving the methyl on the right side a wedge and the ethyl a dash? That would then not provide a meso compound.

I feel I know the reasoning but I couldn't talk myself to it with this example.

Thanks!

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[LightAtTheEnd]

If you switch the ethyl and methyl on the left side, you would simultaneously be switching the R/S configuration, which is essentially changing the molecule.

The only way you can "rotate" about a single bond is if you switch all three groups. So if you put CH3-->dash, ethyl-->on plane, and Cl-->wedge, it would be the same compound.

 

[BobbyBeavis]

If you switch the ethyl and methyl on the left side, you would simultaneously be switching the R/S configuration, which is essentially changing the molecule.

The only way you can "rotate" about a single bond is if you switch all three groups. So if you put CH3-->dash, ethyl-->on plane, and Cl-->wedge, it would be the same compound.

But when adding the Chlorine, what makes you determine where the wedges/dashes go? Say in the Meso compound. The ethyl is down on the left side of the DB, and up on the right side. Why do they both get labeled with a dash?

Also, it says this is an Anti addition. When drawing the product of an Anti addition, you can label both additions inside the plane?

 

[LightAtTheEnd]

Look at the reactant that yields the meso compound. Each carbon is sp2 hybridized right? This means that each group is trigonal planar. This entire molecule lies on the surface of a flat plane.

So draw it out on a piece of paper. Now we need to add two chlorines. Since it's an anti addition one chlorine will add "down" and one will add "up."

Let's add the first chlorine down. Take a pen or pencil and imagine attaching it to the left carbon below the paper. Next imagine a second pen attached to the right carbon above the paper. When this happens, the molecule becomes tetrahedral in shape and the ethyl/methyl are no longer in the plane.

Now look at the paper along its edge. The left Cl is pointing down, the right one is pointing up. Since it's sp3 the methyl will point towards you and the ethyl will point away. The right carbon corresponds with our final drawing, but the left one doesnt. So you need to rotate it about the single bond. If you take the chlorine pointing down and rotate it up to align with the right chlorine, your methyl will be pointing towards you, and the ethyl away from you, which corresponds with the answer.

The wedge/dash is just used to explain the position of a substituent in relation to the others. You can look at the molecule in any view you want and it could be in the planar, wedge, or dash position. What's important is if the ethyl and methyl are in the correct positions around it.

Hope this helps! If it doesn't I'll draw it out for you. Play around with a model kit if you have one.

 

[ph159]

Okay so I have major question..
How are the first 2 answer choices even chiral? The alkene carbon has two methyl groups attached to it..I thought for a chiral center is must have 4 different substituents attached.

Is my thinking wrong? someone please correct me!

 

[LightAtTheEnd]

The definition of a chiral compound is one that has at least one chiral center. Look at the second carbon of the first answer choice and the first carbon of the second answer choice, it has 3 different groups!

 

[ph159]

The definition of a chiral compound is one that has at least one chiral center. Look at the second carbon of the first answer choice and the first carbon of the second answer choice, it has 3 different groups!

THANK YOU!