orgo and GC questions, need help!

[Bigbirdo]

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1. How many valence electrons do Cr have? Are electrons in the d orbitals counted as valence electrons? Chad said that Cr has 6 valence electrons and it includes d orbital electrons, but Br has 7 valence electrons and it excludes the d orbital electrons.

2. This is a question from Bootcamp, In the picture attached, when do the two substituent groups point up and down? And how do you call the up and down position respectively? (I remember something endo..)

 

[aznnytezx]

For your first question, the electron configuration of Cr is [Ar]4s^1 3d^5 , meaning that there are 6 valence electrons from their outer most shell.

For bromine, it is different. The d orbitals are completely filled, meaning we do not count that as our valence electrons. We would only look at the 4th shell, so we would get 2 electrons from the s subshell, and 5 electrons from the p subshell. For transition metals, it is quite different where you cannot follow the same pattern because all transition metals do not have a filled d shell.

 

[saintguitar]

To answer your second question,

first of all, this is a Diels-Alder reaction.

In the second group with CO2ME, which is the same as COOCH3, notice that the two CO2ME groups are in "cis" configuration (or Z). It means that when the Diels-Alder reaction happens, the two CO2ME groups will be in "syn" position, or in the same plane, whether up or down.

Now notice that we have a "peak" in the middle of our Diels-Alder product, and because reactions almost always want to avoid steric hindrance, the two CO2ME groups will attack from the "bottom" in "syn" fashion.

 

[aznriptide859]

Also recall the "endo rule", stating that stereochem of the reactants in a D-A reaction will retain the same stereochemistry as found in the product.