skye_mage - 30-1-2015 at 11:07
I'm seriously stuck on a couple of homework problems. I've indicated what answers I have recently tried with these and I'll explain what I've tried to
do with each.
With the first image it has to add with 1,2 and 1,4 addition and I have to include dashes and wedges on the 1,2 figure. I thought the bromine would
add at the 2 for 1,2 and that it would have the wedge while carbon would have the dash but this was incorrect. With 1,4 I thought there would be a
shift of the double bond to the third carbon and the bromine would attached to the fourth, but this doesn't appear to be the case.
The second image I got one of the structures correct but I keep screwing up on the second one. I'm trying to envision in my head what it means when
they tell me to "flip" di-bromoethene but I can't figure it out. My professor hasn't gone over this kind of thing yet.
Thanks in advance.
Chemosynthesis - 30-1-2015 at 11:38
1. Where did the bromine even come from in the first question? Your 1,4 addition is wrong for two reasons. I don't think you need a bromine (hint: or
hydrogen), unless I am missing part of the question, and you need to have the addition on the first and fourth molecule. Which did you add bromine to?
Now look at the hybridization/valence/molecular orbitals. First, think about your electrons, since organic chemistry notation is essentially shorthand
for electron transfers. You draw an arrow from a nucleophile (double bond) to something. Then you need to consider the intermediate or transition
state. Now see if resonance gives you an alternative. In this case, what are your carbocation options?
Add 1,4 with the most electronegative component (if heterogenous) adding to the most substituted, stable carbocation and look again. See the
difference? This process will help you through any addition in the class. Oh, and dashes and wedges only make a difference on chiral SP3 carbons. Your
problem here, in my opinion is entirely related to not drawing your intermediates/transition states of reactions. You need to work out your mechanisms
if you want to understand or even pass organic chemistry.
2. Check out R/S nomenclature (uses Cahn-Ingold-Prelog). If you are having trouble, try to rotate one of your drawings until you can lay it on top of
the other. You will find it is identical, just viewed from a different perspective; if you notice, the two structures you drew are actually the same
structure, just rotated to view the opposite plane of the ring. If you look into R/S nomenclature (next up in class, I bet), you will see you need to
change the priority.
skye_mage - 30-1-2015 at 11:46
It was a mistake. I've been working on this homework for the last 9 hours, so I confused one question with another.
Chemosynthesis - 30-1-2015 at 15:09
Just making sure. Until you get very familiar with a mechanism, and can do it in your head... or if you are very tired, I highly recommend
procuring a whiteboard of some kind. You "save trees" and can make mistakes, then erase them and pretend they never happened. When you draw out the
full mechanism and intermediates, you should get the correct answer, and reinforce concepts for later use in other mechanisms.