HELP O-CHEM FISCHER´s !

[FutureDental88]

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Can someone help me understand how this works, and how exactly the rules for rotation are, since I just simply can not get it right...

Thanks !

Correct answer is D .


PS; Please try to explain as detailed as possible...

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[rockclock]

OK, explaining this in-person would be ideal, but bear with me here...

Any two bonds in a tetrahedral center are going to form a V (109.5 degrees). In a Fischer projection, if you pick any center (intersection of lines), then the two vertical bonds form a V (with both bonds going into the page), and the two horizontal bonds form a V (with both bonds coming out of the page). The two V's are in perpendicular planes.

Basically, what I would recommend is picking one of those V's (I like to use the vertical bonds going into the page) to get your bearings orientation wise, and then use that to figure out what the other V (in my case, the horizontal bonds coming out of the page) should be. It will either match or be the opposite. You repeat this for all chiral centers and if the all match, you know they're identical molecules.

(C) Start by looking at the -Br and -C coming off that center...they form a V with both bonds going back into the page. Find the corresponding V on the dash-wedge diagram. Picture yourself looking at that V dead-on, i.e. you're looking at the vertex directly and the bonds are going away from you. In that view, you'd have -D on your left and -OH on your right. Match that up to the Fischer projection...not a match, they're switched so those centers have different configurations.

(D) Start by looking at the -Br and -C coming off that center...they form a V with both bonds going back into the page. Find the corresponding V on the dash-wedge diagram. Picture yourself looking at that V dead-on, i.e. you're looking at the vertex directly and the bonds are going away from you. In that view, you'd have -OH on your left and -D on your right. Match that up to the Fischer projection...a match, they're the same so those centers have the same configurations.

Repeat to check all (in this case, both) centers.

Note that you only have to keep track of which way the V is pointing for the first V that you use to get your orientation. When matching up the second pair of bonds, it's only a matter of right and left (the fact that it's a tetrahedral center automatically takes care of the into/out of the page issue for you).

 

[Bis-GMA111]

or you could do the following..

assign R and S values for the molecule and then for the fischer projection..

you always want to make sure your lowest priority is on a dashed line, so for the bottom chiral center swap the H and OH and then assign an R or S value. THEN take the opposite of that and compare.

 

[addis4ever]

(D) Start by looking at the -Br and -C coming off that center...they form a V with both bonds going back into the page. Find the corresponding V on the dash-wedge diagram. Picture yourself looking at that V dead-on, i.e. you're looking at the vertex directly and the bonds are going away from you. In that view, you'd have -OH on your left and -D on your right. Match that up to the Fischer projection...a match, they're the same so those centers have the same configurations.

Repeat to check all (in this case, both) centers.

Note that you only have to keep track of which way the V is pointing for the first V that you use to get your orientation. When matching up the second pair of bonds, it's only a matter of right and left (the fact that it's a tetrahedral center automatically takes care of the into/out of the page issue for you).

OK, i def understand C. but for D, shouldn't compound D be on the vertical line in the Fischer projection? because its wedged in the other one and that means it should be counted as if its in the back of the page. right?

 

[rockclock]

OK, i def understand C. but for D, shouldn't compound D be on the vertical line in the Fischer projection? because its wedged in the other one and that means it should be counted as if its in the back of the page. right?

okay, i'm gonna call [C] the central carbon for that chiral center...

The V formed by Br-[C]-C in the Fischer projection has bonds pointing into the page (by definition), right? Now find that same V in the wedge-dash projection and try to orient yourself visually so that you're looking at the molecule from behind your screen out towards your keyboard. That puts that same V in the same orientation as you saw it in the Fischer projection. From that view, OH is on your left and D is on your right, which matches what the Fischer projection has.

I find it easier to start with the Fischer projection because what's coming out of the page and what's going into the page is fixed by definition...really easy to get your bearings quickly, whereas dash-wedge diagrams can be drawn anyway you like. Then, instead of rotating the dash-wedge in my head, I just picture myself looking at it from different angles. That way all you have to keep track of is where you're standing, and the actual (correctly drawn) molecule is always physically on the paper in front of you...minimizes the things you could confuse. Then its just a matter of figuring out what's left and what's right and seeing if the Fischer projection matches.

 

[Teethpick]

Check out http://www.freelance-teacher.com/videos.htm This guys has great explanations on lots of different topics. He has a video that talks about this. He goes a bit slow through things, but he does that so you thoroughly understand it before moving on. Hope this helps.

 

[AllahIsGreater]

👍

or you could do the following..

assign R and S values for the molecule and then for the fischer projection..

you always want to make sure your lowest priority is on a dashed line, so for the bottom chiral center swap the H and OH and then assign an R or S value. THEN take the opposite of that and compare.

 

[nateguy40]

bottom dashed on the right. top dashed on the left

 

[LetsGo2DSchool]

All you have to do is assign R and S. End of story.

 

[GoBlue24]

I do it a different way, and it's easier to me. Did well on my orgo exam at umich. I didn't read rock clocks explanation (too long for me this early in the morning) but let me know if it's similar. I match fischer projections to structures using a TRAPEZOID-SPINE trick that I made up.

1. First, we see that BR is at the top and F is at the bottom of the Fischer Projection, going down a straight line vertically. Immediate make sure that Br and F on the corresponding structures are on solid lines of the structure as well, not wedges or dashes (its just easier to visualize this way... you could have both on wedges or both on dashes).

-If THEY ARE NOT on solid lines (all the answers above have either Br, F or both on wedges or dashes aka they are not on solid lines), ROTATE THEM so that they are.

Example, choice B) has F already on the solid line so no rotation necessary. However, Br is still on a wedge. Push it down (Making it a solid line), move D to a dash position, and OH to a wedge position.

***if you practice it, this step should take literally seconds to do.

2. Now, (going on with choice B) I call it the Trapezoid Spine rule or trick because you want your newly formed solid line (F-C-C-Br) to form a trapezoid-like spine \____/. Doesn't matter if it's flipped, just make sure the solid lines form a trapezoid. So for example B) again, after rotating Br to a solid line in step one, rotate the entire top chiral center (Br, H, OH) 180 degrees so that the solid line is now shaped like a Trapezoid Spine ( \____/) with dashes and wedges jutting down.

3. Now that the solid lines are pointed up for B), your dashes and wedges should be pointing down. Simply pretend you're lying underneath the structure and determine what is on your left and on your right. Br should be above you and F should be below you. (Choice B doesn't match because it's the wrong answer... try it for D .)

If you practice it and visualize it, it becomes super easy to do (took me 30 secs to find the answer). Might not seem like it... but trust me, it helped a lot of confusing people in my class. I'm not sure about the whole R/S thing and somehow switching OH with H.. but I'm sure that works too.My method seems random but it's actually not. An actually fischer projection's natural form is to be in curved spine like manner with either both dashes and wedges pointing up or both dashes and wedges pointing down. With more and more chiral center added, the structure will continue to curve with both dashes and wedges continuing pointing up or both pointing down.Think of a centipede on it's back with it's lets jutted out (the dashes and wedges). It's just what the fischer projections are actually based off of. Pretty sure thats how I saw it in my book when I took it last year. Much easier to explain in person, lol!


Yeap, I just looked it up on google. It only shows the top view but if you visualized it from the side view, you could def see the trapezoid outline from 1 to 4 with 2,3, 5, 6 jutting up and out. From this angle, you look from the top. In Step 2 that I gave, they were jutting downward so I looked from the bottom. Doesn't matter as long as you look from the side the dashes/wedges come at you. http://www.google.com/imgres?imgurl=http://chemwiki.ucdavis.edu/%40api/deki/files/1034/%3Dethane%2520dash-wedged%3DFischer%2520projections.gif&imgrefurl=http://chemwiki.ucdavis.edu/Organic_Chemistry/Chirality/Fischer_Projections&usg=__XJoqYSAcfTB9FI-NGoB3Y2iwsjs=&h=286&w=557&sz=5&hl=en&start=0&sig2=EV75VtVgt8fTgVP_c0KcrQ&zoom=1&tbnid=0SmB3kKQKZU68M:&tbnh=71&tbnw=139&ei=SdAiTsC1Fsm4tgeeq8DCAw&prev=/search%3Fq%3Dfischer%2Bprojection%26um%3D1%26hl%3Den%26client%3Dfirefox-a%26sa%3DN%26rls%3Dorg.mozilla:en-US😳fficial%26biw%3D1366%26bih%3D529%26tbm%3Disch&um=1&itbs=1&iact=rc&dur=1903&page=1&ndsp=24&ved=1t:429,r:3,s:0&tx=27&ty=51