[susdujcrd]

I was asked if A and B are enantiomers (First Pic)
I answered that it's the same molecule, because rotating A by 180 gives (Second Pic)
and rotation the top and bottom bonds while keeping the carbons stationary gives (Third Pic), which is B

But I was wrong, wheres my mistake?

[furaniki18]

i think you are right...what you are saying is correct. in fischer projection, the steps that you have followed, do not change the configuration. basically they are allowed.
a more definite way to confirm this would be to write R/S configuration of the two chiral carbon atoms in the two molecules..if their R/S configuration is same then they are the same molecules.
for e.g. if i just randomly no. the carbon attached attached to Br, Me and H as C1 and the other carbon attached to F, Et and Cl as C2 and then write R/S configurations for both these carbons separately and for both the molecules then i get- C1 is S and C2 is R for both the molecules.
on the other hand if for the second molecule (B) had i got a configuration as C1 is R and C2 is S, then i would have called them enantiomers.
precaution: always be careful that you number the same carbon as 1 and 2 in the two molecules

[susdujcrd]

Well, the answers are: (regarding the first pic)

1. top carbon for A and B is R    
2. A and B are enantiomers      
3. A and B have different molecular formula    
4. bottom carbon for A is R and for B is S
5. answers 1+2
6. answer 2+4
7. none of 1-4 are correct
8. all of 1-4 are correct

I guess the lecturer made a mistake, because the only correct statement is no.4

[furaniki18]

the answer is correct...the lecturer made no mistake...
the bottom carbon for A is C2 which is R and the bottom carbon for B is actually what we have taken to be C1 which is S...i hope you now got where you are going wrong...remember numbering your carbons correctly!

[furaniki18]

i guess i have interpreted your last statement incorrectly...are you trying to say that the answer according to your lecturer is 4 or you also think the answer should be 4?
anyways..we can easily see they have the same molecular formula and that they are not enantiomers...so whatever the case may be the correct answer should be 4..

[susdujcrd]

I dont know what is the right answer according to the lecturer, but it has to be one of the  statements 5-8, and none of them is correct.

Anyway, you cleared up the subject for me, thanks for your help