[catarina1998]

How many diastereomers exist for the compound below
Link of the image: https://drive.google.com/open?id=0B5797uZ_layldmQxTGZCNUtsd2s
A) 2
B) 4
C) 6
D) 7
E) 8
The answer is C, but I can't understand why, because I only see 2 assymetric carbons, so the formula to calculate the number of isomers is 2ⁿ, so it should be 4 isomers, which means 2 diastereomers? 

[catarina1998]

I have seen many exercises like "Identify all pairs of diastereomers /enantiomers / configurational isomers etc" and then we have the structure of 6 or more molecules...
Like this one: https://drive.google.com/open?id=0B5797uZ_laylTFo0Qmt2a1RmWGs
I know the theory behind this concepts but i find it really hard to just picture the molecule in 3d and rotate it and etc, so i was hoping there was some tips on how to do this?

[sjb]

Carbon 3 is also asymmetric. Consider the meso form 2R, 4S, then isomers at carbon 3

[catarina1998]

ok thanks

[Dan]

The answer is C, but I can't understand why, because I only see 2 assymetric carbons, so the formula to calculate the number of isomers is 2ⁿ, so it should be 4 isomers, which means 2 diastereomers? 

You are right that the compound you posted only has 2 asymmetric centres (the middle one, C3, is indeed symmetric in this isomer). However, the question is a perfect example of how you should not rely blindly on general rules as shortcuts. The reason 2ⁿ does not work here is that C3 is asymmetric (technically pseudoasymmetric) for some pentitol isomers, which complicates matters. Also you must bear in mind that the 2ⁿ rule ignores the possibility of meso compounds, which some pentitols are. So bottom line is: start drawing and understanding the limitations of the 2ⁿ rule.

For getting better at rotation, there is no shortcut, it's about practice. Practice rotating molecules about a vertical axis in the plane of the page and about a horizontal axis in the plane of the page. Doing a couple every day or so will get you up to speed with it in no time.

[catarina1998]

Now I understand, thank you 

[Dan]

Now I understand, thank you 

You're welcome!

[Babcock_Hall]

When you have four groups around a carbon, you can hold one group in place, and "rotate" the other three on paper.  If the four groups are A, B, C, and D, and if I hold A in place, then B moves to where C was, C move to where D was, and D moves to where B was in the new drawing.  I usually chose A as one of the two groups that is in the plane of the paper or screen.  Does that help?

[catarina1998]

Thanks for the tip