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I got tripped up by this question; I thought these were all enantiomers when in fact they were all meso compounds.

How can one prevent this from happening?

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How does one rotate the Fischer projection, for example, in the first problem; I understand that one has to hold one group in place, but then what?

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To rotate on a single bond in a Fischer Projection, all 3 groups move in the same direction. $\frac {1} {2\ \ \ 3} \rightarrow \frac {2} {3\ \ \ 1}$. See this page for more information.

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Exchanging two groups in fischer projection changes it to its enantiomer and further exchange reverts it back.After exchanging as follows gives meso compound with internal plane of symmetry:

Note that these are for the compunds on the left. Do similiarly for others on right

a) $\ce{CH3<->H}$ THEN $\ce{H<->OH}$

b)Rotate nearer $C$ by $\pi/3$ rad. anticlock wise.

c)Imagine rotation about $C_2-C_3$ by $\pi$ rad., the $OH$ in the right back of the plane comes in front projecting out towards us hanging down and thus we have a vertical plane of symmetry.

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