The compound shown is 2,3,4-trichloropentane (no configuration is specified). How many of its stereoisomers are achiral (meso) forms? Give the answer as an integer.

C2 and C4 are stereocentres and C3 is pseudoasymmetric. If C2 and C4 have the same configuration, (R,R) or (S,S), the molecule is chiral and C3 is not stereogenic: this gives one pair of enantiomers. If C2 and C4 are (R,S), the molecule has a mirror plane through C3 and is meso; the Cl on C3 can then be on either side relative to the other two, giving two distinct meso diastereomers. Total stereoisomers = 2 + 2 = 4, of which 2 are meso.