Q.(A) Define Enantiomers.

Chromite ore is fused oxidatively with alkali to sodium chromate, acidified to dichromate, then converted to the less-soluble potassium salt. OR: Grignard/actinoid basics + a magnetic-moment calculation.

(A) Enantiomers: a pair of stereoisomers that are non-superimposable mirror images of each other. They have identical physical and chemical properties (in an achiral environment) except for the direction in which they rotate plane-polarised light (optical activity), and their differing reactivity with other chiral molecules.

(B) Preparation of $K_2Cr_2O_7$ from chrome iron ore (chromite, $FeCr_2O_4$):

Step 1 — Oxidative alkaline fusion: powdered chromite ore is fused with sodium carbonate in the presence of air (or with limestone as an oxidising aid) to give sodium chromate:

$4FeCr_2O_4 + 8Na_2CO_3 + 7O_2 \xrightarrow{\Delta} 8Na_2CrO_4 + 2Fe_2O_3 + 8CO_2$

Step 2 — Acidification: the yellow sodium chromate solution is acidified with sulphuric acid, converting it to orange sodium dichromate:

$2Na_2CrO_4 + H_2SO_4 \rightarrow Na_2Cr_2O_7 + Na_2SO_4 + H_2O$

Step 3 — Conversion to potassium salt: sodium dichromate solution is treated with potassium chloride; since potassium dichromate is less soluble, it crystallises out on cooling:

$Na_2Cr_2O_7 + 2KCl \rightarrow K_2Cr_2O_7\downarrow + 2NaCl$

OR alternative:

(A) Grignard reagent: an organomagnesium halide, $RMgX$, prepared by reacting an alkyl or aryl halide with magnesium turnings in dry ether: $RX + Mg \xrightarrow{dry\ ether} RMgX$.

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