Page:Outlines of Physical Chemistry - 1899.djvu/108

 88 OUTLINES OF PHYSICAL CHEMISTBY

can only be the algebraic sum of the optical effects of the different faces of the dissolved molecules.

It is easy to show, by the theory we are now consider- ing, that the presence of an asymmetric carbon atom is the necessary condition (sine qua non) of optical activity. In a substance of the type C lf u 2 > 3> optical activity is not possible.

In fig. 21, a

The face 2 — 1 — 1. . .is inactive „ „ 3 — 1 — 1 . ' .is inactive

3—2—1 . . . turns to the left ) equal and 3—2—1 (plane of the, paper) „ „ right) opposite effects

The face 2—1—1 is inactive

„ „ 3 — 1 — 1 is inactive

„ „ 3 — 2 — 1 turns to the right

„ „ 3—2—1 (plane of the paper). turns to the left

These two last effects, in each case, being equal and opposite, the resultant is zero.

Let us now examine a molecule containing two asym- metric carbon atoms, and let us first take one of the type C i,2,3 — 1>2 ,3> such as we have in tartaric acid. Four modifications are possible : the two asymmetric systems which together make up the molecule may be

(1) both dextro-rotatory ;

(2) both laevo-rotatory ;

(8) one dextro- and the other laevo-rotatory— the resul- tant effect being zero ; or (4) a solution of two molecules of opposite activity is inactive, and from it an inactive substance crystallises out. But this substance can be decomposed into two active modifications ; it is termed the racemic form. permits of an investigation of the effects of all the tetra- hedral faces (figs. 22, l, 2, 3, and 4).

Each half of the molecule being heavier and more

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