Racemic Mixture Properties Explained

A racemic mixture, also known as a racemate, is a mixture containing equal amounts of two enantiomers (non-superimposable mirror images of a chiral molecule). Understanding the behavior of these mixtures, particularly concerning their interaction with polarized light, is key to identifying incorrect statements.

Optical Activity of Racemic Mixtures

Chiral molecules can rotate the plane of plane-polarized light. This property is called optical activity. Enantiomers rotate plane-polarized light by equal amounts but in opposite directions. For instance, if one enantiomer rotates light clockwise (+ or d-), the other enantiomer rotates it counterclockwise (- or l-) by the exact same magnitude.

In a racemic mixture, since there are equal proportions of both enantiomers, the optical rotation effects cancel each other out. Therefore, racemic mixtures are optically inactive. This means they do not cause any net rotation of plane-polarized light.

Analysis of Options

• Option 1: Racemic mixture causes finite rotation of plane polarized light

  This statement is incorrect. As explained above, racemic mixtures are optically inactive due to the cancellation of rotational effects from equal amounts of opposing enantiomers. They cause zero net rotation, not a finite rotation.

• Option 2: It is often designated as ($ \mp $)

  This statement is generally considered correct in the context of chemical notation. While the symbol ($ \pm $) is more common for indicating a racemic mixture (representing a mix of '+' and '-' enantiomers), symbols like ($ \mp $) can also be used to denote mixtures of enantiomers with opposite configurations, implying optical inactivity. It signifies the presence of both chiral forms.

• Option 3: ($ \mp $)-2-butanol is a racemic mixture

  This statement is correct. 2-butanol is a chiral molecule. The notation ($ \mp $) before the name indicates that the compound is present as a mixture of both its enantiomers in equal proportions, which defines it as a racemic mixture.

• Option 4: Plane polarized light remains invariant inside a racemic mixture

  This statement is correct. "Remains invariant" means unchanged. Since a racemic mixture is optically inactive, it does not alter the plane of polarized light passing through it. The light's polarization plane stays the same.

Conclusion

Based on the analysis, the statement that incorrectly describes a racemic mixture is that it causes a finite rotation of plane-polarized light. Racemic mixtures are characterized by their optical inactivity.