A 1.50 g sample of coniine, the toxic extract of poison...

A 1.50 g sample of coniine, the toxic extract of poison hemlock, was dissolved in $10.0 \mathrm{~mL}$ of ethanol and placed in a sample cell with a $5.00 \mathrm{~cm}$ pathlength. The observed rotation at the sodium $\mathrm{D}$ line was $+1.21^{\circ} .$ Calculate $[\alpha]_{\mathrm{D}}$ for coniine.

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Key Concepts

Observed Rotation

Observed rotation is the angle by which plane polarized light is rotated after passing through an optically active sample. It is measured using a polarimeter and is a direct experimental reading that must be normalized by the concentration and pathlength to obtain intrinsic optical activity properties.

Pathlength

Pathlength refers to the distance the light travels through the sample in the polarimeter. It is a crucial factor in determining the observed rotation because the degree of rotation is directly proportional to how far the light travels in the optically active medium.

Concentration (g/100 mL)

In polarimetric measurements, the concentration of the optically active substance is typically expressed in grams per 100 milliliters (g/100 mL). Converting the actual concentration into this standard unit allows for consistent comparisons and proper normalization when calculating specific rotation.

Specific Rotation

Specific rotation is an intrinsic property of an optically active substance that quantifies the degree to which it rotates plane polarized light per unit pathlength and per unit concentration. It is calculated by dividing the observed rotation by the product of the pathlength and the standardized concentration, providing a standardized measure for comparing optical activities.

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