Question

a. A pure sample of (R)-limonene has a specific rotation of +125.6". If a sample had a specific rotation of +80.4º, how much of each enantiomer is present in the sample? Show your work b. Why is a large excess of water used in the distilling flask in the steam distillation of limonene? c. After your chemical tests how would you expect the IR to change? d. What would be the molecular formula of the product when excess Br2 reacts with limonene? Explain.

Post lab question help. This lab was about extracting limonene from orange peels. Chemical tests were then done with limonene using Br2 and MnO4.

Answer 1

(a) The specific rotation of (R) isomer = +125.6

If equal amount of (R) and (S) are present then the specific rotation is expected to be zero because the (S) isomer will have an opposite value as -125.6

The rotations of the two enantiomers cancel each other, so the rotation of the mixture will be that of the excess enantiomer.

enantiomeric excess = (observed specific rotation / maximum specific rotation) x 100 %

= (80.4 / 125.6)*100 % = 64.01 %

We can calculate the percent of each enantiomer as described below

If we have a mixture of (+) and (-) isomers and (+) is in excess,

%(+)=ee/2+50%

We have a 64 % enantiomeric excess of (+).

∴ %(+)=64%/2 + 50%=(32+50)%=82%

So, the mixture contains 82 % (+) isomer and 18 % of (-) isomer

(b) Steam distillation is used for separating substances which are immiscible with water, volatile in steam & having high vapour pressure at the boiling temperature of water. As limonene boiling point is higher (170 C) large amount of water is required so as to have it distilled at the boiling temperature of water. At times higher concentration of limonene may lead to polymerization causing a jam like structure due to rise in temperature. TO avoid these water quantity should be excess.

(c) As the addition of OH group over the double bond of limonene will lead to the appearance of OH stretching peaks at 3500-3200 cm^-1

(d) LImonene has two double bonds. Addition of excess bromine will add 4 Br groups to liminone

Molecular weight of limonene - 136

Addition of 4 bromo atoms will give the molecular weight of 136 + (4 x79) = 452 g/mol