Question

a)predict the observed rotation of a solution of (S) 2butanol (2g) in 10ml water using a 1dm polarimeter tube and the D line of sodium at 25 °c given that (R)2butanol has [alpa]D 25 =-13.52
b) predict the observed rotation of a solution of (R) 2butanol (4g) and (S) 2 butanol (2g) in 10ml water using a 1dm polarimeter tube and D line of sodium at 25°C .what would be the specific rotation of the mixture if a further 2g of (S) 2butanol was added?

Answer 1

a)Specific rotation of pure R 2-butanol is -13.52

concentration = 2g/10mL = 0.2 g/mL

length of tube = 1 dm

We know

specific rotation = observed rotation /concentration x length

-13.52 = observed rotation/ 0.2g/mL x 1dm

Thus

observed rotation = - 2.704 deg

b) part1)

observed rotation = [concentration of excess enantiomer - concentration of other ] x specific rotation of pure excess isomer/length

= [0.4 -0.2] x +13.52 /1 dm

= +2.704 deg

part ii) If further 2 g of s isommer added , the mixture has 4g of R- and 4 g of S- isomer . thus it is racemic mixture and has 0 specific rotation.