Experiment 5 - Synthesis of Dulcin

criteria for successful recrystallization

• compound to be purified must be:

  • soluble in hot solvent

  • insoluble in cold solvent

• impurities must be either:

  • soluble in cold solvent

    • so that it will remain in the filtrate during cold filtration while compound to be purified is isolated on the filter paper

  • OR insoluble in hot solvent

    • so that it will remain on filter paper during hot filtration while the compound to be purified remains in the filtrate

During hot filtration of a mixture of crude product (acetanilide, sand and salt) dissolved in hot solvent (water) it must be kept warm/hot because:

the desired product (acetanilide) will crystallize before passing through the filter paper if the mixture is cold

How much acetanilide (Solubility of acetanilide is 5.5 g/100 ml water at 90 °C, solubility of acetanilide is 0.53 g/100 ml water at 0 °C) is recovered from 5 g of impure acetanilide (containing 3 g of acetanilide, 1 g of salt and 1 g of sand) when it is heated (dissolving the salt and acetanilide) and filtered to remove the sand and then cooled (recrystallizing the acetanilide and keeping the salt dissolved) and filtered to retrieve the acetanilide? (Assume a constant 100 ml recrystallizing volume)

2.47 g

How much acetanilide (Solubility of acetanilide is 5.5 g/100 ml water at 90 °C, solubility of acetanilide is 0.53 g/100 ml water at 0 °C) is recovered from 8 g of impure acetanilide (containing 6 g of acetanilide, 1 g of salt and 1 g of sand) when it is heated (dissolving the salt and acetanilide) and filtered to remove the sand and then cooled (recrystallizing the acetanilide and keeping the salt dissolved) and filtered to retrieve the acetanilide? (Assume a constant 100 ml recrystallizing volume)

• AND HOW MUCH ACETANILIDE IS DISCARDED?

4.97 g recovered, 1.03g

can acetanilide be separated from the impurities (sand, salt, sucrose, and sawdust)

No, salt, sucrose, and acetanilide will all remain in the filtrate

• thus, acetanilide will not be separated

does hot or cold filtration removes sand, sawdust, salt, and/or sucrose?

• hot filtration removes:

  • sand, sawdust

• cold filtration removes:

  • salt, sucrose

2 major reasons for measuring melting point

• to identify a compound

• to give an indication of a compound’s purity

what holds the molecules in a solid organic compound together

intermolecular forces that bind individual molecules into a crystal lattice

why will rinse-water wash away some of the product (from the reaction of p-phenetidine with cyanate)

due to amine solubility

what is the solubility of acetanilide at 90*C and 0*C

• at 90*C

  • 5.5 g / (100 mL of water)

• at 0*C:

  • 0.53 g / (100 mL of water)

what is the solubility of salt at 0*C

35 g / (100 mL of water)

recrystallization and why its important

used to purify crude solid compounds by removing impurities

describe recrystallization procedure

• principle: organic solids are a lot more soluble in hot solvents than in cold solvents

• steps:

1. dissolve impure solid in a small amount of hot solvent

2. hot filter to isolate from insoluble impurities

3. cool slowly to room temp. then place on ice to form crystals

• pure crystals form while soluble impurities remain in the liquid

4. cold filter (using a Buchner funnel) to isolate the pure crystals from the liquid

why a lot of dulcin might be lost during recrystallization

2 possible errors:

• using excess solvent

  • keeps too much compound dissolved even when its cold

• washing crystals with warm solvent

  • this redissolves the recovered crystals

how can loss of dulcin from recrystallization be minimized?

remember the 3 most important rules:

1. dissolve crude solid in minimal volume of hot solvents hot

2. allow flask to cool SLOWLY at room temp. before icing

3. wash the final crystals with minimal amount of ice-cold solvent

dulcin is known to be an artificial sweetener but it is not used as one today. Why not?

it is chemically modified in the body to p-aminophenol which is toxic