Question

1.00 gram of phenobarbital is soluble in 1 L water, 8 mL of ethanol, 40 mL of Chloroform, 13 mL of diethyl ether, and 700 mL of benzene. Which of the above solvents would be best for recrystallizing phenobarbital? (Remember that it is used as a medicine) Explain your answer and support it with calculations

Answer 1

Answer 2

Given Solubility Data (for 1.00 g phenobarbital):

• Water: 1 L (1000 mL)

• Ethanol: 8 mL

• Chloroform: 40 mL

• Diethyl ether: 13 mL

• Benzene: 700 mL

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Criteria for Ideal Recrystallization Solvent:

1. High solubility at high temperature, low solubility at room temperature (to maximize recovery).

2. Non-toxic (since phenobarbital is a medicine).

3. Volatile (easy to remove after crystallization).

4. 
   

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Step 1: Calculate Solubility Ratios

Recrystallization requires a solvent where the compound is:

• Very soluble when hot, but barely soluble when cold.

Compare solubilities (lower mL/g = better solubility):

• Ethanol: 8 mL/g (best solubility).

• Diethyl ether: 13 mL/g.

• Chloroform: 40 mL/g.

• Benzene: 700 mL/g (poor solubility).

• Water: 1000 mL/g (worst solubility).

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 Step 2: Practical Recrystallization with Ethanol

1. Dissolve 1 g phenobarbital in ~8 mL hot ethanol.

2. Cool slowly → crystals form (solubility drops sharply at lower temps).

3. Filter and dry.

Yield Calculation:
If solubility in cold ethanol is ~1 g/50 mL (estimated), recovery is:

$$\text{Recovery}=1\text{g}-\left(\frac{8\text{mL}}{50\text{mL}}\right)\approx 0.84\text{g}(84\mathrm{\%})\mathrm{.}$$

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Final Answer:

The best solvent is ethanol due to:

1. Optimal solubility (8 mL/g).

2. Safety (non-toxic for medicinal use).

3. High recovery yield (~84%).