Recrystallization of organic solids(ch 3 lab)

What is recrystallization?

A purification method where a solid is dissolved in hot solvent and then crystallized upon cooling based on solubility differences.

Why is recrystallization used?

To purify crude organic solids by removing insoluble, soluble, and colored impurities.

What principle guides recrystallization?

"Like dissolves like" — similar polarity = similar solubility.

What structural features affect solubility?

Polarity, hydrogen bonding ability, and functional groups.

For alcohols with few carbons, what solvents do they dissolve in best?

Polar solvents.

How do alkanes generally behave in terms of solubility?

Soluble in nonpolar solvents; insoluble in polar ones.

What happens if a molecule has >6 carbons and only one OH group?

Solubility is dominated by the alkyl portion → behaves like an alkane (nonpolar).

What are the three solubility behaviors that determine suitability of a solvent?

1. High solubility in hot & cold (bad)

2. Low solubility in hot & cold (bad)

3. High solubility hot, low solubility cold (good)

Which solubility behavior is ideal for recrystallization?

High solubility in hot solvent and low solubility in cold solvent (behavior #3).

Define "Solubility" in recrystallization.

Ability of solute to dissolve in hot solvent to form a saturated solution.

Define "Saturation level."

Point where the hot solvent holds maximum solute; crystals form on cooling.

Define "Exclusion."

Impurities remain dissolved in the solvent and do not crystallize with the solute.

Define "Nucleation."

Initial formation of tiny crystal seeds where larger crystals grow.

What are the ideal characteristics of a recrystallization solvent?

Dissolves solute only when hot

Insoluble when cold

Doesn't dissolve impurities (or dissolves them only cold)

Non-reactive

Volatile, nonflammable, nontoxic, inexpensive

What is the key solvent rule for recrystallization?

"Soluble HOT, insoluble COLD."

Why add boiling chips during heating?

Prevent bumping and ensure even heating.

When do you use activated charcoal?

Only when the solution is colored (removes colored impurities).

Why must charcoal be used sparingly?

Too much can adsorb the product.

What type of filtration removes charcoal or insoluble solids?

Gravity filtration (hot).

What happens if the solution cools too fast?

Powdery crystals form and trap impurities — poor purification.

Why cool slowly first before using an ice bath?

Allows proper crystal lattice formation → purer crystals.

What type of filtration is used to collect crystals?

Vacuum filtration (Hirsch funnel or Büchner funnel).

Why rinse crystals with cold solvent during vacuum filtration?

To remove impurities without dissolving the product.

How are crystals dried?

On the funnel under vacuum or in a warm environment until solvent evaporates.

How is % recovery calculated?

%Recovery=mass of dry crystals/mass of crude solid X 100%

Step 1 of recrystallization?

Choose the appropriate solvent (test solubility).

Step 2?

Dissolve solute in minimal HOT solvent.

Step 3?

Decolorize with charcoal (if needed), then gravity filter.

Step 4?

Filter suspended insoluble solids (hot filtration).

Step 5?

Cool slowly → crystallize the solute.

Step 6?

Collect crystals by vacuum filtration and wash with cold solvent.

Step 7?

Dry crystals and determine % recovery & melting point.