CH 220: Recrystallization & Arenes

Recrystallization: 3 Solvent Rules

1. Cold = compound poorly soluble at room temperature.

2. Hot = compound soluble near the solvent’s boiling point.

3. Impurity solubility should ideally be temperature-independent.

Recrystallization: Solvent rules EXTRA

An impurity that is soluble at both temperatures remains dissolved while the product crystallizes; an impurity insoluble at both can be filtered away.

Recrystallization: Base →

water

Recrystallization: Neutral →

methanol

Recrystallization: Sequence of procedure

crude solid → minimum hot solvent → solid just dissolves → cool slowly → crystals form → cold wash/vacuum filtration → dry → mass + melting point

Recrystallization: Why not too much (hot) solvent

Too much solvent means the product may remain dissolved even after cooling, causing lower recovery.

Recrystallization: Washing Neutral crystals →

~5 mL cold methanol

Recrystallization: Washing Base crystals →

cold water

Recrystallization: The solvent to wash is cold because…

the desired compound should be poorly soluble in cold solvent, so you can wash contaminants away while minimizing product loss.

Melting range =

temperature where first liquid appears → temperature where final solid disappears.

Impurities for melting point…

generally broaden the melting range and lower the onset of melting.

The recrystallized sample is purer because

its melting range became narrower. Its melting-point region remained similar because it is still the same compound.

Recrystallization: Identity comes from

comparing the purified melting range with known/reference values.

Percent Recovery Formula

(mass recovered / theoretical mass available) * 100

Recrystallization: Why cool slowly before using ice during recrystallization?

Allows better/purer/orderly crystal formation; immediately forcing it cold can produce poor/small crystals and hurt purification quality

• Slow cooling → molecules have time to arrange into an organized crystal lattice.

  • Very Fast cooling → rapid precipitation/nucleation → many small, less orderly crystals → greater chance of trapping impurities.

• Ice is used afterward to decrease solubility further and maximize recovery.

Recrystallization: Why heat a melting-point sample only 1–2°C/min?

Gives the sample time to equilibrate with the apparatus so the melting range is measured accurately.

• to maintain thermal equilibrium and accurately observe the temperatures where melting begins and ends.

Arenes: Overall Reaction

It is a free-radical substitution in which H is replaced by Br.

Arenes: Mechanism Steps

1. Initiation

2. Propagation 1

3. Propagation 2

4. Termination

Arenes: Termination possibilities (3)

1. Br2

2. R-Br

3. R-R

Arenes: Second step significance

The second step of propagation regenerates the Br radical, which is why the chain propagates.

Arenes: Aromatic H

H directly attached to an aromatic-ring carbon (red)

Arenes: Aliphatic H

H attached to an sp³ carbon in the alkyl portion (blue)

Arenes: Benzylic H

H on an sp³ carbon directly adjacent to the aromatic ring (yellow)

Arenes: Heirarchy for H’s

aromatic < aliphatic < benzylic

• within benzylic positions: 1° < 2° < 3°

Arenes: why the heirarchy of H’s

Radical stability

• Removing the most reactive hydrogen produces the most stable radical.

Arenes: Toluene (radical produced, relative reaction speed)

• 1° benzylic

• THIRD

Arenes: Ethylbenzene (radical produced, relative reaction speed)

• 2° benzylic

• SECOND

Arenes: Isopropylbenzene (radical produced, relative reaction speed)

• 3° benzylic

• FASTEST/ FIRST

Arenes: t-Butylbenzene (radical produced, relative reaction speed)

• Aliphatic radical (no benzylic H available)

• SLOWEST/ FOURTH

Arenes: Compound heirarchy

isopropylbenzene > ethylbenzene > toluene > t-butylbenzene

Arenes: Statistical factor

how many hydrogens of each type are available. Propane has 6 primary H and 2 secondary H, so a purely statistical reaction would predict roughly a 3:1 primary:secondary product ratio.

Arenes: Energy factor

different hydrogens have different intrinsic reactivities because abstraction creates radicals of different stability. More stable radicals form preferentially.

Arenes: Observed product distribution =

availability of H's (statistical factor) + energetic preference for forming the more stable radical (energy factor).

Arenes: actual chlorination distribution is about ___, rather than the statistical 75/25 expectation

45% primary / 55% secondary

Arenes: Bromination distribution

Even more selective: the professor gives roughly 97% secondary / 3% primary for propane.