recrystallization/arenes

Pure Compounds

homogeneous sample consisting only of molecules having the same structure

Supersaturated Solution

a chemical solution that contains more dissolved solute than the solvent can normally hold at a given temperature

one of the reasons you could have no crystals after recrystallization, either add a crystal of original or take a stir rod/spatula, get solvent, allow it to cool + form crystals, and use that

another reason no crystals = too much solvent used

Rules of a recrystallization solvent

• Compound is insoluble in cold solvent (meaning room temperature)

• Compound is soluble in hot solvent (meaning near/at boiling point)

• Impurities solubility is temperature independent (either completely soluble or completely insoluble)

Process of a recrystallization

• Add solvent slowly until just dissolves

• Filter

• Cool down slowly (why: Rapid cooling by immersing the flask in water or an ice-water bath tends to lead to the formation of very small crystals that may adsorb impurities from solution)

• Filtration

Melting Point

temperature at which the liquid and solid phases exist in equilibrium with one another without change of temperature

Physical Constant

numerical values associated with measurable properties of these substances

useful only in the identification of previously known compounds

Recrystallization Solvent for base

solvent for neutral

base: h2o

neutral: MeOH (methanol)

Taking a melting point

• Heat increase ramp at 1 to 2 degrees per minute (Thermal Equilibrium: Heat takes time to transfer evenly from the heating block or bath through the capillary tube to the solid sample. If heated too quickly, the temperature of the thermometer/sensor rises faster than the sample can absorb the heat, causing the sample to melt at a thermometer reading higher than its actual melting point.)

• Report temperature as a range

Melting point data analysis

• What does melting point data tell you

• Purity - A narrow melting-point range ordinarily signals that the sample is pure

• Identity

bromine safety

• always wear gloves

• rinse skin with water for 15 min if exposed

• if spill, must rinse with sodium thiosulfate solution

Reaction Control

• Statistical Factor

• Energy Factor

statistical factor - determined by the number of hydrogen atoms whose replacement will give a specific constitutional isomer

energy factor - related to the strength of the type of C–H bond being broken and is called the relative reactivity (primary, secondary, tertiary, aka substitution)

• Free Radical Substitution of C-H to C-X (X=halogen)

• Hydrogen Types

  • Aromatic

  • Aliphatic

  • Benzylic

aromatic - hydrogens bound directly to an aromatic ring

aliphatic - hydrogens bound to sp3-hybridized carbons

benzylic - An aliphatic hydrogen atom attached to an sp3-hybridized carbon atom that in turn is bound to an aromatic carbon atom

• Free Radical Mechanism
  
  

  • Initiation

  • Propagation

  • Termination

Initiation - Radicals are formed in low concentration from neutral molecules, resulting in a net increase in the concentration of free radicals within the system

Propagation - Radicals produced in the first step react with molecules to yield new molecules and new radicals; there is no net change in the concentration of radicals

Termination - Various radicals combine to give molecules, resulting in a net decrease in radical concentration and a decrease in the rate of the reaction

• Relative reactivities of each type of hydrogen

  • Aromatic vs Aliphatic vs Benzylic 

  • Primary vs Secondary vs Tertiary

  • What is determining factor as to why some hydrogens are more reactive than others (radical stability)

hyperconjugation for determining factor: tertiary more stable than primary b/c orbitals can spread electron charge