CHEM 43A FINAL

Why can’t you use liquid-liquid extraction for amine compounds?

• they are all organic compounds and highly soluble in organic solution

Why can’t you use acid-base chemistry for these amine compounds?

• they are all amine and when they get protonated, they go to the aqueous layer

Why does water go up in chromatography?

• capillary action

• intermolecular interactions between liquid and surfaces

• if small pores, surface tension and adhesive interaction between liquid and surface causing the liquid to propel forward

Why do the colors go up?

• they are water soluble so with some attraction to water, the water drags them up

Why did the spots separate and not just appear at the top with water?

• attraction to paper

Stationary phase

• absorbent

• paper, silica gel, aluminum gel

Mobile phase

• eluent

• solvent

• the thing moving across the stationary phase

Compound and stationary phase interaction

• POLAR COMPOUND + POLAR S.P. → strong interaction → slower since it sticks to the stationary phase (DIPOLE-DIPOLE)

• NONPOLAR COMPOUND + POLAR S.P. → weak interaction → moves faster (dipole-induced dipole)

Compound and solvent interaction

• POLAR COMPOUND + POLAR SOL. → STRONG (compound interacts with solvent so it moves faster)

• NONPOLAR COMPOUND + POLAR SOL. → WEAK (slight interaction, faster)

• POLAR COMPOUND + NONPOLAR SOL. → WEAK (no rate increase, still reacts with S.P., solvent does not interfere)

• NONPOLAR COMPOUND + NONPOLAR SOL. → WEAK (no rate increase, still reacts with S.P., solvent does not interfere)

Solvent and stationary phase interaction

• POLAR SOL. + POLAR S.P. → STRONG (doesn’t interact with stationary phase, FASTERER)

• NONPOLAR SOL. + POLAR S.P. → fasterer

Which solvents don’t work for chromatography? (too polar and dissolves silica gel)

• water

• ethanol/methanol

• acetone

What are some medium polarity solvents for chromatography?

• ethyl acetate

• diethyl ether

• dichloromethane

What are some nonpolar solvents for chromatography?

• toluene

• hexane

• petroleum ether

TLC

• identification and purity technique

TLC chamber

• wick

  • sucks up solvent and solvent evaporates off wick and fills chamber up with solvent vapor

  • IF NO VAPOR, SOLVENT WILL VAPORIZE OFF PLATE

• remove close to the top/stops advancing

  • don’t want Rf to run off plate

  • stops advancing - evaporating faster than it can be carried with capillary action

What are some ways to visualize a TLC plate?

• iodine

  • sublimes and reversibly stick to organic compounds

• UV light - green UV dye in silica gel

  • if compound absorbs UV, dye doesn’t glow = dark spot

  • conjugated pi-systems absorb light

Retention factor

• distance traveled/distance solvent traveled

• not precise

Column chromatography

• purification technique

• Rf ~ 0.3 for desired compound

• delta Rf > or equal to 0.2 for good separation

Packing column

• wet pack

  • slurry of solvent + silica gel

  • when compound is soluble in solvent

• dry pack

  • add dry silica to column, run solvent through

  • when compound does not dissolve in solvent

  • dissolve in minimum amount of solvent

Why do you start with the nonpolar solvent for column chromatography?

• to maintain a gradient and make sure it elutes slowly to produce good separation

What is distillation?

• separation and purification technique that is based on phase change

• affected by temperature and pressure

Raoult’s law

• Vp = sum of components = PaNa + PbNb

• N = mole fraction

Boiling point of mixtures: NON VOLATILE SOLUTES

• VP SOL < VP H2O

Boiling point of mixtures: MIXTURE OF 2 LIQUIDS

• higher VP, more volatile, low boiling point

• lower VP, less volatile, higher boiling point

Fractional distillation

• multiple boiling and condensation cycles to raise purity

• each cycle results in solution and vapors enriched with more volatile component (theoretical plate)

• fractioning columns

  • extra surface area

  • copper mesh has higher surface area

Gas chromatography

• mobile phase

  • N2, He, Ar (no strong interaction)

• stationary phase

  • polymer/liq

• requires vapor and heat

• more volatile compound goes through faster (less attraction)

• less volatile compound goes through slower (more attraction)

Jones test

• positive: blue-green

• negative: red-orange

• primary/secondary alcohols, aldehydes

Lucas test

• check solubility in water first

• positive: cloudy with 2 layers

• negative: clear

• secondary alcohols (slow), tertiary alcohols (fast)

Tollens test

• positive: precipitate formation (SILVER MIRROR)

• negative: clear

• aldehydes

2-4 DNP test

• positive: precipitate formation ~ red-orange

• negative: clear orange solution

• aldehydes and ketones

ALCOHOL DERIVATIVE (3,5 DNB)

• add pyridine as catalyst

• benzoate ester

ALDEHYDE AND KETONE DERIVATIVE

• 2,4-DNP

  • HCl + ethanol

  • hydrazone

• semicarbazone

  • sodium acetate (NaOAc) + ethanol

  • carbazone

Iodoform test

• positive: pale yellow solid

• negative: dark solution

• methyl ketone