Organic Chemistry Lab

Question 1

procedure of recrystallization

Answer 1

1. “hot” (gravity) filtration
2. “cold” (suction) filtration
3. “washing”

Question 2

true vs capillary melting points

Answer 2

• true melting point - temperature at which solid and liquid phases of compound in equilibrium at one atmosphere
• capillary melting point - temperature range over which crystals of a small sample of compound first begin to liquify until liquefaction completes

Question 3

extraction

Answer 3

partitioning of solutes between two contacting solvents, which must not be appreciably soluble in one another

Question 4

modifying water-solubility of acids

Answer 4

1. induce acid in aqueous layer by adding base (eg sodium hydroxide), so water-insoluble acid converted to water-soluble ion
2. add strong acid (eg HCl), which reacts with ion to reform water and acid in alkaline solution

Question 5

normal boiling point

Answer 5

temperature at which liquid’s vapor pressure equals external pressure

Question 6

simple vs fractional distillation

Answer 6

• simple distillation - single vaporization/condensation cycle (enrichment in more volatile component estimated from application of Raoult’s Law or Dalton’s Law)
• fractional distillation - procedure “redistills the distillate” through several vaporization/condensation cycles (bubble plate or beads); cleaner separation of A and B

Question 7

height equivalent of a theoretical plate (HETP)

Answer 7

vertical length of column required for a single vaporization/condensation

• smaller HETP (better high surface packing), more plates available for a given length of column
• longer column available for given HETP, more plates available for better fractionation
• plates attributed to column (n - 1): one plate used between distilling pot and fractionating column

Question 8

neutralization equivalent calculation

Answer 8

(mg × purity factor of pure acid) / (volume of base titrated to equivalence) × (molarity of base) = molecular weight × number of carboxylic acid groups in compound

Question 9

sodium fusion follow-up tests

Answer 9

1. Pb²⁺ - black precipitate indicates presence of sulphur
2. FeSO₄/Fe³⁺ - deep blue precipitate indicates presence of nitrogen
3. AgNO₃ - precipitate indicates halogen (silver - chlorine; pale yellow - bromine; bright yellow - iodine)

Question 10

stationary vs mobile phase for gas chromatography

Answer 10

• stationary phase - long, coiled metal tube packed with an inert solid support coated in a high-boiling liquid
• mobile phase - inert carrier gas (usually He)

Question 11

“spiking” (enrichment) technique in gas chromatography

Answer 11

• (height of peak) × (width at half height) = area under peak
• assume equal weights of each compound result in equal areas under peaks
• percentage of each compound = (area of peak) / (sum of all peak areas)

Question 12

stationary vs mobile phase for thin-layer chromatography

Answer 12

• stationary - solid; interaction between solute and solvent molecules occurs at the surface
• mobile - liquid (solvent must be reasonable for given compound)

Question 13

specific rotation

Answer 13

observed rotation in degrees read in polarimeter depending on the compound, sample concentration, path length, wavelength of light, temperature, and solvent

• [α]^T_D = (observed rotation) / (path length dm) × (density of liquid)

Question 14

optical purity (entantiomeric excess)

Answer 14

(observed specific rotation) / (specific rotation of pure entantiomer)

Question 15

anyhdrous potassium carbonate (K₂CO₃)

Answer 15

used as drying agent; removes water from compound, which then can be separated via gravity filtration

Question 16

chromic acid test (identifying aldehydes and ketones)

Answer 16

• compounds oxidized by CrO₃ and H₂SO₄
• color change (orange to blue-green) indicates a primary alcohol, secondary alcohol, or aldehyde
• forms carboxylic acid, Cr³⁺

Question 17

Tollen’s test (identifying aldehydes and ketones)

Answer 17

• diamminesilver (I) ion as oxidant - silver deposits as a mirror or collodial black precipitate
• aldehyde specific
• forms carboxylate ion (R-C(=O)-O^-) and Ag⁰

Question 18

iodoform test (identifying aldehydes and ketones)

Answer 18

• oxidation of organic substrate by iodine; yellow precipitate of iodoform (CHI₃) forms
• indicates presence of methyl ketones, secondary methyl carbinols, acetaldehyde, and ethanol
  
  • methyl ketones - carboxylate ion (R-C(=O)-O^-) and CHI₃
  • secondary methyl carbinol - methyl ketone (R-C(=O)-CH₃) and CHI₃
  • ethanol - acetaldehyde (H-C(=O)-CH₃) ⇒ carboxylate ion and CHI₃

Question 19

semicarbazones (indentifying aldehydes and ketones)

Answer 19

• semicarbazide - H₂N-NH-C(=O)-NH₂
• formed by all aldehydes and ketones; melting point characteristic of aldehyde/ketone from which semicarbizone derived
• double-bonded oxygen replaced by N-NH-C(=O)-NH₂