Lab Practical Review

[START HERE] What four questions should you answer for every lab step?

What is its purpose? Why does it work? Where is the desired compound? What happens if the step is done incorrectly?

[START HERE] What does the instructor say to prioritize for the final?

Experimental theory, each reagent's role, apparatus, mechanisms, physical properties, and calculations; exact procedural volumes and times need not be memorized.

[START HERE] Which property controls chromatography?

Different affinities for the mobile and stationary phases.

[RECRYSTALLIZATION] What are the five steps of recrystallization?

Choose solvent; dissolve impure solid in minimum hot solvent; hot gravity filter insoluble impurities; cool to crystallize; collect crystals by suction filtration.

[RECRYSTALLIZATION] What are the four characteristics of a good recrystallization solvent?

Dissolves target hot but poorly cold; impurities stay soluble cold or remain insoluble hot; chemically inert; volatile enough to remove readily.

[RECRYSTALLIZATION] Why use the minimum amount of hot solvent?

Excess solvent keeps more product dissolved after cooling and lowers recovery.

[RECRYSTALLIZATION] What does hot gravity filtration remove?

Insoluble impurities. The desired compound stays dissolved in the hot filtrate.

[RECRYSTALLIZATION] Why cool slowly to room temperature before an ice bath?

Slower crystal growth usually excludes impurities more effectively; the ice bath then improves recovery.

[RECRYSTALLIZATION] How can you induce crystallization?

Scratch the flask's inner wall or add a seed crystal of the desired compound to create a nucleation site.

[RECRYSTALLIZATION] Why wash collected crystals with a small amount of cold solvent?

Remove dissolved impurities while minimizing dissolution and loss of product.

[MELTING POINT] What is a melting point range?

Temperature at first liquid droplet through temperature at complete liquefaction.

[MELTING POINT] How do soluble impurities usually affect melting point?

They lower the melting temperature and broaden the melting range.

[MELTING POINT] What heating rate is used for an accurate melting point near the expected range?

About 1-2 degrees C per minute; slow down about 20 degrees C below the expected melting point.

[MELTING POINT] What is a mixed melting point test?

Mix unknown and known solid approximately 50:50, then measure the melting range to test identity.

[MELTING POINT] What is the eutectic point?

The minimum melting temperature for a particular two-component system, occurring at a specific composition.

[REFLUX AND DISTILLATION] What is reflux?

Boil a mixture while condensing vapor and returning it to the same flask, allowing prolonged heating with little solvent loss.

[REFLUX AND DISTILLATION] How does reflux differ from distillation?

Reflux returns condensate to the reaction flask; distillation collects condensate in a separate receiving vessel.

[REFLUX AND DISTILLATION] Where should condenser cooling water enter and exit?

Enter at the lower port and exit at the upper port, keeping the jacket full.

[REFLUX AND DISTILLATION] Why add boiling chips before heating?

They provide nucleation sites for smooth boiling and reduce superheating and bumping.

[REFLUX AND DISTILLATION] What happens to boiling point when external pressure decreases?

It decreases.

[REFLUX AND DISTILLATION] When does your course recommend simple distillation?

For liquids whose boiling points differ by roughly 100 degrees C or more, or to separate a volatile liquid from nonvolatile solute.

[REFLUX AND DISTILLATION] When does your course recommend fractional distillation?

For miscible liquids whose boiling points differ by less than roughly 100 degrees C; treat this as the course guideline.

[REFLUX AND DISTILLATION] What is an azeotrope?

A constant-boiling mixture whose vapor and liquid have the same composition at that pressure; ordinary fractional distillation cannot cross that composition.

[EXTRACTION] What determines the upper and lower extraction layers?

Density: denser phase is the bottom layer, less dense phase is the top.

[EXTRACTION] Where is methylene chloride relative to ordinary aqueous solution?

Usually the bottom layer because it is denser than water.

[EXTRACTION] What does acid do to an amine during acid/base extraction?

Protonates it to a charged, usually water-soluble ammonium salt: RNH2 + H+ -> RNH3+.

[EXTRACTION] What does base do to a carboxylic acid during extraction?

Deprotonates it to a charged, usually water-soluble carboxylate: RCO2H + OH- -> RCO2- + H2O.

[EXTRACTION] How do you recover an amine from its aqueous ammonium salt?

Add base to regenerate the neutral amine, which may precipitate or be re-extracted into organic solvent.

[EXTRACTION] How do you recover a carboxylic acid from its aqueous carboxylate?

Add acid to regenerate the neutral carboxylic acid, which may precipitate.

[EXTRACTION] What possible acidic components appear on your acid/base slide 7?

Benzoic acid, 2-methoxybenzoic acid, or o-toluic acid. This is the slide's listed set; check your current unknown list.

[EXTRACTION] In the slide 8 extraction flowchart, which reagent is added first?

HCl first extracts benzocaine as its ammonium salt into water; NaOH then extracts the carboxylic acid from the remaining organic layer as carboxylate.

[TLC] What are the stationary and mobile phases in normal-phase TLC?

Polar silica gel or alumina is stationary; an organic liquid solvent or solvent mixture is mobile.

[TLC] Which compound usually has lower Rf on silica with the same eluent: more polar or less polar?

More polar: stronger stationary-phase interactions keep it closer to the origin.

[TLC] What usually happens to Rf when the eluent becomes more polar on silica?

Rf increases because the stronger eluent competes more effectively for silica interactions.

[TLC] How do you calculate Rf?

Distance from origin to spot center divided by distance from origin to solvent front; both measured along solvent travel.

[TLC] Why use pencil rather than ink on a TLC plate?

Ink may dissolve and separate into extra spots; graphite generally stays at the origin.

[TLC] What are the four analgesic standards in your TLC experiment?

Acetaminophen, acetylsalicylic acid (aspirin), caffeine, and ibuprofen.

[GC AND SUBSTITUTION] What are the mobile and stationary phases in your gas-liquid chromatography experiment?

Inert carrier gas, such as helium, is mobile; a nonvolatile high-boiling liquid/polymer film is stationary.

[GC AND SUBSTITUTION] What is retention time?

Time from injection to the detector peak for a component.

[GC AND SUBSTITUTION] What is the purpose of your GC reaction experiment?

Compare Br- and Cl- competing to form tert-butyl halides from tert-butanol under acidic conditions, then analyze the product ratio.

[GC AND SUBSTITUTION] Why is the tert-butanol reaction classified as SN1?

Protonated OH leaves as water to form a tertiary carbocation, then Br- or Cl- attacks; a tertiary center disfavors SN2.

[GC AND SUBSTITUTION] What is the SN1 rate law?

rate = k[substrate]; carbocation formation is the rate-determining step.

[GC AND SUBSTITUTION] What is the SN2 rate law?

rate = k[substrate][nucleophile].

[NUTMEG AND HYDROGENATION] What compound is isolated from nutmeg?

Trimyristin, a triglyceride composed of glycerol esterified with three myristic acid units.

[NUTMEG AND HYDROGENATION] What kind of extraction is the nutmeg experiment?

Solid-liquid extraction of trimyristin using methylene chloride.

[NUTMEG AND HYDROGENATION] Is hydrogenation oxidation or reduction of the alkene?

Reduction, because carbon gains bonds to hydrogen.

[NUTMEG AND HYDROGENATION] What catalyst and hydrogen donor were used for olive-oil hydrogenation?

Pd/C catalyst and cyclohexene hydrogen donor.

[NUTMEG AND HYDROGENATION] What does cyclohexene become, and what drives this transfer?

Benzene; formation of an aromatic ring favors hydrogen donation.

[NUTMEG AND HYDROGENATION] Why rinse the Celite filter with hexane?

To transfer product retained in the filter into the collected solution.

[DIPHENYLACETYLENE] What is the two-stage route from trans-stilbene to diphenylacetylene?

Bromination to meso-stilbene dibromide, followed by two dehydrohalogenations to make the alkyne.

[DIPHENYLACETYLENE] What is PHPB's role?

Pyridinium hydrobromide perbromide provides a bromine equivalent for alkene bromination; it is a solid alternative to handling liquid bromine.

[DIPHENYLACETYLENE] What reagents/conditions convert stilbene dibromide to diphenylacetylene in your lab?

KOH in ethylene glycol with heat/reflux.

[DIPHENYLACETYLENE] What happens in each E2 dehydrohalogenation?

Base removes a neighboring H while the C-H electrons form a pi bond and C-Br breaks to Br-; two eliminations form the triple bond.

[DIPHENYLACETYLENE] How many HBr equivalents are removed to form an alkyne from a vicinal dibromide?

Two; this requires at least two base equivalents in the net stoichiometry.

[CHEMDRAW AND WILLIAMSON] What mechanism is used in Williamson ether synthesis?

An alkoxide or phenoxide nucleophile attacks an alkyl electrophile by SN2 to form a C-O bond.

[CHEMDRAW AND WILLIAMSON] At which carbon does phenoxide attack chloroacetic acid/chloroacetate?

The CH2 carbon bonded to chlorine, displacing Cl- by SN2 and producing an aryloxyacetate framework.

[CALCULATIONS] How do you convert mass to moles?

n = mass / molar mass.

[CALCULATIONS] How do you find the limiting reagent for any balanced reaction?

For each reactant calculate available moles divided by its stoichiometric coefficient; the smallest value limits reaction progress.

[CALCULATIONS] How do you calculate theoretical product mass?

Moles of limiting reagent x (product coefficient / limiting reagent coefficient) x product molar mass.

[CALCULATIONS] How is percent yield calculated?

100 x actual dry product mass / theoretical product mass.

[START HERE] What four questions should you answer for every lab step?

What is its purpose? Why does it work? Where is the desired compound? What happens if the step is done incorrectly?

[START HERE] What does the instructor say to prioritize for the final?

Experimental theory, each reagent's role, apparatus, mechanisms, physical properties, and calculations; exact procedural volumes and times need not be memorized.

[START HERE] Which property controls recrystallization?

The desired solid's solubility changes strongly with temperature: soluble hot, poorly soluble cold.

[START HERE] Which property controls distillation?

Relative volatility and boiling point differences.

[START HERE] Which property controls liquid-liquid extraction?

Relative solubility in two immiscible liquids; acid/base reactions can change solubility by changing charge.

[START HERE] Which property controls chromatography?

Different affinities for the mobile and stationary phases.

[RECRYSTALLIZATION] What are the five steps of recrystallization?

Choose solvent; dissolve impure solid in minimum hot solvent; hot gravity filter insoluble impurities; cool to crystallize; collect crystals by suction filtration.

[RECRYSTALLIZATION] What are the four characteristics of a good recrystallization solvent?

Dissolves target hot but poorly cold; impurities stay soluble cold or remain insoluble hot; chemically inert; volatile enough to remove readily.

[RECRYSTALLIZATION] Why use the minimum amount of hot solvent?

Excess solvent keeps more product dissolved after cooling and lowers recovery.

[RECRYSTALLIZATION] What does hot gravity filtration remove?

Insoluble impurities. The desired compound stays dissolved in the hot filtrate.

[RECRYSTALLIZATION] Where is the desired compound immediately after hot gravity filtration?

In the hot filtrate, which must be kept.

[RECRYSTALLIZATION] Why keep the funnel and solution hot during gravity filtration?

To prevent desired product crystallizing prematurely on the filter or in the stem.

[RECRYSTALLIZATION] Why use a short-stem funnel and fluted paper for hot filtration?

They reduce cooling/clogging and help the hot solution filter quickly.

[RECRYSTALLIZATION] Why cool slowly to room temperature before an ice bath?

Slower crystal growth usually excludes impurities more effectively; the ice bath then improves recovery.

[RECRYSTALLIZATION] How can you induce crystallization?

Scratch the flask's inner wall or add a seed crystal of the desired compound to create a nucleation site.

[RECRYSTALLIZATION] What if scratching, seeding, and cooling still do not produce crystals?

Too much solvent may have been used; evaporate some solvent and cool again.

[RECRYSTALLIZATION] Why wash collected crystals with a small amount of cold solvent?

Remove dissolved impurities while minimizing dissolution and loss of product.

[RECRYSTALLIZATION] Where is the desired compound during final suction filtration?

In the solid crystals retained on the filter paper.

[RECRYSTALLIZATION] Why wet Buchner filter paper before filtration?

To seal it against the funnel so crystals cannot pass underneath it.

[RECRYSTALLIZATION] Why must filter paper cover all holes in a Buchner funnel?

Uncovered holes let crystals pass through, lowering recovery.

[RECRYSTALLIZATION] Why use thick-walled vacuum tubing?

Thin tubing can collapse under reduced pressure and block suction.

[RECRYSTALLIZATION] What is the neoprene adapter's purpose?

It creates an airtight seal between the Buchner funnel and filter flask.

[RECRYSTALLIZATION] Why dry crystals before weighing?

Residual solvent adds mass and artificially increases calculated recovery or yield.

[RECRYSTALLIZATION] How is percent recovery calculated for recrystallization?

100 x mass of dry recovered solid / initial mass of crude solid. This is a purification recovery, not a reaction yield.

[MELTING POINT] What is a melting point range?

Temperature at first liquid droplet through temperature at complete liquefaction.

[MELTING POINT] What melting range is typical for a pure solid?

A narrow range, often about 1-2 degrees C; compare with the literature value.

[MELTING POINT] How do soluble impurities usually affect melting point?

They lower the melting temperature and broaden the melting range.

[MELTING POINT] How does residual solvent affect a solid's melting point?

It acts as an impurity, usually lowering and broadening the range.

[MELTING POINT] How should a melting point sample be prepared?

Dry it, crush to a fine powder, and pack a small 1-2 mm height tightly into the closed end of a capillary.

[MELTING POINT] What errors can too much sample or poor packing cause?

Uneven heating and a broader, less reliable melting range.

[MELTING POINT] What heating rate is used for an accurate melting point near the expected range?

About 1-2 degrees C per minute; slow down about 20 degrees C below the expected melting point.

[MELTING POINT] Why use a fresh sample for each melting point run?

Heating may change or decompose the sample, making reuse unreliable.

[MELTING POINT] What is a mixed melting point test?

Mix unknown and known solid approximately 50:50, then measure the melting range to test identity.

[MELTING POINT] Unknown plus standard A melts sharply with no depression. What does this suggest?

The unknown and A are likely the same compound.

[MELTING POINT] Unknown plus standard B melts lower and broader. What does this suggest?

B is a different compound and acts as an impurity.

[MELTING POINT] What is the eutectic point?

The minimum melting temperature for a particular two-component system, occurring at a specific composition.

[MELTING POINT] Can a mixture ever melt sharply?

Yes. A mixture at its eutectic composition can melt sharply, so a narrow range alone does not prove purity.

[REFLUX AND DISTILLATION] What is reflux?

Boil a mixture while condensing vapor and returning it to the same flask, allowing prolonged heating with little solvent loss.

[REFLUX AND DISTILLATION] How does reflux differ from distillation?

Reflux returns condensate to the reaction flask; distillation collects condensate in a separate receiving vessel.

[REFLUX AND DISTILLATION] Where should condenser cooling water enter and exit?

Enter at the lower port and exit at the upper port, keeping the jacket full.