CHEM 43A Final Exam

What is a compound's melting point?

- it is the temperature at which a solid transition to a liquid

What was the objective of Experiment 0: Introduction to the Organic Lab

- determine the precision and accuracy for the density of water using pasteur pipettes
- determine the identity of an unknown using melting points of pure compounds and mixtures

Narrow vs. broad melting point ranges

- narrow melting point range: compound is pure
- broad/depressed melting point range: compound contains impurities that disrupt the intermolecular interactions from occurring properly

How can you definitively confirm the identity of an unknown compound using melting point determination?

- mix the unknown with a known standard
- sharp melting point range: the unknown and the known are the same
- depressed/broad melting point range: the unknown and the known are different compounds

Why do we calculate the correction factor and how do you calculate it?

- correction factor: literature mp value - observed mp value
- the thermometer is not always accurate
- in order to account for this error, we use the correction factor and either add/subtract it from our observed melting point

What was the objective of Experiment 1: Recrystallization?

- to purify an impure unknown via macro- and microscale recrystallization techniques
- identify the purified unknown using melting point determination

What is recrystallization?

- it is a purification technique that takes advantage of differences in solubility between the desired compound and any impurities that may be present
- insoluble impurities are removed via filtration
- after recrystallization, the crystals are recovered by filtration and the soluble impurities remain in the cold solution

What are the key steps of recrystallization?

- dissolution
- decolorizing
- hot filtration
- crystallization
- recovery

Dissolution

- the minimum amount of solvent is used to dissolve the compound
- too much solvent used = concentration of compound is too low, cooling will not yield a supersaturated solution, and no crystals will form from solution

Why are boiling chips used?

- they are small inert, black pieces of porous carborundum
- helps small bubbles quickly form and prevent bumping (formation of large bubbles that can boil over violently)

Decolorizing

- colored soluble impurities can be removed by the addition of activated carbon
- colored molecules have conjugated double bonds that absorb to the surface of charcoal, which can then be removed during filtration

Hot filtration

- the filtration is used to remove solid impurities
- it is performed hot to prevent premature cooling --> the filtration setup is pre-warmed
- gravity filtration is used to avoid evaporation and cooling that occurs during vacuum filtration

Crystallization

- crystal formation is dependent on solute concentration, temperature of solvent, and cooling rate
- too fast cooling rate = crystals are unstable, rapidly precipitate, and often contain impurities
- solute too soluble in solvent = no crystallization

How can crystal nucleation be induced?

- addition of previously obtained crystals (seed crystals)
- scratching the side of the flask
- increase concentration by reheating solution to remove excess solvent by evaporation

Isolation and recovery: macroscale

- vacuum filtration is used to collect purified compound in a Buchner funnel
- the vacuum trap must be used to prevent liquids from entering the vacuum system
- one rubber tubing connects side arms of flasks and the other connects from the top rubber adapter to the vacuum

Isolation and recovery: microscale

- used for smaller quantities (10-100 mg)
- craig tube is placed in a centrifuge tube and centrifuged to isolate the crystal from the solution
- crystals are clustered on the top of the plunger and whereas the liquid sits in the centrifuge tube

What was the objective of Experiment 2: Isolation of Natural Products

- determine the partition coefficient of benzoic acid in water
- extract caffeine from tea leaves into hot water
- separate caffeine from the aqueous layer, dry, and purify via sublimation

When you first place the tea bag in the hot solvent, what does the initial aqueous extract contain?

- caffeine: moderately polar and can be extracted from water into polar solvent (methylene chloride)
- cellulose: insoluble material
- tea proteins and pigments: water soluble and won't disrupt isolation of caffeine
- saponins and tannins

What are saponins?

- molecules that contain polar, water-soluble group and large, non-polar water-insoluble hydrocarbon group
- can increase the solubility of organic molecules in water

What are tannins?

- has D-glucose core and multiple units of gallic acid via ester bonds
- tannins are soluble in methylene chloride, which complicates isolation of caffeine
- aqueous base (NaCO3) cleaves ester bonds and forms carboxylate anion of gallic acid (gallic acid salt) and glucose --> both are soluble in water and will not be extracted into the methylene chloride

Liquid-liquid extractions

- separation method based upon differential solubility of components in a mixture between two immiscible solvents
- used to isolate and separate a desired compound based on differing solubilities and partition coefficients

What are the steps of liquid-liquid extraction?

- dissolution of mixture in suitable solvent
- addition of an immiscible solvent
- mixing of solvent layers
- separation of layers using funnel and back-extractions of aqueous layer
- drying of organic solution
- isolation of separated component (followed by sublimation)

What is the partition coefficient?

- K --> the ratio of the solubility of the compounds in the two solvents
- [compound] organic / [compound] aqueous

How do you dry an organic solution?

- water and organic solvents can still be partially miscible so organic layer still contains a little bit of water
- organic layer is first washed with brine to remove most of the dissolved water
- drying agents (like magnesium sulfate and sodium sulfate) absorb the last traces of water and can be removed via filtration
- enough drying agent is added when it looks like a snow-globe

What is the difference between magnesium sulfate and sodium sulfate?

- magnesium sulfate --> absorbs less water but rate of absorption is faster
- sodium sulfate --> removes more water but slower kinetics for absorption

What is sublimation?

- process of passing directly from solid to gas without going through the liquid state
- occurs when atmospheric pressure= vapor pressure of compound
- in this experiment, crude caffeine (solid) becomes a gas and goes to the top of the petri dish where it encounters the ice bath and freezes (becomes a solid once again).

What was the objective of Experiment 3: Acid-Base Extractions

- separate a three-component mixture using liquid-liquid extraction and acid-base chemistry

What does changing the pH of the aqueous layer do?

- it changes the composition and solubility of specific organic compounds
- changing hydrophobic, neutral organic compounds into hydrophilic, water soluble cations and anions

Acidic organic compounds

- ex. carboxylic acids or phenols
- converted into water soluble anions
- weak bases like sodium bicarbonate can deprotonate carboxylic acids
- strong bases like sodium hydroxide can deprotonate CA's and phenols

Basic organic compounds

- ex.) amines
- converted into water soluble cations
- acids like hydrochloric acid can protonate amines

What do you do when the organic compound has moved into the acidic/basic aqueous phase?

- if the organic compound is in a basic solution and is deprotonated, an acid is added to neutralize the compound and is moved back into the organic phase
- if the organic compound is in an acidic solution and is protonated, a base is added to neutralize the compound and is moved back into the organic phase
- reversal of acid/base reaction and extracted into organic solvent

What was the objective of Experiment 4: Chromatography

- to determine the relative polarity of hydroxyacetophenone isomers using TLC
- to identify an unknown over-the-counter analgesic using TLC
- to separate a three component mixture by column chromatography and evaluate by TLC

How does chromatography separate compounds?

- separates based on intermolecular interactions between the compound, a mobile phase, and a stationary phase
- chromatography is a separation technique and the resultant optical response must be compared to a known standard for identification

Interactions between stationary and mobile phases

- the stationary phase is often polar and will interact strongly with polar compounds --> nonpolar compounds travel farther and faster whereas polar compounds travel slower
- polar solvents interact with polar stationary phases and allow polar compounds to travel farther than normal

Thin layer chromatography

- stationary phase is affixed in a thin layer to a plate
- placed in solvent reservoir and solvent travels up stationary phase via capillary action
- used for small scale determination of the number of compounds in a mixture and the conditions required to achieve efficient separation

What is the purpose of using filter paper?

- used to equalize the solvent vapor in the chamber
- prevent eluent from evaporating off the plate

Spotting for TLC's

- too small of a spot = no visualization of sample
- too big of a spot = streaking or no distinct spots

What are retention factors?

- calculated because they are characteristic for a compound under specific conditions
- can show compounds' relative polarities under those conditions
- can be used to identify an unknown compound relative to a known compound
- Rf = distance from origin to center of the spot / distance from origin to solvent front

Column chromatography

- stationary phase is packed into a tube
- mobile phase is passed over the stationary phase using gravity or pressure
- used as a large scale separation method
- usually, fractions are collected at regular intervals and tested using TLC plates

Dry packing/loading column chromatography

- dry packing means that adsorbent is not suspended in solvent and is directly added to the column
- dry loading is used when compounds are not very soluble in eluent
- compounds are dissolved in a different solvent and some silica gel is added; solvent is then evaporated to absorb compounds onto silica gel
- cotton --> sand --> adsorbent stationary phase --> mixture for separation --> sand

Solvent system of column chromatography

- we want a solvent system where each compound shows an isolated spot
- typical gradient: nonpolar solvent --> increasing solvent polarity
- thus, the first eluted compound will be the nonpolar compound then in order of increasing compound polarity

What was the objective of Experiment 5: Distillation and Gas Chromatography

- separate a mixture of two hydrocarbons by simple and fractional distillation
- evaluate the separation using gas chromatography

What is distillation?

- it is a separation and purification technique that relies on differences in boiling points and vapor pressures to separate components of a liquid mixture

How does distillation work?

- the mixture of miscible liquids is heated and begins to boil off/vaporize
- at first, the vapors contain the more volatile component (smaller boiling point)
- later on, the vapors will contain the less volatile component (greater boiling point)
- vapor is cooled and condensed in a condensed and falls into a separate flask

How can distillation be used to purify liquids?

- the distillation process is repeated many times, each time increasing the purity of the collected liquid

How is fractional distillation different from simple distillation?

- fractional distillation is a technique for accomplishing a larger number of simple distillations in a single continuous operation

What does the fractionating column increase the efficiency of separation?

- has an extensive surface area that induces vapors to condense before the collection flask
- cooled condensate encounters upward moving hot vapors that cause the condensate to vaporize once again according to their vapor pressures/boiling points
- vapors that rise to the top of the column are enriched in the more volatile component whereas the vapors that recondense back down into the distillation flask primarily contain the less volatile component

What are the components of the distillation apparatus?

- lab jack
- heating mantle/hot plate
- round bottom flask
- fractionating column with copper mesh
- distillation head
- condenser: water out then water in
- bent adapter
- thermometer: positioned just below the side arm of the distillation head
- plastic keck clips

Gas chromatography

- analytical, quantitative technique that uses a gas as the mobile phase
- solution containing the compounds to be separated is injected into GC; sample is heated under reduced pressure to vaporize everything
- an inert gas carries the vaporized compounds through the stationary phase
- detector at the exit determines the time it takes for each component to pass through the column (retention time = response factors of TLC)

How is GC quantitative?

- you can determine the relative amounts of each component injected by calculating the relative area under each peak in the GC trace
- the peak area is proportional to the absolute amount of compound that was injected

Relative response factors

- they are calculated because different compounds can respond differently to every GC detector (even if it is a 1:1)
- detectors can be more sensitive to one compound over the others
- use the smaller peak area and normalize the areas to this one small peak area; divide each area by the reference area
- this "correction factor" is used to adjust the areas (area/correction factor)

Percent of compound in mixture

area a / (area a + area b) x 100
- these area values are all the corrected areas

Volume of a component

volume of total sample x % compound

Moles of a component

volume of component x density (g/mL) / molecular weight (g/mol)

What was the objective of Experiment 6: Fischer esterification

- to identify an unknown alcohol by transforming it into an ester via the Fischer Esterification
- to identify the ester (and thereby the starting alcohol) by IR, GC, and boiling point

How does Fischer Esterification work?

- carboxylic acid + alcohol (H2SO4 and reflux) --> ester + water
- this reaction is reversible and under equilibrium
- only one drop of sulfuric acid is needed to catalyze the reaction

How is the Fischer Esterification reaction maximized?

- we take advantage of Le Chatelier's principle to push the reaction forward
- a large excess of one of the reactants is used
- the byproduct water is removed as it forms throughout the reaction

What is the setup for a Fischer esterification?

- conical vial with mixture solution
- magnetic spin vane
- reflux condenser
- drying tube with CaCl2 on top of reflux condenser

During esterification, what is the point of refluxing and the reflux condenser?

- Because some reactions occur at a slower rate, the purpose of refluxing is to increase the rate of the reaction by heating the mixture in a controlled, constant setting
- the reflux condenser prevents vapor from escaping the system, cools these vapors, and recondenses it back into the original flask

Why is a plug column chromatography used?

- filtration through the silica removes any polymeric by-products, which do not move through the silica

Boiling point determination

- boiling point: temperature at liquids become gases

How do you determine the boiling point of a liquid?

- fill melting point capillary tube with liquid using a TLC capillary tube
- insert glass bell into mp capillary tube with open end pointing down
- raise temperature of melting point apparatus until one bubble/second exits the glass bell
- trapped air exits the bell as bubbles; sample vapors enter the glass bell and displace the trapped air
- at this stage, only sample vapors are in the bell and escape more rapidly --> then, turn off heat
- note the temperature when the last bubble escapes the bell = boiling point
- when the temperature falls below boiling point of sample, the sample vapors inside the glass bell condense and liquid is rapidly pulled into the bell

IR Spectroscopy

- organic molecules absorb infrared light when the wavelength and energy corresponds to specific molecular vibrations and rotations
- IR spectrum shows inverted peaks where wavelengths of light are absorbed
- wave number (cm^-1) = 1/wavelength
- the spectrum can show which distinct functional groups are present/absent

What are the different ways a molecule can vibrate?

- stretching
- wagging (asymmetric stretching)
- bending

What are the distinct regions of the IR spectrum and what wave numbers do they correspond to?

3500 - 2700: O-H, N-H, C-H
2700 - 1900: triple bonds (C---C, C---N, X--Y--Z)
1800 - 1600: double bonds (C--C, C--O, C--N, N--O)
1400-600: fingerprint region (C-C, C-O, C-N) --> contains complex series of absorptions; contains all bending, stretching, and combination vibration bands

How do the GC traces help you determine your unknown ester?

- the retention times of your unknown ester should approximately match one of the four known products
- however, because retention times can vary over time and between runs, a co-spot is done (mixture of your unknown ester with a 1:1:1:1 solution of known ester products)
- only the peak corresponding to your ester product will show an increase in peak area because that ester product constitutes more of the mixture solution

What was the objective of Experiment 7: Identification of an unknown compound

- apply chemical knowledge and logical thinking to efficiently identify an unknown compound
- determine the physical properties of an unknown compound
- identify the functional groups present/absent using IR and chemical tests
- successfully prepare a chemical derivative to confirm identity via melting point

Chemical tests

- these tests are characterized by visual changes that indicate a reaction took place
- visual changes include color changes, formation of precipitate, formation of bubbles
- Jones, Lucas, 2,4-DNP, and Tollens Test

Jones Oxidation Test

- used to test the presence of primary or secondary alcohols or aldehydes
- color change --> red-orange Cr(VI) is reduced to blue-green Cr(III)
- reagents: chromic acid (from CrO3 and H2SO4)

Lucas Test

- used to test the presence of secondary or tertiary alcohols; cannot be a primary alcohol because a primary carbocation is very unstable
- starting alcohol must be water soluble
- solubility change, clear solution becomes cloudy or divides into two layers --> water soluble alcohol becomes immiscible alkyl halide
- reagents: anhydrous Zinc chloride and HCl

2,4-Dintrophenyl hydrazine test

- used to test the presence of aldehydes and ketones; condensation reaction
- precipitate formation --> insoluble red-orange precipitate forms from clear orange solution
- reagent: 2,4-Dintrophenyl hydrazine

Tollens Test

- used to test the presence of aldehydes
- precipitate formation --> silver mirror forms from clear solution
- reagent: AgNO3 and NH4OH

Preparation of chemical derivatives

- the liquid organic compound reacts with specific reagents to generate a new compound, a derivative of the original
- the derivatives can be easily purified via recrystallization and characterized by melting point determination

Preparation of 3,5-Dinitrobenzoate Ester

- derivation for alcohols
- chemical reaction: ester formation
- reagents: 3,5-dinitrobenzoyl chloride and pyridine

Preparation of 2,4-Dintrophenyl hydrazone (DNP) derivatives

- derivative for aldehydes and ketones
- chemical reaction: hydrazone formation
- reagents: 2,4-dinitrophenylhydrazine, HCl, and ethanol

Preparation of semicarbazone derivatives

- derivative for aldehydes and ketones
- chemical reaction: semicarbazone formation
- reagents: semicarbazide hydrochloride, sodium acetate, and ethanol