Orgo Lab Final comprehensive deck

Why are melting points carried out?

1.Characterizes known compound (among several known compounds)
2. Records physical characteristic of new compound for future characterization by others
3. Establishes purity of known compound (if an experiment reliably forms this known compound or if you are taking the MP of a known compound)

Definition of the melting point of a pure compound

the temperature at which the vapor pressures of the solid and liquid are equal

What is the criteria for impurity to depress melting point? What would one observe when taking a melting point of an impure sample?

Impurity must be soluble in the compound, and can be a solid, a liquid, or an organic solvent. An impure compound will melt over a wide range of temperature.

Examples of impurities that don't depress melting point.

Sand and charcoal are insoluble impurities that won't depress the melting point; will widen the range.

The impact of rate on taking melting point - how to take melting point accurately

Determination must be slow (temp increase should be no greater than 1 C) to give enough time for the heat to be transferred equally from the heating block to the sample to the thermometer. If you go too fast, temperature at sample is very different from temperature at thermometer and sample appears to have a wide range. Also, heat will reach the sample before the thermometer and the observed mp temp will be lower than expected temp.

What forces affect melting point? (Also, consider the effect on the melting point of larger
molecules, symmetrical molecules, ability of molecules to pack well, R,S isomers, and hydrogen
bonding molecules)

-intermolecular forces - ionic attractions, VdW forces, dipole-dipole interactions, hydrogen bonds
-efficient stacking maximizes Vander Waal's forces (higher mp)
-larger molecules - higher mp than smaller molecules
-symmetrical molecules - higher mp than non symmetrical molecules
-R,S isomers\-- R and S enantiomers will have the same melting points but the racemate will have a different mp
-Hydrogen bonding molecules- have higher mp than their counterparts of similar molecular weight

Define eutectic point (two ways mentioned in notes) and eutectic temperature
What are features of a eutectic point?

Eutectic point
-lowest point on melting point composition diagram where a mixture has a very narrow melting range
-point at which the two solids and their liquid solutions are in equilibrium

Features: narrow melting range but lower than pure melting point

Eutectic Temperature: the lowest temperature at which a mixture at a certain concentration will start melting

Melting point composition graph - how to read and interpret

Mixed melting points experiment - procedure, meaning of results

Mixed MP experiment is a way to confirm identity of an unknown. Take the unknown's melting range. Mix the unknown with a possible standard (that has a MR close to the unknown's MR) it could be. Take the MR again. If the MR stays the same, then unknown \= known standard. If MR decreases, then unknown \=/\= known.

How sample is prepared? Why sample should be prepared in a specific manner?

1.Sample is dry
a. Moisture from solvents like water will depress the MP and have a wide MR
2.Sample is finely powdered
a. If sample is in chunks, sample will not pack well causing air pockets that slow heat transfer
b. Air pockets are not good heat conductors and sample will appear to have a wide MR
c. Ensure efficient heat transfer throughout sample
3.Sample is tightly packed
a. By rapping sharply on a hard surface
b. To avoid air pockets
c. Ensure efficient heat transfer throughout sample
d. If not followed, will act like an impurity and widen MR
4.Sample must not be more than 2-3mm high
a. Too much sample in capillary will cause a differential in temperature throughout sample
b. Ensure efficient heat transfer throughout sample
c. If not followed, will act like an impurity and widen MR

Criteria for sample before packing and how it should be packed.
Melting point capillary

Shake the sample into closed end of capillary & pack tightly by rapping sharply on hard surface. If not packed tightly, then there might be air pockets causing inefficient heat transfer throughout sample.

How should sample be heated and why? What may happen if you heat the sample too quickly?

Rapidly heat the sample to within 20 degrees C of the MP of sample. Determination should be slow (less than 1 deg C/min). Determination should be slow to give enough time for the heat to be transferred equally from heating block to sample to thermometer. If you go too fast, temperature at sample will be very different from temperature at thermometer and sample will appear to have a wide range. Also, heat will reach the sample before the thermometer and the observed mp temp will be lower than expected temp

Reasons for doing melting point determination of an unknown sample twice

Run a very fast determination on the first sample to ascertain the approximate mp and then make a slow, careful determination using the other sample.

Definition of melting range

the temperature from when the first drop of liquid is observed (in the sample) to the temperature when sample is completely liquefied

What happens if you have too much sample

There will be a differential in temperature throughout the sample and inefficient transfer of heat.

What happens if sample is coarse (not powdered)

If sample is in chunks, there will be air pockets that will act like impurities by widening the MR by not allowing the sample to be packed properly. Also, it will result in inefficient transfer of heat throughout the sample.

What might happen if you take a melting point of a sample that had already melted and why?

the sample may decompose, so second mp will be lower than the first

Sources of error when taking a melting point

1. Moisture in air absorbed by sample
2. Thermometer in Mel-Temp not calibrated
3. Determination made too fast
4. Criteria for packing sample not adhered to
a. Sample not dry
b. Sample not packed tightly
c. Sample not powdered
d. Sample more than 2-3mm high in capillary
5. Contamination of sample

Expected observations and impact on result when sources of error are in play and when
precautions are not adhered to.
MP Experiment

Error 1 will depress MP and widen MR
Error 2 can depress or elevate MP
Error 3 will widen MR
Error 4a will depress MP and widen MR
Error 4b-4d will widen MR
Error 5 will depress MP and widen MR

Details on when to slow down heating and why

the mel-temp made the temp increase in a steady rate, but i think they want us to know that we should pre-set it to 20deg lower than the known mp, and then set it to increase at 1degC per minute
why: same reason you explained about slow rate

Impact on experiment and observations if criteria not adhered to

1) if not dry, moisture will act as impurity
2) if not sufficiently crushed, air pockets
3) if not packed tightly, air pockets
4) if higher than 3mm, will have a differential in temp

Impact on melting point and range if criteria not adhered to

1) if not dry, mp will be depressed and wide
2) if not sufficiently crushed, large range
3) if not packed tightly, large range
4) if higher than 3mm, large range

Strategy for identifying unknowns - once you take its melting point, how do you narrow down possibilities for the identity of the unknown.

Choose a possible standard (a known compound) that has a MR similar to the MR you determined for the unknown.

Reason for carrying out recrystallization

a method to purify solids

Concepts involved in recrystallization and how they are employed (including the concepts of
saturation and nucleation involving kidney stones)

-solubility: a solution with dissolved solute in hot solvent
-saturation level: solution becomes saturated with respect to the solute, which then recrystallizes
-exclusion: impurities are excluded
-nucleation- initiation of recrystallization; which can be a seed crystal, a speck of dust, or a scratch on the wall of the test tube.

kidney stones
- saturation level goes up with low urine volume and high concentration of stone promoters
-Nucleation on calcium phosphate - presents solid surface on which seed crystals form and eventually grows

Picking solvent: like dissolves like, balance of polarity that is needed for recrystallization

Nonpolar solvent dissolves nonpolar solutes and polar solvents dissolve polar solutes. If solute and solvent are too similar in polarities, then the solute will dissolve even when solvent is cold. If solute and solvent are too different in polarities, then solute will not dissolve at all in the solvent.

Requirements for solvents with respect to solute and impurities (and the level of solubility for
solutes as compared to impurities) and results if these requirements are not satisfied

- Requirements for solvents with respect to solute: Dissolves solute completely when solvent is hot (@ boiling point of solvent). Precipitates solute completely when solvent is cold. Doesn't react w/ solute

- Requirements for solvents with respect to impurities: Either does not dissolve impurities at all, so impurities can be filtered off from hot solution. Or dissolves impurities very well (even when solvent is cold), so impurities can be removed from crystals via filtration. If a solvent does neither one of these (wrong solvent), the purity is suspect.

How different kinds of impurities can be separated from solute

can be insoluble in solvent and filtered from hot solution or can be dissolved in solvent and removed via filtration

Requirement for solvents for solvent pairs

Both solvents have to miscible in each other. One solvent has to be a poorer solvent for the solute than the other solvent. Used when no one solvent has ideal properties with respect to solute.

Why solvent pairs are used and procedure of how to use solvent pairs

Solvent pairs are used when no one solvent has ideal properties with respect to solutes. To use a mixed solvent pair, dissolve the crystals in the better solvent (more solubilizing) and add the poorer solvent (less solubilizing) to the hot solution until it becomes cloudy, and the solution is saturated with the solute. The two solvents must, of course, be miscible with each other. If the crystals dissolve instantly at about 22°C, that solvent cannot be used for recrystallization because too much of the solute will remain in solution at low temperatures.

What happens in the process of using solvent pairs and precautions when using them

solution becomes cloudy which indicates the initial crystal formation
if 2nd solvent has a lower boiling point than first, can cause sudden vigorous boiling of mixture and hot solvent and may spew from apparatus
- remedy: lower temp slightly and then add 2nd solvent
if you use too much of the first solvent, you will have to use large amounts of 2nd solvent to start crystallization

Desirable and undesirable qualities of water, ethanol and ethyl acetate and acetone (common
crystallization solvents)

Water:
-desirable: cheap, nonflammable, nontoxic, dissolves a large variety of polar organic molecules
-undesirable: high bp and high heat of vaporization makes it difficult to remove from crystals

ethanol:
-desirable: high bp makes it a better solvent for less polar molecules, evaporates readily from crystals

ethyl acetate:
-desirable- right combination of moderately high bp and the volatility needed to remove it from crystals

acetone:
-desirable: excellent solvent
-undesirable: low bp means there is not much differences in the solubility of compound at its bp compared to about 22 C.

How to heat and all precautions while heating

All the steps that need to be taken to ensure a saturated solution

- Dissolve in minimum amount of hot solvent to ensure solution is saturated (watch carefully).
- Between adding the drops, to ensure only minimum amt of solvent is used: allow sufficient time for dissolving to happen
- It is best that solid be crushed to powder to speed up dissolving process
- If adding solvent fails to dissolve any more solid, it is likely that insoluble impurities are present. These can be removed in the filtering suspended solids step.

How to minimize loss of product in crystallization, collecting and washing steps - why does
product get lost in these steps

Cannot get 100% recovery at crystallization stage because there will always be some solute soluble in cold solvent. The colder the solution, the better recovery of crystals. Limitation would be if solvent would freeze when solution is at a cold temperature. Some crystals will be lost because they adhere to filter a paper (at the collecting step) and some crystals dissolve during the washing process.

Requirements when cooling, how to maximize % recovery in the cooling step

- Cool slowly; if cooled too fast, solid will come "crashing out" of solution as powder precipitating impurities along with it
- To ensure slow cooling, use paper towels to insulate test tube
- Don't disturb crystallizing solution- If disturbed, too many nuclei for crystallization form, leading to small crystals that come out of solution along with impurities

Remedies for unsaturated solution, supersaturation

Solution gets supersaturated: saturated solution cools & no crystals form
To induce crystallization, use seed crystals (should save some impure crystals as seeds before recrystallization)
Scratch bottom of test tube w/ glass rod
- Unsaturated Solution - solvent can dissolve more solute
• boil off solvent under aspirator tube
• or blow off solvent using gentle steam of air or nitrogen

Definition of superheating and prevention - how boiling sticks and stones achieve their purpose.

To prevent superheating (heating solution above its boiling point w/o actually boiling, which occurs w/ explosive violence (bumping)), add boiling stick or boiling chip. Air trapped in the wood comes out of the stick and forms the nuclei on which even boiling can occur.

Why crystals oil out - remedies and prevention

Oiling out - crystals come out of solution as an oil
Occurs when:
• TSaturated solution or BPsolvent \> MPsolute
• Melting point of solute is depressed to point such that low melting eutectic mixture of solute & solvent comes out of solution
Remedy: lower temperature at which solution becomes saturated with solute by adding more room-temp. solvent

Small powdery crystals - why they form and how to prevent, why large crystals are preferred

Large crystals are preferred over small crystals because they can be easily separated by filtration and they can be easily washed free of adhering impure solvent.

Activated charcoal - purpose and how it works

Used when a chemical reaction will produce high molecular weight byproducts that are highly colored. the impurities can be adsorbed onto the surface of activated charcoal by simply boiling the solution with charcoal.

Why is it important that boiling stick is removed during cooling process and why would crystals
form so easily on boiling stick?

It is important that boiling stick is removed during cooling process so that crystals don't form on the boiling stick, which can become a landing spot for crystals to grow on. This could lower the % recovery because you're losing crystals.

Filtering of suspended solids - when is it carried out, why vacuum filtration not used.

- Vacuum filtration is not used because hot solvent will cool & product will recrystallize in filter
- Removing solid impurities or charcoal from hot, saturated solution by gravity filtration, pressure filtration, decantation, or removing solvent w/ Pasteur pipette

Collecting crystals - why gravity filtration not used, why filter paper necessary for frit

- Shouldn't use gravity filtration because vacuum filtration is more efficient in recovering solvent from crystals
- Collect crystals using Hirsch funnel (vacuum filtration); used to remove dissolved impurities
-the frit will not be clogged with insoluble impurities

Drying crystals - why important

a. Necessary since solvent can act as impurity
b. If recovery \> 100%, product is wet
c. Dry by keeping on Hirsch funnel for a few minutes
d. Complete drying by squeezing crystals in between filter paper & letting crystals air-dry on watch glass

Sources of error in recrystallization and expected observations and impact on result due to those

- Material left behind after each transfer (on test tube, on spatula, on weighing paper) → lowers % recovery
- Solution being cooled too fast
- Moving test tube while solution is recrystallizing
- Contamination due to airflow
- Crystals are not dry

Precautions to take in all steps of recrystallization (especially dissolving, crystallization, collecting, washing, drying) - what observations expected if precautions not adhered to, impact on % recovery and purity if precautions on adhered to.


Expected observations and impact on result when precautions not adhered to

a. Dissolve in minimum amount of hot solvent to ensure solution is saturated (watch carefully). If too much solvent is added, evaporate solvent to point of saturation. If not saturate, not much will come out of solution (lower % recovery)
b. If boiling stick not added superheating will occur (heating solution above its boiling point w/o actually boiling, which occurs w/ explosive violence)
c. Boiling stick/chip should not be added to hot solution, which may be superheated and boil over or bump
e. When using solvent pairs:
ii. If 2nd solvent has lower boiling point than 1st, can cause sudden vigorous boiling of mixture & hot solvent might spew from apparatus
iii. Remedy: cool solution w/ 1st solvent slightly before adding 2nd solvent; if you use too much of 1st solvent, undesirably large amounts of 2nd solvent must be used to start crystallization
a. Make sure boiling stick is removed from crystallization solution so that crystals don't form on boiling stick
c. Cool slowly; if cooled too fast, solid will come "crashing out" of solution as powder precipitating impurities along with it
d. To ensure slow cooling, use paper towels to insulate test tube
e. Don't disturb crystallizing solution
i. If disturbed, too many nuclei for crystallization form, leading to small crystals that come out of solution along with impurities
ii. Crystals are not dried properly and still wet - higher % recovery and lowers the purity b/c solvent can act as an impurity

Which sources of error (or precautions not adhered to) affect purity and which affect % recovery

-crystals are still wet (affects % recovery and purity b/c solvent can act as an impurity)
-material left behind after each transfer (affects % recovery)
-solution cooled to fast (affects purity)
-contamination from airflow (affects purity)
-moving the test tube during crystallization (affects purity)

Role of each step in recrystallization (look at Hints for Preparation to consider each step and why those steps are important)

1. choosing the solvent and solvent pairs
a. must be balance in polarity
b. if solute and solvent have very different polarities, solute will not dissolve at all
c. if solute and solvent are too similar in polarities, solute will dissolve even in cold solvent
2. dissolving the solute - to saturate solution
3. decolorizing the solution - remove impurities
4. filtering suspended solids - remove insoluble impurities
5. recrystallizing the solutes - forming pure crystals
6. collecting and washing the crystals - remove adhering soluble impurities from crystals
7. drying crystals

How melting point affected if purity is of question after recrystallization

The melting point will be depressed because impurities depress and widen MP of the compound.

Reason for carrying out distillation

a way to purify liquids based on different bp's

Insulation of apparatus

Wrap sides of flask and distillation head (up to level of sidearm) with foil to make process as adiabatic as possible - want to keep this area as warm as possible so that liquid vaporizes.

Wrapping with a wet paper towel - why?

Want to keep sidearm as cool as possible - wrap with paper towel to make sure vapor condenses.

Position of the thermometer

just below the level of the sidearm
- So it accurately records the temperature of the vapor distilling through sidearm
- Bulb must not touch glass of apparatus. do not want to record temp. of glass

Position of end of sidearm

inside receiver vial to minimize release of vapors into the hood (loss of liquid)

How to control heat

scrape away sand from flask to cool down / pile up sand around flask to heat up using spatula

What may occur if the distillation occurs too quickly - why

Will not get equilibrium of descending liquid and ascending vapor
Will not maintain the proper temperature gradient in the column (to ensure liquid separation)

What may occur if the fractional distillation occurs too slowly why

Flooded column - characterized by column of liquid observed in the fractional column at the joint between column and reaction flask

Boiling point of mixture of a homogeneous mixture and a heterogeneous mixture

Bp of MISCIBLE mixture: bp will be between bp's of pure compounds

Boiling point composition graph - Fig. 5.3 and 5.4 - phases in the graph and how to interpret data from the graph (liquid and vapor composition of mixture - see graph and explanation on pg. 91. and 92 and lecture)

Effect of changing atmospheric pressure on boiling point

Bp \= When vapor pressure of liquids \= atmospheric pressure as the liquids are heated
The lower the atmospheric pressure, the lower the boiling pt.
The higher the atmospheric pressure, the higher the boiling pt

Uses of simple and fractional distillation - what kinds of mixtures are suitable for each kind of
distillation

Simple distillation:
- When bp difference btwn two liquids is LARGE (at least 75 deg C difference)
- When one liquid composition is < 10% (ex. One liquid \= 95%, other liquid \= 5%)
- simple distillation: not a complete separation (both liquids purer than before but not completely pure)
Fractional distillation:
- When bp difference btwn two liquids is small (prefer 20-30 deg C diff)
- When both liquids are present in substantial amounts
- Fractional distillation: series of continuous distillations (after a fraction vaporizes, some compound condenses and gets redistillied)

Requirements for a successful fractional distillation -including problems and how to solve

1) significant contact between liquid and vapor phases in the column
-achieved by filling the column with packing material that has a large surface area
2)maintenance of the proper temperature gradient along the column - dependent on the rate of distillation (hotter at bottom, cold at top)
3) sufficient length of column (figured out by trial and error)
4) sufficient difference in boiling points of the two liquids being separate (prefer 20-30 degrees difference)
Problem: flooded column (don't go too slow)
-characterized by a column of liquid observed in the fractional column at the joint between column and reaction flask
-avoided by having rate of distillate collected adjusted so that the column does not get flooded

Behavior of temperature during a simple or fractional distillation of a mixture.

temperature increases throughout distillation because composition of vapor that is distilling varies throughout
-some lower boiling point component comes of first
-then vapor is enriched with higher boiling point component

Definition of azeotrope (minimum and maximum) - recognize azeotrope on boiling point composition graph, behavior of azeotrope

Azeotrope: mixture of 2 liquids with a fixed boiling pt and fixed composition that cannot be altered by simple or fractional distillation
- Minimum boiling azeotrope: bp of mixture is below pure bp's of both liquids
- Maximum boiling azeotrope: bp of mixture is above pure bp's of both liquids

Definition of hold up

Definition of holdup: unrecoverable distillate that wets the column packing (only for fractional distillation)

Use of a packing material and criteria for choosing, why should the packing material not be too dense or have too large a surface area

Packing material (only for fractional distillation) purpose: to exchange heat btwn ascending vapor and descending liquid
Criteria:
1) Easily insertable into column
2) Doesn't fall out of column like glass beads do
3) Large SA for heat exchange
4) Good heat transfer characteristics
5) Low holdup
6) Can be used in microscale and macroscale experiments
Not too dense: Pressure may change within column, causing non-equilibrium conditions
Not too wide a SA: will holdup much of material to be distilled (also will lose heat to environment)

Purpose of vacuum distillation

Distillation carried out under reduced pressured - used when neither ordinary distillation nor steam distillation is practical
- For compounds that decompose at boiling pts or are sensitive to oxidation
- LIQUIDS MUST BE MISCIBLE

What kind of compounds need to be purified by vacuum distillation

For compounds that decompose at boiling pts or are sensitive to oxidation

Steam distillation - uses, what kind of mixture used in steam distillation - difference between
vacuum distillation and steam distillation

Uses: can volatilize liquids and solids below their boiling pts by passing steam into boiling flask
Mixtures: high-boiling substances that decompose before bp is reached
- Used with immisicible liquids (one of which is water)
- Bp of mixture is below bp of each pure component
- Only works on VOLATILE COMPOUNDS (b/c the pt. of steam distillation is to volatilize the liquid)

Diff btwn vacuum and steam dist:
Vacuum - used on miscible liquids; good for compounds decomposing at their bp
Steam - used on immiscible liquids; good for compounds decomposing before their bp reached

Vacuum distillation - uses, what kind of mixture used in vacuum distillation

Uses: to separate miscible mixtures under reduced pressure for compounds that decompose at their bp's or are sensitive to oxidation

What requirement is necessary for boiling point in steam distillation to be constant?

Adequate amounts of water and organic component (must be volatile) present to saturate the vapor space

Precautions about distilling to dryness, distilling in closed system

1) Never distill in airtight system - apparatus will rupture
2) Never distill to dryness - dry liquid residues can be explosive
3) Do not fill flask more than 2/3 full - too much causes liquid to bump and contaminates distillate

Sources of error in a distillation - what happens and why it happens

1) Loose connectors cause vapors to escape (loss of liquid)
2) Temperature variations due to airflow in hoods
3) Large surface area of microscale apparatus - heat lost to env
4) Thermometer calibration not done properly
5) Fast distillation rate
6) Position of thermometer and sidearm not adhered to

Expected observations and impact on result from sources of error and when precautions not
adhered to

Precautions:
-never distill in an airtight system (can cause apparatus to rupture)
-never distill to dryness (dry residue of liquids can be explosive)
-do not fill flask more than 2/3 full (too much would cause liquid in flask to bump and contaminate distillate)

Sources of error:
-loose connectors (will cause vapor to escape - loss of liquid)
-temperature variations (due to airflow in hoods)
-large surface area of micro scale apparatus (easy to lose heat to environment)
-thermometer calibration
-positioning of thermometer and sidearm of apparatus not adhered to

Reasons for not recovering all liquid in distillation and why

-positioning of sidearm
-hold up in fractional distillation
-significant amount of distillate left adhering to glass surface of apparatus
-loose connectors/adaptors

Precautions to take when spotting, developing and visualizing - how to ensure experiment
proceeds efficiently, what may occur if precautions not adhered to and why

Spotting Precautions:
1. Lightly touch capillary tube to TLC plate (or adsorbent will flake off and stop flow of fluent or cause fluent to run crookedly)
2. Draw spotting line lightly in pencil
3. USE PENCIL, NOT PEN. (pen will chromatograph)
4. Make spots small and compact and not too concentrated (streaking will occur)
5. allow solvent to dry completely between applications to the same spot (spots will become too large and run in to each other)
6. Don't put spots too close together on spotting line (may bleed into each other)
7. Don't put spots too close to edge of TLC plate (leads to inaccurate Rfs b/c none enough solvent/adsorbent surrounding spot)

Developing Precautions:
1. Make sure solvent is BELOW spotting line (otherwise samples will dissolve in solvent)
2. Hold plate by the sides (oils from fingers sometimes smear a TLC plate)
3. Make sure bottom of TLC plate is level with bottom of developing chamber (solvent will rise up unevenly)
4. Refrain from moving/bumping jar while mixture is eluting (solvent will rise up unevenly)
5. Do not let solvent run past solvent front (5 mm line) (Rf value will be too high)
6. Make sure that TLC plate is not touching sides of filter paper (fluent will be adsorbed by the adsorbent and interfere with the ascending eluent)

Visualizing Precautions:
1. Hold UV lamp so that light is pointing straight down onto table
2. Dry TLC plate by placing it in hood before touching it for visualization step
3. Immediately circle exposed spots with pencil after visualizing with UV light

Why should bores of capillaries be small

Capillary bores should be small so that once a liquid is drawn into them, it will not flow out to form a drop.

What is capillary action and when it is observed (all instances) in the TLC experiment

Definition: ability of a liquid to fill spaces between solid particles in opposition to gravitational force
Occurrences in TLC:
- When spotting TLC plate: solvent flows into spotting capillary via cap action
- During plate developing step: solvent travels up TLC plate via capillary action

How to ensure enough sample has been spotted on spotting line (vs. having too concentrated a
sample - what is the problem with this?)

To ensure enough sample on spotting line - spot the TLC plate several times
Too concentrated solution causes streaking.

What are typical characteristics for solvents used as eluents

- Volatile
- Good balance in polarity with solute - ensures spot differentiation
- Mixture must be soluble in solvent
- Solvent should be able to make concentric rings with sample between origin and solvent front
-low viscosity

Characteristics of solvents used with respect to samples

Polar solvent will carry polar substances up the TLC plate.
Nonpolar solvent will carry nonpolar substances up the TLC plate.
If mixture components stay at bottom of TLC plate, add more of POLAR solvent to pull up the substances.
If mixture components travel to top of TLC plate, add more of NONPOLAR solvent to stunt their travel

Characteristics of solvents that allow them to migrate rapidly

- Low boiling point, low viscosity

Adsorbent materials and specific uses (what kind of adsorbents would be better for what kind of
substances)

Adsorbent \= stationary phase \= material of TLC plate
Alumina:
- More active
- Use for nonpolar substances will adsorb (relatively "unreactive") nonpolar molecules more
- If used for polar compounds, polar compounds will stay at bottom of TLC plate very low Rf values
Silica:
- Less reactive
- Use for polar substances will not adsorb (relatively "reactive") poplar molecules as much
- If used for nonpolar compounds, nonpolar compounds will run to top of TLC plate very high Rf values

Table 8.2 (Order of solute migration)

Uses of TLC and how it is used- all 6

1) Determine \# of components in a mixture
2) Determine identity of 2 substances
3) Monitor rxn progress
a. Spot standards on sides and rxn mixture in the middle, can see where reaction stands in terms of \# reactants compared to \# of products that show up in rxn mixture component
4) Determine appropriate solvent and adsorbent for column chromatography
5) Monitor column chromatography rxn progress
6) Determine effectiveness of purification (single spot does not guarantee single substance if both substances share similar polarities)

Effects of using too polar or too nonpolar a solvent

If use too polar solvent mixture components run to TOP of TLC plate high Rf values
If use too nonpolar solvent mixture components will stay at BOTTOM of TLC plate low Rf values

Visualization techniques - why certain kind of visualization technique was used for certain types
of compounds

Conjugated compounds - use UV light
- TLC plates contain a fluorescent indicator that makes them glow green under UV light of wavelength 254 nm. Conjugated compounds adsorb UV light at this wavelength and so will quench the green fluorescence, thus appearing blue under UV lamp
Unconjugated/most other compounds - use I2 vapors
- Alkanes/alcohols/ethers do not absorb UV light enough to quench fluorescence of TLC plate (do not show up udner UV lamp). But they do adsorb iodine vapors and can be detected by placing plate in bottle of iodine crystals brown spots

Why is a pencil always used to mark a plate?

Graphite (carbon) is inert, will not chromatograph.

Why is ink never used to mark the plate?

Pen will chromatograph along with the rest of the reaction mixture's components.

Sources of error for TLC

- Not closing the developing jar (volatile solvent evaporates)
- Contamination
- Too much eluent in jar - goes above spotting line
- Spots put too closely to each other on the spotting line - may run into each other
- Spots put too close to edge of TLC plate
o Unequal solvent forces for each spot (not enough solvent surrounding spot)
o Leads to inaccurate Rf's
- TLC plate leaning against filter paper
o Eluent on filter paper can seep through sides of TLC plate can interfere with the movement of ascending eluent

Expected observations and impact on result from sources of error and when precautions not
adhered to

Being able to see results on a TLC plate and predict what precaution/step was not adhered to

Column chromatography - why used, precautions, use of TLC in column chromatography

Why used: for separating and purifying both solids and liquids when carrying out microscale experiments

Precautions:
- Separation begins with low polarity solvent to allow compounds to adsorb to stationary phase
o Polarity of solvent slowly increased
o Don't change polarity too suddenly - heat will evolve. Alumina or silica gel will vaporize the solvent. This causes channels to form in column that reduce its separating power.
o Don't use water to elute a column because may dissolve silica gel
- If precautions not adhered to: fractions may be too large (contain more than one compound). Only way to fix this is to rerun the column chromatography.

Use of TLC in column chromatography:
- Components are separated by collecting small fractions that contain each compound.
o Can determine which compound is in each collected fraction by spotting multiple fractions on TLC plate
- TLC used to identify appropriate solvent and adsorbent

Reason for carrying out extraction

To separate solutes by using solvents of different densities
To remove a compound of interest from a solution or solid mixture

Definition of washing and why used

Washing: removing impurities from compound of interest

Definition of backwashing, why used and impact of not carrying it out.

Backwashing: washing test tube contents with ether in order to remove any organic material that might contaminate tube contents
If not carried out: could have unwanted organic material in tube, will lower purity and may cause oily crystals, impure crystals, diminish return/recovery

How acid base reactions work (what is used to extract what species and what occurs after
extraction)

NaHCO3 (weak base): used to move ion of strong acid to aqueous layer
NaOH (strong base): used to move ion of weak acid to aqueous layer
HCl (strong acid): used to move ion of base to aqueous layer
Evaporation: used to isolate neutral compound

What occurs after extraction:
If separating an acid, must reprotonate the conjugate anion with HCl
If separating base, must deprotonate the conjugate cation using NaOH
BASE BEING USED TO EXTRACT ACID MUST HAVE A CONJUGATE ACID THAT IS WEAKER THAN THE ACID BEING EXTRACTED

Flowchart on how to separate strong acids, weak acids, bases and neutral compounds

Why extractions are carried out in the sequence they are (weak base then strong base)

If use NaOH first in presence of a weak acid and a strong acid will move BOTH acids to aqueous layer BECAUSE the conjugate acid of NaOH is H2O, which is a very weak conjugate acid. H2O is probably weaker than both the starting weak and strong acids.