What do you need to bring to lab?
Lab Coat, Goggles, Pen, Lab Notebook, Stapler, Appropriate Attire
How do you define the melting point range?
It starts when the first drop of liquid appears and stops when the sample is totally liquefied.
What happens when you heat the sample too fast?
The melting point range tends to be too high and too wide.
How do impurities affect melting point?
They lower the melting point and/or increase the range.
How can you use melting point to determine if two compounds are identical?
By checking their melting point ranges independently and then mixing them and making sure the range does not change.
What is a simple rule for solubility?
Like dissolves like
How is polarity determined in organic molecules?
By the presence of polar bonds (N,O) in functional groups, like -NH2 and -OH.
How do halogens affect polarity?
Despite their high electronegativities, halogens do not significantly increase the polarity of organic compounds.
How does adding carbon atoms to an organic chain affect polarity?
It decreases the polarity of the compound.
When can H-bonding occur?
When there is a H atom bonded to N, O, or F.
How do organic compounds get converted to ions?
In acid-base reactions, such as RCOOH + NaOH or RNH2 + HCl.
How can you separate an acid and a neutral compound?
Add NaOH, then organic layer = neutral and aqueous layer = acid. Add HCl to aqueous layer to return to insoluble acid.
What is the distribution coefficient K?
The ratio of solute concentration in both phases. K = C1/C2 = g/mL organic / g/mL aqueous.
What is the threshold for K for a separable mixture?
If K>1.5, the solute can be separated from water into an organic layer.
What is an emulsion layer?
It's a layer between the aqueous and organic layers that is a milky mixture of the two solvents. Common in chlorinated solvents.
What should you do if an emulsion forms?
You can filter it via suction filtration or just wait it out.
How does Kbrine compare to Kwater?
Generally Kbrine > Kwater for a given compound.
Which layer do ionic compounds prefer?
Aqueous. Always.
How do you separate a carboxylic acid and an alcohol/phenol?
Deprotonate it with NaHCO3, which is unreactive with alcohols and phenols.
What is a drying agent?
An inorganic salt that doesn't dissolve in organic solvents but is very water soluble. It is used to dry organic compounds.
What are three of the most common drying agents?
CaCl2 & Na2SO4 (decantable), MgSO4 (fine powder, filtration).
How do you know if you've added enough drying agent?
Add drying agent and swirl around. When it stops clumping and spreads out, you know you've added enough.
When do you write a separation scheme?
When you perform: extraction, washing, precipitation, recrystallization, distillation, evaporation, drying agent, filtration.
What is reflux?
A method for heating a mixture without any loss of material. The mixture is heated and the vapor produced is continuously condensed and the liquid is returned to the flask.
What special equipment is needed for reflux?
A condenser- the same one that is used in distillation.
When do you need a water cooled condenser?
If the boiling point of the solvent is >150 C. Otherwise, air is sufficient.
What is a reflux ring?
The hovering ring you sometimes see where the vapor is condensing.
What is the Finkelstein Reaction?
Used to get alkyl iodides from chlorides/bromides:
RX + NaI -> RI + NaX.
What solvents are used in the Finkelstein Reaction?
Acetone or methyl ethyl ketone.
Why can't you make RI from alkane halogenations?
I2 reacts too slowly.
What is the mechanism for the Finkelstein Reaction?
SN2, so methyl>1>2>3
When is I- a better nucleophile than Cl-?
In polar protic solvents. It's the reverse in polar aprotic solvents.
Why use polar aprotic solvent acetone for the Finkelstein Reaction?
Because only NaI (not NaBr or NaCl) is soluble in acetone, so the precipitation of NaBr and NaCl drives the reaction to the right.
Which is denser, DCM or H2O?
DCM or methylene chloride is denser than H2O.
What are the products of AgNO3 + RX reaction?
AgX(s) + R+ + NO3-
What are the colors of the AgX precipitates?
AgCl = white, AgBr = beige (offwhite), AgI = yellow.
What is the mechanism of the AgNO3 reaction?
SN1, so 3>2>1>methyl
How do you know if you went too far in the Finkelstein Reaction?
If the liquid turns pink/purple, this means the RI is decomposing.
What is distillation?
A method for purifying liquids based on their boiling points. A liquid is heated to its boiling point and the pure vapor is condensed into another container, leaving behind impurities.
What is the boiling point?
The temperature where a liquid's vapor pressure equals atmospheric pressure. At bp, liquid and gas are in equilibrium.
What are the four types of distillation?
Simple- best for liquids with a bp difference ~100C or solid contaminant.
Fractional- best for liquids with close bps (~25 C).
Vacuum- for liquids with high bps that are at risk of decompositon.
Steam- for water insoluble liquids with high bps ^.
How does vacuum distillation work?
In addition to increasing temperature, it lowers the pressure inside, lowering the bp of the liquid.
Where should you put the thermometer in distillation?
The tip should be slightly below the center of the condenser to obtain accurate vapor temperatures.
What should you avoid with a closed system?
Heating it- fixed volume means increased pressure = explosion risk.
What distillation head did we use?
The Hickman head.
What is Raoult's law?
P = p1 + p2, where p1 = y1P, etc.
What's different about fractional compared to simple distillation?
Many "equilibrium stages," so the columns tend to have lots of surface area.
How did we increase surface area for fractional distillation?
We packed the column with steel wool.
How can you improve fractional distillation accuracy?
Use a longer column, more surface area, heat more slowly.
How do you interpret a distillation graph?
Looks almost cubic, inflection point is where it switches from compound A to compound B.
What is an azeotrope?
When x1 = y1 and x2 = y2. Then the mixture behaves as a pure liquid and distills at a constant temperature and concentration. Impossible to separate via distillation.
What is "bumping?
When a liquid gets superheated- above its boiling point but does not boil, becoming volatile. Use boiling chips or scratched glass to prevent.
What causes bumping?
The surface tension of the liquid is high enough to prevent phase change. It's harder to create the first bubble than to expand existing ones. Boiling chips can trap bubbles, allowing the liquid to boil.
What is recrystallization?
A common method for purifying solids. The solid is dissolved in a hot solvent and as it cools the impurities stay in the solvent and the solid crystallizes.
How does temperature affect solubility?
Liquid and solid solutes are more soluble in hot solvent, whereas gaseous solutes are more soluble in cold solvent.
What is the procedure for recrystallization?
Heat solvent to boiling. Place impure solid in scratched flask. Pour small amount of solvent into flask. Swirl flask and keep warm on hot plate. Filter out remaining solid impurities. Leave on bench to slowly cool to room temperature. Place in ice bath. F
What is the most common recrystallization mistake?
Adding too much hot solvent. Add only until barely dissolved.
What is the tradeoff from recrystallization?
Trading yield for accuracy.
Why do we cool slowly?
More purity. Fast cooling can trap impurities.
What makes a good solvent?
1) Very soluble at high temperatures 2) Very insoluble at low temperatures 3) Dissolves impurities at low temperatures 4) Cheap
What functional groups are on aspirin?
Carboxylic acid, ester, phenyl groups.
How do you remove high MW colored impurities?
By adding activated charcoal powder that strongly adsorbs colored impurities to the hot solution, and using filtration.
What is thin layer chromatography (TLC)?
A method for separating compounds based on differential solubilities (polarity) in stationary and mobile phases.
What are TLC plates made of?
Glass/plastic/Al coated with a polar adsorbent, usually silica (SiO2), alumina (Al2O3), or cellulose.
How do you see colorless compounds on a TLC plate?
Using a UV light- the TLC dye glows and the compounds appear as dark spots.
Which compounds move further down a TLC plate?
Nonpolar compounds travel further down TLC plates.
What role does the solvent (mobile phase) play?
If it's too polar, all the compounds will be carried down the TLC plate. If it's too nonpolar, the compounds will all separate out early.
What is the retention factor (Rf)?
Rf = distance moved by compound / distance moved by solvent front
How do identical compounds appear on TLC?
Identical compounds travel the same distance. If there's a mixture of different compounds they'll separate into different spots.
How do you make a developing chamber for TLC?
A beaker with a watchglass on top and a strip of paper along the bottom with some solvent.
Where should you draw the starting line on a TLC plate?
About 5 mm from the end with a pencil.
How much sample does TLC require?
~1 microgram
How do you develop a TLC plate?
Put it in the developing chamber with the solvent below the starting line and let the solvent carry the compounds up. Stop it ~5 mm from the other end and mark the solvent front with a pencil.
How do you view a TLC plate without UV?
You can use I2 to produce brown spots.
What is column chromatography?
A mixture is passed through a packed column with a solvent. The compounds will elute at different rates.
What do we pack the column with?
Silica, which is converted to gel with solvent (hexane) addition.
What did we separate with packed column chromatography?
Ferrocene and acetylferrocene in TBME.
Which one eluted first?
Ferrocene, it was yellow. Acetylferrocene came next and it was red/orange.
How can you tell if Br2 has formed from HBr and H2O2?
HBr and H2O2 are both clear and Br2 is dark reddish-brown.
What is the stereochemistry of the product of cyclohexene and Br2?
Racemic, trans-1,2 dibromocyclohexane.
When you rinse the product of bromination with DCM, what happens?
2 layers- DCM + dibromocyclohexane is organic, denser, aqueous layer has HBr, Br2.
Why do you wash the organic layer with NaHSO3?
To remove any last impurities that should be in the aqueous layer.
What are common types of NMR?
Proton, Carbon, DEPT, COSY, HSQC.
What does H-NMR tell you?
The number of signals = number of different H's.
Location shows shielding/functional group.
Intensity or integration shows number of H's of that type.
Splitting shows number of H's on adjacent atoms.
Which way is more shielded?
Upfield or to the right, downfield or to the left is less shielded, closer to O or N.
What are the rules of splitting?
Equivalent don't split. If on the same atom and not equivalent, split. O and N don't split. Splitting = neighbors + 1.
How do you read COSY?
Draw a line down the diagonal. Any two spots that are horizontal, one on the line and one to the left, are adjacent carbons.