Organic chemistry I Lab Final
When is Fractional distillation used instead of simple distillation? Which method is more efficient?
When the boiling points of two compounds differ by less than 40 °C, they cannot be separated by simple distillation. Fractional distillation is more efficient
What is the function of the glass beads packed in the fractionating column during fractional distillation?
They provide a larger surface area to perform a series of repeated distillations in the column for separation of compounds.
Where should the bulb of the thermometer be placed during distillation?
Right below the side-arm of the distilling head.
List 4 errors that can lead to incorrect melting points?
- The packed sample is not finely powdered.
- The sample is heated too rapidly, leading to inaccurate melting point reading.
- Poor heat transfer due to the placement of too much sample into the capillary tube.
- Mistaking sweating for melting of the sample. Melting is the first appearance of liquid within the bulk of the sample itself.
Describe an ideal solvent for recrystallization of compound X?
-X is insoluble at room temperature but soluble at the solvent’s boiling point
- The solvent must either completely dissolve all the impurities at all temperatures or it must not dissolve the impurities at all
- The melting point of X is higher than the solvent’s boiling point.
What is the purpose of using activated carbon during recrystallization?
Activated carbon is used to remove colored impurities because it can absorb the impurities on its surface and then can be left behind in the filter paper.
If a student uses too much activated carbon, what can be the consequences?
Reduction of percent recovery since it can also absorb the compound to be purified.
During recrystallization, what are some options if you see no crystal after cooling down the solution to room temperature?
-Cool in ice bath
- Scratch surface with glass rod - Add a pure seed crystal
- Concentrate by evaporation
Indicate at 2 ways to visualize a TLC spot which is invisible under UV light.
Iodine Chamber or Stain (Dye)
What is characteristic of compounds that travel far on TLC plates?
They are less polar
What is characteristic of compounds that do not travel far on TLC plates?
They are more polar
When reacting Br2 with Hexane, what is observed?
Solution remains red
When reacting Br2 with Cyclohexene, what is observed?
Solution becomes colorless
When reacting KMnO4 with Hexane, what is observed?
Solution remains purple
When reacting KMnO4 with Cyclohexene, what is observed?
A dark brown or black precipitate is formed
When reacting H2SO4 with Hexane, what is observed?
No heat is generated
When reacting H2SO4 with Cyclohexene, what is observed?
Heat is generated
In Gas Chromatography, how does Boiling point influence retention time?
Compounds with low boiling points have faster retention times
In Gas Chromatography, how does polarity influence retention time?
More polar compounds have slower retention times
Define Theoretical Plate
Each condensation and revaporization that occurs on a fractioning column is a theoretical plate
What is the retention time trend for the following compounds: butane, diethyl ether, and butanol in a Carbowax (medium-high polarity) column?
Butanol > diethyl ether > butane
What is the reaction rate order for the Lucas test?
What is the reaction rate order for the TCICA test?
(3o is unreactive)
What is the Lucas reagent?
ZnCl2 in HCl and H2O
What is formed in the TCICA reaction?
A Isocyanuric acid precipitate and a carbonyl group
What are Derivatives?
Unknown alcohols can be reacted to give stable, crystalline compounds called derivatives
Why are derivatives useful?
Stable, crystalline derivatives can be used to identify unknown alcohols by comparing their derivative melting point with literature values.
What is the purpose of Trituration in the Grignard lab?
Trituration is grinding of the crude product using petroleum ether in order to remove Biphenyl.
Why does the addition of sodium or potassium Iodide catalyze SN2 reactions of alkyl halides?
Because Iodide is both a good leaving group and a good nucleophile.
Selectively dissolves the desired compound into an appropriate solvent.
The desired compound is in the extract
Removes impurities by dissolving them in an appropriate solvent.
The desired compound is left behind
Alcohol ease of dehydration dehydration order
Why is a slow rate of heating necessary to obtain an accurate melting point?
-It will let the apparatus, capillary, and compound all reach the same temperature at the same time.
-A slow rate of heating will spread heat evenly throughout the sample to reach an accurate mp
What is the difference between evaporation and boiling?
-Evaporation occurs below the BP.
-Boiling occurs when the pressure of evaporating gas equals the pressure of the surrounding gas
Why should you not add boiling chips to an already hot liquid?
Boiling will be too vigorous and out of control
Describe the recrystallization procedure.
- Choose correct solvent
- Dissolve crude product in just enough of boiling hot solvent with 5% excess
- Cool filtrate to RT using an ice bath.
- Filter, separate and wash crystals on filter.
In a solution of acetanilide, p-toluic acid, and p-tert-butylmethyl ether, what can be extracted using NaHCO3?
Define Eutectic Composition
Both compounds are in equilibrium with the liquid
Define Eutectic temperature
Lowest possible melting point for a mixture of X and Y
Why would you choose methanol over water for a recrystallization solvent?
The boiling point of methanol is lower
In the Extraction lab, what is the purpose of heating up and cooling down the mixture before adding HCl?
It helps get rid of trace amounts of TBME which can prevent recrystallization
Which law takes into account the mole fractions of the compounds?
When 2 compounds are miscible, what law should be used to determine overall pressure?
When 2 compounds are immiscible, what law should be used to determine overall pressure?
Why is steam distillation used preferable to simple distillation for isolating natural product with a high boiling point?
It allows for distillation at lower temperatures which avoids decomposition of the product
How should be characteristic of a solvent for gas chromatography?
The solvent should be more volatile than the compound of interest.
What is the difference between steam distillation and codistillation?
-Simple distillation is for miscible liquids and does not involve water, while steam distillation is for immiscible liquids and uses water.
-Steam distillation is a form of codistillation that uses water.
What form of distillation is used for immiscible liquids?
Codistillation. Steam distillation is a type of codistillation.
For extraction, which solvents are more dense than water?
For extraction, which solvents are less dense than water?
Hydrocarbons and ethers
What solvents are typically used for Sn1?
Polar protic (MeOH, EtOH)
What solvents are typically used for SN2
Polar aprotic (Acetone, DMSO, DMF, MeCN)
When removing impurities during the recrystallization lab, what are the consequences of not preheating the gravity filtration apparatus?
It may cause premature crystal formation on the filter paper.
In the recrystallization lab, what are the consequences of adding too much boiling solvent?
It will inhibit crystal formation because there will be difficulty in forming the crystal lattice if solvent particles are getting in the way.
What is the purpose of boiling chips?
Boiling chips facilitate a smooth boil, preventing boiling over and uneven bubbles.