chem 2211L review

1. choose the appropriate solvent

2. dissolve impure compound in hot solvent

3. remove the insoluble impurities from hot solvent via suction filtration

4. cool to crystallize/precipitate the pure compound from the solution

5. isolate the pure compound via suction filtration

--> compound has to be fully soluble at elevated temperature and not room temperature --> impurities should not be soluble either at elevated temperatures or room temperature --> chemically inert with the compound and impurities --> suitably volatile so it can be removed easily once pure crystals have been isolated

dissolution is necessary so that the crystalline lattice can be broken down and purified. if the compound is soluble at room temperature, it will be difficult to retrieve the product from the solution

if the solvent reacts with the compound or impurities, a chemical transformation will occur rather than a purely physical dissolution/recrystallization

--> if the solvent is not volatile enough, it will linger in the sample and affect the percent yield and melting-point determination --> if the solvent is too volatile, it will evaporate before the compound is fully dissolved in the first place

the compounds have similar solubilities at both temperatures, or the compounds will dissolve at room temperature, or the solubilities don't increase with temperature

--> hot solvent is added to the solid to dissolve it --> only use sufficient amount of solvent

if you use too much solvent, it will be far more difficult to retrieve the pure solid at the end. you will have to wait more time for all of the solvent to evaporate

--> place a funnel and filter paper on top of erlenmeyer flask --> add few milliliters of solvent with boiling chips and place on hotplate --> cover with watch glass --> once vapors of solvent permeated through filter, remove watch glass and filter the hot solution -->pour small amount of boiling solvent to dissolve any solid that may have crystalized during filtration

--> crystals will begin to form as flask reaches room temperature --> ice bath

--> scratch the inner wall of the flask with a stirring rod --> introducing a seed crystal that matches the compund to the solution

too much solvent during dissolution.. heat more more time to evaporate the solvent.

--> suction filtration for several minutes -->remove filter paper from the funnel and place it on a watch glass and scrape off the crystals from the filter paper --> weigh the product

The temperature at which the sample first shows signs of melting and the temperature at which the sample becomes fully liquified

impurities broaden and lower (depress) the range

as the mole percentage of compound A continues to increase, a minimum melting point value for the two-component mixture is attained. at the specific composition of mixture A:B both compounds will melt simultaneously.

--> difficult to load the sample into the capillary tube --> the solvent acts as an impurity and lower and broaden the range

--> make sure sample is dry --> crush a small portion into a fine powder on you watch glass --> press the open end of the capillary tube into the crystals so that a small portion of the sample is forced into the tube --> settle the sample into the tube and use a condenser to drop the tube and pack the sample into the closed end of the capillary

thin, glass tube with an opening at one end

consists of heating block, thermometer well, a capillary holder, a temperature/voltage control, a light source and an observation window.

first attempt - a higher setting and a heating rate of 6-8ºC per minute. second attempt - heat the sample quickly within 20ºC of original melting point range and lower the heat rate to 1-2ºC per minute

--> sample is not dry --> heat too quickly --> use a new sample on a hot apparatus, it most likely melt immediately

--> choose two most likely compounds that resemble the melting points (same or higher) --> prepare two samples - the first has 50:50 of unknown sample and first likely compound and second has 50:50 of the unknown sample and second likely compound

there will be no noticeable change from the melting point of the mixture and the previously observed melting point range of just the unknown sample.

recrystallization, melting point determination, gravity filtration, suction filtration

use the ratios given and set up equation to solve for the unknown value

simple distillation, fractional distillation, and boiling point determination

simple and fractional

--> separate/ isolate two volatile and miscible liquids --> isolate a volatile liquid from a nonvolatile, but soluble solid

boiling point differences

used to define the composition of the solution vapor for the mixture of miscible liquids Ptot = Pa + Pb + Pc +...

partial vapor pressure of each component is equal to the vapor pressure of the pure component multiplied by its mole fraction within the mixture Pa = (Pºa)(mol fraction)

Ptot = Pºa + Pºb + Pºc +... helps to understand how the two primary methods of distillation are used to purify mixtures containing liquids of varying boiling points

when solution is heated, the temperature of the vapor rises to the boiling point temperature of the lower boiling point component. then the solution and vapor are in thermal equilibrium and the temperature remains constant. this plateau is good indicator of a successful separation

--> simple used when the liquids have boiling point difference of 100ºC or more --> fractional distillation is used when the liquids have boiling point difference less than 100ºC

involves a series of concurrent simple distillations occurring within the same apparatus that is achieved by adding a fractionating column in between the distillation flask and still head

glass tube filled with tightly packed media such as steel wool or glass beads. as the vapor comes into contact with the packing material, it condenses and re-vaporizes slowly makes it way up the column (column increases surface area). with each condensation and vaporization, the vapor becomes more and more enriched in the lower boiling component of the mixture

measured in the number of theoretical plates that it can generate. measured in column length and density of packing material

the mini distillations that occur within the fractionating column. as the number of theoretical plates increase, the efficiency of the fractionating column increases

the vapor and liquid concentrations are the same. the two liquids form constant boiling points. separation via distillation would not work for these compounds.

less water would be present and the condenser would not be cooled enough and the vapor will remain as vapor and will not condense back to liquid state.

the experiment must be run at a moderate pace using appropriate temperature settings. the boiling must be steady and slow to achieve the desired result --> if the distillation is rushed, the individual mini-distillations in the fractionating column will decrease and the resulting separating will be poor

(mL of compound passed at boiling point/ mL of total solution) x 100 = % of compound in solution

solid/liquid extraction, gravity filtration, and simple distillation

a physical process by which a single component is removed from a mixture of others

solid/liquid and liquid/liquid

facilitates the removal of a pure substance from a permeable, composite solid. this separation is achieved based on a difference in solvent solubility between the desired compound and the bulk of the components comprising the complex solid

trimyristin is extracted from nutmeg (20-25%)

separates the Trimyristin from the residue of the nutmeg after reflux

removes the solvent (methylene chloride) from the extracted product. the distillation was complete when the temperature reached the boiling point of methylene chlorine and plateaued

isolates the pure crystals

heating mantle, round bottom flask, condenser (water inlet at the bottom)

process of boiling reactants wile continually cooling and condensing the vapor thereby returning it back to the boiling flask so there is no loss of solvent

heat mixtures for extended periods of time without solvent loss to extract compounds or carry out reactions

melting point was used to characterize the recovered trimyristin product

methylene chloride, acetone

used because it's nonpolar so it could dissolve the nonpolar trimyristin compound and not the other compounds in nutmeg

rinse/wash flask and crystals and wash away impurities

technique used to separate non volatile mixtures

--> identifying individual components of a mixture --> compound purity determination --> compound/ multi-component mixture identification --> monitoring the progress of a chemical reaction

stationary phase and mobile phase. the more strongly a compound is absorbed on the stationary phase, the slower it moves through the system, separating the components of the mixture from each other

silica gel, alumina (polar)

low BP organic solvent (ethyl acetate with 0.5% acetic acid)

as the solvent travels up the plate through the capillary action, it passes over the compounds absorbed on the solid. a partitioning sets up between the absorptivity of the compounds on the solid absorbent and their solubility in the solvent

--> polar compounds means strong interactions with polar stationary phase so they elute slower --> the molecules that moved the most towards the top of the plate were the most nonpolar --> like dissolves like

--> the polarity of the solvent ultimately determines the distance from the origin that a compound move up the plate --> solvent system is chosen that will move the desired component between 1/3 to 2/3 of the way up the plate --> ethyl acetate containing 0.5% acetic acid was the solvent because it's non polar and the silica gel is polar

--> Rf = distance traveled by solute/ distance traveled by solvent --> Rf = 0 means the compound doesn't move from the original position (polar) -->Rf = 1 means the compound traveled along with the solvent front because the compound is completely soluble in the solvent --> if two substances have the same Rf values it doesnt mean they are same

ultraviolet lamp (dark spots)

1:1 methylene chloride: ethanol

ibuprofen, acetaminophen, caffeine, aspirin (acetylsalicylic acid)

all the compounds would be displaced, causing them to travel up the plate near the solvent front with no separation. the plate would continue developing and there would be no way to determine how far the spots traveled, so we could not calculate the Rf values.

the distance traveled by the compounds would not be accurate. there isn't enough solvent to move the compounds fast enough for separation

result in one spot rather than two distinct spots. there is not enough time for the compounds to separate. Rf values cannot be accurately calculated

the component of the mixture would dissolve into the developing solution instead of traveling up the TLC plate. there would be no spots after the plate because the sample got lost in the solvent

Rf values are based on ratio so the plate size is not important

liqui/liquid extraction, separatory funnel, IR spectroscopy, 1H NMR spectroscopy, suction filtration

three component mixture containing an acidic, basic, and neutral component

liquid/liquid extraction --> relative solubilities in two different immiscible liquids each compounds acid/base properties will be exploited to move the components between immiscible aqueous and organic layers

the reactions are simple proton transfers that lead to significant shifts in compound stability. the changes allow to the sequential isolation of each compound

--> an aqueous solvent-organic solvent --> two organic solvents of differing densities/polarities

--> aqueous solvent-organic solvent --> two organic solvents of different densities/polarities

--> organic solvents are not miscible with water so they form two different layers when mixed in the separatory funnel --> the denser liquid is the bottom layer

methylene chloride (organic, nonpolar)

--> less polar than water --> immiscible with water --> more volatile than liquid component of the original solution --> non-toxic

2M HCl (acid) --> 6M NaOH was used to neutralize the positively charged benzocaine

1M NaOH --> 6M HCl was used to protonate the neutralized benzoic acid

water and brine

protonated to form a water-soluble ammonium salt. the acidic compound remains unaffected

deprotonated to form a water soluble carboxylate salt. the basic component remains unaffected

--> aqueous on top - acidic and organic on bottom - basic --> layer separation is based on differing densities

--> benzocaine: basic --> benzoic acid: acidic --> diphenylmethanol: neutral