chem 2211L review

5 steps for recrystallization

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

recrystallization graphic

step 1: how to choose a solvent for recrystallization

--> 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

why should the compound not dissolve in the solvent at room temperature (recrystallization)

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

why must the solvent be chemically inert (recrystallization)

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

why must the solvent be suitably volatile (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

examples of a bad solvent for recrystallization

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

step 2: dissolving the impure solid for recrystallization

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

What are the consequences of using too much solvent for recrystallization

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

step 3: separation of impurities via hot filtration

--> 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

Step 4: cooling the solution and crystallizing the solid

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

how to induce crystallization

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

why are the crystals not precipitating out of the solution

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

step 5: isolating the pure compound via suction filtration

--> 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

suction filtration graphic

Melting point range of a chemical substance

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

what effect do impurities have on melting point range of a substance

impurities broaden and lower (depress) the range

eutectic point of the system

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.

why do you need a dry sample?

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

how to prepare a melting point sample

--> 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

what is a capillary tube?

thin, glass tube with an opening at one end

Mel-Temp apparatus

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

how to set the temperature for Mel-Temp

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

sources of error for melting point determination

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

50:50 mixture method for compound determination steps

--> 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

how to determine which compound from 50:50 mixture method

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.

techniques used in recrystallization experiment

recrystallization, melting point determination, gravity filtration, suction filtration

calculate the volume necessary to recrystallize a certain amount of solid

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

techniques used for distillation experiment

simple distillation, fractional distillation, and boiling point determination

what are the two types of distillation

simple and fractional

what is the purpose of distillation

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

what is the distillation separation based on

boiling point differences

dalton's law of partial pressures

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

Raoult's Law of partial pressures

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)

Combined law

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

Simple distillation

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

difference between simple and fractional distillation

--> 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

fractional distillation

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

fractionating column

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

fractionating column efficiency

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

theoretical plate

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

azeotrope

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

what is the consequence of reversing the water hose inlet and outlet

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.

simple distillation apparatus

fractional distillation apparatus

how to make sure fractional distillation is successful

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

how to calculate the composition through the data graph

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

techniques used in nutmeg extraction

solid/liquid extraction, gravity filtration, and simple distillation

Extraction

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

two types of extraction

solid/liquid and liquid/liquid

solid/liquid extraction

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

what was extracted from what in the solid/liquid extraction

trimyristin is extracted from nutmeg (20-25%)

gravity filtration in the nutmeg extraction

separates the Trimyristin from the residue of the nutmeg after reflux

simple distillation in nutmeg extraction

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

suction filtration in nutmeg extraction

isolates the pure crystals

reflux apparatus

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

reflux

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

reflux purpose in nutmeg extraction

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

melting point determination in nutmeg extraction

melting point was used to characterize the recovered trimyristin product

solvent used in nutmeg extraction

methylene chloride, acetone

methylene chloride purpose in nutmeg extraction

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

acetone purpose in nutmeg extraction

rinse/wash flask and crystals and wash away impurities

nutmeg extraction reaction

Thin layer chromatography

technique used to separate non volatile mixtures

What is thin layer chromatography used for

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

What is the separation based on for TLC

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

solid stationary phase

silica gel, alumina (polar)

mobile liquid phase

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

how does the TLC plate work

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

what is the effect of polarity for TLC

--> 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

choice of solvent for TLC experiment

--> 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

calculating Rf value

--> 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

what were the TLC plates visualized with

ultraviolet lamp (dark spots)

what was the solvent system used to dissolve the compounds in preparation for TLC spotting

1:1 methylene chloride: ethanol

what were the 4 ingredients used in the TLC experiment

ibuprofen, acetaminophen, caffeine, aspirin (acetylsalicylic acid)

what if you remove the TLC plate before the solvent reaches the top of the plate

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.

what if the solvent was only allowed to travel halfway up the plate before it was removed (TLC experiment)

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

what if you apply a too large of a spot to the TLC plate

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

what if the origin line is too low on the TLC plate

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

does TLC plate size make a difference in Rf values

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

acid/base extraction experiment techniques

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

acid/base extraction

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

what is the separation based on for acid/base extraction

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

how do the acid/base reactions work

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

ways of liquid/liquid extraction

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

two types of liquid/liquid extraction

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

Separatory funnel in the liquid/liquid extraction

--> 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

separatory funnel apparatus

what was the solvent in the liquid/liquid extraction experiment

methylene chloride (organic, nonpolar)

characteristics of an extraction solvent

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

what solvent was used to isolate the basic component

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

what was used to isolate the acidic component

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

what was used to isolate the organic component

water and brine

what happens to an amine in acidic conditions

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

what happens to carboxylic acid in basic conditions

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

which layer is on the top/bottom and based on what

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

what are three components

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

benzoic acid structure

diphenylmethanol structure

benzocaine structure