OChem Lab - Extraction

technique for separating and isolating individual compounds from a mixture, relies on there being a difference in solubility b/w each component of mixture

extraction

since the solvents are _____ w/ initial solvent, can separate two solvents by draining the bottom layer

immiscible

four criteria for choosing an extraction solvent

1) solvent must be immiscible w/ initial solvent

2) extraction solvent shd not react w/ compound such that it cannot easily be reversed

3) shd pick an extraction solvent that is as specific w/ compound you are trying to extract as possible

4) shd choose solvent that can easily be removed once the extraction is finished

general theory of extraction

by exposing a mixture of compounds to 2 diff solvents w/ 2 diff polarities at the same time, compounds will migrate into solvent that best matches the polarity of individual compound

recrystallization

method of purification that again relies on the diff solubility of pure compound & that of an impurity

recrystallization - how?

crude sample dissolved in small amt of solvent

solvent chosen such that desire compound not very soluble at room temp, but at higher temps it will dissolve

conversely - any impurities shd be soluble at any temp, ideally

heats the solution to boiling to ensure all of sample dissolves, then let solution cool slowly to room temp

desired compound precipitates as crystalline solid, while impurities remain in solution

filitration - used to isolate solid product

determining melting point of compound 

involves heating small amount of solid and determining temperature at which it melts

steps in determining a melting point

1) transferring sample into melting-point capillary tube (one sealed end)

2) heat capillary tube until solid melts - melting point

3) most reproducible and accurate results obtained by heating sample at rate of 1-2 degrees C/min to ensure heat is transferred to sample at same rate as temp increases (and that mercury in thermometer & sample in capillary tube in thermal equilibrium)

loading sample into capillary tube

1) place small amt of solid on clean watchglass and press the open end of tube into solid to force small amt of solid into tube

2) take piece of tubing (6-8 mm) abt 1 m long, hold it vertically on hard surface & drop capillary tube down larger tubing w/ sealed end down

3) packs solid sample at closed end of capillary tube `

melting-point range of compound

melting occurs over range of a degree, perhaps slightly more - typically reported w/ lower temp being that at which 1st tiny drop of liquid appears & higher temp is that at which solid completely melted

thiele tube

simple type of melting-point apparatus; tube shaped such that heat applied to heating liquid in sidearm by burner distributed evenly to all parts of vessel by convection currents, so stirring not required

electric melting-point devices

more convenient to use; Thomas-Hoover unit f

filtration

primary technique to separate solids from liquids 

filter paper

most common of materials to produce barrier that is key to separating solids from liquids; more porous the paper, faster the rate of filtration (greater the possibility that solid particles will pass thru paper)g

alternative filters that are more porous than paper

glasswool and cotton

inert, finely divided solids used along w/ filter paper, glasswool, or cotton

silica gel, Celite - generally function by adsorption of solid and collodial materials from solution being filtered

gravity filtration

most commonly used to remove solids such as impurities, decolorizing carbon, or drying agents from liquids prior to crystallization, evaporation, or distillation; only filter paper, stemmed funnel, ring clamp, receiver needed

rate and ease of filtration increased if piece of _________ used

fluted filter paper - increases surface area of paper in contact w/ liq being filtered

hot filtration

when necessary to remove solid impurities from an organic compound that is only sparingly soluble in solvent at room temp; to remove the insoluble solids w/o leaving desired material in funnel s

separatory funnels

may be equipped w/ either glass or Teflon stopcock, commonly used during work-up procedures after completion of chemical rxn

how to use separatory funnels

1) filling separatory funnels - stopcock shd be closed & clean beaker placed under funnel before any liquids added; upper opening stoppered after liquid 3/4ths full

2) holding and using separatory funnels - if contents to be shaken, held in specific manner w/ base and stopcock held closed

3) shaking separatory funnels - increases surface area of contact b/w immiscible liquids so that equilibrium distribution of solute b/w 2 layers will be attained quickly - only vigorous or lengthy shaking may produce emulsions

4) layer identification - imp to ascertain which of 2 layers in funnel is aqueous and organic w/ denser solvent on bottom

5) emulsions - if two layers don’t separate 

pure compound

homogeneous sample of molecules having the same structure 

alternate techniques for purifying solids

sublimation, extraction, chromatography

solution recrystallization

involves dissolving the solid in an appropriate solvent at an elevated temperature and allowing crystals to re-form on cooling, so that any impurities remain in solution

alternative approach of recrystallization

melting solid in absence of solvent and then allowing the crystals to re-form so that impurities are left in the melt - often not used in organic lab bc the crystals often form from viscous oil contains impurities and from which it is difficult to separate desired pur solid

why is it important to determine the purity of the sample

even after solid has been recrystallized, may still not be pure - so need to determine the melting point of solid

steps of solution recrystallization

1) selection of an appropriate solvent

2) dissolution of the solid to be purified in solvent near or at its BP

3) decoloration w/ an activated form of carbon, if necessary, to remove colored impurities & filtration of hot solution to remove insoluble impurities and decolorizing carbon

4) formation of crystalline solid from solution as it cools

5) isolation of purified solid by filtration

6) drying the crystals

solvent should satisfy certain criteria for use in recrystallization:

a) desired compound shd be reasonably soluble in got solvent (abt 1g/20mL) and insoluble in cold solvent (room temp) - favorable temperature coefficient

b) impurities shd either be insoluble in solvent at all temperatures or must remain at least moderately soluble in cold solvent - temperature coefficient unfavorable

c) boiling point of solvent shd be low enough so that it can be readily removed from crystals

d) BP of solvent shd be generally be lower than melting point of solid being purified

e) solvent shd not react chemically w/ substance being purified

physical constants

numerical values associated w/ measurable properties of these substances

properties invariant & are useful in identification and characterization of substances encountered in lab so long as accurate measurements made under specified conditions such as temp and pressure

useful in identification of previously known compounds - not possible to predict values of such properties accurately

most frequently measured physical properties of organic compound

melting point, boiling point, index of refraction, density, specific rotation, and solubility

presence of an impurity generally ______ MP of pure solid

decreases

eutectic point

determined by equilibirum composition at which A and B melt in constant ratiob

mixed melting point

broadening of melting-point range that results from introducing an impurity into pure compound may be used to advantage for identifying a pure substance c

calibration

use of standard substances for the measurement of temp at series of known points w/in range of thermometer and comparison of observed readings w/ true temperatures

partitioning phenomena

involves difference in solubilities of substance in 2 immiscible solvents - selective dissolutiona

adsorption phenomena

based on selective attraction of substance in liquid or gaseous mixture to surface of solid phase

liquid-liquid extraction

one of the most common methods for removing an organic compound from mixture; involves distributing solute A, between 2 immiscible liquids (extracting phase and original phase)

partition coefficient

at given temp, the amt of A in each phase is expressed quantitatively as this constant

extraction procedure - initial

1) place 20 mL of NaOH in erlenmeyer flask & place on ice

2) each person weighs out 0.25 g of unknown mixture and dissolve in 10 mL of ethyl acetate

3) place separatory funnel in ring stand, close stopcok, add small portion of water to ensure no leaks

4) add ethyl acetate solution to separatory funnel & add 10 mL of HCl

5) put glass stopper on top of funnel, remove funnel from ring stand, and carefully shake the funnel while firmly holding glass stopper on w/ one finger

6) stop shaking, invert funnel, slowly open stopcock to vent separatory funnel - may hear small hissing sound as gas escapes

7) close stopcock, place funnel upright back in ring stand and remove glass stopper

8) drain aqueous (bottom) layer into Erlenmeyer flask - acidic extract

9) combine ethyl acetate solutions together w/ partner and combine HCl solutions together

isolating unknown base - procedure

1) place erlenmeyer flask w/ acidic extract into ice bath for 10 min beside NaOH

2) while still in ice bath, neutralize acidic extract w/ cold 10 mL of NaOH - check pH; unknown base shd hv precipitated out of solution

3) to isolate unknown base - filter solution w/ Buchner funnel & scrape solids into labeled weigh boat - store in locker till next week

isolating unknown neutral

1) drain the remaining ethyl acetate solution into small Erlenmeyer flask; add approx 4 spatulas worth of sodium sulfate

2) remove sodium sulfate by pouring your ethyl acetate solution through glass funnel w/ small piece of cotton at base letting it drain into clean beaker

3) remove solvent by simple distillation, add ethyl acetate to 50 mL round bottom flask w/ stir bar; use 100 mL round bottom flask as receiving flask and submerge it in ice water

4) set hotplate to abt 200 C and stir plate to between 4 and 5

5) store 50 mL round bottom flask unstoppered in locker for recrystallization next week

recrystallization procedure

1. Remove the aluminum block from the hotplate, and turn the hotplate on to 250 ºC

2. Add roughly 75 mL of water to your largest beaker and place it on the hotplate.

3. Add roughly 20 mL of methanol to your second largest beaker and place it on the hotplate.

4. Add your crude base and neutral compounds to separate, smaller beakers. Preferably, place the neutral compound in the smallest of the three beakers and place the base in the biggest of the three beakers.

5. Obtain two plastic pipettes.

6. The methanol will begin to boil first. If the boiling is very vigorous turn the hotplate down or move the beaker to the edge of the hotplate where it is cooler.

7. Add a pipette of hot methanol to the beaker containing the neutral compound. Keep adding hot methanol until there is enough liquid to cover the bottom of the beaker.

8. Place the beaker containing the neutral product onto the hotplate to reheat the solvent.

9. If the solvent begins to boil but there is still solid floating around, add another pipette of hot methanol.

10. Once all of the solid dissolves, remove the beaker containing the neutral compound off of the hot plate and cover with a watch glass to prevent the solvent from evaporating.

11. Remove the beaker containing pure methanol from the hotplate. This is now waste.

12. Repeat this procedure with the base but use boiling water as the solvent. Once you remove these solutions from the hotplate, you do not need to cover them with a watch glass, since the water does not evaporate quickly. Don't forget to turn off the hotplate after you have finished dissolving all three compounds.

13. Once the solution containing the neutral has cooled to room temperature, you should see some precipitate form. Put the beaker into an ice-water bath while you set up you Buchner funnel.

14. After a minute or two in the ice-water bath, pour the neutral compound into the Buchner funnel and vacuum filter. Wash the crystals with ~5 mL of cold methanol. Then move the crystals to labeled weigh boat.

15. Repeat this process for the base compound. Wash these compounds with cold water instead of methanol.

16. Once you have your two crystalline products, you can clean up your glassware and let your compounds air dry until the next week.

17. The following lab period, you need to obtain the mass and melting point of each of your two compounds using a Mel-Temp apparatus.

if base doesn’t precipitate - solution

try blowing air over top of solutions; sometimes ethyl acetate floats on top of water & dissolves all of desired product; so blowing - evaporates ethyl acetate & forces product to precipitate

4-nitroaniline hazards

considered irritant for skin, eyes, and ingestion; inflammation of eye, itching, water - flush w/ water and apply emollient and seek medical attention if needed; DO NOT induce vomitting

NaOH hazards

corrosive, irritant, permeator in terms of skin; irritant, corrosive in terms of eye, hazardous for inhalation and ingestion; flush if in contact w/ skin or eyes with water for at least 15 min; cover irritated skin w/ emollient, if inhaled move to fresh air; DO NOT induce vomiting unless directed to do so by medical personnel

HCl hazards

corrosive, irritant, permeator in terms of skin; irritant, corrosive in terms of eyes, slightly hazardous for inhalation - non-corrosive for lungs; flush w/ water for eyes and skin, cover irritated skin w/ emollient; if inhaled, remove to fresh air, DO NOT induce vomiting unless directed

ethyl acetate hazards

severe fire hazard, inhalation - irritation, headache, dizziness, narcotic effects, eyes and skin - irritation, redness, defatting action on skin - remove contaminated, thoroughly wash w/ water & seek medical attention if in eyes; if ingested - contact physician, give water or milk