chem 261 lab final
what technique would you use to purify two liqiud with differening bp of 25 celsius or more?
simple distillation
what technique would you use to purify two liqiud with differening bp of less than 25 celsius
fractional distillation
what are the pros of simple distillation
it simple hence the name and easy to step up, its a quicker process and works well for substances that have compounds with large difference in bp, its effective for seprating voltaile and non volatile liquids (w a significant difference in bp)
what are the pros of fractional distillation (vigreux column)
it provides a more refined seperation of mixtures with compounds with closer boiling points meaning purer fractions. utiltizes a bigreux column that inc sa for condensation and vaportization, so liquid goes undergo many of these cyycles improving seperation effciency. less volatile compound are left behind and seprate from more volatile compounds in the indentations
how to set up a simple distilation 8 steps
1) set mantle on jack, plug matle into variac into outlet
2) rbf secured with clamp above, use glass funell to pour sample into flask with boiling stones
3)add still head on top of flask, add condensor (water flow?tubing secure?) to arm of still head and secure with joint clamp
4) support body of condesor with a universal clamp
5) add receiver adpater to other end of condesnor use a joint clamp to secure
6) push thermometer into red thermometer adapter and connect this to the still, bulb should be in opening of still head arm
7) raise lab jack so rbf can take a seat
8) label test tubes and set up to receive the liquid-->
turn on variac till orange light turns on
how do you know when a compound has switched over in distillation
liquid in rbf will boil, ring of condesation travels up inc is temp should be observed as vapour reaches the thermoeter bulb, temp remains constant as compound distills over should decrease when over and inc for the nect compound,
what should you NOT do in distillation
DO NOT distill to dryness of round bottom flask
fractional distillation steps (6)
1) set mantle on labjack, plug mantle into lvariac and variac into outlet
2) Add sample and boiling stones to flask
3) Add vigreux column on top
4) Put thermometer into red thermometer adapter and attach that to the still head, put still head on vigreux column
5)Attach condenser (make sure water flow is good) to the arm and secure with a universal clamp on Body and joint clamps at connection, add receiver adapter to condenser and secure that in place with another joint clamp
6)Turn on variac till orange light on
If you were asked to extract trimyristin from ground nutmeg, what technique would you use?
reflux
reflux set up steps
1)set heating mantle onto the lab jack, plug heating mantle into the variac and the variac into the electrical outlet
2)Clamp the round bottom flask above mantle and adjust lab jack
3)Add reagent and boiling stone to rbf and then condenser into rbf (make sure no leaks/tubing secured)
4)Raise lab jack until flask is sitting into the heating mantle
5)Turn variac dial till orange light turns on
percent recovery- reflux (numeg) equation
mass of trimyrstin (wanted)/ mass of nutmeg (used) X 100%
If you were given an unknown solid and asked to purify by recrystallization, how are you going to determine an optimal solvent? (4)
-dissolves the solid when heated near its boiling point but not at room temp
-have a high bp
-does not react with the compound
-use a solvent that can be easily evaporated.
recrystalization steps
1) label test tubes, add solid into each (20mg)
2) pippette w bulb, pippete different solvents into each test tube (,hexane, water, DCM-1ml), shake each tube and record solubilities (if it dissolves- no good)
3)Hot water bath, water in beaker on hot plate, place undissolved test tubes in bath and record solubility
* Optimal solvent: dissolves in heat but not at room temp, highest bp, doesn’t react and ,easily evaporated
4) weigh desired crude solid, put in erlenmeyer
5) put solvent in another erlenmeyer flask with boiling stones and heat on hot plate
6) Pipette and Swirl minimum amount of hot solvent into crude solid just to dissolve (clear) can keep recrystallization flask on hot plate too. Remove flask and let cool (should be cooled in an ice bath as well)
7) perform vaccum filtration
calculate percent purity
percent purity equation (recrystallization)
% purity= (mass of purified compound- after recrystallization/ mass of crude) x 100 percent
How would you use TLC to determine the identity of an unknown compound?
By using the Rf values (rf= distnace travelled by compound/ distance travelled by solvent front). this value shows the relative polarity of the compound and can be used to determine the indetity of a compound by comparing Rf values agaisnt standards on the same plate.
what has a higher rf values polar or non polar compounds
non polar compounds have higher rf values becuase it less attracted to the silica gel unlike polar compounds that stick to the stationary phase for longer due to molecular forces
What do you need to change if the eluent system used to develop the plate resulted in Rf values what were too close to the baseline
if Rf values are too close to the base line the eluent polarity is too high use a less polar eluent system. to DECREASE polarity - use more hexane since it is non polar
- hence 4:1 hexane ethyl acetate to 5:1 hexane: ethyl aceate
what do you need to change if if the eluent system resulted in rf values too close to the solvent front
the eluent system is too non polar, increase polarity, add more ethyl acetate
-4:1 hexane: ethyl acetate to 3:2 hexane: ethyl acetae or 1:1
What if your spots were not visible under the UV lamp, what would you do next?
Spots not visible under UV light aren’t UV active (don’t absorb UV light) this is common for non aromatic/non conjugated compounds like alkanes and ethers. Use a dye (potassium permagnate KmNo4), dip and heat with a heat gun
If you compared a compound containing a carboxylic acid with a compound that contains an ether, would you expect one to have a higher Rf and why?
Carboxylic acid is more polar (-COOH) group, moves slower and interacts with the silica gel more (lower RF). Ethers are less polar meaning they'll have a higher Rf value
TLC steps (9)
1) Grab eluent (hexane: ethyl acetate) (4:1)
2) ½ filter paper in medium baker, wet filter paper with eluent, leave 0.5cm at bottom of chamber, cover with watch glass
3) lightly draw baseline on TLC with pencil 1cm, ticks spaced out (labelled). Indicate tick of solvent front.
4) Dissolve 20g sample/dilute sample in 5-10 drops of dichloromethane/ or w eluent
5) Spot sample with micropipette w capillary action (rinse between w acetone), spot standards (write down)
6) Place plate with forceps, remove plate when solvent front 1cm from top, mark w pencil, allow TLC to dry, use UV light and circle spots with pencil
7) If stain needed, dip and heat with gun holding onto plate.
8) Acetone—>acetone waste, TLC—> TLC waste, liquid—> organic liquid waste
9) Calculate RF
how to prepare a solid IR sample
1) put DCM onto paper towel clean salt plates by rubbing and dry
2) Place 1 salt plate on paper towel, using a glass pipette dissolve solid in DCM (5mg of slid= 10 drops of DCM), make sure its fully dissolved
3)Dip just the glass Pippette (capillary action), place into centre of salt plate lightly tapping and letting it spread
4) Solvent should evaporate, crystallized sample on the plate
how to prepare a liquid Ir sample
1) put DCM onto paper towel clean salt plates by rubbing and dry
2) Plate two salt plates on paper towel, put pipette no bulb in sample and put in centre of plate and allowing solution to flow, lightly tapping
3) Place second plate and sandwhich
ir spectrum operation (6)
1) background air sample (collect-background)
2) Lay metal holder down, Insert sample (collect-sample)
3) Correcting baselines (process-correct baseline)
4) Peak pick (analyze- find peaks), use mouse to set the
boundary of wanted peaks, sensitivity gauge to 20, click ok
5)Printing (file-print-ok)
6) Remove sample, clean salt plates, dry on their side.
annotating ir spectrum
abosption maxima, relative intensity (s,m,w,), shape (s,b), functional group
How do you know if solvent has fully evaporated from your sample? (reduced pressure evaporation)
You can visually inspect the sample or weigh the flask then keep preforming the evaporation and weigh it again. If the weight stays the same the solvent has been fully removed.
redcued pressure evaporation steps
1) Weigh filter flask, with a glass funnel transfer liquid sample
put a stopper and clamp it, attach tubing, slowly turn on vacuum
2) When near ending, remove tubing and turn off vacuum, weigh (REMOVE STOPPER) and then revaccum and weigh again (REMOVE STOPPER)
when do you do vaccum filtration?
when you need to speed up the filtration process and separate large amounts of solid from liquid quickly. If the solid is relatively coarse, filtering viscous liquids/ need a higher pressure to push the liquid through the filter paper, need to isolate precipitate for crystallization.
when do you do gravity filtration
uses the force of gravity to pull the liquid through the filter paper, used for separating solids from liquids when the filtrate is needed in relatively large quantities, if you don’t need to remove all the liquid rapidly, when the solid particles are relatively fine and if you need to use the liquid for further use like recrystallization
gravity filtration steps
1) make fluted filter paper into 1/16 back and forth
2) Ring clamp, place glass funnel with filter inside of it within ring with stem inside the Erlenmeyer flask, wet filter paper with the solvent used in your sample
3) Swirl mixture and pour into the funnel, when all liquid is drained, filtration is complete.
vaccum filtration steps (recrystallization)
1) clamp filter flask, adapter and Büchner funnel
2) Please correct sized filter paper in funnel
3) Connect vacuum tubing and turn on
4) Wet filter paper with Pippette cold solvent
5) Swirl recrystallization liquid and pour into funnel, rinse erlenmeyer with some solvent and dump again and wash crystals with cool solvent
6) Vacuum 5-10 min
7) Record weight of filter paper and Record final mass of dry compound using spatula.
melting point steps
1) have an approximate range for chemical (literature)
2) If you don’t have literature, create an approximate melting range by doing mp determination twice.
3) Grind solid on watch glass
4) Put crushed solid my tube tapping (1-2mm)
5) Put in gallenkamp, turn on thermometer and gallenkamp(switch down and dial point to 5)
6) When reaching 15°C to literature turn down dial
* Range= first sign of liquid to fully liquid
7) Turn rheostat to 0, turn off gallenkamp, turn of thermostat, unplug
8) Dump liquid in organic waste and melting tube in glass waste
- If you don’t know literature
1) Get an approximate range the first time, Turn off, take tube out, let cool to 20°C less than recorded MP
2) Put new tube and turn on, and record 2nd mp range precisely
pippette chromatrography
1) Get a clean pippete, roll up a ball of cotton and push it into the stem on the pippete to stop from any sand/ silica from entering
2) Clamp the column inside the fume hood while the stop cock is closed use a glass funnel and funnel 1-2 cm of sand, lightly tap the pippete to even out the sand
3) Dry load: measure amount of silica needed half way of pipette, tap lightly to been out
3) Open stop cock and start adding eluent (4H:1E) with a pipette onto the walls of the “column”, continue till whole column is wet, once the eluent is added faster than it can saturate the column it will start building up (buffer layer) once it gets to 8cm high you can put the eluent directly from the beaker using a glass funnel, use an air adapter to push the eluent down faster
4) Push eluent down until it is 2cm above the silica
5) Dry load 100mg sample, pippette eluent, using adapter collect 2ml Labelled test tubes, make sure to not let sillica get dry
6) combine fractions with same rf after a tlc and complete a reduced pressurve evap and mp test
will dichloromethane be above or below water
dichloromethane is polar and denser than water so it will go below water
the denser layer goes in the top or bottom
denser layer goes in the bottom
will ethyl acetate go below or above water
ethyl acetate is less dense than water so it will go above the water
the salt will always be in the what layer
the aqueous layer becaise it is ionic and very polar and will dissolve in water
what do you have to remeber to do before draining a liquid from the sep funnel
TAKE OFF THE STOPPER