Organic Chemistry Lab Exam

A mixture of (S)-2 butanol and (R)-2-butanol in water shows a 50% of enantiomeric excess in favor of (S) isomer. What's the mass percent composition of the two enantiomers

%ee= (%major E)-(%minor E) Therefore, (S)-2-butanol is the major enantiomer because it is in excess %Major E= 50% + ((100-50)/2) mass percent of (S)-2-butanol= 75% %MinorE= (100-50)/2 mass percent of (R)-2-butanol= 25%

A sample of carvone was found to be 39% optically pure in favor of the dextrorotatory (+) enantiomer. The specific rotation for enantiomerically pure carvone is 61 degrees. The polarimetry experiment that led to this result was performed utilizing a 2.5 dm polarimeter tube and a solution was prepared by dissolving 3.49 g of carvone in 55 mL of ethanol. What was the observed rotation for this experiment?

(+) enantiomer is major 0.39 = ([a] mixture/ +61 degrees) [a] mixture is 24 degrees 24 degrees= (a/(2.5dm * 3.49g/55mL)) a= +3.8 degrees observed rotation= +3.8 degrees

enantiomer composition of a mixture

-%Major Enantiomer= %ee + (100%-%ee)/2) -% Minor Enantiomer= (100%-%ee)/2

Duels-Alder Reaction Procedure

-1,3-butadiene is a gas at room temperature, which makes it a difficult reagent to measure and handle in the lab. to get around this, 1,3-butadiene will be generated in situ (in the reaction medium) from the thermal decomposition of 3-sulfolene (butadiene sulfone). while 3-sulfolene is relatively stable solid at room temperature, when heated it decomposes to give 1,3-butadiene and sulfur dioxide -butadiene sulfone is an irritant, it emits toxic corrosive sulfur dioxide when heated -maleic anhydride is toxic and corrosive -xylene is flammable -clamp and immerse a 25 mL round-bottom flask in a sand bath on top of a hotplate/stirrer -measure out by weight approximately 0.85 g of 3-sulfolene and 0.45 g of malice anhydride sequentially and place them in this 25 ml round bottom flask -add 0.5 ml of xylene using a plastic disposable pipet followed by a boiling stone to the flask -attach a condenser (clamp on condenser is not necessary) with appropriate water hoses (water in the bottom out of the top) and turn on a gentle stream of water. water hoses are found in the box on top of the yellow solvent cabinet by the windows -turn on heat to setting of 8 and heat the reaction mixture until it is refluxing. keep the sash down as the reaction generates SO2 -maintain the reaction at reflux for 30 mins (start timing when the reaction begins to boil), then allow it to cool to room temperature (about 5 mins of cooling) -add 4ml of toluene to the cooled solution in the 25 mL round-bottom flask and then add hexane drowse until a slight cloudiness persists. roughly 2.5-3.5 mL of hexane will be needed -reheat the solution until it becomes clear and then cool it in an ice bath for 10 minutes. during the recrystallization step, the side of the flask may have to be scratched with a glass rod to induce the recrystallization -collect crystalline product by vacuum filtration. wash solid with about 1-2 ml of cold hexane and allow it to air dry for 2 mins. transfer product into beaker/watch glass -obtain weight an IR spectrum for product and submit sample for NMR

Column Chromatography Procedure

-obtain chromatography column and clamp it securely and straight to the metal support rod on a hot plate/stirrer -place small wad of cotton in the bottom of the column, enough to cover the hole in the outlet, tamp it down gently with glass rod -add about 1 cm layer of sand to column and even it out -fill the column half way with hexane, tap column to even out sand if needed NOTE: do not use sand from the heating baths, which is likely contaminated; use the clean sand found in a flask on the white shelves -slowly add alumina to the column, allowing it to gently float to the bottom, add enough to fill column half way -once column is half filled with alumina, wash the sides of the column down with a small amount of hexane -add enough sand to form a 1 cm layer on top of the alumina -allow the level of hexane to fall to a point just below the top of the sand. close the valve. it is important not to let any portion of the alumina dry out -weigh out 0.75 g of the bibenzyl/benzyhdrol mixture and dissolve it in as little methylene chloride as possible! (about 2 mL) -add methylene chloride solution to the column with a pipet, allowing the solution to flow down the sides of the column. add solution evenly down all sides of the column NOTE: ONCE THE SOLUTION OF THE MIXTURE IN METHYLENE CHLORIDE HAS BEEN PREPARED, ADD IT TO THE COLUMN IMMEDIATELY. IF IT IS ALLOWED TO STAND AROUND FOR A WHILE THE METHYLENE CHLORIDE EVAPORATES AND THE COMPOUND BEGINS TO PRECIPITATE OUT. IF THIS HAPPENS BRING THE VOLUME OF THE SOLUTION BACK UP TO 2ML BY ADDING MORE METHYLENE CHLORIDE BEFORE APPLYING THE SOLUTION TO THE COLUMN -once the entire solution has been added to the column, wash down the sides with a minimal amount of methylene chloride (no more than 1.5 mL). open the valve and allow the liquid to fall to a point just below the top of the sand -add about 1 mL of hexane and wash down sides of column, open valve and allow liquid to fall just below top of the sand -fill column to top with hexane -open valve and collect elutant in large test tubes, allowing each to fill about 90% (NOTE: the column will have to be refilled periodically with eluting solvent; do not let the solvent level fall below the top of the sand) -collect about 3 tubes then test each tube by TLC using a 3:1 hexane/ether solvent system in the TLC developing jar, in order to monitor the progress of the column -if the 3rd fraction still shows a spot, collect more fractions (one or two) using hexane as the eluting solvent, testing each by TLC. continue this procedure until a fraction is obtained that does not produce a spot on the TLC plate -when the first compound has been eluted from column, switch eluting solvent in column from hexane to a 1:1 hexane/ether solvent system, its okay t mix this in with leftover hexane -collect four fractions and test by TLC as above -once the column is complete, prepare two screw cap jars by taping a label on each. record weight of each labeled, empty jar -combine all the fractions that contain the first component in the jar labeled Compound 1 and combine all of the fractions that contain the second component in the jar labeled Compound 2 -place jar in plastic tub to allow solvent to evaporate, however DO NOT leave the jars until next week, when all solvent has evaporated, cover each jar with a lid -next week obtain a recovered weight and an IR spectrum of each compound and submit a sample of each for an NMR spectrum -to clean the column, open the valve and allow it to drain as much as possible, then empty the contents in waste contained -wash column with water then acetone

Preparation of a Vicinal Bromide Procedure

-performing two different bromination reactions that start with either E or Z stilbene -bromine is highly corrosive and irritating, pyridinium tribromide will be used as the source of the bromine, the solid reagent is safer and easier to handle than elemental bromine -place a crystallizing dish on a hotplate/stirrer and fill it half-way with water. turn on the heat to a setting of 4 and regulate the water temperature to approximately 50-55 degrees C -while the water bath is heating, measure out approximately 0.4 grams of your assigned stilbene isomer and place it in a 25 mL round-bottom flask -clamp round bottom flask above the water bath on a hotplate/stirrer, add 4 mL of glacial acetic acid and a magnetic stirring bar -measure out 0.8g oh pyridinium tribromide and add it to the round-bottom flask in one portion. wash down the side of the round-bottom flask with an additional 3 mL of glacial acetic acid -place the round-bottom flask in the water bath and turn on the stirring to get efficient mixing -place round bottom flask in the water bath and turn on the stirring to get efficient mixing -after 10 minutes of stirring (orange-yellow color of pyridinium tribromide fades to yellow), cool the round-bottom flask in the air for about 5 min then add 6 mL of water to the reaction mixture, and place the round-bottom flask on an ice bath for 10 mins -collect the solid product by vacuum filtration, wash solid with 5 mL of water and allow it to air dry for 2 min. transfer the solid into a beaker/watch glass and place the open beaker/watch glass in drawer to dry until next week -next week obtain weight and melting point

Diets-alder reaction

-reaction of a conjugated or 1,3-diene (a 4 pi electron system) with a substituted alkene or alkyne, the dienophile (a 2 pi electron system), was discovered -forms a 6-membered ring from 2 pieces: 'conjugated diene' (which provides four of the ring atoms, should be electron-rich, has to be able to achieve the s-cis conformation) and a 'dienophile' (which provides two of the ring atoms, must have an electron withdrawing group such as carbonyl or nitrile to activate the double bond ) -cyclic dienes are locked in the necessary s-cis confirmation so are especially reactive -the two new sigma bonds and one new pi bond appear to form simultaneously with the breaking of the original pi bonds, leading to a cyclic transition state and therefore results in high stereoselectivity -concerted process -4+2 cycloaddition reaction

TLC applications

-to determine whether a reaction has gone to completion by seeing if the starting material is no longer present -to determine how many different products were produced in a reaction -if standards are available for comparison, to determine the identity of the compound(s) -to determine an elution solvent for column chromatography and to analyze the fractions resulting from the column -determines how many compounds need to be separated, provides a clue to the elution solvent needed to effect the separation by column chromatography, and also is used to analyze the fractions resulting from the column to determine where each compound

general notes on running TLC plates

-use a different capillary tube for each solution and be careful not to mix them up -be sure the solvent level in the jar is below the line on the plate when it is placed in the jar. if the solvent level is at or above the line, the spotted compounds will dissolve into the solvent and no spots will be seen under the UV light because there will be no compounds on the plate -only spots on the same plate can be safely compared -when visualizing plates under UV light, if the spots are too light, make a more concentrated solution of the sample (add more sample to solution). if they are too dark and/or large, dilute the solutions with more acetone -touch the surface of the plate very lightly, be sure not to gouge out the silica gel -when choosing solvent systems, if the spots are running too high, make the developing system (in the jar) less polar by adding more of the less polar solvent. If the spots are running too low, make the developing system more polar by adding more of the more polar solvent -if a mixed solvent system is being used the solvent in the jar should be changed after every other plate is run because if one solvent evaporates quicker, the solvent system will be changed. Do not rinse the jars with acetone between uses, since the same solvents are used each time, just pour out contents into waste and add freshly made solvent -do not disturb jar while plate is developing -Rf values are a measure of how far up the plate a solvent went

Polarimetry Experimental Procedure

-weigh out 1.5 grams of substance "Pure (+) enantiomer" and dilute to 25 mL with ethanol -obtain 2 dm polarimetry tube, remove cap and carefully pop out the glass cover slip, but leave the black rubber washer in the cap -clamp tube on ring stand -fill tube with solution -carefully and quickly slide the glass cover slip onto the tube, being careful not to get a large air bubble in the tube, then securely cap the wide end of the tube. If there is an air bubble, and a dark shadow/distortion(similar to partial solar eclipse) is seen the bubble is too large, if a relatively clear view of a full orange circle is seen the bubble is OK. NOTE: before placing a tube in the polarimeter be sure to wipe off all solvent from the outside of the tube with a kim wipe -with nothing in polarimeter, set scale to zero. it should consist of two halves of equal intensity (orange in color) -place polarimetry tube into chamber and close. The two halves of the circle are no longer of equal intensity- one is light, and one is dark, note whether the dark is on the left or the right. rotate the scale until the two halves are of equal intensity again and light orange in color, be careful not to rotate too far- if you rotate too far the dark half of the circle will jump from the right to the left or vice versa; if this happens go back to zero and repeat, rotating the arm less. if dark half is on right, rotate arm clockwise, if dark half is on left, rotate arm counterclockwise -read and record the observed rotation. if the dark half of the circle had been on the right, the solution is dextrorotatory (+); if it had been on the left, the solution is levorotatory (-) -pour contents of tube into waste bottle and rinse tube with ethanol DO NOT USE ACETONE -repeat for the unknowns which is a mixture of (-) (+) enantiomers of the compound analyzed in the first part -repeat with another unkown

An 8.05g of (S)-butanol in 10.00 mL of water was placed in a 10.0 cm polarimeter tube. The observed rotation at 20 degrees C was a= +104 degrees 1. What is the specific rotation of (S)-butanol in water? 2. What is the specific rotation of (R)-butanol in water?

1. 10.0 cm = 1 dm [a]= +104/(0.805g/mL * 1 dm) [a]= +129.19 degrees 2. [a]= -129.19 degrees

Boiling Points/Distillation/Spectroscopy (IR and NMR) Procedure

1. obtain an unknown and measure out the entire volume of the unknown, place in a 25 mL round bottom flask 2. add magnetic stir bar to flask, clamp neck of flask to metal support rod of hotplate, place sand bath on hot plate and immerse the round bottom flask in it 3. assemble a distillation apparatus. top of the bulb should be even with the bottom of the sidearm of the distillation flask. water flows in the bottom and out the top 4. turn on gentle flow of water (just a trickle) and set heating to 7 *the liquid will not begin to distill immediately nor will the temperature rise immediately. a temperature rise will not be seen until the liquid is ready to distill over* 5. as liquid begins to boil, it will condense onto the thermometer bulb and the inside walls of the distillation head. at this point, the temperature will begin to rise, often rather quickly, and eventually will stabilize. the correct time to record the boiling point is when a few mL of liquid have already collected in the receiving flask and the temp is remaining steady 6. continue distillation until at least 10 ml of liquid has been collected. NEVER distill to dryness; always leave some liquid in the distillation flask, distilling certain liquids to dryness results in explosion! while this practice means a 100% recovery is impossible, it is standard practice for safety reasons. recoveries of 100% are rarely achieved in organic chemistry with any technique 7. turn off heating, remove receiving flask and set aside in a safe place, and replace it with a small beaker to catch the last few drops of liquid coming out of the condenser. raise the entire distillation apparatus out of the sand bath *it is important not to leave the round bottom flask in the sand bath, it will go dry. Also, do not disassemble the apparatus piece by piece, keep the condenser on at this time. vapors that come out of the round bottom flask/distillation are flammable and could lead to a hazard situation 8. obtain a weight of the compound, prepare a sample for NMR and record an IR

(S)-2-bromobutane has an optical rotation of [a]= +23.1 degrees. A sample of mixture of the R and S isomers has a rotation of [a]mixture= -17.7 degrees 1. calculate the optical purity (enantiomeric excess, %ee) of the mixture 2. For a 5.00 gram sample of this material, what would be the enantiomer composition by weight? (%R and %S)

1. optical purity= (-17.7 degrees/ +23.1 degrees) * 100 %ee= 76.6% optically pure in favor of levorotatory (-) enantiomer 2. the major E is (R)-2-bromobutane (-) %major E= 76.6 + ((100-76.6)/2) = 88.3% %minor E= (100-76.6)/2 = 11.7% composition by weight for 5.00 grams: (R)-2-bromobutane (-)= 5.00g * 0.883 = 4.42 g (S)-2-bromobutane (+)= 5.00g* 0.117 = 0.580g

Reduction of acetophenone to 1-phenylethanol with Noyorei's BINAP/diamine-Ru catalyst resulted in a mixture of the alcohol enantiomers. The reaction was performed on 3.85 g of acetophenone and produced the enantiomeric alcohol products in 95% overall yield. The entire product was dissolved in 75 mL of ethanol and analyzed by polarimetry in a 3 dm polarimeter tube. The observed rotation was -1.4 degrees. What is the optical purity of the product? What is the enantiomer composition of the product (%+ and %-) given that the specific rotation of enantiomerically pure 1-phenylethanol is 10 degrees?

3.91g theoretical yield 3.91 * 0.95= 3.71 g [a] of mix= (-1.4 degrees/(3 dm* 3.71g/75mL)) = -9.43 degrees optical purity=( -9.56 degrees/ 10 degrees) * 100= 95.6% optical purity= 96% in favor of (R)-1-Phyenylethanol enantiomer composition: % major E (R)-1-Phyenylethanol= 96 + ((100-96)/2) = 98% % minor E (S)-1-Phyenylethanol= (100-96)/2 = 2.0%

For a given number of carbons, it can be seen that alkanes have the lowest BP, alcohols the highest, and 1-chloroalkanes fall in the middle. Explain.

Alkanes are non polar, and thus only have dispersion intermolecular forces acting on them, which are weak and easier to overcome, thus giving them the lowest boiling points. the compound 1-chloroalkanes are slightly polar, so in addition to dispersion forces, they also have the stronger dipole-dipole intermolecular forces acting on them, so they have boiling points that are higher than alkanes. Alcohols have the highest boiling point because they are polar substances with dipole-dipole intermolecular forces and they have H-bonds, the strongest type of intermolecular forces that need to be overcome to transition to the gas state, so more thermal energy is needed thus explaining the higher boiling point

What is the relationship between the number of carbons and boiling point, regardless of the series? What is the reasoning behind this trend?

As the number of carbons increases, the boiling point increases. The reason for this trend is because as the number of carbons increases, the molecular weight of the compound also increases. Compounds with higher molecular weights have stronger intermolecular forces acting on them because *****!!!! So, they have more forces that need to be overcome for it to transition to the gas phase, so, as a result, they have higher boiling points

If proper safety precautions are taken a distillation will never afford a 100% recovery. Why?

Because some liquid must always be left in the distillation flask, distilling certain liquids to dryness results in explosion!

Consider the two compounds of bibenzyl or benzhydrol Which compound has a lower Rf value when the mixture is developed in a 1:1 hexane/ether solvent system? Explain.

Benzhydrol will have the lower Rf value because the alumina adsorbent used is polar, so it will strongly adsorb polar molecules like benzhydrol but will not have much attraction for the non polar bibenzyl. Therefore, bibenzyl will be eluted first while benzhydrol will be eluted later. Since benzhydrol is eluted later, it will not travel as far as bibenzyl resulting in a lower Rf value

Consider the two compounds of bibenzyl or benzhydrol Which compound would you expect to come off first from the column when the eluting solvent is hexane first? Explain.

Bibenzyl would be expected to come off first when eluting in hexane first because bibenzyl is non polar so it is not strong adsorbed by the alumina but it will be highly soluble in the non polar hexane solvent. Therefore, it will move down the column at a quick rate to be collected first. Benzhydrol is polar so it is attracted more to the absorbent alumina than to the non polar solvent and is thus selectively help back on the column

You suddenly notice you have forgotten to add boiling stones to your round bottomed distillation flask but the distillation is now in progress. What should you do?

Boiling stones should never be added to a hot liquid because it could cause the liquid to boil very violently. Now that the distillation is in progress, the solution has to be cooled down if the boiling stones need to be added

TLC Procedure

EXPLORING EFFECT OF SOLVENT AND COMPOUND STRUCTURE ON RF VALUES -dissolve tiny amount of diphenylmethane in a small amount of acetone in small test tube. In another test tube, dissolve tiny amount of benzoic acid in small amount of acetone -obtain three 3x5 cm TLC plates -on each spot, make a spot of each solution (diphenylmethane and benzoic acid) side by side on the spotting line -develop one plate in pure hexane -develop one plate in pure ether -develop one plate in 3:1 hexane/ether mix change the solvent after every two plates run -record the distances traveled by each spot and the solvent and calculate the Rf value for each. each distance is measured from the starting line and for each spot the distance is measured to its midpoint -discard TLC plates and put solutions in waster bin UNKOWN IDENTIFICATION -obtain a test tube with unknown solution -unknown consists of two of the following: Acetaminophen (Tylenol), Caffeine, Acetylsalicylic acid (Aspirin), Naproxen (Aleve) -prepare a solution in absolute ethanol of each of the drugs above found in the lab -prepare a dilute solution of the unknown in ethanol -run a plate consisting of the unknown and two of the standard samples in a solvent system of 200:1 ethyl acetate: acetic acid -run a second plate with the unknown and the remaining two standards -plate one: 0.5% acetic acid in ethyl acetate with tylenol, unknown, and caffeine -plate two: 1:1 hexane:ethyl acetate with aspiring, unknown, and naproxen (aleve)

What would you expect to happen to the Rf values if you used acetone instead of hexanes as the eluting solvent? Explain your answer.

If acetone was used as the eluting solvent instead of hexane, I would expect the Rf values to be higher. The less polar compound does not stick to the polar silica gel so it still is eluted at a fairly rapid pace, and now that the solvent is polar instead of non polar, the more polar compound will be soluble in the solvent and it will be eluted faster and will be able to travel a farther distance. Since both compounds will travel a further distance, the Rf value will be higher.

Consider the two compounds of bibenzyl or benzhydrol What happens if you use hexane as an only eluting solvent throughout the chromatography? Explain.

If only hexane was used, the bibenzyl component would be separated from the mixture but the benzhydrol would remain on the column. The two compounds would be separated but there would be no way to recover the benzhydrol

A student who was performing a distillation for the first time failed to position the thermometer correctly. The bulb was set too high. What effect would this have on the observed boiling point of the liquid being distilled?

If the bulb was set too high, the observed boiling point of the liquid would be lower than the actual boiling point. When the bulb is positioned too high, it will not be in the vapor path of the boiling liquid, and therefore, the liquid will start to condense and collect in the receiving flask but the temperature will not actually reflect the boiling point because the thermometer is not close enough to get the temperature of the actual vapor as it is boiling.

1. During a separation with column chromatography, why must the level of the solvent be kept above the top of the stationary phase once the procedure is started (i.e. why must the column never go dry?)?

If the level of solvent drops below the stationary phase, alumina, air pockets and cracks would develop in the alumina which would lead to uneven elution of the compounds. The mobile solvent phases could fall into the cracks or air bubbles, preventing them from being eluted at the proper speed for successful separation.

When analyzed by TLC, a compound exhibited an Rf value of 0.48 when diethyl ether was utilized as the developing solvent. In this analysis the solvent was allowed to travel a distance of 7.8 cm up the plate. If the solvent had been allowed to travel 11.3 cm up the plate how high up the plate would the compound have traveled? What would the Rf value of the compound be for the trial in which the solvent was allowed to travel 11.3 cm up the plate? Explain.

If the solvent had been allowed to travel 11.3 cm up the plate, the compound would have traveled 5.42 cm up the plate because if the Rf value was 0.48 when the solvent was allowed to travel a distance of 7.8 cm up the plate, then the compound would have traveled 3.74 cm up the plate. If the experimental conditions were held constant, then the Rf value for the TLC run in which the solvent was allowed to travel 11.3 cm up the plate would also be 0.48 and the Rf, in this case, could be considered like a constant value for the compound

Commercial Naproxen is usually sold as its sodium salt (see structure below) in drug stores. Why does naproxen need to be protonated before analyzing by TLC?

Naproxen is a salt ionic compound is even more polar !!!!!CHECK!!!!

Would the separation of the two compounds (bibenzyl and benzhydrol) in this experiment have been successful if the eluting solvent order had been reversed (1:1 hexane:ether first, hexanes second)? Explain.

No, separation would not have been successful if the eluting solvent order had been reversed. The 1:1 hexane:ether solution is more polar than a hexane solution. Benzyhdrol is soluble in the 1:1 hexane:ether solvent so it will be eluted faster in this solution than in the nonpolar hexane solution. The alumina still absorbs the benzhydrol and not the bibenzyl, so bibenzyl is still eluted at a faster pace than benzhydrol. However, with the polar solvent being used first, although bibenzyl still is eluted faster, benzhydrol is also eluted at a moderately faster pace, so with both compounds being eluted rather quickly, it would be difficult to effectively separate the two compounds without using hexane as the solvent first.

Bromination of 2,3-Dimethylbutane

Note: bromine causes severe burns and should be dispensed in the hood with gloves. the by-product HBr is also corrosive and causes burns. The brominated alkanes should be handled with gloves -measure out and transfer approximately 2.0 mL of 2,3-dimethylbutanewith grad cylinder into a medium-sized test tube -add 0.25 ml of bromine using a plastic disposable pipet to the test tube containing 2,3-dimethylbutane above. immediately rinse the disposable pipet by drawing with approximately 2 mL of 1 M sodium bisulfite or sodium thiosulfate solution a few times in a 10mL graduate cylinder -the reaction mixture is swirled briefly and the test tube is put in a 250ml beaker and placed near the light source in the reagent hood -after a few minutes, the bromine color should fade to a light yellow indicating the reaction is complete -loosely cover the test tube with a cork and take it back to your hood. cool the test tube with an ice-water bath -after complete crystallization of the product, collect it by vacuum filtration. allow the solid to air dry in the Buchner funnel -obtain weight and melting point

Rationalize the order of elution of the two compounds from the column in this experiment (bibenzyl and benzhydrol)

The first eluting solvent used was the nonpolar hexane, which allowed the bibenzyl to be separated from the mixed compound first. The nonpolar bibenzyl was not strongly absorbed by the alumina in the column but it was highly soluble in the nonpolar hexane solvent. Therefore, it eluted at a rapid pace. The benzhydrol component, however, is polar, so it was strongly absorbed by the alumina and was not soluble in the nonpolar hexane, allowing it to remain in the column while bibenzyl was separated out. The 1:1 hexane:ether was the second eluting solvent used, and this polar solvent allowed the polar benzhydrol to be separated now that no bibenzyl remained in the column. The hexane/ether solution is more polar, so it dissolved the benzhydrol, eluting it from the column at a fairly rapid pace.

A student spots an unknown sample on a TLC plate. After developing in hexanes/ethyl acetate 50:50, he/she saw a single spot with an Rf of 0.55. Does this indicate that the unknown material is a pure compound? What can be done (using TLC) to verify the purity of the sample?

The presence of a single spot could indicate that the unknown material is pure. however, it is also a possibility that two compounds in the mixture traveled the same distance and are appearing as a single spot. to verify the purity of the sample, a TLC should be performed with a different hexane:ethyl acetate concentration. Making the eluting solvent more or less polar overall would help cause a difference in Rf of the unknown mixture if it happened to contain two compounds of varying polarity. However, if another test is done and only one spot appears, then that would further help prove that the compound is pure

simple distillation

a general technique used for removing a solvent, purifying a liquid, or separating the components of a liquid mixture. In distillation, a liquid is vaporized by boiling, then condensed back to a liquid, called the distillate or condensate, and collected in a separation flask (receiving flask) -compounds with lower boiling points will vaporize more quickly than those with higher boiling points and hence will distill first -can be used to purify a liquid that is contaminated by high boiling, non-volatile compounds and can also be used to separate 2 liquids that have very different boiling points

a chemist wishes to carry out a gravity chromatographic separation using diethyl ether and ethanol as the eluting solvents a. with which solvent should the chemist begin the elution? b. what would happen if the chemist started with the other solvent?

a. b.

Consider the two compounds of bibenzyl or benzhydrol Which compound is more polar?

benzhydrol group is more polar because it has an alcohol functional group present

chiral compounds

capable of rotating plane polarized light and enantiomers rotate the light in equal but opposite directions

halogenation of alkenes

electrophilic addition mechanism -the products obtained from alkene halogenation are called vicinal dihalides because the two halogen substituents are attached to adjacent carbon atoms -when halogen used is bromine or chlorine, halogenation of alkenes occurs rapidly at room temperature, and the resulting vicuna dihalides are stable -alkene pi bond is replaced with two sigma bonds -anti addition, trans vicinal dihlaides on cyclic alkenes

Consider the two compounds of bibenzyl or benzhydrol What happens if you use ethyl ether as an only eluting solvent throughout the chromatography? Explain.

fractions containing bot bibenzyl and benzhydrol components would be eluted and obtained and separation would not be effective. Ethyl ether is polar so benzhydrol will be soluble in the solvent so it will be eluted faster than when a non polar solvent is used. the alumina still adsorbs the benhydrol and not the bibenzyl so the bibenzyl still is eluted at a moderately fast pace. With both compounds being eluted rater quickly, it is difficult to collect two separately

optical purity

gives the composition of a mixture of enantiomers, also known as %ee (% enantiomeric excess): %ee= %Major Enantiomer- %Minor Enantiomer and is related to the specific rotation: %ee= (specific rotation [a] for mixture/ specific rotation [a] for pure enantiomer) * 100%

solvents used in column chromatography

least polar alkanes toluene halogenated hydrocarbons diethyl ether ethyl acetate acetone alcohols acetic acid most polar

expected elution order of organic classes in column and thin-layer chromatography

least polar/fastest alkanes alkenes ethers halogenated hydrocarbons aldehydes and ketones esters alcohols amines carboxylic acids most polar/slowest

optical activity

one of the properties in which enantiomers behave differently; thus it can be used to study enantiomers. It can be used for identification purposes since the specific rotation of a compound is a physical constant. It can also be used to measure the enantiomeric ration in a mixture of enantiomers --the rotation of the plane of polarization of linearly polarized light as it travels through certain materials

specific rotation

specific rotation= (observed rotation)/(path length in decimeter(l)*concentration in g/mL (c))

thin layer chromatography

the adsorbent is coated on one side of a strip or plate of glass, plastic, or aluminum -solvents and compounds travel up the plate by capillary action -different compounds in the sample are carried different distances up the plate because of variations in their adsorption on the adsorbent coating -more polar compounds towards the bottom of the plate and the less polar compounds toward the top

Consider the following boiling points (all in oC): hexane: 69 2-methylpentane: 62 2,2-dimethylbutane: 48 Although all of these compounds have the same number of carbons and are all alkanes, their boiling points are different. Explain.

the compounds have the same number of carbons and are all alkanes however they differ in their structure. the compound 2,2-dimethylbutane is the most highly branched of the three whereas hexane is the least branched compound. branching affects boiling point because of surface area. when a compound has higher branching, it has less surface area, and therefore weaker intermolecular forces to overcome in transition to the gas phase, resulting in a lower boiling point. a compound with less branching has more surface area and stronger intermolecular forces, so more energy is required to overcome the liquid phase into the gas phase, resulting in a higher boiling point.

retention factor (Rf value)

the distance that the spot of a particular compounds moves up the plate relative to the distance moved by the solvent front =(distance traveled by the compound)/(distance traveled by the solvent)

If 2 compounds showed two different Rf values on a silica gel TLC plate, which compound is more polar?

the less polar compound is the compound that moves the fastest and thus will be the closest to the solvent front. The polar compound moves slower and will be closer to the original spot and farther from the solvent front.

polarimetry

the method used to measure optical activity

halogenation of alkanes

the replacement of one or more hydrogen atoms in alkanes by a halogen -a simple substitution reaction in which a C-H bond is broken and a new C-X bond is formed -exothermic -reactivity decreases in the following order: F2>Cl2>Br2>I2 -energy input in the form of heat or light is necessary to initiate these halogenations -one complication is that all the hydrogen atoms of an alkane may undergo substitution, resulting in a mixture of products

observed rotation

the rotation measured in the lab using a polarimeter and is used to calculate a physical constant for the compound (specific rotation)

boiling point

the temperature at which its vapor pressure equals the external pressure acting on the surface of the liquid. the external pressure is usually the atmospheric pressure -when vapor pressure equals atmospheric pressure, the liquid will boil -compounds with higher vapor pressures will boil at lower temperatures

column chromatography

used to separate, and thus purify the components of a mixture -a mixture of compounds is applied to the top of the column and then solvent is added -the sample on the column is subjected to two opposing forces: the solvent dissolving it and the adsorbent absorbing it

apparatus for simple distillation

when liquid in the distillation flask boils, vapor rises to the top of the flask, through the distillation head, past the thermometer, and out the sidearm into the condenser