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The Diels-Alder Reaction: Qualitative Test for Conjugated Diene Binghamton University Section A-52 Jacob Kalmanovich

Introduction In this lab, the Diels-Alder Reaction was introduced which is a very versatile and very important reaction used in organic chemistry. The reaction is concerted which means it has no intermediates and its important because it provides a simpler way to adding a 1,4 alkene (dienophile) through [4+2} cycloaddition to a conjugated diene, and forming a substituted

cyclohexane.

The diene must be

conjugated and must be in a cis conformation in order for this reaction to go to completion which can be seen in Figure 2. On top of this the Diels-Alder reaction is site-specific with respect to both the diene and the dienophile. Addition in the reaction has to be syn on both reactants. What these means is the two new sigma bonds would both form on the same face of the reactant molecule. Also if the diene has substituents on it that have the same stereochemistry they will end up on the same face of the molecule while if the substituents have opposite steochemistry, then they end up on opposite faces of the product. On top of this cyclic dienes can give different stereoisomeric products depending on whether the dienophile lies under or away from the diene in the transition state. The endo product/dienophile lies under/in is usually the major product. The reason this reaction is thermodynamically favorable due to the break of 3 π-bonds and the creation of 1 π-bond and 2 new stronger σ-bonds. There are other things that can impact

the reaction other than just the diene and dienophile. Substituent groups don’t affect the rate of the reaction. Electron-withdrawing groups increase the rate of the Diel-Alder reactions. This is why in the experiment maleic anhydrous is used; it has very strong withdrawing groups to increase the effectiveness of the reaction. It is important to use dry glassware in this reaction in order to exclude moisture during the reaction which could cause hydrolysis of maleic anhydride to maleic acid. To specifically identify the adduct formed, the qualitative property of melting point was used. Even though the dienophiles have the same molecular formula, there structures are different which lead them to having different melting points. Also gas chromatography is used during this reaction. Gas chromatography is used to identify how much of the diene is in the starting reaction which can help quantify the amount of limiting reagents (maleic anhydride) is needed. In addition, reflux of the unknown Diene B and Maleic Anhydride mixture was done. Refluxing is a technique that involves the condensation of a vapor and the return of the condensate to the system from which it originated after cooling. Procedure: Pa r t1 :Ga sChr o mat o g r a ph i cAna l y s i s 1 )Anun kn o wns a mp l eofo i lBwa so b t a i n e da l o n gwi t hac h r o ma t o g r a mo ft h eo i lmi x t u r e 2 )Th ep e r c e n t a g eo fd i e n ewa se s t i ma t e dq u a l i t a t i v e l yb a s e dont h ec hr o ma t o g r a m. I ti st h e ma j o rc o mp o ne n ta n dc or r e s p o nd e dt ot h el on g e s t p e a k . 3 )Th ep e r c e n tc o mpo s i t i ono fd i e newa su s e dt oc a l c ul a t et h ea mo u n to ft hea mo u n td i e n e i nt he2 . 5go fu n kno wno i l

Pa r t2 :Pr e p a r a t i o no fDi e l s Al d e rAd d uc t

1 )Th ea mo u n tofma l e i ca n h y d r i d e , t h el i mi t i n gr e a g e n t ,t h a ti se s s e n t i a lt oc o mp l e t e l yr e a c t wi t h2. 5 gs a mp l eo fo i l , wa sc a l c u l a t e d . 1. 2 4gr a mso fma l e i ca c i dwa sne e d e d . 2 )2. 5 go ft h eu nkn o wno i lBwa sa d d e dt ot h er e a c t i onfla s k , a n dwa st h e nmi x e dwi t h5 ml a n h y d r o u se t h y le t h e r . 3 )1. 2 4g r a mso fma l e i ca c i dwa sa d d e d . 4 )Th er e a c t i o nfla s k , c o n d e ns e r , a n dd r y i n gt u b ef orr e flu xwe r ea l lp u tt o g e t h e ra s s e mb l e d . 5 )Ar o u n d b o t t o mfla s kwa sa l s ot i g h t l yc l a mp e dt ot her i n gs t a n d . Th er o u n db o t t o mfla s k wa sa t t a c h e dt i g h t l y . I twa sg e n t l yh e a t e dus i n gah o t t a pwa t e rba t h . 6 )Awa t e r c oo l e dc o nd e n s e rwa st h ep u to nt o po ft her o u ndb o t t o mfla s ka n da t t a c h e dt o t h er i n gs t a n dvi aa n o t h e rc l a mp . 7 )At u bewa sa t t a c he dt ot h en o du l eo nt h ec o n d e n s e rc l o s e s tt ot h er o u n db o t t o m fla s ka s t h a tn o d ul ei st h ewa t e ri nn od u l e . 8 )Th et o pno d u l ewa sa t t a c h e dt oat u b ea n dt hi sa c t e da st h ewa t e ro u tt ub e . 9 )Ad r y i n gt u b ewa sp l a c e do nt o po ft h ec o n d e n s e ri no r d e rt ok e e pmo i s t ur ef r omc o mi n g i n . 1 0 ) Th emi x t u r ewa sr e flu x e dg e n t l yf o r4 0 45mi n u t e s . Th er e flu xwa sk e p tg o i n gb yh e a t i n g c a r e f u l l ywi t hah o t t a pwa t e rb a t h . 1 1 ) Se t u pi ss e e ni nFi g u r e4 1 2 ) Af t e rt h er e flu xwa sc o mp l e t e ,t h ewa r mr e a c t i o nmi x t u r ewa smo v e dt oa5 0 mlbe a k e r , c o v e r e dwi t hawa t c hg l a s s , a n dc o o l e dt or o o mt e mpe r a t u r e . 1 3 ) Du r i n gt hec o o l i n gp r o c e s s , c r y s t a l l i z a t i o no ft h ea d d u c tb e g a n .Th eb e a k e rwa so n l y pl a c e di nt oa ni c eb a t ha f t e ri th a dr e a c h e dr o o mt e mp e r a t ur e . Th i sh e l p e dc o mp l e t et h e c r y s t a l l i z a t i o n .

1 4 ) As u c t i o nfil t r a t i o na p p a r a t u swa ss e t u p . 15) The crystals were filtered using vacuum filtration by using a Buchner funnel with a piece of circular filter paper placed inside and where a tube connected the first filter to another filter flask which was used as a vacuum source to create the suction and allowed for the water to be sucked out of the product. 1 6 ) Th es o l i dwa sfil t e r e du s i n gaBu c h n e rf u n n e la n dr i n s e dwi t h5 mlh e xa n e . 1 7 ) Th efin a l p r o d u c t swe r edr i e db yp u l l i n ga i ro v e rt h e mf o rs e v e r a lmi nu t e s . Th e ywe r et h e n we i g h e d . 1 8 ) Th eme l t i n gp o i n twa si d e nt i fie du s i n gDi g i me l t# 5. 19) Digimelt#5 was warmed up for 15 minutes. 20) Digimelt#5 was set up by pushing start temp and using the up arrow to set the starting temperature at 60°C. Then by pushing stop temp and the up arrow the stop temp was set at 160°C. 21) The substance was placed in a capillary tube and the initial fast rate melting point detection was done with a ramp speed of 20°C. The start button was pushed to allow the Digimelt to preheat and once the Digimelt went from preheat to ready the start button was pushed again to begin melting 22) Then the substance then was placed in 3 new capillary tubes and a slow melting point detection was done with a however the ramp speed was set to 5°C by pushing the ramp rate button and the down arrow to move the ramp rate from 20°C to 5°C. Also the starting temperature was raised to 90°C and the stop temp was set at 140°C Table of Reagents Na me

Me l t i n g Mol e c ul a r Sa f e t y

Co mp ou n dSt r u c t u r e

β My r c e n e

Po i nt

We i g h t

( ° C) 38 48

( g/ mol ) 1 3 6 . 2 4

Fl a mma b l e l i q u i da n d v a p o r , Ma y b ef a t a li f s wa l l o we d a n de n t e r s a i r wa y s , Ca u s e ss ki n a n de y e

Al l oo c i me ne

8 3 8 4

1 3 6. 2 4

i r r i t a t i on Fl a mma b l e l i q u i da n d v a p o r , Ma y b ef a t a li f s wa l l o we d a n de n t e r s a i r wa y s , Ca u s e ss ki n i r r i t a t i on , Ca u s e s s e r i ou se y e

αt e r p i n e n e

6 0 6 1

1 3 6 . 2 4

i r r i t a t i on Fl a mma b l e l i q u i da n d v a p o r , Ha r mf u li f s wa l l o we d , Ma yb ef a t a l i fs wa l l o we d a n de n t e r s

αp he l l a n dr e n e 12 6 -

1 3 6 . 2 4

1 2 7

a i r wa y s Fl a mma bl e l i q u i da n d v a p o r , Ma y b ef a t a li f s wa l l o we d a n de n t e r s a i r wa y s

Ma l e i c An h y d r i d e

5 3

9 8. 0 6

Ha r mf u li f s wa l l o we d , Ca u s e s s e v e r es k i n b u r n sa n d

e y ed a ma g e , Ca u s e s s e r i ou se y e

Et h y l Et h e r

1 1 6

3 4 . 6

d a ma g e Ex t r e me l y fla mma b l e . I nh a l a t i o n c o ul dc a u s e d r o ws i ne s s , h e a d a c h e , s o r e t hr o u g h , s ki na n de y e i r r i t a n t . Do n o ti n g e s t

Data Tables Percent Composition of Diene (%) Mass of Unknown Sample (g) Mass of Maleic Anhydride (g) Melting Point Range (Fast) (°C) Melting Point Range (Slow- 3 Trials) (°C)

62.4- converts to 1.56 grams of diene 2.77 1.24 Crumble: 100.9 Melt: 113.8 Crumble: 111.9, 111.2, 110.4

Final Weight Unknown B Identity

Melt: 113.0, 112.3, 110.9 1.79g a ph e l l a n d r e n e

Table 2: Height, Width and percent composition of peaks A-E seen in Figure 1 Height (cm) 1.7 14.1 6.6 1 6.0

Width at Half Height (cm) .5 .9 .5 .5 .6

Percent Composition (%) 4.1 60.6 15.8 2.4 17.2

Observation 

Oil is impure



Ether smelled strongly and was clear liquid



Maleic anhydride was solid. Limiting reagent



When the maleic anhydride was added into the round-bottom flask with the ethyl ether and the oil, it turned yellow almost instantaneously.



After the reflux the solution was completely yellow



The crystals after suction were initially yellow



When the hexane was added during the suction filtration the crystals immediately turned form yellow to white



Slow cool developed some crystals. Most crystals formed during ice bath

Calculations: Area of chromatogram peak: H x 1/2W For Peak 2: 14.1cm X 0.90 cm = 12.69 cm²

Sum of Peak areas: Diene peak+P1+P2+P3+P4 12.69 cm² + 0.85 cm² + 3.30 cm² + 0.50 cm² + 3.60 cm² = 20.94 cm²

Percentage of Diene:(Area of Diene Peak/Sum of Peak areas) x 100% (12.69 cm²/20.94cm²) x100% = 60.6%

Mass of Diene is 2.5g sample: 2.5g x percentage of diene 2.77g x 0.606mol = 1.68g

Amount of Maleic Anhydride Needed: Mass of diene in sample x %comp * (1/molar mass of diene) x (molar mass of Maleic anhydride) 2.77g x (1 mol/136.24g) * .606 x (98.06g/1 mol) = 1.21g

Theoretic Mass of Adduct: Moles of maleic anhydride x (molar mass of diene + molar mass of maleic anhydride) 1.21g x (1 mol/ 98.06g) = 0.0123 moles of maleic anhydride 0.0123 moles x (98.06g/mol + 136.24g/mol) = 2.88g

Percent yield of Crystal Adduct: (Actual Mass/Theoretical Mass) x 100% (1.79g/2.88g) x 100% = 62% Graphs/Figures: Figure 1: Chromatogram of unknown oil B

Figure 2: Diels Alder Reactions of with β My r c e n e , Al l o o c i me nea n dαt e r p i n e n e

Figure 3: Diels Alder Reaction of a p h e l l a n d r e ne

C10H16 + C4H2O3  C10H10O3 Figure 4: Reflux set-up

Discussions The main goal of this lab was to perform a Diels-Alder reaction, which is a cycloaddition reaction where a Diene and Dienophile come together. No intermediate is formed in this sort of reaction because it is concerted, which means it only needs one step to fully react. The experiment was done with unknown sample B. As seen in Figure 1, the gas chromatograph of the sample was given an analyzed. The largest peak on the graph was the Diene peak. It had a height of 14.1 cm and a width at half height of .9 cm. Thus the area underneath the peak was calculated to be 12.69 cm². Then, the total percent of diene in the mixture was calculated by taken the areas

of all the peaks which were 12.69 cm², 0.85 cm², 3.30 cm², 0.50 cm², 3.30 cm² summing them together to the total and dividing 12.64 by that total. That gave 60.6% as the percent of diene in the mixture. The quantity of diene was then calculated as 1.68 grams out of the 2.5 gram mixture. The percent of diene was then used to calculate the mass of maleic anhydride needed for the reaction, which was found to be 1.21 grams. An important thing to note about this reaction is that maleic anhydride is the limiting reagent which means that it is the concentration of maleic anhydride that determines how much product is formed. After the reflux was finished, the final weight of the product was 1.79grams. This gives a percent yield of 62%. While, this is an ok yield it is not a good one which indicates potential errors. This low percent yield can be due to moisture getting into the solution and hydrolyzing maleic anhydride, which can end up reducing the amount of total product made. Some crystals were also lost right before weighing them as there was some air flow that blew some crystals off. Another potential error could have come from the scale. The scale kept fluctuating in values, which meant that both its accuracy and precision are not at 100%. Perhaps with a more accurate scale that can separate the substance being weighed from the outside would yield better results. The crystals were then put into a Digimelt in order to find the melting point, to find the melting point of the anhydride adduct of the diene, which in turn would help identify the unknown diene B that was used in the experiment. First a fast trial was conducted to narrow down to melting point range. During the fast melting point determination trial the crumbling temperature was found to be 100.9°C and the melting point was 113.8°C. Afterwards, three trials of the slow melting point determination were done. For the 3 trials, the crumbling temperatures were 111.9°C, 111.2°C and 110.4°C and the melting point was at 113.0°C, 112.3°C and, 110.9°C respectively. While this isn’t the exact melting point it is closest to the melting point of the

anhydride adduct of alpha-phellandrene which is at 126-127°C. The other closest melting point was the one from Allo-ocimene which was 83-84°C which doesn’t correlate with the results from the Digimelt. One possible reason that the melting point was not closer to the actual range is that the unknown solution mixture B given could have been impure and impurities lower the overall melting point of the main substance inside. Due to the finding that the unknown diene was alphaphellandrene as seen in Figure 3 the Diels-Alder reaction of alpha-phellandrene and maleic anhydride was drawn out and the balanced equation is put underneath. The reason that the structure of the product is correct is because the cyclic diene (alpha-phellandrene) when reacting with a dienophile favors its endo form which means that the dienophile would point inwards towards the alkene which is what is drawn in Figure 3. The substituent groups face the way they do because the dienophile was cis and a cis dienophile gives of cis substituents which are why they are facing the same direction. Conclusion: The main objective of this lab was to find out the unknown diene used in a Diels-Alder reaction. After looking at the chromatogram for unknown substance B which is seen in Figure 1, the area underneath the peak was calculated to be 12.69 cm². This was used to find the percent of diene in the mixture which was 60.6%. The amount of diene was then calculated as 1.68 grams out of the 2.5 gram total gram of the mixture. To perform the reaction the amount of maleic anhydride had to be calculated which was found to be 1.21 grams. The final weight after the redox, crystallization and suction filtration was 1.79 grams which led to a percent yield of 62%. The melting point of the unknown crystals where found in order to ultimately find the unknown diene used in the reaction by finding the melting point of the adduct. The fast and the crumbling temperature were found to be 100.9°C and the melting point was 113.8°C. This was used to

readjust the temperature for the slow trail and after three trials of the slow melting point determination. After 3 trails of the slow melting point determination the crumbling temperatures were 111.9°C, 111.2°C and 110.4°C and the melting point was at 113.0°C, 112.3°C and, 110.9°C respectively. While this isn’t the exact melting point it is closest to the melting point of the anhydride adduct of alpha-phellandrene which is at 126-127°C. Due to this the reaction mechanism was drawn out in Figure 3. The product was drawn correctly due to it being in the endo formation which is favored when a cyclic diene reacts with a dienophile and the substituents of the dienophile are cis because that is where the dienophile had them originally in the cis formation. This is a very versatile reaction and is used in a plethora of different ways outside of an organic chemistry lab in ways such as the creation and manufacture of active pharmaceutical ingredients, agrochemicals, flavors, and fragrances that are used commercially. References: Organic Chemistry Lab Manual Handbook of Chemistry and Physics 93rd Edition pdf Pubchem.ncbi.nlm.nih.gov http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch10/ch10-5.html...