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die Diene dienophile ccloadduct The Diels-Alder Reaction and the determination of an unknown compound using NMR and IR Aim The aim of this experiment is to synthesise cyclopenadiene from dicyclopentadiene using a reverse Diels-Alder reaction. Using the cyclopentadiene and a forward Diels-Alder reaction cis-Norbornene -5,6- endo-dicarboxylic anhydride will be synthesised. A hydrolysis reaction on the anhydride will be performed to yield cis-Norbornene -5,6-endo- dicarboxylic acid . H 2 SO 4 will then be used to dehydrate the dicarboxylic acid to give an unknown compound (compound X). NMR and IR spectroscopy will be used to identify compound X Introduction A Diels-Alder reaction is an example of a pericyclic reaction. A pericyclic reaction (or cycloaddition reaction) is one where ‘a flow of electrons move around a circle’ [1] with no intermediates and no positive or negative charges. The reaction is a single step reaction and ‘proceeds through a cyclic transition state in which two or more bonds are broken and C-C bonds are formed at the same time’ [2]. The Diels-Alder reaction is an important method for makings six membered carbon rings [3] Fig 1: The diene and dienophile, showing the transition state and the product formed The Diels-Alder reaction occurs between a conjugated diene (two C=C separated by a single bond), this provides four of the 6 membered ring atoms and an alkene termed the dienophile which provides the other two atoms for the ring. The diene must be in the s-cis arrangement (s referring to the sigma bond and cis referring to on the same side of the Dieneophi Transition
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Page 1: The Diels Nmr, Ir Report Repaired) Repaired)

dieDiene dienophile c cloadduct

The Diels-Alder Reaction and the determination

of an unknown compound using NMR and IR

Aim

The aim of this experiment is to synthesise cyclopenadiene from dicyclopentadiene using a

reverse Diels-Alder reaction. Using the cyclopentadiene and a forward Diels-Alder reaction cis-

Norbornene -5,6-endo-dicarboxylic anhydride will be synthesised. A hydrolysis reaction on the

anhydride will be performed to yield cis-Norbornene -5,6-endo-dicarboxylic acid . H2SO4 will then

be used to dehydrate the dicarboxylic acid to give an unknown compound (compound X). NMR

and IR spectroscopy will be used to identify compound X

Introduction

A Diels-Alder reaction is an example of a pericyclic reaction. A pericyclic reaction (or cycloaddition

reaction) is one where ‘a flow of electrons move around a circle’ [1] with no intermediates and no

positive or negative charges.

The reaction is a single step reaction and ‘proceeds through a cyclic transition state in which two

or more bonds are broken and C-C bonds are formed at the same time’ [2]. The Diels-Alder

reaction is an important method for makings six membered carbon rings [3]

Fig 1: The diene and dienophile, showing the transition state and the product formed

The Diels-Alder reaction occurs between a conjugated diene (two C=C separated by a single

bond), this provides four of the 6 membered ring atoms and an alkene termed the dienophile

which provides the other two atoms for the ring. The diene must be in the s-cis arrangement (s

referring to the sigma bond and cis referring to on the same side of the single bond) like the one in

fig 1. , If in the trans arrangement the Diels – Alder reaction will not proceed. Cyclic dienes that are

permanently in the s cis arrangement are ‘exceptionally good at Diels-Alder reactions.’[1]. The

diene must also be electron-rich. The dienophile must have a two atom pi system and be electron

withdrawing (the example in fig.1 would be a ‘poor reaction’ because there is no electron

withdrawing groups on the dienophile) [1]. The product of a Diels-Alder reaction is termed an

adduct [4] and is one molecule made from the diene and the dienophile (no atoms are lost to form

other compounds). The product contains two new sigma bonds and one new pi bond.

The Diels –Alder reaction is stereospecific, although the diene must be in the s cis arrangement,

the dienophile can have cis and trans conformations. A cis dienophile will give cis substituent’s in

the adduct, trans dienophiles will present trans substituent’s in the adduct.

DieneophileTransition state

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(1)

(2)

Figure 2:- (1)- cis dienophile producing cis adduct, (2)- trans dienophile producing trans adduct

Cyclic dienes sometimes give stereoisomeric products. ‘The orientation in the transition state’ [4]

of the diene and the dienophile gives rise to this. If, in the transition state the diene and dienophile

are aligned directly over each other, yields the ‘Endo’ product. If, in the transition state the diene

and the dieonphile are staggered to each other yields the ‘Exo’ product.

Fig 3.1 Fig 3.2

Figure 3.1: The orientation in the transition state is directly over each other. Fig 3.2: The ‘Endo’ product

Fig 4.1 Fig 4.2

Fig 4.1: showing the orientation in the transition state staggered. Fig 4.2 The ‘Exo’ productFigures modified from ‘http: //itech.pjc.edu/tgrow/2211L/dielsalder’[5]The endo product is favoured as it gives maximum overlap of the p orbitals in the transition state.

[3]

A reverse Diels alder reaction starts with the dimer (in this case dicyclopetadiene which is then

‘cracked’ (split in two) to give 2 monomers of cyclopentadiene.

Interpreting NMR

NMR (Nuclear Magnetic Resonance) is the ‘determination of chemical structures by probing the

environments of individual elements’ [6], namely 1H and 13C nuclei. NMR occurs when certain

nuclei are put in a static magnetic field and then are exposed to a second magnetic field [7]. The

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protons within the atoms sometimes posses nuclear spin which mean the nuclei behave like bar

magnets with an N and S. The spin causes the nuclei to produce an NMR signal. For nuclei to

posses spin there must be an odd number of protons, odd number of neutrons or both [6]. When

an atom is placed in a magnetic field, the electrons around the atom rotate around the direction of

the magnetic field. The circulation of the electrons causes a magnetic field within the nucleus

(termed the effective field) [7] that oppose the externally applied magnetic field.

The electron density around each nucleus in a molecule will be different depending on what types

of bonds and nuclei are in the molecule, the opposing field and therefore the effective field will

vary. This is called the ‘chemical shift’ (δ) [7]

The chemical shift of a nucleus is the difference between the resonance frequency of the nucleus

and a standard divided by the standard (standard usually tetramethylsilane) [6] The numbers are

reported in ppm. The chemical shift is used as a scale to determine the chemical environment of a

nucleus and is the position on the scale where the peak occurs.1H nuclei range from 0-12ppm normal range and 13C ranges from 0-220ppm. The types of H or C

nuclei are indicated by the chemical shift of each group. Low numbered ppm chemical shifts are

termed ‘high field’ and high ppm chemical shifts are termed ‘low field’ 1H NMR spectroscopy.

If two 1H nuclei have the same chemical shift then they are ‘magnetically equivalent’ to each

other.

Fig 5.1: CH3 has 3 magnetically equivalent H nuclei Fig 5.2: CH3CH2OH has 3 magnetically inequivalent H

Fig 6: An example of 1H NMR spectra taken from www.chem.ucalgary.ca/courses/350/Carey/Ch13/ch13hnmr.html[9]

The integration of the spectral peak shows how many H there are of this kind. The area of the

peak is proportional to the number of H the peak represents. [9]

The number of groups of signals there are on a spectra, indicates how many types of magnetically

inequivalent H there are in the molecule. In Fig 6, there are 5 magnetically inequivalent H.

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The closeness of other H atoms around the nuclei being observed causes the signal on the

spectra to split. This is termed ‘coupling’ and its coupling that gives rise to splitting (multiplicity) in

the spectrum. The signal splits into two, (termed a doublet) if a C-H (1 H) is adjacent. The signal

splits into three, (termed a triplet) if a CH2 (2 H’s) is adjacent and splits into 4 (termed a quartet) if

a CH3 is adjacent to the observed H nuclei. Sextets are also witnessed for example if the hydrogen

being observed is between two CH2 groups- each CH2 group will split the signal into 3, totalling 6

splits. The multiplicity (no of splitting) = number of H + 1.

Using chemical shift charts the H can then be assigned to the peaks.13C NMR spectroscopy.

The number of peaks there are on the spectra indicates how many magnetic inequivalent 13C

nuclei there are in the molecule. If a molecule is symmetrical or has some symmetry, the number

of peaks on the spectra will be less.

Fig.7.1 The molecule is symmetrical and has 3 Fig.7.1 The molecule is unsymmetrical and has 5 magnetically inequivalent 13C nuclei magnetically inequivalent 13C nuclei.

The external magnetic field felt by the carbon nuclei is affected by the electronegativity of the

atoms attached, this increases the chemical shift [10] larger chemical shifts are to the left (low field

end) so a carbon with an oxygen attached will have a peak that is shifted to the low-field end. Of

the scale. Quaternary C (carbon with no hydrogen attached) has peaks of low intensity (small

peak)

A DEPT (Distortionless enhancement by polarization transfer) is a different type of 13C spectra.

With a DEPT experiment the peaks on the spectra appear pointing down (negative) as well as

pointing up (positive)

A DEPT spectrum helps to identify which peak belongs to which C. CH3 and CH groups are

positive, CH2 groups are negative and quaternary carbons (C) disappear from the spectra.

Chemical shift charts also help with the assignment of peaks.

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Experimental

Method as script: - no changes made

The apparatus was set up as fig.8 for the preparation of cyclopentadiene.

Fig.8 Set up of apparatus used for ‘cracking’ dicyclopentadiene to cyclopentadiene.

The cyclopentadiene was used immediately in the preparation of cis-5-norbornene-endo-2,3-

dicarboxylic anhydride. 6cm3 of cyclopentadiene added to 6g maleic anhydride in a QUICKFIT

conical flask and heated to dissolve the anhydride. 20cm3 of ethyl acetate added to flask. The

reaction was exothermic and the flask got hot, this was cooled in ice until the reaction stopped.

The solution in the flask formed a white precipitate The solution was then heated until the solution

went clear. After approx 15 minutes the solution was still slightly cloudy but was placed in ice for

crystals to develop. The white crystals were then filtered under suction with a Buckner funnel

Yield of cis-5-norbornene-endo-2,3-dicarboxylic anhydride = 9.79g (wet)

= 8.20g (dry)

4g of the anhydride was used in the preparation of cis-5-norbornene-endo-2,3 dicarboxylic acid.

This was added to 50cm3 of water in a flask. The solution was then heated until the solution was

clear. Once clear, the flask was placed in ice to allow cooling to 10°C. The crystals were filtered

using a Buckner funnel and dried by suction

Yield of cis-5-norbornene-endo-2,3 dicarboxylic acid = 2.92g

1g of the diacid was used in the preparation of compound X. 5cm3 of concentrated H2SO4 was

added to the diacid in a graduated conical flask. The solution was then heated to dissolve the

diacid. Once dissolved the solution was put in ice to cool. Ice was then added to the solution to

make the volume to approx 30cm3. As the ice was added the flask got hot indicating an exothermic

reaction. The solution was then heated to boiling then allowed to simmer for 5mins. The flask was

Dicyclopentadiene 20cm3

Calcium chloride guard tube.

Vigreaux tube

Condenser

Round bottomed collection flask for distilled cyclopentadiene. Placed in ice to prevent dimerisation

Thermometer

Water in

Water out

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then put in ice and scratched to induce crystal formation. The formed crystals were collected by

Buckner funnel then recrystalised from hot water.

Yield of compound X = 0.11g

Results

Yield and percentage yield of cis-5-norbornene-endo-2,3-dicarboxylic anhydride:

Cyclopentadiene – molecular weight (MW) = 66.10g/mol [11]

Density = 0.81g/cm3[11]

Mass used = density x volume = 4.86g cyclopentadiene used

Moles = mass / MW = 0.07moles

Maleic anhydride - MW = 98.06g/mol [12]

6g used

Moles = mass/MW = 0.06moles maleic anhydride used.

Limiting reagent = maleic anhydride.

Theoretical yield of cis-5-norbornene-endo-2,3-dicarboxylic anhydride (MW – 164.16g/mol)[13]

= 0.06moles x 164.16g/mol =9.85g theoretical yield of anhydride.

Percentage yield =( 8.20g (dry) / 9.85g) x 100 = 83.2%

Yield and percentage yield of cis-5-norbornene – endo-2,3-dicarboxylic acid.

Cis-5-norbornene-endo-2,3-dicarboxylic anhydride

Mass used = 4g

MW = 164.16[13]

Moles = mass /MW = 0.02moles of anhydride used

Water

Volume used = 50cm3

MW = 18 g/mol

Density = 1g/cm3

Mass used = 1 x 50cm3 = 50g of water used

Moles used = mass / MW = 2.77moles used

Limiting reagent = Cis-5-norbornene-endo-2,3-dicarboxylic anhydride

Theoretical yield of cis-5-norbornene-endo-2,3-dicarboxylic acid (MW-182.17g/mol) [14]

= 0.02moles x 182.17g/mol = 3.64g theoretical yield of diacid

Percentage yield = (2.92g / 3.64g) x 100 =80.2%

As stated in the lab script, compound X is a structural isomer of the diacid, therefore has the same

molecular weight of 182.17g/mol.

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Melting points for products

Compound Literature range &

ref

Melting point 1(°C) Melting

point 2 (°C)

Melting

point 3

(°C)

cis-5-norbornene-endo-

2,3-dicarboxylic anhydride

165-167 °C [15] 165.7 165.4 165.5

cis-5-norbornene – endo-

2,3-dicarboxylic acid

175°C [15] 173.2 173.8 174.3

Compound X 199.6 201.3 204.8

IR and NMR spectra for the products

Fig.9.1 IR spectra for anhydride.

The two peaks at 1851cm-1 and 1781cm-1 confirm that the compound is a anhydride. The two

peaks at 1234 and 1089cm-1 indicate a C-O bond

Fig.9.2. structure of cis-5-norbornene-endo-2,3-dicarboxylic anhydride

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Fig.10.1 IR spectra for cis-5-Norbornene-endo-2,3 dicarboxylic acid

The broad peak at 3084cm-1 indicates the acid group O-H. The very intense peak at 1711cm-1

shows the C=O of the acid groups. The intense peak at 1235cm-1 indicates C-O bonds.

Fig.10.2 Structure of cis-5-Norbornene-endo-2,3 dicarboxylic acid

Fig.11. IR spectra for compound X

The absorption band at 3433cm-1 indicates that an O-H group is present. The peak at 1113cm-1

shows that the compound has a secondary alcohol group. The intense band at 1773cm-1 indicates

that the compound is a 5 membered lactone (cyclic ester). The two bands at 1179 and 1251cm-1

indicate shows C-O bonding in the cyclic ester. The absorption band at 1694cm-1 shows C=O

bonding.

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Fig.12. 1H spectra for compound X

The 1H spectrum for compound X indicates that there are 10 hydrogen atoms in 7 magnetically

different environments in the ratio of 1:1:1:1:2:3

Fig.13. 13C spectra for compound X

The spectra indicates that there are 8 magnetically inequivalent 13C nuclei suggesting no

symmetry within the molecule The small peak at 171.60ppm indicates 1 quaternary carbon or

more because it is not intense and to the low field end.

Fig.14. DEPT spectra for compound X

The DEPT spectra for the compound indicates that there are 2 CH2 groups, this is shown as the

two peaks that point down. The missing peak at 171.6ppm proves that this is a quaternary carbon

with no hydrogen bonded to it (a C group)

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The proposed structure of compound X can be deduced by knowing that the compound is a

structural isomer of the diacid, therefore contains only C9H10O4. The H2SO4 served as a

dehydration agent when added to the diacid, and dehydrated one of the acid groups. The water

then hydrated the double bond on the diacid and created an alcohol group. The newly formed

alcohol group reacted with the remaining acid group in the presence of H2SO4 catalyst to produce

a five membered cyclic ester (termed a gamma lactone) (see reaction mechanism 4)

Fig 15.

By comparing the proposed structure of compound X to the NMR spectras and the IR spectra the

structure can be confirmed. From the DEPT spectrum, the proposed structure does have 2 CH2

groups. The structure also has 2 quaternary carbons. The H spectra shows that there are 10

hydrogen’s in the molecule The IR spectra suggests that the compound is a gamma lactone- the

proposed structure does have a gamma lactone ring

Discussion.

A reverse Diels-Alder reaction was carried out to produce cyclopentadiene from

dicyclopentadiene. (see mechanism 1). Cyclopentadiene was freshly distilled because it

undergoes dimerisation at room temperature to produce dicyclopentadiene. It does this where one

molecule of the cyclopentadiene acts as a diene and another molecule acts as the dienophile in a

forward Diels-Alder reaction.

The cyclopentadiene was then used to prepare the anhydride. Maleic anhydride was dissolved in

ethyl acetate and cyclopentadiene was added. The cyclopentadiene acting as the conjugated

diene and the maleic anhydride acting as the dienophile in the forward Diels-Alder reaction (see

mechanism 2) to produce cis-5-norbornene-endo-2,3-dicarboxylic anhydride. The melting point

and the IR spectra confirmed that the product was the anhydride.

The anhydride product was then hydrolysed to give cis-5-Norbornene-endo-2,3 dicarboxylic acid

(see mechanism 3) the product was confirmed by the IR spectra and the melting point.

In the preparation of compound X, the diacid product was first dehydrated using H2SO4. The water

then went on to hydrate the alkene double bond and created an O-H alcohol group. The O-H

group with a H2SO4 catalyst went on the react via an intramolecular reaction with the remaining

acid group to produce a 5 membered cyclic ester. Unfortunately no literature values for the melting

point of compound X (IUPAC name 5-Oxo-4-oxa-tricyclo[4.2.1.03,7]nonane-9-carboxylic acid)

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could be found. The book ‘Organic Experiments’ [16] states that the melting point of compound X

is 203°C and from the experimental results (average 201.9°C) it would seem plausible to assume

that the compound made was name 5-Oxo-4-oxa-tricyclo[4.2.1.03,7]nonane-9-carboxylic acid. The

NMR and IR spectra also helped to confirm this.

Corresponds to peak at 1.6ppm

CH2 corresponds to peak at 2.0-2.2ppm

Corresponds to peak at 4.7-4.8ppmthe chemical shift is at the lowend field as its near an O atom

The H on the O-H group ‘is sometimes observed, but is often not’[17]. The chemical shift for the O-

H splitting is variable from 0.5-5ppm and so it difficult to place. This could explain why the integral

on the peak 1.6 – 1.8ppm is 3. On the proposed structure for compound X, there is not 3

magnetically equivalent Hydrogen nuclei, so the peak for the O-H splitting could be within this

group of peaks for the CH2 on the bridge, which would give an integral of 3.

The carbon NMR spectra, the 2 quaternary C can be assigned to the peak at 171.6ppm and are

identified by the small intensity at the low field end. The 2 CH2 groups can be assigned to the

peaks at 32.55 and 37.26ppm using the DEPT spectra. The peak at 79.88ppm can be assigned to

the C attached to the O atom on the bottom left of the proposed molecule.

Conclusion

A forward and reverse Diels Alder reaction was carried out successfully. The product made was 5-

Oxo-4-oxa-tricyclo[4.2.1.03,7]nonane-9-carboxylic acid. The ambiguity of the O-H splitting on the 1H spectra could be resolved by adding D2O, which converts O-H to O-D. This would cause the O-

H peak to disappear and remove any uncertainty regarding the integral on the first peak at the

high field end of the 1H spectra.

Corresponds to peak at 3.2-3.3ppm – shifted to low end field due to O

Corresponds to peak at 3.0-3.1ppm. shifted to low end due to O

Fig 16. The structure of compound XShowing H,s allocated to peaks on NMR

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Mechanisms

References.

[1] Clayden, Greeves, Warren &Wothers.(2001) Organic Chemistry. Chpt 35, Pericyclic reactions. P905. Oxford university press.

[2] Hornback. J (2006) ‘Organic Chemistry’ 2nd edt .Books/cole

[3] Parsons.A (2003) ‘keynotes in Organic chemistry’ pg 95, Blackwell publishing.

[4]Roberts. M, Gilbert.J, Rodewald.L, Wingrove.A. (1979) ‘Modern Experimental organic chemistry’ 3rd edt. Chpt 8, Dienes, p199, Holt, Rinehart & Winston.

[5] ‘Diels-Alder Condensation Reaction Organic Chemistry’ [WWW] http://itech.pjc.edu/tgrow/2211L/dielsalder.doc. (last accessed 15/02/09)

[6]Nuclear Magnetic Resonance Spectroscopy, (15/01/09) Belt.S

[7] ‘The basics of NMR’ [Ebook] Hornak.J [www] 7 http://www.cis.rit.edu/htbooks/nmr/ (last accessed 15/02/09)

[8] Harris.R Nuclear Magnetic resonance spectroscopy. (1983) Pitmann, London

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[9] Carey. F, ‘Online learning centre for organic chemistry’ chpts13, spectroscopy. [WWW] http://www.chem.ucalgary.ca/courses/350/Carey/Ch13/ch13-hnmr.html (last accessed 15/02/09)

[10] Clark.J ‘Interpreting C13 NMR spectra’ (2007) [WWW] http://www.chemguide.co.uk/analysis/nmr/interpretc13.html#top (last accessed 15/02/09)

[11] ‘Cyclopentadiene’ [WWW] http://en.wikipedia.org/wiki/Cyclopentadiene (last accessed 15/02/09)

[12] Maleic anhydride, [WWW] http://en.wikipedia.org/wiki/Maleic_anhydride (last accessed 15/02/09)

[13] ‘cis-5-norbornene-endo-2,3-dicarboxylic anhydride, [WWW] Chemexper.com/ cis-5-norbornene-endo-2,3-dicarboxylic anhydride. (last accessed 15/02/09)

[14] ‘cis-5-Norbornene-endo-2,3-dicarboxylic acid [WWW] sigmaaldrich.com/catalog/ 216704 cis-5-Norbornene-endo-2,3-dicarboxylic acid. (last accessed 15/02/09)

[15] Sigma Aldrich Handbook of fine chemicals.

[16] Fieser.L, Williamson.K, ‘Organic experiments 3rd edt’ (1975) chpt 19 -Cis-Norbornene 5,6-endo-dicarboxylic anhydride. D.C Heath & co [17] ‘Interpreting proton NMR spectra’ [WWW] columbia.edu/itc/chemistry/c3045/client_edit/ppt/13_06_13_files/13_06_13.html. (last accessed 15/02/09)