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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
(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
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.
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.
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
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.
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
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.
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)
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)
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
Mechanisms
References.
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[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
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[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)