The Diels Nmr, Ir Report Repaired) Repaired)

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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 cisNorbornene -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 a cid to give an unknown compound (compound X). NMR and IR spectroscopy will be used to identify compound XIntroduction

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]

die Diene

d i en o p h Dieneophileile

Transition u ct c cloa dd state

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 except ionally 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 substituents in the adduct, trans dienophiles will present trans substituents in the adduct.

CH2

CO2ME CO2ME CO2ME CO2ME CO2ME

(1)CH2 CO2ME CO2ME MEO2C

(2)

CH2

CH2

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 th e diene and the dieonphile are staggered to each other yields the Exo product.H H O O O O O

Fig 3.1

Fig 3.2

O

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

O O H O H

O O H O

Fig 4.1

Fig 4.2

H

Fig 4.1: showing the orientation in the transition state staggered. Fig 4.2 The Exo product Figures 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 ( ) [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.1 13

H nuclei range from 0-12ppm normal range and

C 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 field1

H NMR spectroscopy.

If two 1H nuclei have the same chemical shift then they are magnetically equivalent to each other.H H H H H H H O H H

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 Hs) 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.13

C NMR spectroscopy.13

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

C

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.CH3 C C C C CH3 Fig.7.1 The molecule is symmetrical and has 3 13 magnetically inequivalent C nuclei C C CH3 Fig.7.1 The molecule is unsymmetrical and has 5 13 magnetically inequivalent C nuclei. C C C C CH3 C C

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 of13

C 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.Thermometer Vigreaux tube Condenser

Calcium chloride guard tube. Dicyclopentadiene 20cm3Water in

Water out

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

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-2,3dicarboxylic anhydride. 6cm3 of cyclopentadiene added to 6g maleic anhydride in a QUICKFIT conical flask and heated to dissolve the anhydride. 20cm 3 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 50cm 3 of water in a flask. The solution was then heated until the solution was clear. Once clear, the flask was