Both molecules are locked in an s- cis conformation, which is favourable. I'm thinking that it's due to the extensive conjugation in 9-anthracenemethanol? Yes, you are exactly right. Anthracene is better described by this structure where there are no fixed single and double bonds, but rather a delocalized aromatic structure.
So in anthracene, the aromatic character must be disrupted to some degree in order to undergo the Diels-Alder reaction. In cyclopentadiene, there is no aromaticity to disrupt, the double bonds are basically fixed like in buta-1,3-diene and, as you point out, locked in the required s- cis conformation.
A final note, benzene won't undergo a Diels-Alder under normal conditions because its aromaticity is completely disrupted and lost during the process. In anthracene the aromaticity remains in two of the rings, a much smaller price to pay. Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. Diels-Alder rate of reaction Ask Question. Asked 5 years, 1 month ago. Active 5 years, 1 month ago. Viewed 2k times.
Edward Edward 7 7 silver badges 20 20 bronze badges. Active Oldest Votes. Anthracene is better described by this structure where there are no fixed single and double bonds, but rather a delocalized aromatic structure than a structure like this where the single and double bonds are fixed and distinct from one another.
I'm still co fused about the need for 3 molar equivalents of n-methylmaleimide versus 1 molar equivalent of 9-anthracenemethanol? Are you running your reaction in water? The reactants are less soluble in water and therefore tend to cluster around themselves, effectively increasing their concentration and accelerating the reaction.
Because the maleimide does have some solubility in water an excess is used so that there is 1 equivalent of maleimide that is not dissolved in the water and available to react with the 9-anthracenemethanol outside of the water. Sign up or log in Sign up using Google.
Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers, and students in the field of chemistry. It only takes a minute to sign up. In the nitration of compounds such as naphthalene and anthracene, how can you determine which would be the major product? I tried drawing resonance structures for the intermediate formed because stability of intermediate determines which product will be major, but this became too hard for me as there were too many resonance structures and I couldn't really compare them.
Can someone please help? In the electrophilic substitution of polycyclic aromatics, when drawing resonance structures keeping as many benezene rings intact as possible is important. Note too that a naphthalene ring isn't as "good" as two separate benzene rings. Look at the structures below, electrophilic attack at the 1 position in naphthalene top row of drawing allows you to draw 2 resonance structures with a benzene ring remaining intact 4 if we count structures with the double bonds simply shifted in the intact benzene ring ; attack at the 2 position middle row in figure only has 1 resonance structure 2 if we count structures with the double bonds simply shifted in the intact benzene ring with a full benzene ring intact.
Hence, electrophilic attack at the 1-poisition is preferred in naphthalene. Applying the same considerations to anthracene, we see that only attack at the central ring position allows 2 full benzene rings to remain intact in the possible resonance structures.
Consequently electrophilic attack at anthracene's 9-position is preferred. When you compare the stability of the carbocation intermediates of polycyclic aromatic compounds, you have to consider if the aromaticity of one of the rings is sacrificed in order to delocalize the positive charge.
If this is the case, the intermediate is less stable. For naphthalene, for example, substitution at C1 is favored, because the cation is stabilized by allylic resonance and the aromatic character of the second ring is maintained. Hence, this C2-substituted intermediate is less stable source. You can make analogous considerations for anthracene, except that there is a third position for substitution, namely C9 in the middle ring.
Delocalization of the charge on this position creates a cation with two intact benzene rings, which favors substitution at this position. To supplement Jannis Andreska's excellent answer, you can consider the total number of resonance structures available to both carbocation intermediates.
Nitration at position 1 produces a carbocation that has 7 total resonance structures, 4 of which appear to preserve the aromaticity of the second ring.
Nitration at position 2 produces a carbocation that has 6 total resonance structures, 2 of which appear to preserve the aromaticity of the second ring. You can do the same analysis for anthracene, and you will probably find that nitration at position 9 on the middle ring is favored. Sign up to join this community.
The best answers are voted up and rise to the top.Although naphthalene, phenanthrene, and anthracene resemble benzene in many respects, they are more reactive than benzene in both substitution and addition reactions. This increased reactivity is expected on theoretical grounds because quantum-mechanical calculations show that the net loss in stabilization energy for the first step in electrophilic substitution or addition decreases progressively from benzene to anthracene; therefore the reactivity in substitution and addition reactions should increase from benzene to anthracene.
In considering the properties of the polynuclear hydrocarbons relative to benzene, it is important to recognize that we neither expect nor find that all the carbon-carbon bonds in polynuclear hydrocarbons are alike or correspond to benzene bonds in being halfway between single and double bonds. The 1,2 bonds in both naphthalene and antracene are in fact shorter than the other ring bonds, whereas the 9,10 bond in phenanthrene closely resembles an alkene double bond in both its length and chemical reactivity.
Orientation in the substitution of naphthalene can be complex, although the 1 position is the most reactive. Some examples follow. Sometimes, small changes in the reagents and conditions change the pattern of orientation. One example is sulfonation, in which the orientation changes with reaction temperature. Another example is Friedel-Crafts acylation; in carbon disulfide the major product is the 1-isomer, whereas in nitrobenzene the major product is the 2-isomer.
Substitution usually occurs more readily at the 1 position than at the 2 position because the intermediate for 1-substitution is more stable than that for 2-substitution.
The reason is that the most favorable resonance structures for either intermediate are those that have one fully aromatic ring. We can see that 1-substitution is more favorable because the positive charge can be distributed over two positions, leaving one aromatic ring unchanged. Only one resonance structure is possible for the 2-substitution intermediate that retains a benzenoid-bond arrangement for one of the rings.
The reactions of the higher hydrocarbons with electrophilic reagents are more complex than of naphthalene. For example, phenanthrene can be nitrated and sulfonated, and the products are mixtures of 1- 2- 3- 4- and 9-substituted phenanthrenes:.
However, the 9,10 bond in phenanthrene is quite reactive; in fact is is almost as reactive as an alkene double bond. Addition therefore occurs fairly readily; halogenation can give both 9,addition and 9-substitution products by the following scheme:.
Anthracene is even more reactive than phenanthrene and has a greater tendency to add at the 9,10 positions than to substituted. However, the addition products of nitration and halogenation readily undergo elimination to form the 9-substitution products:. John D.Dennis, N. Tetrahedron48 Synthesis Synlett Tetrahedron53 Tetrahedron57 The Diels-Alder reaction also known as the Diene Synthesis is the reaction of a 1,3-butadiene with an alkene to form a cyclohexene.
One of the first cycloadditions performed by Diels and Alder Nobel Prize was the reaction of cyclopentadiene with p-benzoquinone Diels, O.
Liebigs Ann. Quinone cycloadditions have been frequently used in natural product syntheses, including the cyclopentadiene-benzoquinone adduct iteself in the synthesis of epi-Epoxydon. Nobel-Prize-winning chemistry O. Diels, K. Alder, Dendrobine : Kende, A. Miltirone : Lee, J. The scope of the Diels-Alder reaction is very broad, including not only many substituted dienes and alkenes, but numerous heteroanalogs of both the diene and the dienophile. Stereospecificity: cis-dienophilies go to cis cyclohexenes, trans-trans dienes go to cis-1,4-cyclohexenes.
Conformation of Diene : Only dienes which can adopt the s-cis conformation undergo facile Diels-Alder reactions. Any substitution pattern which favors the s-trans isomer slows down the cycloaddition.
Especially unfavorable are cis-substituents on the diene, which usually prevent successful cycloadditions from being performed. Electronic Effects : Most successful Diels-Alder reactions involve an electronic imbalance between diene and dienophile - usually the diene is electron rich the donor and the dienophile is electron poor the acceptor. Generally, 1-substituents have a larger reactivity effect than 2-substituents.
It is also possible to have Diels-Alder reactions with inverse electron demand, where the diene is electron deficient. Steric Effects : Because of the compact cyclic nature of the Diels-Alder transition state, the reaction is very sensitive to steric effects at all positions except the two central positions on the diene. Alkyl groups on the dienophile are especially problematic because the electronic and steric effects are in the same direction. The Diels-Alder reaction involves a stereospecific cis addition suprafacial to both the diene and dienophile.Aromatics & Cyclic Compounds: Crash Course Chemistry #42
Existing stereochemical relationships in the dienophile cis or trans and the diene trans-trans or cis-trans are translated into stereochemical relationships in the product.
The Alder Endo Rule : the unsaturated substituent on the dienophile generally prefers the endo position in the transition state. The endo-selectivity is not usually very large, with values between and being common.
Diels-Alder help, URGENT!!?
Facial Selectivity : If the diene or dienophile does not have planar symmetry, then there may be substantial face-selectivity in Diels-Alder reactions resulting from combinations of steric and electronic effects. Intermediate in prostaglandin synthesis: Corey, E. The regioselectivity of the Diels-Alder reaction of unsymmetrical dienes with unsymmetrical dienophiles can be predicted by the ortho-para rule.
I'm wondering why maleic anhydride adds to the middle cycle of anthracene, and not the outer two. I would have expected that a Diels—Alder with the outer ring would be better, because I expected a naphtalene part to be lower in energy than two benzene parts more resonance stabilisation.
I guess it has to do with reactant based arguments that the atomic coefficients for the two center carbon atoms C-9 and C are higher than from the outer cycle C-1 and C For the Diels—Alder reaction, you may imagine two different pathways. I invite you to draw the mechanisms by yourself:. Yet gradually, as experimentally found, in this group of three, benzene is the most, anthracene the least aromatic compound.
Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. Why is it the middle ring of anthracene which reacts in a Diels—Alder? Ask Question. Asked 4 years, 11 months ago. Active 2 years, 6 months ago.
Viewed 9k times. Active Oldest Votes. I invite you to draw the mechanisms by yourself: A reaction that involves carbon atoms 1 and 4 or 5 and 8. Possible, by mechanism. There are five double bonds remaining in conjugation, and you count one six-membered ring in the state of "a benzene ring" the very left one. In the very right six-membered ring, there is only a single double bond, too. Alternatively, a Diels—Alder reaction with carbon atoms 9 and It is so much favourable to the former, that this is the reaction observed.
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Offer an explanation of why anthracene preferentially forms a Diels-Alder adduct at 9,10 positions. There are four reasonable resonance structures for anthracene. Draw them. A large number of polycyclic benzenoid aromatic hydrocarbons are known. From the literature, locate and then draw the structure of this hydrocarbon. Can you suggest other sources where this material might be expected to be present? Outline a synthetic reaction scheme for the preparation of triphenylmethanol from: a.
Methyl benzoate b. Diethyl carbonate 5. In the experiment, ligroin may be used as a solvent for the separation of the product from biphenyl. What is ligroin? Can you suggest an alternative solvent that might be used in this step?
Give the reaction scheme, showing the products formed before hydrolysiswhen one equivalent of ethylmagnesium bromide is treated with one equivalent of 5-hydroxypentanone. Does addition of two equivalents of the Grignard reagent to this yield a different product s? If so, give structure s. What would be the final product of the reaction between methyl benzoate and two equivalents of ethylmagnesium bromide? Consider the same reaction as in the previous question 8 except that in this case it is carried out with ethyl benzoate.
What product would be expected in this case? Grignard reagents may be used to prepare other organometallic reagents, for example, ethylmagnesium bromide reacts with cadmium chloride to yield diethylcadmium. There are numerous condensations that are closely related to the Perkin reaction. What general class of compounds can be prepared using each of these well-known reactions?
Why it is important that any aldehyde used in Witting reaction be free of carboxylic acid impurities?Anthracene is a solid polycyclic aromatic hydrocarbon PAH of formula C 14 H 10consisting of three fused benzene rings. It is a component of coal tar. Anthracene is used in the production of the red dye alizarin and other dyes.
Coal tar, which contains around 1. Common impurities are phenanthrene and carbazole. The mineral form of anthracene is called freitalite and is related to a coal deposit.
Hydrogenation gives 9, dihydroanthracenepreserving the aromaticity of the two flanking rings. Anthracene photodimerizes by the action of UV light:.
Substituted anthracene derivatives behave similarly.
The reaction is affected by the presence of oxygen. Chemical oxidation occurs readily, giving anthraquinoneC 14 H 8 O 2 belowfor example using hydrogen peroxide and vanadyl acetylacetonate. Electrophilic substitution of anthracene occurs at the 9 position. For example, formylation affords 9-anthracenecarboxaldehyde.
Substitution at other positions is effected indirectly, for example starting with anthroquinone. Anthracene is converted mainly to anthraquinonea precursor to dyes.
22.8: Substitution Reactions of Polynuclear Aromatic Hydrocarbons
Anthracene, a wide band-gap organic semiconductor is used as a scintillator for detectors of high energy photonselectrons and alpha particles. Plastics, such as polyvinyltoluenecan be doped with anthracene to produce a plastic scintillator that is approximately water-equivalent for use in radiation therapy dosimetry. It is also used in wood preservativesinsecticidesand coating materials. Anthracene is commonly used as a UV tracer in conformal coatings applied to printed wiring boards.
The anthracene tracer allows the conformal coating to be inspected under UV light. A variety of anthracene derivatives find specialized uses. Derivatives having a hydroxyl group are 1-hydroxyanthracene and 2-hydroxyanthracene, homologous to phenol and naphtholsand hydroxyanthracene also called anthrol, and anthracenol   are pharmacologically active. Anthracene may also be found with multiple hydroxyl groups, as in 9,dihydroxyanthracene.
Anthracene, as many other polycyclic aromatic hydrocarbonsis generated during combustion processes. Exposure to humans happens mainly through tobacco smoke and ingestion of food contaminated with combustion products.
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