The following diagram illustrates how the acetyl group acts to attenuate the overall electron donating character of oxygen and nitrogen. This page titled Reactions of Fused Benzene Rings is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by William Reusch. This apparent nucleophilic substitution reaction is surprising, since aryl halides are generally incapable of reacting by either an SN1 or SN2 pathway. As expected from an average of the three resonance contributors, the carbon-carbon bonds in naphthalene show variation in length, suggesting some localization of the double bonds. Because of their high nucleophilic reactivity, aniline and phenol undergo substitution reactions with iodine, a halogen that is normally unreactive with benzene derivatives. Stability can be compared only for isomeric or related compounds or at best for unsaturated hydrocarbons it is compared only when they give same hydrogenated products. This increased reactivity is expected on theoretical grounds because quantum-mechanical calculations show that . The next two questions require you to analyze the directing influence of substituents. rev2023.3.3.43278. Which is more reactive naphthalene or anthracene? You should try to conceive a plausible reaction sequence for each. energy released on hydrogenation) of benzene than naphthalene according to per benzene ring Both are aromatic in nature. CHAT. ASK AN EXPERT. Correct option is C) Electrophilic nitration involves attack of nitronium ion on a benzene ring. In most other reactions of anthracene, the central ring is also targeted, as it is the most highly reactive. Acylation: Electrophilic substitution reaction is a reaction where an electrophile substitutes some other species in the given chemical compound. School of Chemistry, University of Sydney Recap benzene Benzene is planar with a symmetric hexagonal shape. Accessibility StatementFor more information contact us atinfo@libretexts.orgor check out our status page at https://status.libretexts.org. Acylation is one example of such a reaction. Naphthalene has two aromatic rings, but only 10 pi electrons (rather than the twelve electrons that it would prefer). Accessibility StatementFor more information contact us atinfo@libretexts.orgor check out our status page at https://status.libretexts.org. Consider napthalene, anthracene, and phenanthrene (if you add one benzene ring to the upper-right of phenanthrene, you have pyrene): The resonance stabilization that one benzene ring gets is #"36 kcal/mol"#. The potential reversibility of the aromatic sulfonation reaction was noted earlier. We have already noted that benzene does not react with chlorine or bromine in the absence of a catalyst and heat. The structure and chemistry of more highly fused benzene ring compounds, such as anthracene and phenanthrene show many of the same characteristics described above. Stack Exchange network consists of 181 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. Legal. The zinc used in ketone reductions, such as 5, is usually activated by alloying with mercury (a process known as amalgamation). In this instance, it is more beneficial than "the ring" symbolizing the delocalised electron system, as this helps you to account for the precise number of -electrons before the reaction (starting materials), during the reaction (the mechanism), and after the reaction (the product). Hence, order of stability (or RE): Benzene > Phenanthrene ~ Naphthalene > Anthracene. We also know that Anthracene is a solid polycyclic aromatic hydrocarbon compound. b) Friedel-Crafts alkylation of benzene can be reversible. 4 Valence bond description of benzene. So electrophilic substitution reactions in a haloarenes requires more drastic conditions. I'm wondering why maleic anhydride adds to the middle cycle of anthracene, and not the outer two. Why is the endo product the major product in a Diels-Alder reaction? The product is cyclohexane and the heat of reaction provides evidence of benzene's thermodynamic stability. These include zinc or tin in dilute mineral acid, and sodium sulfide in ammonium hydroxide solution. Substitution reactions of compounds having an antagonistic orientation of substituents require a more careful analysis. (more on that in class) and the same number of electrons (4n+2) as the -system of benzene, it is aromatic. Although naphthalene, phenanthrene, and anthracene resemble benzene in many respects, they are more reactive than benzene in both substitution and addition . The LibreTexts libraries arePowered by NICE CXone Expertand are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. Learn more about Stack Overflow the company, and our products. Aromatic hydrocarbons are cyclic, planar compounds that resemble benzene in electronic configuration and chemical behavior. The two structures on the left have one discrete benzene ring each, but may also be viewed as 10-pi-electron annulenes having a bridging single bond. . NH2 group is the most activating group which is present in aniline (C6H5NH2) hence it is the most reactive towards electrophilic substitution reaction. The procedures described above are sufficient for most cases. We can see then that the HOMO-LUMO gap converges as the number of rings increases, i.e. Explain why fluorobenzene is more reactive than chlorobenzene toward electrophilic aromatic substitution but chloromethylbenzene is more reactive than fluoromethylbenzene. EMMY NOMINATIONS 2022: Outstanding Limited Or Anthology Series, EMMY NOMINATIONS 2022: Outstanding Lead Actress In A Comedy Series, EMMY NOMINATIONS 2022: Outstanding Supporting Actor In A Comedy Series, EMMY NOMINATIONS 2022: Outstanding Lead Actress In A Limited Or Anthology Series Or Movie, EMMY NOMINATIONS 2022: Outstanding Lead Actor In A Limited Or Anthology Series Or Movie. Such oxidations are normally effected by hot acidic pemanganate solutions, but for large scale industrial operations catalyzed air-oxidations are preferred. The structure and chemistry of more highly fused benzene ring compounds, such as anthracene and phenanthrene show many of the same characteristics described above . when the central ring opened, two benzene ring had been formed, this action leads to increase the stability (as we know the benzene . Making statements based on opinion; back them up with references or personal experience. The modifying acetyl group can then be removed by acid-catalyzed hydrolysis (last step), to yield para-nitroaniline. Is it suspicious or odd to stand by the gate of a GA airport watching the planes? Many reactions of these aryl lithium and Grignard reagents will be discussed in later sections, and the following equations provide typical examples of carboxylation, protonation and Gilman coupling. The major product obtained for DHA was anthracene (80% yield) as analyzed by gas chromatography (GC, Figure S22). Reduction is easily achieved either by catalytic hydrogenation (H2 + catalyst), or with reducing metals in acid. 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. Well, the HOMO and LUMO are both required in electrophilic addition reactions. Why can anthracene, but not phenanthrene, take part in DielsAlder reactions? the oxidation of anthracene (AN) to 9,10 . Symmetry, as in the first two cases, makes it easy to predict the site at which substitution is likely to occur. Collectively, they are called unsaturated hydrocarbons, which are defined as hydrocarbons having one or more multiple (double . In case of acylation, the electrophile is RCO +. . 12. Note that if two different sites are favored, substitution will usually occur at the one that is least hindered by ortho groups. To learn more, see our tips on writing great answers. ENERGY GAPS AS A FUNCTION OF VOLUME (AND ENTROPY). Did any DOS compatibility layers exist for any UNIX-like systems before DOS started to become outmoded? order of stability (or RE): Benzene > Phenanthrene ~ Naphthalene > Anthracene. Are there tables of wastage rates for different fruit and veg? Is it possible to form an 8 carbon ring using a Diels-Alder reaction? Science Chemistry Give the diene and dienophile whose reaction at elecvated temperature produces the adduct shown below: I x OA. Anthracene, however, is an unusually unreactive diene. But you can see in the above diagram that it isn't: From this, we could postulate that in general, the more extended the #pi# system, the less resonance stabilization is afforded. The reason is that the most favorable resonance structures for either intermediate are those that have one fully aromatic ring. Connect and share knowledge within a single location that is structured and easy to search. Orientation in the substitution of naphthalene can be complex, although the 1 position is the most reactive. Yet gradually, as experimentally found, in this group of three, benzene is the most, anthracene the least aromatic compound. It should now be apparent that an extensive "toolchest" of reactions are available to us for the synthesis of substituted benzenes. The following problems review various aspects of aromatic chemistry. MathJax reference. Electrophilic substitution occurs at the "9" and "10" positions of the center ring, and oxidation of anthracene occurs readily, giving anthraquinone . ISBN 0-8053-8329-8. In the last example, catalytic hydrogenation of one ring takes place under milder conditions than those required for complete saturation (the decalin product exists as cis/trans isomers). Electrophilic nitration involves attack of nitronium ion on benzene ring. Why is thiophene more reactive than benzene? In the last example, catalytic hydrogenation of one ring takes place under milder conditions than those required for complete saturation (the decalin product exists as cis/trans isomers). I would have expected that a DielsAlder 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). How many of the following compounds are more reactive than benzene towards electrophilic substitution. 22.8: Substitution Reactions of Polynuclear Aromatic Hydrocarbons. Hence, pyrrole will be more aromatic than furan. In terms of activation, notice that maleic anhydride is a highly reactive dienophile, due to the presence of two electron- withdrawing carbonyl substituents. When two electrons are removed, i.e., dicationic systems are analyzed, the reverse trend is obtained, so the linear isomer is more stable than the kinked one. For the two catafusenes 2 and 3, both of which have 14 electrons, the result is presented in Fig. Why is the phenanthrene 9 10 more reactive? EXAMINING THE EXTENSIVITY OF RESONANCE STABILIZATION. The structure on the right has two benzene rings which share a common double bond. Browse other questions tagged, Start here for a quick overview of the site, Detailed answers to any questions you might have, Discuss the workings and policies of this site. Explain why polycyclic aromatic compounds like naphthalene and anthracene are more reactive toward electrophilic aromatic substitution reactions than benzene. 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. 05/05/2013. Naphthalene is more reactive than benzene, both in substitution and addition reactions, and these reactions tend to proceed in a manner that maintains one intact benzene ring. However, for polycyclic aromatic hydrocarbons, stability can be said to be proportional to resonance energy per benzene rings. Surly Straggler vs. other types of steel frames. Now these electrons can overlap with the electrons in the benzene ring and if we look at the molecule as a whole, the oxygen shares these electrons with the rest of the system and so, increases the electron density. . Why is phenanthrene more reactive than anthracene? The possibility that these observations reflect a general benzylic activation is supported by the susceptibility of alkyl side-chains to oxidative degradation, as shown in the following examples (the oxidized side chain is colored). The reaction is sensitive to oxygen. The alpha position is more prone to reaction position in naphthalene because the intermediate formed becomes more stable due to more diffusion of charges through the adjacent pie electrons. This provides a powerful tool for the conversion of chloro, bromo or iodo substituents into a variety of other groups. The fifth question asks you to draw the products of some aromatic substitution reactions. This is illustrated by clicking the "Show Mechanism" button next to the diagram. Hence, order of stability (or RE): Benzene > Phenanthrene ~ Naphthalene > Anthracene. In anthracene the rings are con- The structure and chemistry of more highly fused benzene ring compounds, such as anthracene and phenanthrene show many of the same characteristics described above. From this, we could postulate that in general, the more extended the pi system, the less resonance stabilization is afforded. Bulk update symbol size units from mm to map units in rule-based symbology, Identify those arcade games from a 1983 Brazilian music video, Trying to understand how to get this basic Fourier Series. However, ortho-chloroanisole gave exclusively meta-methoxyaniline under the same conditions. The toxicity of different crude oils and refined oils depends not only on the total concentration of hydrocarbons but also the hydrocarbon composition in the water-soluble fraction (WSF) of petroleum, water solubility . Similar exquisite degree of control at the individual polymeric chain level for producing functional soft nanoentities is expected to become a reality in the next few years through the full development of so-called "single chain technology&amp . Three canonical resonance contributors may be drawn, and are displayed in the following diagram. Similarly, alkenes react readily with halogens and hydrogen halides by addition to give alkyl halides, whereas halogens react with benzene by substitution and . Thus, resonance energy per ring for anthracene(3 rings) = 84 3 = 28kcal/mol. Example 6 is interesting in that it demonstrates the conversion of an activating ortho/para-directing group into a deactivating meta-directing "onium" cation [NH(CH3)2(+) ] in a strong acid environment. Examples of these reactions will be displayed by clicking on the diagram. They are described as polynuclear aromatic hydrocarbons, the three most important examples being naphthalene, anthracene, and phenanthrene. Question 6. A smaller HOMO-LUMO gap means a more reactive system, despite it having resonance throughout. Why is there a voltage on my HDMI and coaxial cables? The intermediate in this mechanism is an unstable benzyne species, as displayed in the above illustration by clicking the "Show Mechanism" button.