Describing a molecule as a blend of different resonance structures is similar to describing a paint color as a blend of primary colors. Fig. This shows that double bonds in benzene differ from those of alkenes. Benzene does not do this. Any of these structures alone cannot explain all the properties of benzene. It is this stabilisation due to resonance which is responsible for the aromatic character of benzene. Year: 2011. Thus, absorption of 420-430 nm light renders a substance yellow, and absorption of 500-520 nm light makes it red. That no resonance structures can be drawn for benzene. The concentrations were adjusted so that the absorption intensities of the components were roughly the same. I P Carrier T 40. We know that benzene has a planar hexagonal structure in which all the carbon atoms are sp2 hybridized, and all the carbon-carbon bonds are equal in length. 1 shows the structures of benzene, naphthalene, and anthracene. Building the orbital model. Owing to the oxidation reaction with ozone, the structure of the substance was established. The value of resonance energy has been determined by studying the enthalpy of hydrogenation and enthalpy of combustion of benzene. 2. The remaining one sp2-hybrid. They are mainly produced as intermediate in the production of surfactants, for use in detergent. The six-membered ring in benzene is a perfect hexagon (all carbon-carbon bonds have an identical length of 1.40 Å). Hydrocarbons which contain only single bonds are called alkanes. This has an important effect on both the properties of the ring and of the -OH group. Benzene Vs Cyclohexene - - Cyclohexene has single double bond while benzene has 3 delocalized double bonds. Procedure: Benzene (\(C_6H_6\)) is a planar molecule containing a ring of six carbon atoms, each with a hydrogen atom attached. Legal. (b) The ozone molecule is a blend of two resonance structures. Here, complementary colors are diametrically opposite each other. Thus, manufacturers of ethylbenzene are the major buyers of benzene, claiming more than half of total output. Benzene causes harmful effects on the bone marrow and can cause a decrease in red blood cells, leading to anemia. See section 8.06. as described below: 1. Its structure and formula reveal benzene to be an aromatic hydrocarbon, which is defined as a compound that is composed of hydrogen and carbon that has alternating double bonds forming a ring. However, it has been determined experimentally that all six bonds on the ring are identical. This is depicted as a cloud of negative electron density above and below the carbons. • Benzene shows resonance and can exist in different forms depending upon the the position of the double bond, making it extremely stable. Fig. Moreover, two isomers should result in a ‘ 1, 2 disubstituted benzene as shown in Fig. The simplest way to draw the structure of phenol is:. C-C bond length in benzene is 140 pm and C-H bond length is 109 pm. Here, two structurally and energetically equivalent electronic structures for a stable compound are written, but no single structure provides an accurate or even an adequate representation of the true molecule. The compound appears like a colourless liquid having a characteristic odour. Two sp 2 -hybrid orbitals of each C-atom overlap with two sp 2 -hybrid orbital of two other C-atoms to form sigma bonds. The experimental structure of benzene shows that all six C C bonds are of equal length, 1.40 Å, intermediate between the values for a C C single bond (1.54 Å) and a C C double bond (1.34Å). This relationship is demonstrated by the color wheel shown on the right. This compound is primarily utilized for producing polystyrene. orbital on each carbon atom overlaps axially with 1s orbital of hydrogen atom to form C-H sigma bond. Download our FREE Guide. Sidewise overlapping of orbitals. This nitration reaction, in the presence of activated benzene ring, forms yellow product. Both structures have -bonds. In basic solutions, both groups are deprotonated. The six-membered ring in benzene is a perfect hexagon (all carbon-carbon bonds have an identical length of 1.40 Å). Some women who breathed high levels of benzene for many months had irregular menstrual periods and a decrease in the size of their ovaries. Evidence for the enhanced thermodynamic stability of benzene was obtained from measurements of the heat released when double bonds in a six-carbon ring are hydrogenated (hydrogen is added catalytically) to give cyclohexane as a common product. Benzene resists addition reactions because those reactions would involve breaking the delocalization and losing that stability. The experimental structure of benzene shows that all six C C bonds are of equal length, 1.40 Å, intermediate between the values for a C C single bond (1.54 Å) and a C C double bond (1.34 Å). That only one Lewis structure can be drawn that satisfies the octet. J. König, in Colour Additives for Foods and Beverages, 2015. Several structural formulae can be written for a colored compound, since the molecule usually contains an alternation of single and double bonds. In cases such as these, the electron delocalization described by resonance enhances the stability of the molecules, and compounds composed of such molecules often show exceptional stability and related properties. Hydrogen lies on the outer side of the ring structure. The structures of cyclohexene and cyclohexane are usually simplified in the same way that the Kekulé structure for benzene is simplified - by leaving out all the carbons and hydrogens. Furthermore, molecular orbital theory predicts that those cyclic molecules which have alternate single and double bonds with 4n + 2 (n = 0, 1, 2, 3 etc.) Ethylbenzene is produced in on a large scale by combining benzene and ethylene in an acid-catalyzed chemical reaction: C 6 H 6 + C 2 H 4 → C 6 H 5 CH 2 CH 3. The resonance hybrid is a weighted average of the resonance forms 1a and 1b. We cannot describe the ozone molecule in terms of a single Lewis structure. Benzenesulfonic acid [ACD/Index Name] [ACD/IUPAC Name] [Wiki] ... NO Hydrocarbon Biodegradation (BioHCwin v1.01): Structure incompatible with current estimation method! 15.3: Structure and Stability of Benzene Formula: C 6H 6, four degrees of unsaturation (section 6.2) three double bonds + one ring The π-bonds of benzene are resistant to the normal reactions of alkenes and alkynes Br Br Cl CHO CHO Br 2 HCl O 3 No Reaction Benzene’s cyclic conjugated structure gives it … RCRA waste no. Apply this test to tyrosine, tryptophan, phenylalanine and glutamic acid. structural changes in an amino acid or protein that take place at different pH values alter the relative solubility of the molecule. You may wish to review Sections 1.5 and 14.1 before you begin to study this section. 2 shows the absorption spectra obtained by dissolving these compounds in ethanol and analyzing the resulting solutions. It has a gasoline-like odour and is a colourless liquid. Addition of hydrogen to cyclohexene produces cyclohexane and releases heat amounting to 28.6 kcal per mole. Each carbon atom uses two hybrid orbitals for axial overlap with similar orbitals of two adjacent carbon atoms on either side to form C-C sigma bonds. Benzene contains a main C 6 axis which contains S 6 and S 3 axes. Fig. The nitrogen has a lone pair of electrons perpendicular to the ring. 43.2. The resonance hybrid is more stable than any of the contributing (or canonical) structures. The molecular orbital containing n electrons spreads uniformly over the entire carbon skeleton and embraces all the six carbons as shown in Fig. This delocalisation of 1t-electrons, results, in the decrease in energy, and hence, accounts for the stability of benzene molecule. Let's go ahead and draw our resonance structure. Bromine in CCl4 is red. Furthermore, we found a new phenomenon that the silicon-cored structure could efficiently enhance emission intensity and adjust the emission colors of dendritic benzene. Missed the LibreFest? We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. When bromine reacts with an alkene or an alkyne, the alkyl halide formed is colorless; hence the disappearance of the red color is a positive test for unsaturation. Structures A and B have same arrangement of atoms and differ only in electronic arrangement. The formula of benzene (C 6 H 6), caused a mystery for some time after its discovery, as no proposed structure could take account of all the bonds (carbon usually forms four single bonds and hydrogen one).. 2,3 An extraordinary example is 5-methyl- 2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile, which occurs Structures A and B have same arrangement of atoms and differ only in electronic arrangement. Benzene Benzene can only be fully depicted with all of its resonance structures, which show how its pi-electrons are delocalized throughout its six-carbon ring. There is one unhybridised p-orbital having two lobes lying perpendicular to the plane of hybrid orbitals. Fig. Hence Benzene belongs to the D 6h point group.. Improper Rotations Explained Inversions Explained. In the hybrid structure on the right, the dashed hexagon replaces three double bonds, and represents six electrons in a set of three molecular orbitals of π symmetry, with a nodal plane in the plane of the molecule. The "localised" structures 1a and 1b above are resonance forms of benzene: neither 1a nor 1b are a true representation of benzene on their own; they are hypothetical, or "theoretical", structures. This section will try to clarify the theory of aromaticity and why aromaticity gives unique qualities that make these conjugated alkenes inert to compounds such as Br2 and even hydrochloric acid. By altering the structure of the compound, colour chemists can alter the wavelength of visible light absorbed and therefore the colour of the compound. 15.2: Structure and Resonance Energy of Benzene: A First Look at Aromaticity, information contact us at info@libretexts.org, status page at https://status.libretexts.org. 5-cyclohexatriene (Kekule benzene). what is ?) That would mean too many bonds to this carbon. The yellow color of the complex, in contrast to the colorless solvent-free crystals, is attributed to the planar geometry of the C=N-N=C moiety. derivatives. You might ask yourselves how it's possible to have all of the bonds to be the same length if the ring is conjugated with both single (1.47 Å) and double (1.34 Å), but it is important to note that there are no distinct single or double bonds within the benzene. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. Benzene has a cyclic structure with C-C double bonds. An orbital model for the benzene structure. Benzene undergoes substitution reactions in spite of the high degree of unsaturation. . One such reaction is the nitration of a benzene ring with nitric acid. The one in red and so, we can go ahead and draw a resonance structure and we take these electrons in magenta and move then into here. There is an interaction between the delocalized electrons in the benzene ring and one of the lone pairs on the oxygen atom. Nitrobenzene is a colourless to pale yellow, oily, highly toxic liquid with the odour of bitter almonds. Structure definition, mode of building, construction, or organization; arrangement of parts, elements, or constituents: a pyramidal structure. The unusual stability of benzene makes it resistant to the usual addition reactions of alkenes. Crystal Structure of Benzene-Solvate of Bis(benzophenone) Azine: A Color Polymorph . Dr. Dietmar Kennepohl FCIC (Professor of Chemistry, Athabasca University), Prof. Steven Farmer (Sonoma State University), William Reusch, Professor Emeritus (Michigan State U. There are delocalized electrons above and below the plane of the ring, which makes benzene particularly stable. According to orbital structure, each carbon atom in benzene assumes sp2-hybrid state. An alternative representation for benzene (circle within a hexagon) emphasizes the pi-electron delocalization in this molecule, and has the advantage of being a single diagram. These heats of hydrogenation would reflect the relative thermodynamic stability of the compounds. Benzene is highly toxic and carcinogenic in nature. This diagram shows one of the molecular orbitals containing two of the delocalized electrons, which may be found anywhere within the two "doughnuts". Iodobenzene is an organoiodine compound consisting of a benzene ring substituted with one iodine atom. Aliphatic Hydrocarbons hydrocarbons which do not contain a benzene ring are called aliphatic hydrocarbons. Benzene is among the 20 … benzene 1,4-dichlorobenzene: Supplementary data page; Structure and properties. In benzene the two cyclohexatriene Kekulé structures, first proposed by Kekulé, are taken together as contributing structures to represent the total structure. Building the orbital model. describe the structure of benzene in terms of resonance. The structure of benzene molecule is best described in terms of molecular orbital treatment theory. BibTex; Full citation; Publisher: Japan Society for Analytical Chemistry. The unhybridised p-orbital on each carbon atom can overlap to a small but equal extent with the p-orbitals of the two adjacent carbon atoms on either side to constitute n bonds as shown in Fig. Benzene is the simplest organic, aromatic hydrocarbon. Sorption to aerosols (25 Dec C)[AEROWIN v1.00]: Vapor pressure (liquid/subcooled): 0.0054 Pa (4.05E-005 mm Hg) Log Koa (Koawin est ): 5.817 Kp (particle/gas partition coef. This video shows how benzene is structured by resonance,orange is Carbon, the purple is the p-orbital, and the yellow dots are electrons. The remaining carbon valence electrons then occupy these molecular orbitals in pairs, resulting in a fully occupied (6 electrons) set of bonding molecular orbitals. compare the reactivity of a typical alkene with that of benzene. The structure on the right has two benzene rings which share a common double bond. Structure of the Benzene Molecule. lines, the color differences of polymorphic crystals are of considerable interest.2,3 An extraordinary example is 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile, which occurs ... this paper, we describe the crystal and molecular structures of benzene-solvated BBA and explain the structural and color differences. In other words, there are six C-H bonds, and any dipole formed by these bonds is balanced by carbon and hydrogen itself. + g e Deduce how many electrons are involved in the π-bonding in each structure and describe how their arrangements are different. In this case, benzene is resistant to oxidants, such as potassium permanganate and nitric acid. Based upon observable facts given above and the tetravalency of carbon, the following open chain structures were proposed for benzene. In this case, both forms contribute equally to the hybrid. Eventually, the presently accepted structure of a regular-hexagonal, planar ring of carbons was adopted, and the exceptional thermodynamic and chemical stability of this system was attributed to resonance stabilization of a conjugated cyclic triene. 1 shows the structures of benzene, naphthalene, and anthracene. 17 18. Benzene is embedded in a channel running along the c-axis. Structure of Aromatic Compounds. HSDB 55. Chemical bonding - Chemical bonding - Resonant structures: The description of the planar hexagonal benzene molecule, C6H6, illustrates another aspect of VB theory. Furthermore, this kind of dendritic benzene derivatives exhibited interesting optical properties with increased concentration. By Kazunori IMAI, Mai TAKAHASHI, Rie ISHII and Keiji KOBAYASHI. Watch the recordings here on Youtube! Benzene sulphonic a cid. It is this completely filled set of bonding orbitals, or closed shell, that gives the benzene ring its thermodynamic and chemical stability, just as a filled valence shell octet confers stability on the inert gases. Thus, the expected enthalpy of hydrogenation for benzene if it were to be represented hypothetically as 1, 3, 5- cyclohexatriene is- 360 kJ mol-1 The experimental value of enthalpy of hydrogenation of benzene has been found be – 208 kJ mol-1 Thus, 152 kJ mol-1 less energy is produced during hydrogenation of benzene than the expected for hypothetical 1, 3, 5-cyclohexatriene. Benzene 71-43-2 Hazard Summary Benzene is found in the air from emissions from burning coal and oil, gasoline service stations, and motor vehicle exhaust. lines, the color differences of polymorphic crystals are of considerable interest. It is naturally present in crude oil although it can be synthesized from petroleum products. Principles of Colour Chemistry 1.1 Basis for colour Unlike most organic compounds, dyes possess colour because they 1) absorb light in the visible spectrum (400–700 nm), 2) have at least one chromophore (colour-bearing group), 3) have a conjugated system, i.e. Monochloorbenzeen [Dutch] CP 27. Kekulé structure contained three carbon double bonds, cyclohexa-1,3,5-triene. After completing this section, you should be able to. 43.4. IP Carrier T 40. UNII-K18102WN1G. 43.1. German chemists Joseph Loschmidt (in 1861) and August Kekule von Stradonitz (in 1866) independently proposed a cyclic arrangement of six carbons with alternating single and double bonds. It is common to represent benzene by omitting the hydrogen atoms attached to carbon and showing only the carbon-carbon framework with the vertices unlabeled. It contains a methyl group (-CH 3) in the benzene ring, instead of the hydrogen atom. It behaves both as saturated and unsaturated hydrocarbon. Affordable and search from millions of royalty free images, photos and vectors. This is the resonance energy of benzene. structures of some common aromatic hydrocarbons are shown below. (a) Green paint is a blend of blue and yellow. Dinitrobenzenes are chemical compounds composed of a benzene ring and two nitro group (-NO 2) substituents.The three possible arrangements of the nitro groups afford three isomers, 1,2-dinitrobenzene, 1,3-dinitrobenzene, and 1,4-dinitrobenzene.Each isomer has the chemical formula C 6 H 4 N 2 O 4 and a molar mass of about 168.11 g/mol. Benzene (its electronic structure and propertiesdescribed in the article) is a highly flammable substance. Toluene: Toluene has the molecular formula of C 7 H 8. Benzene, an aromatic organic compound, is often represented as a hexagon with a circle in it to emphasize which of the following? From heats of hydrogenation or combustion, the resonance energy of naphthalene is calculated to be 61 kcal/mole, 11 kcal/mole less than that of two benzene rings (2 * 36). As shown below, the remaining cyclic array of six p-orbitals ( one on each carbon) overlap to generate six molecular orbitals, three bonding and three antibonding. Benzene (C6H6) may be assigned following two structures A and B. 2.1 Introduction. Hence the above structure of benzene is defective. It is useful as a synthetic intermediate in organic chemistry.It is a volatile colorless liquid, although aged samples appear yellowish. X-ray studies indicate that all the carbon-carbon bonds in benzene are equivalent and have bond length 140 pm which is intermediate between C-C single bond (154 pm) and C=Cbond (134 pm). The C n H 2n+1 chain is unbranched. Phase behaviour solid–liquid–gas Spectral data. The six-membered ring in benzene is a perfect hexagon (all carbon-carbon bonds have an identical length of 1.40 Å). In other words benzene molecule is more stable by 152 kJ mol-1 than 1, 3. In acidic solutions, both amino and carboxylic groups are protonated. The presence of three double bonds should make the molecule highly reactive towards addition reactions. But contrary to this, benzene behaves like saturated hydrocarbons. The Science Museum, South Kensington, London Sir I. Newton showed that white light is not a simple colour, but can be split up by means of a prism into the spectral colours : red, orange, yellow, green, blue, indigo and violet. This video will show you how to draw the ‘circle’ of resonance for benzene, as well as resonance intermediates for substituted aromatic compounds including Electron Donating Groups EDG which resonate into the ring and Electron Withdrawing Groups EWG which cause resonance out of the ring. Finally, there are a total of six p-orbital electrons that form the stabilizing electron clouds above and below the aromatic ring. The actual structure of benzene lies somewhere in between A and B and may be represented as C, referred to as resonance hybrid. Benzene is a liquid chemical that has a characteristic of sweet smell and has no color. It will also go into detail about the unusually large resonance energy due to the six conjugated carbons of benzene. Structures A and B are known as resonating or canonical structures of benzene. The color is attributed to absorption bands corresponding to transitions between the levels which arise from a resonance among these different possible structures. The evidence for stability of benzene is obtained by comparing experimental and calculated values of enthalpies of hydrogenation of benzene. Resonance Structure of Benzene. Monoclorobenzene [Italian] NSC 8433. If benzene is forced to react by increasing the temperature and/or by addition of a catalyst, It undergoes substitution reactions rather than the addition reactions that are typical of alkenes. The structure of phenol. They are called saturated hydrocarbons because there is a hydrogen in every possible location. That lone pair is next to the pi bond. Resonance energy of benzene has been found to be 152 kJ/mole. describe the geometry of the benzene molecule. Benzene, however, is an extraordinary 36 kcal/mole more stable than expected. In practice, 1,3-cyclohexadiene is slightly more stable than expected, by about 2 kcal, presumably due to conjugation of the double bonds. This sort of stability enhancement is now accepted as a characteristic of all aromatic compounds. The conceptual contradiction presented by a high degree of unsaturation (low H:C ratio) and high chemical stability for benzene and related compounds remained an unsolved puzzle for many years. Structure of benzene is a controversial topic because benzene has outstanding characteristics. An orbital model for the benzene structure. There is a σ h, 3 σ v and 3 σ d planes, as well as an inversion centre. The cyclohexatriene contributors would be expected to show alternating bond lengths, the double bonds being shorter (1.34 Å) than the single bonds (1.54 Å). Rather, the delocalization of the ring makes each count as one and a half bonds between the carbons which makes sense because experimentally we find that the actual bond length is somewhere in between a single and double bond. In terms of resonance structure, benzene prefers to undergo substitution reactions because during addition reactions the resonance stabilised benzene ring would be destroyed. We take the electrons in red and we push them off onto this carbon. During this colour of Br2/CCl4 disappears. Linear alkylbenzenes (sometimes also known as LABs) are a family of organic compounds with the formula C 6 H 5 C n H 2n+1.Typically, n lies between 10 and 16, although generally supplied as a tighter cut, such as C 12-C 15, C 12-C 13 and C 10-C 13, for detergent use. The formula of benzene (C 6 H 6), caused a mystery for some time after its discovery, as no proposed structure could take account of all the bonds (carbon usually forms four single bonds and hydrogen one).. It is used in the manufacture of aniline, benzidine, and other organic chemicals. • Benzene is a liquid, colorless aromatic compound which has an aromatic odor. Fig. but to understand phenol properly, you need to dig a bit deeper than this. 1,3-Dinitrobenzene is the most common isomer and … Structure of the Benzene Molecule. So, in the air it quickly lights up, and a mixture of its vapors with oxygen generally explodes. Orbitals with the same energy are described as degenerate orbitals. If either structure were correct, benzene would consist of alternating long single bonds and short double bonds. Benzene is a unique molecule when it comes to resonance structures. A calculation is made for the levels of benzene, cyclohexadiene, fulvene, and … The concentrations were adjusted so that the absorption intensities of … To indicate two structures which are resonance forms of the same compound, a double headed arrow is used as shown in Fig. describe the structure of benzene in terms of molecular orbital theory. 4-chlorobenzene. UN1134. Benzene is formed from natural processes, such as volcanoes and forest fires, but most exposure to benzene results from human activities. Typically, synthetic food colours are synthesized by chemical modification of several precursor compounds, for example, by diazotization of amino benzenesulfonic acid using hydrochloric acid and sodium nitrite. Chemists generally used the Kekule's structure as late as 1945. All of the carbon-carbon bonds have exactly the same lengths - somewhere between single and double bonds. Click the Symmetry Operations above to view them in 3D. Among the many distinctive features of benzene, its aromaticity is the major contributor to why it is so unreactive. Benzene is a colorless, flammable liquid with a sweet odor. AI3-07776. Protecting both people and the environment whilst meeting the operational needs of your business is a very important role and, if you have operations in the UK you will be well aware of the requirements of the Control of Substances Hazardous to Health (CoSHH) Regulations and likewise the Code of Federal Regulations (CFR) in the US 1. Thermodynamic data. Here we discuss different aspects of the structure of benzene. Download benzene stock photos. Amino acids are essentially soluble in water. Perpendicular to the C 6 axis are 6 C 2 axes. 43.5. Benzene, cyclohexadiene and cyclohexene yield cyclohexane on hydrogenation. It evaporates quickly when exposed to air. It is primarily used in the production of polystyrene. The plus and minus signs shown in the diagram do not represent electrostatic charge, but refer to phase signs in the equations that describe these orbitals (in the diagram the phases are also color coded). Benzene is one of the organic and simplest aromatic hydrocarbon and the parent compound of which has about a numerous number of important aromatic compounds. Have questions or comments? The delocalized structure of benzene also accounts for the X-ray data (all C-C bond lengths equal) and the absence of the type of isomerism shown in Fig. CCRIS 1357. Benzene is built from hydrogen atoms (1s 1) and carbon atoms (1s 2 2s 2 2p x 1 2p y 1).. Each carbon atom has to join to three other atoms (one hydrogen and two carbons) and doesn't have enough unpaired electrons to form the required number of bonds, so it needs to promote one of the 2s 2 pair into the empty 2p z orbital. If we take this value to represent the energy cost of introducing one double bond into a six-carbon ring, we would expect a cyclohexadiene to release 57.2 kcal per mole on complete hydrogenation, and 1,3,5-cyclohexatriene to release 85.8 kcal per mole. Here, two structurally and energetically equivalent electronic structures for a stable compound are written, but no single structure provides an accurate or even an adequate representation of the true molecule. This spreading of 1t electrons in the form of ring of n-electrons above and below the plane of carbon atoms is called delocalisation of n-electrons. This gives them a general formula CnH2n+2. The benzene ring and the ring in the intermediate formed after step 2 have different structures shown below. . In 2012, more than 99% of ethylbenzene was produced in this way. Any of these structures alone cannot explain all the properties of benzene. The amino acids that have activated benzene ring can readily undergo nitration. Here, two structurally and energetically equivalent electronic structures for a stable compound are written, but no single structure provides an accurate or even an adequate representation of the true molecule. Enthalpy of hydrogenation of cyclohexene is – 120 kJ mol-1, Enthalpy of hydrogenation of 1 ,4-cyclohexadiene is – 240 kJ mol-1, Thus, the calculated or expected value of enthalpy of hydrogenation of 1, 3, 5-cyclohexatriene is -360 kJ mol-1. 16 17. III. Each carbon has three sp2-hybrid orbitals lying in one plane and oriented at an angle of 120°. The remaining light will then assume the complementary color to the wavelength(s) absorbed. π1) being lowest in energy. Draw the pi-orbitals for this compound. Benzene is a naturally occurring substance produced by volcanoes and forest fires and present in many plants and animals, but benzene is also a major industrial chemical made from coal and oil. Since the contributing structures (A) and (B) are of exactly same energy they make equal contribution to the resonance hybrid and also stabilisation due to resonance should be large. Thus, the net dipole becomes zero. That the hydrogen atoms need not be shown. -When an alkene reacts with hydrogen the enthalpy change for one double bond is -120kJmol-1 -Kekulé benzene with three double bonds would expect to have an enthalpy change of -360kJmol-1 When benzene is hydrogenated the enthalpy change is -208kJmol-1. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. The molecule shown, p-methylpyridine, has similar properties to benzene (flat, 120° bond angles). The molecules of most coloured organic compounds contain two parts: (i) a single aryl (aromatic) ring such as benzene or a benzene ring with a substituent. Structure of benzene can be explained on the basis of resonance.