Plymstock School. Arenes. P.J.McCormack

Transcription

1 Plymstock School 1 A2 Chemistry F324: Rings, Polymers & Analysis Arenes Arenes P.J.McCormack

2 Arenes Objective Checklist Draw the structure of benzene Explain the terms arene and aromatic State the empirical and molecular formula of benzene State five properties of benzene State the formula of a phenyl group Explain the models used to describe the structure of benzene Explain the term pi ( ) bond Explain what the term delocalised electrons means Explain in terms of bond lengths why Kekule s benzene is incorrect Compare the Kekulé and delocalised models for benzene in terms of p-orbital overlap forming pi ( ) bonds Write an equation for the hydrogenation of cyclohexene Explain using thermodynamic evidence why benzene cannot have alternating single and double bonds State the shape and bond angles around the carbon atoms in the benzene ring State the mechanism by which benzene reacts State the formula of a nitronium ion Write a balanced symbol equation to show the formation of a nitronium ion Write an equation for the nitration of benzene State the conditions required for the nitration of benzene Draw the mechanism for the reaction of benzene with a nitronium ion State the meaning of the term halogen carrier Write an equation to show the halogenation of benzene Explain why a halogen carrier is needed to halogenate benzene State the formula of three halogen carriers Draw the mechanism for the monohalogenation of benzene State the molecular formula of phenol Draw the skeletal formula of phenol State the physical appearance of phenol at room temperature and pressure Write an equation for the reaction of phenol with sodium hydroxide Write an equation for reaction of phenol with sodium State the uses for compounds containing phenol Write an equation for the reaction of phenol with bromine to form 2,4,6- tribromophenol Explain the relative ease of bromination of phenol compared with benzene State three uses of phenols

3 3 Introduction. The simplest arene is benzene. Arenes are compounds that contain benzene or derivatives of benzene obtained by replacing hydrogen s by other groups or fusing benzene rings together. Arenes are characterised by high stability, non-reactivity, and are unsaturated hydrocarbons but are unlike alkenes. All arenes contain a delocalised system of electrons. Benzene has the empirical formula C a molecular mass of 78 and molecular formula of C 6 6. Benzene ring C 6 6 In 1825, Michael Faraday analysed an oily residue formed inside a gas lamp. e found that the residue contained previously unknown hydrocarbon, whose molecular formula was later shown as C 6 6.This structure must be unsaturated. August Kekulé proposed a ring structure. The idea was attributed to a dream he had about a snake biting its tail. Kekulé drew the structure of benzene as cyclohexa-1,3,5-triene. C C C C C C Sometimes Kekulé s structure is shown without the carbon and hydrogen atoms being drawn. Derivatives of Benzene. Substitution Group Systematic Name Other Name Methyl C 3 Methylbenzene Toluene Chloro, -Cl Cholorbenzene - Nitro, -NO 2 Nitrobenzene - ydroxy, -O Phenol - Amino, -N 2 Phenylamine Aniline Carboxylic acid, -COO Benzenecarboxylic acid Benzoic acid

4 4 Naming. Methyl groups occupy the 1 position and the ring is numbered clockwise. The lowest position of the substituted groups are used. -C 6 5 this is a phenyl group, so called from the Greek pheno I bear light as Faraday isolated benzene from illuminating gas. Methylbenzene (toluene) 1,2-dimethylbenzene Ethylbenzene Bromobenzene Phenol Phenylamine 2-methylphenol Nitrobenzene Benzenecarboxylic acid (benzoic acid) 2,4,6-trinitrotoluene (TNT) Aspirin

5 5 Properties of Benzene. 1. Planar 2. ighly symmetrical 3. Non-polar 4. Lack of polarity results in benzene being a liquid at r.t.p. 5. Immiscible with water 6. Boiling point = 80 C 7. Melting point = 6 C 8. Reacts by electrophilic substitution. 9. Carcinogenic The high melting point is due to the ease at which the highly symmetrical benzene ring can pack into a crystal lattice. Methylbenzene m.p.t = -95 C (a) Compare the Kekulé and delocalised models for benzene in terms of p-orbital overlap forming pi ( ) bonds. Kekulé s structure suggests that benzene has three single bonds and three double bonds, so would react in the same way as cyclohexene and undergo electrophilic addition. It further suggests that the single bonds are long and the double bonds are short bonds. Problems with Kekulé s Model. 1. X-ray diffraction shows that benzene is planar which concurs with Kekulé s structure but:- The molecule is a regular hexagon of carbon atoms, with six equal bond lengths. C-C in cyclohexane = 0.154nm C=C in cyclohexene = 0.133nm C-C bond in benzene = 0.139nm Kekulé s structure would look like the diagram on the right with differing bond lengths. Benzene is highly symmetrical so cannot have alternating double and single bonds. 2. We find that benzene has a constant bond length, somewhere between a single and double bond length. 3. Benzene does not behave like an alkene. It reacts by electrophilic substitution rather than by electrophilic addition, even though it is unsaturated.

6 6 4. The theoretical enthalpy of hydrogenation of Kekulé s benzene is -360 kjmol -1. The experimental value is -208 kjmol -1. The structure is more stable (endothermic) than Kekulé s structure. The extra stability and equivalent carbon-carbon bond length can be explained by delocalisation. Bonding in Benzene. 1. The carbon atoms in the ring are bonded to one another and to their hydrogen atoms by sigma bonds (fig. 1.0). 2. This leaves one unused p orbital on each carbon, each containing a single electron. These p orbitals are perpendicular to the plane of the ring, with one lobe above and one below the plane (fig. 2.0). 3. Each p orbital overlaps sideways with two neighbouring orbitals to form a single bond that extends as a ring of charge above and below the plane of the molecule (fig.2.1). 4. The electrons in the bond cannot be said to belong to any particular carbon atom. Each electron is free to move throughout the entire system, so the electrons are said to be delocalised. It is this delocalisation that gives benzene its extra stability. Any system in which electron delocalisation occurs is stabilised. : 1s orbitals C C C C C C atomic orbitals -bonds are formed by side-by-side overlap of all six 2p atomic orbitals. Figure 2.1 Figure 1.0 Figure 2.0 Electrons tend to repel one another, so a system when they are far apart as possible will involve minimum repulsion and will therefore be stabilised. Delocalisation of the electrons has a profound effect on the both physical and chemical properties. The Delocalised Theory of Benzene. This theory suggests that instead of three double and three single bonds in fixed positions (localised), the six p-orbitals overlap and the six pi ( ) electrons are free to move within this system, creating a ring of delocalised electrons. This explains why benzene is highly symmetrical and more stable than predicted from the Kekulé s structure.

7 (b) Evidence for a delocalised model of benzene in terms of bond lengths, enthalpy change of hydrogenation and resistance to reaction. Thermodynamic Stability Evidence. When an alkene (double) bond is reacted with hydrogen the energy change is called the enthalpy of hydrogenation. The theoretical enthalpy change when one double bond is hydrogenated is -120 kjmol -1. Kekulé s structure of benzene has three double bonds therefore the total theoretical enthalpy value is -360 kjmol = -120 kjmol -1 Cyclohexene Cyclohexane 32 = -360 kjmol -1 When benzene is hydrogenated experimentally the value for the enthalpy change is -208 kjmol -1. This is 152 kjmol -1 less than the expected enthalpy change of hydrogenation of cyclohexa-1,3,5-triene. From this it can be concluded that: Cyclohexa-1,3,5-triene Cyclohexane 1. Benzene is more stable than the Kekulé structure (it has a lower enthalpy of hydrogenation value) 2. The bonding in benzene cannot be composed of alternating double and single bonds.

8 8 The diagram on the right shows the theoretical values for the hydrogenation of Kekule s benzene with a comparison of the delocalised model. The delocalised model of benzene is more stable than the Kekule s structure so has a lower enthalpy value (c) Electrophilic substitution of arenes with concentrated nitric acid in the presence of concentrated sulfuric acid. Electrophile a species that accepts a pair of electron Electrophilic Substitution. The benzene ring has a high electron density associated with the delocalised electrons. ence an attacking reagent that is attracted by this negative charge is needed an electrophile. Nitration The substitution of a hydrogen atom for a nitronium ion (NO 2 +). When benzene is treated with a mixture of concentrated nitric and concentrated sulfuric acid and gently refluxed at 50 C nitrobenzene is produced. NO 2 + NO 3 c. 2 SO 4 50 C + 2 O This reaction occurs in several steps. First the nitronium ion is formed. NO 3 + 2SO 4 2NO SO 4-2NO 3 + NO O

9 (d) Outline the mechanism of electrophilic substitution in arenes, using the mononitration and monohalogenation of benzene as examples The mechanism by which arenes react is electrophilic substitution. A hydrogen on the benzene ring is replaced (substituted) for an electrophile such as NO 2 + or Cl +. The general mechanism is: Mechanism for the Nitration of Benzene. NO 2 NO 2 NO 2 + Step 1 Step 2 + Intermediate + + Electrophilic attack by the nitronium ion takes place as the positively charged ion is attracted to the delocalised electrons. A covalent bond is formed to one of the carbon atoms disrupting the delocalised system. The intermediate has a high activation energy as a considerable amount of energy is required to break the delocalised system. alogenation. Benzene does not react with chlorine, bromine, or iodine on their own because the non-polar halogen molecule has no centre of positive charge to initiate electrophilic attack therefore a catalyst is needed. The catalyst is called a halogen carrier, and is thought to work by accepting a lone pair from one of the halogen atoms. This induces polarisation in the halogen molecule. Typical halogen carriers are iron, iron(iii) halides or aluminium halides. Cl---Cl:---FeCl The dotted lines show bonds breaking and bonds forming between a chlorine atom and the iron(iii) chloride. The positive end of the halogen is now an electrophile and can attack the benzene ring.

10 10 + Cl 2 + Cl Cl Alkylation: Friedel-Crafts Reaction. Alkylarenes are made using a halogen carrier and a halogenoalkane to bring about substitution of a delocalised ring. C 3 + C 3 Cl AlCl 3 eat Methylbenzene As in the reaction with halogens, the halogen carrier (aluminium chloride) accepts an electron pair from the chlorine atom, polarising the chloromethane molecule. C 3---Cl:---AlCl The positively charged methyl group attacks the delocalised ring and electrophilic substitution occurs. This is an example of a Friedel-Crafts reaction (e) Explain the relative resistance to bromination of benzene, compared with alkenes, The Kekulé structure of benzene with its alternating carbon-carbon bonds would suggest that benzene might readily undergo an addition reaction with dihalogens. When bromine is added to benzene without a halogen carrier no reaction occurs. This suggests that benzene is not composed of alternating double and single bonds, but of a delocalised system. Addition of halogens to benzene is very hard to achieve. This is surprising if we represent benzene by the Kekulé structure. + 3Br 2 Br Br Br Br Br Br Addition of bromine to an alkene such as cyclohexene requires mild conditions (room temperature and pressure). Cyclohexene produces 1,2-dibromocyclohexane on shaking with bromine water. The mechanism for this reaction is electrophilic addition. + Br 2 Br Br

11 11 Alkanes have a localised electron system with four electrons spread over only two carbon atoms. Benzene has a lower electron density than alkenes as the 6 delocalised electrons are spread over six carbon atoms in the ring. The resistance of benzene to undergo bromination compared to cyclohexene can be explained in terms of the delocalisation of the electrons in the ring. Benzene requires more vigorous conditions (more energy to overcome the stability) before undergoing addition reactions because of the chemical stability of the electron system, which must be broken for an addition reaction to occur. When a non-polar molecule approaches benzene there is insufficient -electron density to polarise the bromine molecule so electrophilic attack does not take place. Phenol. Phenols are a class of compounds where a phenyl group is directly attached to a hydroxyl group (-O). Phenol is a white crystalline solid at room temperature and pressure. Phenol is only slightly soluble in water and is slightly acidic. Phenol and derivatives of phenol are used as antiseptics, dyes and are key components of many pharmaceutical drugs such as paracetamol.

12 (f) Reactions of phenol with aqueous alkalis; Phenol is a weak acid so can be neutralised with aqueous sodium hydroxide. The product formed is called is a salt sodium peroxide. C 6 5 O + NaO C 6 5 O - Na O 4.1.1(f) Reactions of Phenol with Sodium to form Salts; 2 C 6 5-O + 2Na 2 C 6 5O - Na When reactive metals react with phenol, effervescence is observed due to the production of hydrogen gas. The organic product is the salt sodium phenoxide C 6 5O - Na (f) Reactions of phenol with bromine to form 2,4,6-tribromophenol;

13 (g) Explain the relative ease of bromination of phenol compared with benzene. Bromine reacts readily with phenol to form 2,4,6-tribromophenol a white precipitate. This reaction takes place at room temperature and pressure. In order for benzene to react with phenol a halogen carrier is required to polarise the bromine molecule. Bromine reacts much more readily with phenol than with benzene as the lone pair on the oxygen in phenol becomes withdrawn into the benzene ring, increasing the electron density, activating the ring. The increased electron density is able to heavily polarise the bromine molecule which is then attracted to the benzene ring. One lone pair of electrons of the oxygen p-orbital becomes drawn into the delocalised ring of (h) State the uses of Phenols Phenols are used in production of plastics, antiseptics, disinfectants and resins for paints 2,4,6-trichlorophenol (TCP) 4-chloro-3,5-dimethylphenol (Dettol)

14 14 Glossary. Acylation Substitution of one of the hydrogen atoms of a benzene ring by an acyl group from an acyl (acid) chloride. Alkylation Substitution of one of the hydrogen atoms of a benzene ring by an alkyl group from a halogenoalkane. Aluminium chloride The catalyst used in Friedel Crafts alkylation and acylation reactions, chemical formula AlCl 3. In Friedel Crafts alkylation, AlCl 3 reacts with a halogenoalkane to form a carbocation, which is able to react with the benzene ring in an electrophilic substitution reaction. Arene A compound containing a benzene ring. Arenes are also called aromatic compounds. Benzene A cyclical, aromatic hydrocarbon with the formula C 6 6. Despite being unsaturated it does not readily undergo addition reactions but does undergo electrophilic substitution reactions. Its stability is due to a cloud of delocalized pi electrons above and below the carbon ring. Carbocation An ion containing a positively-charged carbon atom. Carbocations are intermediates in electrophilic substitution reactions, for example in Friedel Crafts alkylation. Cyclic structure A compound whose structure consists of a ring of atoms. Kekulé first proposed that hydrocarbon chains might form a ring in his cyclic structure for benzene. Delocalized electrons Electrons that are not attached to one particular atom, but are shared between several atoms. Electronegativity The power of a bonded atom to draw electron density, for example to attract the pair of electrons in a covalent bond. Electrophile A species attracted to regions of high electron density, where it accepts a lone pair of electrons to form a covalent bond. Electrophilic substitution A substitution reaction in which an electrophile attacks an electron-rich centre (such as a benzene ring) and accepts a pair of electrons to form a single covalent bond. An example is the nitration of benzene, in which the nitronium ion NO 2+ acts as the electrophile, and substitutes itself for an + ion in connection to the benzene ring. Enthalpy A measure of the heat energy stored in a chemical system, given the symbol. Friedel Crafts reactions Electrophilic substitution reactions useful in organic synthesis for adding alkyl or acyl groups to a benzene ring. An aluminium chloride catalyst is required. ydrogenation The addition of hydrogen across a carbon carbon double bond. Nitrating mixture A mixture of concentrated nitric and sulfuric acids, used to form nitronium ions for the nitration of benzene. Nitronium ion The NO 2+ ion formed by refluxing a mixture of concentrated nitric and sulfuric acids, which acts as an electrophile in the nitration of benzene. Pi bond A type of covalent bond formed by the sideways overlap of p atomic orbitals. It is involved in double bonds, and is represented by the symbol π. Sigma bond The strongest type of covalent bond, formed by the end-to-end overlap of atomic orbitals and represented by the symbol σ.

15 TNT Trinitrotoluene or 2,4,6-trinitromethylbenzene. An explosive formed by nitrating methylbenzene (also called toluene) at a high temperature. It is stable to shock and friction and therefore safer to handle than many other explosives. Unsaturated A hydrocarbon that contains one or more carbon carbon multiple bonds, for example the alkenes. 15

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17 17 Further Reading & Web Links. Delocalised Model of Benzene ChemGuide Chemistry

18 18 Knock ardy Benzene Notes OCR Textbook pages