UEC Chapter 30 Aromatic Compounds Chemis y tr 1 JLWong Arenes • • • • • • • aromatic hydrocarbons. The simplest and most important Arenes are __________ benzene aromatic hydrocarbon is ___________, C6H6. planar non-polar symmetrical and _____________. Benzene molecule is __________, ______________ π electrons. Each Benzene is electron rich due to the delocalised ____ π bond. The π bonds do carbon atom contributes one electron to a ____ not lie between the two adjacent carbon atoms, but is spread over the delocalized six carbon atoms. Hence, they are said to be ______________. Due to its electron richness, benzene is often reactive with electrophile ___________, electrophilic substitution commonly undergoing ____________________________. Benzene can be drawn a number of different ways, as the delocalized electrons move from one carbon to the next. Resonance Theory, it is suggested that the structure According to the ____________ Kekulé forms, of benzene is a hybrid of 2 ___________ The hybrid structure is often represented by a hexagon containing an delocalized electrons inscribed circle, where the circle represents the _____________ of the benzene ring. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Organic_Chemistry_(McMurry)/ 15%3A_Benzene_and_Aromaticity/15.02%3A_Structure_and_Stability_of_Benzene https://www.britannica.com/science/theory-of-resonance 2 Kekulé structure • • • • Molecular Orbital Theory, all 6 carbons in the ring According to the ___________________ 2 sp use ____ hybrid for bond formation. The sp2 orbitals are involved in forming 3 localized sigma ______ bonds (2 C-C singly occupied p orbital on each carbon atom. and 1 C-H), leaving a ________ The p orbital overlaps with the adjacent orbital on either side of it to form π electrons above and below the plane of the a ring of delocalised ___ π bond. ring, that is a ___ Thus, a very stable structure is obtained. The 6 delocalised electrons are free to move over the entire ring and any substituent attached to it. 3 Naming Aromatic Compounds Monosubstituted benzene ring: • In general, for monosubstituted benzenes, benzene is the parent name and the substituent is indicated by a pre x. bromobenzene chlorobenzene nitrobenzene methylbenzene ethylbenzene (C6H5Br) (C6H5Cl) (C6H5NO(toluene, C6H5CH (C36)H5CH2CH3) 2) When the hydrocarbon chain attached to the benzene ring is small, the hydrocarbon is named as a benzene derivative. benzoic acid benzenesulphonic acid benzenonitrile (C6H5COOH) (C6H5SO3H) (C6H5CN) 4 fi • benzaldehyde (C6H5CHO) In some instances, the name is based on the pre x phenyl. Phenyl group has the formula C6H5. phenol (C6H5OH) phenylamine (C6H5NH2) phenylethanoate (CH3COOC6H5) The pre x “phenyl” is also used in cases where the group substituted is more complex (e.g. containing other functional group). phenylmethanol (C6H5CH2OH) phenylethene (C6H5CH=CH2) 2-phenylheptane (CH3CH(C6H5)CH2CH2CH2 CH2CH3) 5 fi • phenylethanone (C6H5COCH3) fi • • The group C6H5CH2 — is called the benzyl group. benzylchloride benzylamine benzylalcohol (C6H5CH2Cl) (C6H5CH2NH2) (C6H5CH2OH) Multiple substituted benzene ring: • When two or more substituents are present, their relative positions are indicated by numbering carbon atoms of the benzene nucleus. For example, the 3 isomers of dichlorobenzenes are named as follow: 1,4-dichlorobenzene 1,2-dichlorobenzene1,3-dichlorobenzene 6 • • When two substituents attached to a benzene ring are different, the parent name of the benzene numbering system is based on the ______________ derivative, the carbon atom attached to the functional group is designated ____. C1 lowest possible The numbering is then carried out such that the _________ alphabetically numbers are used and the substituents are listed _________________ when writing the name. 2,4-dinitrobenzoic acid 4-amine-3,5-dinitrobenzoic acid 2,4-dichloro-3,5dinitrobenzoic acid 2-bromophenol 3,5-dinitrotoluene 7 • When there are multiple functional groups based upon which a parent name can be written, note the priority order as follow: OH carboxyl (–COOH) H > aldehyde > (–CHO) • > sulfonic acid (SO3H) R’ ketone (R’COR) > ester (–COOR) alcohol (-OH) > > Cl acyl (–COCl) thiol (-SH) > > amide > (–CONH2) (nitrile) > amine (-NH2) When no simple parent name is appropriate, the lowest possible numbers are used and the substituents are listed alphabetically. 1-chloro-2,4-dinitrobenzene NOT 4-chloro-1,3-dinitrobenze 1-bromo-4-chlorobenzene 8 Ortho-, Meta-, Para- (OMP) Nomenclature for Disubstituted Benzenes • Instead of using numbers to indicate substituents on a benzene ring, ortho- (o-), _______ meta- (m-), or _______ para- (p-) can be used in place of _______ positional markers when there are two substituents on the benzene ring (disubstituted benzenes). • They are de ned as follow: 1,2 - (next to each other in a benzene ring) • ortho- (o-): ______ • meta- (m): ______ 1,3 - (separated by one carbon in a benzene ring) • para- (p): ______ 1,4 - (across from each other in a benzene ring) o-dinitrobenzene m-dinitrobenzene p-dinitrobenzene fi 9 o-chloronitrobenzene m-chloronitrobenzene p-chloronitrobenzene Physical Properties of Benzene • • • • • • 10 fl • Colourless liquid at room temperature, which gives out an aromatic odour. 80.1 ºC Boiling point: ________ 5.5ºC Melting point: ______ 0.88g/ml Density: __________ Immiscible with water but soluble in all organic solvents. An organic solvent and can be used to dissolve non-polar inorganic molecules like iodine and sulphur. Highly ammable and burns with a yellow smoky ame. Both the liquid and vapour of benzene are highly poisonous. fl • Chemical Properties of Arenes 1. Halogenation: Cl2 or ________ Br2 • Reagents: _______ halogen carrier (such • Condition: in the presence of catalyst called _________________ Fe _______, FeCl3 _______, FeBr3 _______ AlCl3 or ______) AlBr3 and at room temperature. as ____, halogenobenzene • Product: ____________________. • Example: Benzene reacts with Cl2 in the presence of AlCl3 to give chlorobenzene and steamy fumes of HCl can be seen. • substitution The halogenation of benzene proceeds via electrophilic ________________________ mechanism, involving the following steps: 11 ✴ Step 1: Production of electrophile Br+ from Br2, and a Lewis acid (FeBr3). FeBr3 is used to polarize the Br-Br molecule. + + ✴ Step 2: Formation of a carbocation intermediate due to the attack of Br+ on the benzene ring. ✴ Step 3: Expulsion of H+ from the intermediate and reforming of delocalized π electron clouds, i.e. benzene ring is reestablished. + 12 2. Nitration: conc. H2SO4 conc. HNO3 • Reagents: ________________ and _________________ • Condition: re ux at about _______ 50-60 ºC. • Product: ____________________. nitrobenzene • Example: Benzene reacts with a mixture of conc. H2SO4 and conc. HNO3 at 55ºC to give nitrobenzene, a yellow colour oil. • Nitration also occurs through electrophilic substitution mechanism: ✴ Step 1: Production of the electrophile, NO2+ (nitronium ion or nitryl cation) Note: HNO3 acts as a base in the presence of the stronger H2SO4. The nitronium ion formed is a strong electrophile. fl 13 ✴ Step 2: Formation of a carbocation intermediate due to the attack of NO2+ on the benzene ring. ✴ Step 3: Expulsion of H+ from the intermediate and reforming of delocalized π electron clouds. Note: If temperature rises above 100 ºC, nitrobenzene is formed rst and then further substituted to form 1,3-dinitrobenzene. 2 fi 14 3. Alkylation of benzene ring (Friedel-Crafts reaction) RX; R = alkyl, X = halogen • Reagents: ____________________________ re ux in the presence of ________. AlCl3 • Condition: heat under __________ alkylbenzene • Product: __________________ • The electrophile, ____, R+ is formed from the reaction of RX with AlCl3. • Example: Benzene reacts with CH3Cl to give methylbenzene. A hydrogen atom is replaced by an alkyl group (CH3 –). • The mechanism of the reaction is as follow: ✴ Step 1: Formation of the electrophile CH3+. fl 15 ✴ Step 2: Formation of a carbocation intermediate due to the attack of CH3+ on the benzene ring. + ✴ + Step 3: Expulsion of H from the intermediate and reforming of delocalized π electron clouds. + ✴ + Step 4: H reacts with AlCl4 to give back AlCl3. 16 • Example: Benzene reacts with isopropyl chloride to give isopropyl benzene. 17 4. Acylation of benzene ring (Friedel-Crafts reaction) • Reagents: __________________________ RX; R = acyl, X = halogen • Condition: heat under __________ re ux in the presence of ________. AlCl3 • Product: ____________________. acylbenzene • The electrophile, ____, R+ is formed from the reaction of RX with AlCl3. • Example: Benzene reacts with CH3COCl to give C6H5COCH3. A hydrogen atom is replaced by an acyl group (– COCH3). • Friedel-Crafts reaction is used to substitute a hydrogen in the benzene alkyl group ( – CH3 , – CH2CH3) or an _______ acyl group (– ring for an _______ COCH3, – COCH2CH3 ). side chain into a Friedel-Crafts reactions result in the introduction of a ___________ benzene ring. They are also called alkylation or acylation reaction. electrophilic substitution In Friedel-Crafts reaction proceeds via __________________________. R+ while in acylation it is an acylium ion. alkylation, the electrophile is ___, • • fl 18 5. Sulphonation of benzene ring sulphuric acid • Reagents: ________________ • Condition: ________ 80ºC benzenesulphonic acid • Product: ________________________ SO3 formed from the dissociation of H2SO4. • The electrophile is ______, ❖ Step 1: Production of electrophile. ❖ Step 2: Formation of carbocation. ❖ Step 3: Loss of a proton. 19 ❖ Step 4: Reaction with hydronium ion. General Mechanism of Electrophilic Substitution in Benzene • Step 1: Formation of an electrophile. • Step 2: Electrophilic attack on a pi bond of benzene by an electrophile to form a carbocation. • Step 3: Deprotonation of the carbocation to restore its aromaticity, resulting in a substituted product. 20 6. Addition Reaction of Benzene Due to the stability of the aromatic ring, benzene typically undergoes substitution reaction, but addition reaction can occur under suitable conditions. (a) Hydrogenation excess hydrogen • Reagents: ____________________ • Condition: ________ heat and ______________________ nickel or palladium as catalyst. • Product: ____________________. cyclohexane • Example: Benzene is heated with excess hydrogen gas in the presence of catalyst. 21 21 heat (b) Halogenation Halogen • Reagents: ________________ uv light • Condition: ________________ cyclohalohexane • Product: ____________________. • Example: Benzene is allowed to react with chlorine under uv light. Chemical Tests for Benzene Br2 water and _____________ • ____________ H+/KMnO4 can be used to distinguish benzene from unsaturated compounds such as alkenes, cycloalkenes or decolourizes alkynes. Alkenes, cycloalkenes or alkynes ________________ these reagents but benzene gives no reaction. • Benzenes can also be distinguished from alkanes or alkenes by nitration reactions. Benzene, but not alkanes or alkenes, reacts ____________ nitrating mixture to produce nitrobenzene which is a yellow, with a ___________ oily liquid with a bitter almond smell. 22 Methylbenzene • • • toluene Methylbenene, also known as _________, is found naturally occurring at low level in crude oils. It is a byproduct in the production of gasoline and may be extracted from the byproducts of coal tar or crude oil. Like benzene, methylbenzene is a colourless liquid immiscible with water, and has a density lower than water. Chemical Properties of Methylbenzene • Methylbenzene comprises a benzene ring and a methyl side chain, it will undergo reactions involving: alkanes (a)Typical reactions of __________. Electrophilic substitution of benzene ring, e.g. nitration and (b)__________________________ halogenation. 23 1. Reactions of the –CH3 group a) Free radical substitution by halogen • Reagent: Cl2 or Br2 sunlight/uv light • Condition: presence of __________________ halogenomethylbenzene • Product: ____________________________ Example: Note: A mixture of products are formed. 24 b) Oxidation of the methyl side chain Acidi ed KMnO4 (H+/K2Cr2O7 is not strong enough) • Reagent: ______________________ Re ux • Condition: __________________ Benzoic acid • Product: ____________________ Example: Oxidation of methyl side chain to give benzoic acid. H+/KMnO4 Note: Any number of carbon side chain on the benzene ring can be oxidised to —COOH by prolonged re ux with acidi ed KMnO4. fi fl fi fl 25 In uence of Substitution Groups on Benzene Ring • When substituted benzenes undergo further substitution, the substitution group already present in the benzene derivative will in uence reactivity and _____________. orientation electrophilic substitution in two ways: ____________ • The substitution group can be classi ed into two groups, namely ring activating group and _______________________. ring deactivating group ________________________ • Ring Activating Groups ✴ They are ____________________ electron-donating groups, which intensify _________ the electron electrophilic density in the ring, activating the ring towards ____________________ substitution ______________. ✴ They ____________ increase the rate of reaction. ✴ They are ____ ortho-para C2 and ____ C4 directing, and hence are _____________ directors. ✴ Generally, electron donating groups have _______ lone pair of electrons. ✴ Examples of ring activating groups include: alkyl group (e.g. –CH3, –CH2CH3 etc), amine group (–NH2), hydroxyl group (–OH) and ether group (e.g. –OCH3) ❖ F, Cl, Br, I are electron-___________, withdrawing Exception: Halogen atoms (e.g. _________) deactivating but are ____ C4 directing. C2 and _____ and hence ring ____________, fl fi fl 26 • Ring Deactivating Groups electron-withdrawing groups, which _________ reduce electron ✴ They are _______________________ electrophilic availability in the ring, deactivating the ring towards ________________ substitution ______________. ✴ They ____________ decrease the rate of reaction. C3 directing, and hence are _________ meta directors. ✴ They are ____ ✴ Examples of ring deactivating group include: Halogens: Cl OH amide ester carbonyl acyl carboxyl (–CONH2) (–COOR) (–CO) (–COCl) (–COOH) ✴ (nitrile) (nitro) Note that, except for the halogens, the above examples of ring deactivating groups comprise at least one highly electronegative _______________ not directly attached to the benzene ring atoms (N or O), which are _____________ withdrawing property. and which contributes to the electron-_____________ Think! Is a sulphonyl group (–SO3R) electron-donating or withdrawing? 27 • Substituent group effects on electrophilic aromatic substitution: 28 the benzene ring Product: A mixture of _______________________ more electron-donating carrier such as FeCl3 and AlCl3 2/4- halomethylbenzene dark Note: A steamy fume of ______ is produced. 2 HCl 29 b) Nitration • Reagent: ______________ conc. H2SO4 and _______________ conc. HNO3 30 ºC Condition: ____ 2/4-nitromethylbenzene • Product: A mixture of __________________________ Example: CH3 CH3 CH3 At higher temperature, 2 and then 3 —NO2 groups can be substituted in the benzene ring. 2,4,6-trinitromethylbenzen or 2,4,6-trinitrotoluene (TN Note: TNT is a pale yellow solid used as explosives. 30 Summary of Benzene Reaction uv 31 Summary of Methylbenzene Reaction 32 Ex 1: Arrange benzene, methylbenzene and nitrobenzene order of reactivity towards electrophilic substitution. in increasing Ex 2: Simply outline a route to synthesize 4-nitrobenzoic acid from methylbenzene. 33 Ex 3: Simply outline a route to synthesize 3-nitrobenzoic acid from methylbenzene. Ex. 4: Compound A has molecular formula C9H8, and decolourizes bromine water and acidi ed potassium permanganate rapidly. Under certain conditions, it reacts with one molecule of hydrogen to produce Compound B (C9H10) and under more vigorous conditions, it reacts with hydrogen to produce Compound C (C9H16). In the presence of strong oxidizing agent, A can be oxidised to produce phthalic acid (1,2benzenedioic acid). Identify the molecular structure of Compounds A, B and C. fi 34 Ex 5 35 36