arenes

C6H6

physical properties: benzene is a colourless liquid at rtp. it is highly volatile and flammable, and is toxic and carcinogenic melting & boiling points: melting point ⮕ 5.5C boiling point ⮕ 80C. as benzene is a non-polar molecule, only a small amount of energy is needed to overcome the weak dispersion forces between molecules, so benzee has low melting & boiling points solubility: being a non-polar molecule, benzene is insoluble in polar solvents but soluble in non-polar solvents. benzene can also be used as a non-polar solvent

sp² hybridisation in benzene • each carbon is bonded to 2 other carbons and 1 hydrogen. (3 bond pairs) • each carbon atom is sp² hybridised, as the mixing of 1 s orbital and 2 p orbitals gives 3 sp² hybrid orbitals oriented at 120° to one another. one p orbital remains unhybridised, and is arranged perpendicular to the 3 sp² hybrid orbitals. • the geometry around each carbon atom in benzene is trigonal planar with C-C-C bond angle of 120° bonds in benzene • there are 6 C-C sigma (σ) bonds in benzene, each of which is formed from the head-on overlap of one sp² hybrid orbital of one carbon atom with one sp² hybrid orbital of the adjacent carbon atom. • there are also 6 C-H single bonds, each of which is an s-sp² σ bond formed from head-on overlap of the 1s atomic orbital of H and the sp² hybrid orbital of C. • the remaining unhybridised p orbitals of the 6 carbon atoms are all parallel to one another due to the planarity of the benzene molecule. all 6 unhybridised p orbitals overlap side-on with each other equally to produce 2 continuous rings of π electrons above and below the plane of the benzene ring, also known as the delocalised π electron cloud. • the 6 electrons found in this delocalised π electron cloud are free to move throughout the entire π electron cloud, so the electrons are delocalised.

• the delocalised π electron cloud causes all carbon-carbon bond lengths to be equal, creating a planar, regular hexagonal shape • the delocalised π electron cloud prevents benzene from undergoing any of the typical addition reactions that alkenes show for pt 1: all 6 carbon-carbon bonds of benzene are identical in length and intermediate between a C-C bond length and a C=C bond length. the carbon-carbon bond energy in benzene is also between that of a C-C bond and a C=C bond

• like electron-rich alkenes, the π electron cloud in benzene attracts electrophiles. however, the delocalisation of electrons in the π electron cloud in benzene results in extra stability of benzene. • as a result, the π electron cloud in benzene is less susceptible to attack by electrophiles compared to the C=C double bonds in alkenes. hence, benzene requires stronger electrophiles to react as compared to alkenes. • benzene undergoes electrophilic substitution instead of electrophilic addition, as electrophilic addition destroys the delocalised π electron cloud, which requires a significant amount of energy which is highly unfavourable. benzene preferentially undergoes electrophilic substitution reactions which preserves its aromaticity

reagents & conditions: - Cl2 (g), FeCl3 (s) as Lewis acid catalyst, warm OR - Cl2 (g), Fe (s), warm observations: - decolourisation of greenish-yellow Cl2 (g) AND - white fumes of HCl (g) Lewis acids accept electron pairs

reagents & conditions: - Br2 (l), FeBr3 (s) as Lewis acid catalyst, warm - Br2 (l), Fe (s), warm observations: - decolourisation of reddish-brown Br2 (l) AND - white fumes of HBr (g) Lewis acids accept electron pairs

reagent: - concentrated HNO3 conditions: - concentrated H2SO4 as Bronsted-Lowry acid catalyst - maintained at 55C observation: - pale yellow oily liquid formed Bronsted-Lowry acids donate protons

reagent: - chloroalkane conditions: - AlCl3 as Lewis acid catalyst - warm observations: - white fumes of HCl (g) Lewis acids accept electrons

reagents & conditions: - Cl2 (g), FeCl3 (s), room temperature, absence of UV observations: - decolourisation of greenish-yellow Cl2 (g) AND - white fumes of HCl (g) FeCl3 (s) acts as a Lewis acid catalyst room temp & absence of UV prevents free radical sub on alkyl side chain

reagents & conditions: - Br2 (l), FeBr3 (s), room temperature, absence of UV observations: - decolourisation of reddish-brown Br2 (l) AND - white fumes of HBr (g) FeBr3 (s) acts as a Lewis acid catalyst room temp & absence of UV prevents free radical sub on alkyl side chain

reagent: - concentrated HNO3 conditions: - concentrated H2SO4 as Bronsted-Lowry acid catalyst - maintained at 30C observation: - yellow oily liquid formed Bronsted-Lowry acids donate protons

reagent: - chloroalkane conditions: - AlCl3 (s) as Lewis acid catalyst - room temperature observation: - white fumes of HCl (g) Lewis acids accept electrons

• inductive effect • delocalisation electron-donating groups increase the electron density in the benzene ring, while electron-withdrawing groups decrease the electron density in the benzene ring.

inductive effect arises from the polarisation of electron density in a bond due to the electronegativity of nearby atoms. electron-withdrawing groups via inductive effect: - electron-withdrawing groups like -OH, -NH2, -Cl, inductively withdraw electron density from the benzene ring via the σ bond as O, N and halogens are more electronegative than the C atom in the benzene ring. electron-donating groups via inductive effect: - electron-donating alkyl groups like -CH3, CH2CH3, inductively donate electron density into the benzene ring via the σ bond.

delocalisation occurs when p orbitals on 3 or more adjacent orbitals overlap, forming a π electron cloud. for substituents with a p orbital on the atom joined to the benzene ring, the p orbital can overlap with the π electron cloud of the ring. electron-donating groups via delocalisation - electron-donating groups like -OH, -NH2, -Cl, have a lone pair of electrons in the p orbital of th atom directly joined to the benzene ring, and can thus donate the lone pair of electrons into the benzene ring via delocalisation electron-withdrawing groups via delocalisation - in electron-withdrawing groups like -CHO, COOH, NO2, the atom directly joined to the benzene ring forms double/triple bonds to electronegative atoms like O or N, thus electron density in the benzene ring is drawn away by these electronegative atoms via delocalisation

• inductive effect and delocalisation do not necessarily act in the same direction. when 2 effects are opposing, the stronger effect dominates • eg the -OH group is electron-withdrawing via inductive effect but electron-donating via delocalisation. • substituents that are overall-electron-donating towards the benzene ring will increase the electron density of the benzene ring, making it more reactive towards electrophilic attack. they are called activating groups. eg -OH, -NH2, alkyl groups • substituents that are overall electron-withdrawing from the benzene ring will decrease the electron density of the benzene ring, making it less reactive towards electrophilic attack. they are called deactivating groups. eg halogens, -CHO, -COOH, -NO2, -CN

• since the -CH3 group is activating, the conditions for it to undergo electrophilic substitution are milder compared to those for benzene • activating groups like -CH3 increase the electron density of the benzene ring, making it more reactive towards electrophilic attack, hence milder conditions are needed

the position of the incoming group is determined by the nature of the substituent/group already bonded to the ring, NOT by the nature of the incoming group!!! activating groups are 2,4-directing groups while deactivating groups are 3-directing groups. if there are 2 groups already on the benzene ring, - if both groups direct to the same position, substitution will occur there - if each group directs the incoming electrophile to a different position, the major product follows the directing effect of the more strongly activating group - further subsitution rarely occurs between 2 groups in 1,3-disubstituted benzene rings as the site is too sterically hindered the data booklet contains a table that summarises the effects and position of substitution by he group alreaedy on the benzene ring (don’t have to memorise!)

reagents & conditions: - Cl2 (g), UV light/heat observation: - greenish-yellow Cl2 (g) decolourises slowly

reagents & conditions: - Br2 (l), UV light/ heat observation: - reddish-brown Br2 (l) decolourises slowly

oxidation to benzoic acid reagents & conditions: - KMnO4 (aq), dilute H2SO4, heat observations: - purple KMnO4 is decolourised - white precipitate of benzoic acid is formed oxidation to benzoate salt reagents & conditions: - KMnO4 (aq), dilute NaOH, heat observations: - purple KMnO4 is decolourised - brown precipitate of manganese dioxide, MnO2 (s) is formed

procedure: add bromine in tetrachloromethan dropwise with shaking to 1cm3 of each compound in separate test tubes observations: for benzene, Br2 in CCl4 remains reddish-brown. for the alkene, reddish-brown Br2 in CCl4 decolourises.

procedure: add aqueous bromine dropwise with shaking to 1cm3 of each compound in separate test tubes. observations: for benzene, BR2 (aq) remains yellow-orange. for the alkene, yellow-orange Br2 (aq) decolourises note that for aqueous bromine, if qns ask for equation, must include H2O!! so reactants would be Br2 + H2O