arenes Flashcards
what is the molecular formula of benzene?
C6H6
what are the properties of benzene?
mp, bp, solubility
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
describe the structure of benzene
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.
what are the 2 implications of the π electron cloud delocalisation?
- 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
why does the delocalised π electron cloud prevent benzene from undergoing any of the typical addition reactions that alkenes show?
i.e. why is there a difference in reactivity between benzenes & alkenes?
- 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 reactons which preserves its aromaticity
what are the reagents, conditions and observations for electrophilic substitution of benzene with chlorine (chlorination)?
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
what are the reagents, conditions and observations for electrophilic substitution of benzene with bromine (bromination)?
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
what are the reagents, conditons and observations for electrophilic substitution of benzene with concentrated nitric acid (nitration)?
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
what are the reagents, conditions and observations for electrophilic substitution of benzene with halogenoalkanes (Friedel-Crafts alkylation)?
reagent:
- chloroalkane
conditions:
- AlCl3 as Lewis acid catalyst
- warm
observations:
- white fumes of HCl (g)
Lewis acids accept electrons
what are the reagents, conditions and observations for electrophilic substitution of methylbenzene with chlorine?
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
what are the reagents, conditions and observations for electrophilic substitution of methylbenzene with bromine?
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
what are the reagents, conditions and observations for electrophilic subsitution of methylbenzene with concentrated nitric acid?
reagent:
- concentrated HNO3
conditions:
- concentrated H2SO4 as Bronsted-Lowry acid catalyst
- maintained at 30C
observation:
- yellow oily liquid formed
Bronsted-Lowry acids donate protons
what are the reagents, conditions and observations for electrophilic substitution of methylbenzene with halogenoalkanes? (friedel-crafts alkylation)
reagent:
- chloroalkane
conditions:
- AlCl3 (s) as Lewis acid catalyst
- room temperature
observation:
- white fumes of HCl (g)
Lewis acids accept electrons
what are the 2 ways substituents on a benzene ring affect the reactivity of the benzene ring towards electrophilic substitution?
- 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.
how does the inductive effect arise?
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.
how does delocalisation arise?
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
how does delocalisation and inductive effect affect the reactivity of the benzene ring?
- 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
why is there a difference between conditions for the same electrophilic substitution reactions for benzene and methylbenzene?
- 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
how do substituents on a benzene ring affect the position of the incoming electrophile of monosubstitued arenes?
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!)
what are the reagents, conditions & observations for side chain free radical substitution of methylbenzene with chlorine?
reagents & conditions:
- Cl2 (g), UV light/heat
observation:
- greenish-yellow Cl2 (g) decolourises slowly
what are the reagents, conditions & observations for side chain free radical substitution of methylbenzene with bromine?
reagents & conditions:
- Br2 (l), UV light/ heat
observation:
- reddish-brown Br2 (l) decolourises slowly
what are the reagents, conditions & observations for side chain oxidation of methylbenzene?
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
how can bromine in tetrachloromethane be used to differentiate between alkenes and arenes?
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.
how can aqueous bromine be used to distinguish between alkenes and arenes?
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
how to distinguish betwen certain alkenes and alkylbenzenes?
eg: hexene, benzene, methylbenzene, ethylbenzene
procedure: KMnO4 (aq), dilute H2SO4, heat
observations: for hexene (oxidative cleavage), effervescence of CO2 and decolourisation of purple KMnO4 is observed. no white ppt. for benzene, purple KMnO4 not decolourised, no white ppt, no effervescence. for methylbenzene, purple KMnO4 decolourises and white ppt of benzoic acid forms. no effervescence. for ethylbenzene, purple KMnO4 decolourises, white ppt of benzoic acid forms, effervescencec of CO2 gas is observed.
