C25 - Aromatic compounds Flashcards
What is benzene
C6H6
What are properties of benzene
Colourless
Sweet smelling
Highly flammable
Liquid
Carcinogen
Aromatic
What are the two models of benzene
Kekule’s model
Delocalised model
What is an arene
Aromatic (containing one or more benzene rings) hydrocarbon
What is Kekule’s model
Hexagonal carbon ring, joined by alternate double and single bonds
Evidence to disprove Kekule’s model
-Lack of reactivity of benzene: it does not undergo electrophilic addition reactions or decolourise bromine suggesting lack of double bonds
-Lengths of C-C bond lengths: bond lengths should be alternating but they are uniform
-Hydrogenation enthalpies: enthalpy should be three times cyclohexane, but isn’t
What is the delocalised model of benzene
-Planar, cyclic hexagonal hydrocarbon (C6H6)
-Each C atom uses its available 4 electrons in bonding to 2 other C atoms and 1 hydrogen atom
-Each C atom has 1 electron in a p-orbital, perpendicular to the plane
-Adjacent p-orbital overlap sideways, above and below plane to form ring of e- density
-Overlapping of p-orbitals creates system of pi-bonds, 6 delocalised e- in system of pi-bonds
Naming aromatic compounds with one substituent group
-Alkyl groups, halogens, nitro groups are prefixes
-Benzene substituent to alkyl chain (phenyl is prefix)
What is benzoic acid
What is phenylamine
Naming aromatic compounds with more than one substituent group
Alkyl group takes priority (alkylbenzene becomes parent chain)
What is benzaldehyde
Benzene electrophilic substitution
H atom on benzene ring is replaced by another atom/group of atoms (electrophile)
Nitration of benzene
C6H6 + HNO3 –> C6H5NO2 + H2O
Conditions- H2SO4 + 50C
What happens if nitration of benzene occurs at a higher temp
Further substitution, forming dinitrobenzene (1,3)
Mechanism of nitration of benzene
What does the halogenation of benzene require
Require halogen carrier catalyst (can be generated in situ)
Eg. AlCl3, FeCl3, AlBr3, FeBr3
Bromination of benzene
C6H6 + Br2 —> C6H5Br + HBr
Conditions- RTP, FeBr3/AlBr3
Bromination of benzene mechanism
Chlorination of benzene
C6H6 + Cl2 —> C6H5Cl + HCl
RTP, FeCl3/AlCl3
Alkylation of benzene
Benzene + Haloalkane —> Alkyl benzene + HCl
Conditions - RTP, AlCl3
Acylation
Benzene + acyl chloride —> Aromatic ketone + HCl
Conditions - AlCl3 RTP
Do alkenes decolourise bromine
Yes
What happens in the reaction of cyclohexene and bromine
Pi bond in alkene contains localised electrons above and below the plane of the two carbon atoms in a bond
Produces region of high e- density
Localised e- in pi bond induce a dipole in non-polar bromine
Slightly +ve bromine atom enables the bromine molecule to act like a dipole
Does benzene react with bromine
No, unless a halogen carrier catalyst is present
Delocalised electrons spread below and above the plane
Lower electron density
Cannot polarise non polar bromine molecule
What is phenol?
Organic chemical containing hydroxyl group bonded to aromatic ring
Are phenols more or less soluble than alcohol
Less due to the presence of non-polar benzene ring
What happens when phenol is dissolved in water
Partially dissociates forming phenoxide ion and proton
Is phenoxide a strong or weak acid
Weak, it partially dissociates in water
Phenols acidity compared to alcohols an Carboxylic acids
More acidic than alcohols, less acidic than carboxylic acids
What’s Ka
Dissociation constant
Reactivity of ethanol, phenol and carboxylic acids
-Ethanol does not react with NaOH (strong base) or NaCO3 (weak base)
-Phenols react with NaOH (strong base) but not NaCO3 (weak base)
-Carboxylic acids react with NaOH (strong base) and NaCO3 (weak base)
What reaction can be used to distinguish between phenol and a carboxylic acid
Reaction with NaCO3
Phenol won’t react
Carboxylic acid reacts to produce CO2 gas
Reaction of phenol with sodium hydroxide
C6H5OH + NaOH –> C6H5NaO + H2O
What type of reactions does phenol usually undergo
Electrophilic substitution
-Milder conditions than benzene
Bromination of phenol
C6H5OH + 3Br2 —> C6H2OHBr2 + H2O
Conditions - RTP, no catalyst
Nitration of phenol
C6H5OH + HNO3 (dilute) —> C6H4OHNO2 + H2O
Conditions - RTP, no catalyst
Reactivity of phenol vs benzene
Bromine and nitric acid react more readily with phenol
Phenol reacts with dilute nitric acid rather than conc
Why is phenol more reactive than benzene
Lone pair of e- on p-orbital of O atom is donated into pi system
Inc electron density, attracts electrophiles more strongly
What happens when phenols undergo further substitution with nitric acid
Form two isomers:
2-nitrophenol
4-nitrophenol
Reaction of phenylamine and bromine
Occurs more rapidly than benzene
Reaction of nitrobenzene and bromine
Occurs more slowly than benzene
What does it mean to activate
A group causes the aromatic ring to react more readily with an electrophile
What does it mean to deactivate
A group causes the aromatic ring to react less readily with an electrophile
What is a directing effect
Different groups attached to benzene rings cause further substituents to attach at a specific location, affecting whether it’s an an activating or deactivating group
Which directing groups are activating groups
2- and 4- directing groups (orthodo and para directors)
EXCEPT halogens
Which directing groups are deactivating
3 directing groups (meta directing)
Directing group table
