25 Aromatic compounds Flashcards
State the 3 pieces of evidence to disprove Kekule’s model
- Lack of reactivity of benzene
- The lengths of the carbon-carbon bond in Benzene
- Hydrogenation enthalpies
What is Kekule’s model?
6 members ring of carbon atoms joined by alternate single and double bonds
Describe how the lack of reactivity of benzene is evidence to disprove Kekule’s model
Kekule’s: Should decolourise bromine in electrophilic addition
Reality:
Does not undergo electrophilic addition
Does not decolourise bromine under normal condition
Hence cannot have any C=C bonds in its structure
Describe how the lengths of C-C bonds in benzene is evidence to disprove Kekule’s model
X-ray diffraction determined lengths of bonds
0.139nm = bonds in benzene (in between)
0.153nm = single bond
0.134nm = double bond
Describe hydrogenation enthalpies of benzene is evidence to disprove Kekule’s model
Kekule’s: enthalpy change of hydrogenation 3x cyclohexene (-120) : expected -360kJ
Reality: -208kJ, more stable than Kekule’s model
Hence, delocalised model
What are the 6 main features of the delocalised model?
- Planar, cyclic, hexagonal hydrocarbon containing six carbon atoms and six hydrogen atoms
- Each carbon atom uses 3 of 4 Avaliable electron in bonding to 2 other carbon and 1 hydrogen
- Each carbon atom has one electron in a p-orbital at right angles to the plane of the bonded carbon and hydrogen atoms
- Adjacent p-orbitals electrons overlap sideways, in both directions, above and below the plane of the carbon atoms to form a ring of electron density =
- System of pi bonds
- 6 electrons in this system are delocalised
How do you name aromatic compounds?
Suffixes: Alkyl groups, halogens and nitro
Prefix: If there is a functional group or an alkyl chain with 7 or more carbon atoms
What is the prefix of benzene?
Phenyl
What are the common compounds you need to learn which break the rules of the naming rules?
Benzoic acid
Phenylamine
Benzaldehyde
State the 4 reactions of benzene
Nitration
Halogenation
Alkylation
Acylation
Nitration of benzene reactants, conditions and catalysts and draw full equation
Benzene + nitric acid —> nitrobenzene
Catalysts: sulfuric acid
Conditions: heated to 50C (water bath)
What happens if nitration occurs above 50C?
Further substitution to dinitrobenzene
Alkylation reaction, reagents, catalysts
Electrophilic Substitution of a hydrogen atom in the benzene ring
Benzene + halooalkane —> alkyl + HR
Catalyst: AlCl3
Acylation reaction, reagents, catalysts
Electrophilic substitution
Benzene + Acyl chloride —> aromatic ketone + HR
Catalyst: AlCl3
What happens when cyclohexene reacts with bromine?
Electrophilic addition
1. The pi bond in Allene contains localised electrons above and below the plane of the 2 carbon atoms in the double bond - high electron density
2. Localised electrons in pi bond induce dipole in the non polar bromine molecule so 1 bromine atom of the Br2 molecule slightly positive and the other bromine atom slightly negative
3. Slightly positive bromine atom enables the bromine molecule to act like an elecrophile
Draw the mechanism for cyclohexene + bromine
