Aromatic Chemistry Flashcards
Classes of organic chemicals
aliphatic : straight or branched chain organic substances aromatic or
arene: includes one or more ring of six carbon
atoms with delocalised bonding.
Benzene structure
The simplest arene is benzene. It has the molecular formula C6H6 Its basic structure is six C atoms in a hexagonal ring, with one H atom bonded to each C atom.
Each C atom is bonded to two other C atoms and one H atom by single covalent σ-bonds. This leaves one unused electron on each C atom in a p orbital, perpendicular to the plane of the ring.
The six p electrons are delocalised in a ring structure above and below the plane of carbon atoms.
Planar molecule
Benzene is a planar molecule. The evidence suggests all the C-C bonds are the same and have a length and bond energy between a C-C single and C=C double bond.
H-C-C bond angle
120
Theoretical double bond
Theoretically because there are 3 double bonds in the theoretical cyclohexa-1,3,5-triene one might expect the amount of energy to be 3 times as much as cyclohexene.
However, in actual benzene the amount of energy is less. The 6 pi electrons are delocalised and not arranged in 3 double bonds.
cyclohexa-1,3-diene
some delocalisation and extra stability as the pi electrons are close together and so overlap. The hydrogenation value would be less negative than -240 kJ mol-1
cyclohexa-1,4-diene
not be delocalisation as the pi electrons are too far apart and so don’t overlap. The hydrogenation value would be -240 kJ mol-1
Benzene- addition
Benzene does not generally undergo addition reactions because these would involve breaking up the delocalised system. Most of benzene’s reactions involve substituting one hydrogen for another atom or group of atoms. Benzene has a high electron density and so attracts electrophiles. It reactions are usually electrophilic substitutions.
Toxicity of Benzene
Benzene is a carcinogen (cancers causing molecule) and is banned for use in schools.
Methylbenzene is less toxic and also reacts more readily than benzene as the methyl side group releases electrons into the delocalised system making it more attractive to electrophiles.
Electrophilic substation/nitration
Change in functional group: benzene -> nitrobenzene
Reagents: conc nitric acid in the presence of concentrated sulfuric acid (catalyst)
Mechanism: Electrophilic substitution
Electrophile: NO2+
Overall nitration
⏣ + NO2+ -> ⏣-NO2 + H+
importance of nitration
Nitration of benzene and other arenes is an important step in synthesising useful compounds e.g. explosive manufacture (like TNT, trinitrotoluene/ 2-methyl-1,3,5- trinitrobenzene) and formation of amines from which dyestuffs are manufactured. (The reaction for this is covered in the amines section.)
Formation of electrophile NO2+
HNO3 + 2H2SO4 -> NO2+ + 2HSO4- + H3O+
This is an acid base reaction.
Friedel Crafts Acylation
Change in functional group: benzene->phenyl ketone Reagents: acyl chloride in the presence of anhydrous aluminium chloride catalyst
Conditions: heat under reflux (50OC)
Mechanism: Electrophilic substitution
Acyl chloride
Any acyl chloride can be used RCOCl where R is any alkyl group e.g. –CH3 , -C2H5. The electrophile is the RCO+.
