Organic Chemistry
Lecture 4th
Benzene (aromatic character)
•Benzene undergoes substitution rather than addition.
•structure of Benzene is one that we would call "cyclohexatriene."
•Structure of benzene
•Benzene has the molecular formula: C6H6.
•Resonance structure of benzene, The stability of benzene is the result of
electron delocalization
•Aromatic character.
(1) cyclic (2) conjection bonds (3) submit Huckel 4N + 2 rule
4N + 2= π e- , N is an integer, commonly 0, 1, 2, or 3.. the system
is aromatic, if N ≠ integer number the system is anti aromatic.
•Stability of Benzene Ring
Benzene is actually much more stable than Alkenes (like; cyclohexene) When reacted
potassium permanganate (KMnO4) with cyclohexene to form a glycol, The purple
potassium permanganate color disappears, and a precipitate of manganese dioxide
forms. When permanganate is added to benzene however, no reaction occurs.
Most alkenes (like; cyclohexene) decolorize solutions of bromine in carbon tetrachloride
(Br2/CCl4), The red bromine color disappears as bromine adds across the double bond.
When bromine is added to benzene, no reaction occurs, and the red bromine color remains.
Nomenclature of Aromatic Compounds
• The naming of monosubstituted benzenes, that is, benzenes in which
one H atom has been replaced by another atom or a group of atoms, is
quite straightforward, as shown next:
If more than one substituent is present, we must indicate the location
of the second group relative to the first. The systematic way to
accomplish this is to number the carbon atoms as follows: (The
prefixes o- (ortho-), m- (meta-), and p- (para-) are also used to denote
the relative positions of the two substituted groups)
•Reactions of benzene:
Electrophilic aromatic substitution is the most important method for
synthesis of substituted aromatic compounds
The most important reactions are shown below:
•Nitration of benzene
•Sulfonation of benzene
•Halogenation of benzene
•Friedel–Crafts alkylation.
Ex:
•Friedel–Crafts acylation