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Special points of emphasis

Atomic Theory

The Chemical Bond

Thermochemistry

Conjugated π Systems and Aromaticity

Reactions of Alkenes

Hydrogenation of the first double bond in a conjugated diene evolves less heat than hydrogenation of an isolated double bond. Similarly, the enthalpy change of the catalytic hydrogenation of benzene is significantly lower than would be expected with a hypothetical cylohexatriene that is not aromatic. What leads to the lower than expected internal energy of resonant and aromatic systems is resonance stabilization. Aromatic systems are especially stable. The 2p orbitals of benzene carbon atoms are in alignment for π overlap, which generates the π system of delocalized electron density above and below the plane of the ring. The MCAT is fond of testing whether students are aware of the connection between greater stability and reduced heat of hydrogenation.



Conjugated π Systems and Aromaticity

Magnetism

Molecular Spectroscopy

One of the significant complexities of aromatic compounds in the context of nuclear magnetic resonance spectroscopy is that an external magnetic field causes the electrons of an aromatic π system to circulate and generate an induced field that opposes the applied field inside the ring and adds to it outside the ring. The hydrogens on an aromatic ring are outside the ring, of course, so on an NMR spectrograph, these protons are shifted downfield due to the support of the applied field by the induced field.



Conjugated π Systems and Aromaticity

Reactions of Aromatic Compounds

Important reactions with aromatic compounds include the variations of electrophilic aromatic substitution: Halogenation, Nitration, Sulfonation, Friedel-Crafts Alkylation, Friedel-Crafts Acylation. Further reactions of aromatic compounds include Nucleophilic Aromatic Substitution and the side-chain reactions Alkylbenzene Oxidation and Alkylbenzene Halogenation.



Conjugated π Systems and Aromaticity

Nucleic Acids

Bioenergetics and Cellular Respiration

Aromatic compounds are crucial in biochemistry. NAD+, and the purines and pyrimidines of nucleic acids are examples of important aromatic species in biochemistry, with their aromatic nature playing a significant roll in the biochemical behavior. For example, the oxidized form of nicotinamide adenine dinucleotide (NAD+) contains an aromatic pyridinium ring, while the reduced form (NADH) does not. The aromaticity of NAD+ serves to decreases the oxidation potential of NADH, making the substance an ideal electron donor and receiver. With the purines and pyrimidines in nucleic acids, their aromaticity is also very significant because their aromaticity causes the molecules to be planar, which is very helpful for base-pairing. Pyrimidines are aromatic. The two nitrogens are of the sp2 hybridized 'pyrolle type'.







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