Chemistry · MU Pleven

CONJUGATED SYSTEMS. AROMATICITY

Covers conjugated systems, starting with open-chain dienes and their classification as conjugated, unconjugated or cumulated, then the resonance energy of 1,3-butadiene derived from heats of hydrogenation. It goes on to the orbital picture of benzene, Hückel's (4n+2) rule with carbocyclic examples such as cyclobutadiene, cyclooctatetraene and annulenes, polycyclic aromatic hydrocarbons with their delocalisation energies per ring and carcinogenicity, and finally heterocyclic aromatics such as pyridine, pyrrole, furan and thiophene.

Inside: 4 sections · 50 labelled pictures · 21 flashcards · 18 min

Topic 5 of 47 in the MU Pleven syllabus for Chemistry (Medicine, Year 1). The free AceMed note covers it in 4 sections, about 2,652 words (18 min of reading), with 21 flashcards.

In this note

  1. CONJUGATED SYSTEMS. AROMATICITY
  2. II. Conjugated systems with cyclic chain (aromatic systems)
  3. AROMATICITY OF ARENES, NONBENZENES AND HETEROCYCLIC COMPOUNDS
  4. III. Heterocyclic aromatic compounds

CONJUGATED SYSTEMS. AROMATICITY

I. Conjugated systems with open chain. “Conjugate” is a Latin verb meaning “to link or yoke together’, and chloroethene and conjugated dienes are examples of conjugated systems.

Conjugated dienes are those in which the two doubles bonds are separated by a single bond.

3,5-octadiene

Dienes in which two or more single bonds separate the double bonds are called unconjugated dienes. The double bonds in unconjugated dienes are called isolated double bonds.

2,5-heptadiene

Cumulated dienes have the double bonds sharing a common sp-hybridized carbon atom

2,3-hexadiene

A compound with conjugated double bonds is 15 kJ/mol more stable than similar compound with isolated double bonds. This 15 kJ/mol of extra stability in the conjugated molecule is called the resonance energy of the system. Other terms favored by some chemists are conjugation energy, delocalization energy, and stabilization energy.

If conjugated bonds confer extra stability, 1,3-butadiene (the most important representative of the dienes with conjugated double bonds) should show appreciable delocalization energy.

The heat of hydrogenation of 1,3-butadiene is indeed about 15 kJ/mol less than twice that of 1-butene. 1,3-Butadiene is therefore about 15 kJ/mol more stable than a hypothetical diene with two isolate double bonds.

1,3-butadiene CH₂=CH-CH=CH₂ + 2 H₂ → CH₃-CH₂-CH₂-CH₃ + 237 kJ/mol

1-butene CH₂=CH-CH-CH₃ + H₂ → CH₃-CH₂-CH₂-CH₃ + 126 kJ/mol

Hypothetical diene with two isolated double bonds + 2 H₂ → 2 x 126 = 252 kJ/mol

Delocalization energy of 1, 3-butadiene = 252 - 237 = 15 kJ/mol

This relatively small but significant energy difference is attributed to two factors:

First, the two double-bond distances are essentially normal, but a C₂-C₃ single bond is shorter than the 0.154 nm distance normally associated with carbon-carbon single bonds.

This decreased bond length results partly from the increased s-character of the carbon orbitals comprising this bond. The single bond between the double bonds may be described approximately as C_{sp^2} - C_{sp^2}. This shorter bond is somewhat stronger than carbon-carbon bonds having less s-character.

The other reason why the central single bond is so short is that this bond has some pi-bonding character and some double-bond character.

All carbon atoms in the molecule of 1,3-butadiene are in sp² hybrid state and lie in one plane.

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