Chemistry · Ch 15 — Hydrocarbons
Benzene
Benzene
Benzene, molecular formula C6H6, is the parent compound from which most other aromatic compounds are conceptually derived. It occurs alongside related aromatic compounds such as toluene, phenol and naphthalene in the two large-scale industrial sources of aromatics, coal tar and petroleum. Physically, benzene is a colourless liquid with a characteristic odour, boiling at 353 K. It was first synthesised in the laboratory by Berthelot in 1870, starting from acetylene (a synthesis that foreshadows the ethyne-trimerization preparation route covered in section 15.4.4). Benzene's original historical name was 'phene', and this is the direct source of the still-current name 'phenyl' for the C6H5- substituent group. Table 15.6 draws out six systematic points of contrast between aromatic and aliphatic compounds -- carbon content, flame character, ring-vs-chain structure, resistance to normal oxidising/reducing agents, resistance to addition reactions (no decolourisation of KMnO4 or Br2/CCl4 despite an ap …
Aromatic compounds vs aliphatic compounds, point by point: (1) Aromatic compounds contain a higher percentage of carbon, while aliphatic compounds contain a lower percentage of carbon. (2) Aromatic compounds burn with a sooty flame; aliphatic compounds burn with a non-sooty flame. (3) Aromatic compounds are cyclic with alternating single and double bonds; aliphatic compounds are open-chain. (4) Aromatic compounds are not attacked by normal oxidising and reducing agents; aliphatic compounds are easily attacked by them. (5) Aromatic compounds do not undergo addition reactions easily and do not decolourise dilute alkaline aqueous KMnO4 or Br2/CCl4 even though their structure appears to contain double bonds; unsaturated aliphatic compounds undergo addition readily and do decolourise both reagents. (6) Aromatic compounds prefer substitution reactions; saturated aliphatic compounds give substitution reactions too, but unsaturated ones prefer addition. The table also notes exceptions to a naive 'aromatic = fragrant' assumption: anthra …