Unit 14: Applications of Organic Chemistry Learning Aim C: understand types, structures, reactions, uses and properties of isomers. This assignment and unit was graded an overall DISTINCTION. It is an extremely detailed assignment that meets all criteria in the best of detail, everything has been a...
The accepted structure of benzene
● The molecule known as benzene is made up of a hexagon-shaped
ring made up of six carbon atoms connected to one hydrogen atom
apiece. Three of the four bonding electrons that each carbon has are
utilised to create sigma bonds with two other carbons and one
hydrogen atom. The remaining six electrons in the p-orbital of
carbon. Six p-electrons are delocalized because of the sideways
overlap of the p-orbitals with the pi orbitals above and below the
carbon plane.
The kekule structure of benzene
● As to Kekulé's model, benzene is a cyclic, planar molecule made up
of six carbon atoms arranged in a hexagonal ring with single and
double bonds going back and forth between them. This configuration
showed that each carbon atom was coupled to one hydrogen atom
and one neighbouring carbon atom, which explained the chemical
formula of benzene, C6H6.
Explanation
Since each carbon atom connects with three other atoms during its outer orbital hybridization
process before bond formation, it only needs to hybridise three orbitals instead of all four. In
this process, the remaining 2p electron remains unchanged while two 2p electrons and one
2s electron are used. Each carbon atom has a p electron that overlaps with electrons on
either side, creating a strong sideways overlap that forms a system of pi bonds covering the
entire carbon ring. This phenomenon, where electrons spread over the whole ring rather
than being confined between two carbon atoms, is known as delocalization. Sigma orbitals
are located between atoms, while pi orbitals extend above and below the atoms. In benzene,
the pi orbitals of all the carbon atoms overlap, creating a continuous region that stretches
above and below the molecule. Within this overlapping region, the electrons are free to move
in any direction, which is why they are called delocalized. This overall structure is referred to
as the "delocalized pi system."
Evidence
IR spectroscopy
● Infrared (IR) spectroscopy provides evidence for the current structure of benzene by
comparing the expected bond lengths of the Kekulé structure with those of the
delocalized π-electron model. In the Kekulé structure, benzene is depicted with
alternating single and double bonds, resulting in three longer C=C bonds and three
shorter C-C bonds. However, the experimental IR spectra of benzene show only one
absorption peak for C-C stretching vibrations, indicating that all the carbon-carbon
bonds are of equal length. This observation supports the delocalized π-electron
model, which proposes that the electrons are equally shared among all carbon atoms
in the ring, leading to uniform bond lengths.
X ray crystallography
● X-ray crystallography offers definitive proof of benzene's current molecular
arrangement by precisely determining atom positions and bond lengths. Through this
technique, it has been consistently revealed that all six carbon-carbon bonds within
benzene share the same length, measuring 0.139 nm. This observation aligns with
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