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Aromatic Chemistry

AQA Content

Use thermochemical evidence from enthalpies of hydrogenation to account for this extra stability
Explain why substitution reactions occur in preference to addition reactions.

Specification Notes

The nature of the bonding in a benzene ring, limited to planar structure and bond length intermediate between single and double.
Delocalisation of p electrons makes benzene more stable than the theoretical molecule cyclohexa-1,3,5-triene.


Understanding Electronegativity and Bond Polarity

Electronegativity is the power of an atom to attract the bonding pair of electrons in a covalent bond. The most electronegative element is fluorine. In general, the electronegativity of elements increases from left to right along a period, and up a group (ignoring noble gases).

Polar and Non-Polar Bonds

Non-Polar Covalent Bond:
Both atoms have equal electronegativity, and the electrons are held in the middle of the bond. An example is chlorine gas.

Polar Covalent Bond:
Hydrogen Fluoride (HF) is an example of a polar covalent bond where F has a higher electronegativity than H, and therefore has a stronger pull on the electrons. It has a permanent Dipole.

Permanent Dipoles:
When the electronegativities are different, the electrons are pulled more to one end, leading to a permanent dipole across the covalent bond – a difference in charge between the two atoms.
Polar and Non-Polar Molecules
An entire molecule will be polar overall if it contains asymmetrical polar bonds.
The molecule will be non-polar if either:
It contains no polar bonds
It contains polar bonds but is symmetrical, so the polar bonds cancel out.


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