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