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Module: BIOM - 1007

Lecturer: Dr Bhambra

Date: 14/10/16

Hypervalency, Aromatic Compounds, Polyatomic Compounds and Non-Covalent Interactions
o

Valency of an electron tells us the number of electrons it must share with other atoms if it is going to fulfil
its outer shell
 This can be done by single, double or triple bonds
 For example, carbon can make single, double or triple bonds depending on the adjoining
atoms
 Some elements however, do not obey the octet rule (8 electrons)
 These elements possess an expanded valence shell and so can form more bonds than
expected
 Phosphorus Trichloride shows the typical bonding expectancy of phosphorus
 Phosphorus however, can show Hypervalency by forming 5 bonds in molecules
such as ATP
o Phosphorus can recruit d orbitals in order to house the additional 2
electrons
 Hypervalency can also be caused by the electrons donating themselves to more than 1
atom and so contributing to more than one covalent bond
 Some of the most important biological molecules such as DNA and ATP wouldn’t be
available without Hypervalency

o

Many compounds contain not just single or double bonds, but a
combination of both
...
e
...
5 bonds
 This is only possible with alternating single and double
bonds (must be conjugated)
 Benzene is the most common aromatic compound:
 The carbons share valence electrons with 1 hydrogen, 2
neighbouring carbons and 1 electron in a p orbital
 The p orbitals in each carbon overlap in order to form a
larger combined orbital (see below)
 The electrons are now de-localised

o

There are
that
covalent
bonds,
known as
ionic compounds
 The molecule is comprised of anions and cations
 The anion is usually the polyatomic one
 One example is NaOH – Na+ cation and OH- anion

o

The unequal distribution of electrons in a covalent bond, due to the electronegativity of an atom, causes
polarised bonds, an example is HCl

molecules
contain both
and ionic
these are
polyatomic



o

The atom with the greatest attracts the bonding electron
...
This is termed a dipole
moment

Bonds that are not covalent, non-covalent bonds, are also important in determining the characteristics of a
molecule, they can be intramolecular (within) or intermolecular (in between), here are some noncovalent interactions:
 Hydrogen bonds
 A hydrogen bond is a special type of dipole interaction with one hydrogen on a
molecule to another electronegative atom on another molecule, or a sufficient
distance away on the same molecule
 The most common acceptors for a hydrogen bond are oxygen, nitrogen or fluorine
 The atom that the hydrogen is bonded to already I known
as the hydrogen bond donor
 The hydrogen can only bond with a pair of non-bonding
electrons on a separate donor atom
 The 3 nuclei of the participating atoms must lie in a
straight line for the bond to be at its strongest
 The image to the right is two water molecules exhibiting a
hydrogen bond:
 Ionic interactions
 Ionic forces occur when there are two ionic species, each carrying a full
positive/negative charge
 The main example of ionic forces are ionic bonds, however these are not the only type of
ionic interaction
 They are also prevalent in amino acids
...
Instead, they are determined by a molecules behaviour when
exposed to water
 The polarity of a molecule is what determines whether or not something is hydrophobic or
hydrophilic
 Polar molecules are more likely to be hydrophilic and non-polar molecules are
more likely to be hydrophobic
 Hydrophobic forces arise when portions of a molecule are moved away from their
aqueous surroundings by structural realignment
 Clustered molecules are stabilised by dispersion forces acting between hydrophobic
portions
 Dispersion forces
 Dispersion forces are weak molecular interactions that occur regardless of the presence
of an electrical charge
 This occurs when the electron distribution is uneven around the electron
orbitals and so, there will be a constant repelling from slightly positive and
slightly negative electrons
 The formation of a dispersion force is affected by the size and shape of molecules
 It is a force of attraction that occurs with two areas of opposite charge
 This forms an induced dipole
 Dispersion forces have 3 characteristics
 They are very short lived (10-16 seconds)
o This is because an area that is slightly positive may become slightly negative
due to the constant moving of electrons
 They are extremely weak





o

 They operate over very short distances
Permanent dipole interactions
 Permanent dipole interactions occur with opposite partial charges on polar molecules
 The most electronegative atom has a partial negative charge whereas the least
electronegative atom carries a slight positive charge
Steric repulsion
 Steric repulsion is different from dispersion forces and dipole interactions

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