Aromatic Chemistry & Intermolecular Forces

Question 1

Define & describe electronegativity.

Answer 1

• ability of an atom to pull e- towards itself.
• periodic property so depends on nuclear charge, atomic radius & e- shielding.
• higher nuclear charge, smaller atomic radius & low e- shielding gives an atom a higher electronegativity.
• allows predictions of type of chemical bond.

Question 2

Define & describe polar bonds.

Answer 2

• a polar covalent bond has a permanent dipole (partial charges)
• if different atoms form bonds there is an asymmetric distribution of e- in molecular orbitals because they have different electronegativities

Question 3

Give an example of the polarisation of organic molecules.

Answer 3

• carbon chlorine bond is polar because the chlorine is more electronegative than carbon.
• this gives an unsymmetrical bonding molecular orbital, illustrated through partial charges.
• so the bond has a dipole

Question 4

define a permanent dipole.

Answer 4

small charge difference across a bond that results from a difference in electronegativities of the bonded atoms.

Question 5

If there are more than 2 polar bonds in a molecule then the overall dipole can be either reinforced or reduced, depending on…

Answer 5

the orientation of the bonds.

Question 6

Strong intermolecular forces =

Answer 6

high melting/boiling point.

Question 7

Describe dipole-dipole interactions

Answer 7

• molecules with a permanent dipole attract each other
• permanent dipole due to electronegativity difference between C and O.
• relatively weak but sufficient enough to have an impact on boiling points & solubility

Question 8

Describe Van der Waals forces.

Answer 8

-interactions between molecules which do not have a permanent dipole as a result of induced dipoles.
-e- never stationary
e- distribution around atoms in a molecule can be asymmetrical at any given time.
this can lead to a small, temporary dipole (because charge distribution is uneven).
this dipole can induce another small, temporary dipole in an adjacent molecule, the temporary dipoles then attract.
-larger molecules have more e- so more temporary dipoles form, hence greater attractive forces.
-the closer molecules can get to each other, the stronger the attractive forces and so branched molecules have lower bp than straight chain molecules.
-weaker than dipole-dipole interactions
-hold lipid bilayers together, occur between amino acids with alkyl side chains.

Question 9

Describe H bonds.

Answer 9

• special case of dipole-dipole interaction.
• occurs when H is attached to an electronegative atom, usually N/O/F
• strongest intermolecular force
• can affect bp

Question 10

Give examples of the biological importance of H bonds.

Answer 10

• complementary base pairing in DNA

- protein structure, ES complexes

Question 11

2 substances will mic completely if…

Answer 11

sum of intermolecular forces between molecules after mixing > sum of intermolecular forces between molecules before mixing.