Bonding Introduction

Organic compounds are actually molecules. And these molecules are just made up of atoms that are stuck to each other by covalent bonds.

Just like atoms, these molecules can bond with each other, and the general name for bonding between molecules is called intermolecular forces.

This basic diagram shows the location of covalent bonds and intermolecular forces.
(Notice that there are two types of intermolecular forces, but more about these a little later.)



When molecules are bonded to each other, they become liquids or solids.
If they are not bonded, then the molecules move away from each other and the substance becomes a gas.

Strength of the Intermolecular Forces (bonds)

Depending on the type of molecule you have, the bond strength between them can be very different, ranging from weak bonds to very strong.
There are basically four factors to consider when comparing intermolecular bond strength between molecules.
  1. molecular size
    (weak van der Waals)
  2. branching
    (weak van der Waals)
  3. presence of carbonyl =O
    (weak van der Waals)
  4. presence of hydroxyl -OH
    (strong hydrogen bonds)


1. Molecular Size

Larger molecules have stronger bonds between them.
Study this example.



Will you find stronger bonds between molecules of A or B?
Therefore there will be stronger bonds between molecules of B. Compound B is much larger than compound A.
These bonds are called van der Waals forces.

The reason for this is that compound B, being larger, is able to form better dipoles than compound A. Thus the attraction between molecules of B is greater.


2. Branching

The more branched a molecule is, the weaker are its intermolecular bonds.

Study Compound A and Compound B.
Both molecules are the same size.
Both have a molecular formulae of C5H12.



Which compound would have weaker intermolecular force between its molecules?
B is more branched, B will have weaker intermolecular bonds.

Explanations
Molecules of A, due to their shape, have a larger area for contact. Hence a stronger bond results between them.


Molecules of B, due to their branching :


These bonds are called van der Waals forces.


3. Presence of carbonyl =O

The presence of the carbonyl increases bond strength.

Study these two compounds. They are of similar size, and they are both unbranched.
Compound B has a carbonyl, the = O



Which compound would have stronger bonds?
Compound B would have stronger intermolecular forces than compound A.

The carbonyl allows compound B to become a stronger dipole than compound A. This happens because the oxygen atom has a high electronegativity, and thus creates a stronger dipole.


4. Presence of hydroxyl -OH

The -OH allows for hydrogen bonding, which is the strongest intermolecular force type.

Study compounds A and B.
They are of similar size, and unbranched and both have carbonyls.
But compound B has an -OH group.



Which compound would have stronger intermolecular forces?
Compound B would have stronger intermolecular forces (hydrogen bonds) than compound A.

The presence of the -OH allows compound B to have hydrogen bonds between these molecules. These are much stronger bonds.




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