I hope chapter 1 wasn’t too bad. Now lets look at the structure and Properties of Organic molecules.
Looking back at general chemistry we like to simplify the model of an atom’s electron as a ladder or planetary orbits around the sun.
As explained in freshman chemistry the model we like to present is not entirely accurate. Going back to the history for a moment electrons were thought to behave as particles since there initial discovery in the late 19 th century. This particle theory was the central theme for scientist until Louis de BRAY introduced his dissertation entitled “Research on the Theory of the Quanta” based on the work of two renounce scienctist Max Planck and Albert Einstein This research culminated in the de Bray’s hypothesis stated that any moving particle or object had an associated wave . For this he won the Nobel Prize in Physics in 1929 and resulted in scientist now seeing electron in a wave and particle or wave-particle duality.
Since De Bray stated that waves behaves as waves instead of particle lets take a look a how electron be have as a wave. There are two kinds of waves we must appreciate. The first ones are traveling waves.
Traveling waves as there name states are moving waves we can thing of the ocean as a example. Traveling waves are important in physics but not so much in explaining electron properties in chemistry.
Standing waves on the other had are of interest. Standing waves can be seen as a vibration occuring at a fixed position. As the examples indicate a vibrating air column in an organ or a guitar which I think better explains standing waves.
So lets look at a guitar string to explain molecular orbitals. I call this guitar orbital theory.
If you think of a string at a fixed position (or resting position) consider that the wave we will manipulate. If you pluck the string it will vibrate creating a sound. If we could fix time you could see two of displace for this string during it vibration. When the displacement is positive or upward we could designate at a plus sign. When the displacement is negative or downward we could designate this as a minus sign. A molecular orbital can describe like this with exception we are looking at a 3 dimensional figure as seen on the next slide.
The s orbital you came across in general chemistry is designate being represented by a positive or negative sign. Don’t confuse this with charge as this is just explaining to you where your electron density is. As seen in the figure the dot (nucleus) represent the region where your protron and neutron resides. The blue line or electron density scale up the wave function as you go left to right. The important thing to note is that as your approach the nucleus of decrease the electron density increases. You would expect this as the opposite charge of the protron attracts the electron.
Now as the slides mentions what happens if you place your finger on a guitar cord and pluck. If it is directly in the middle of the chord the string would occilate on both side with the mid point not moving since your finger hold it. This mid point in a electron is called a node and it is a area with no electron density. Because of this node the wave begins to occilate out of phase with respect of each other.
This occultation represents the p orbital and why the p orbital shape is represented as such. Keep in mind that the wave function signs are opposite with respect to one another.
To summarize what we just covered. The electron density or orbit is limited to the orbital (the mathematical construct) of the proximity of the electrons being present. In organic chemistry we will need to understand s and p orbitals behave to predict chemical reactions. More will be explained later.
LCAO or linear combination of atomic orbitals just the operation of combining orbitals to form new orbitals to allow reactions to occur in organic. The items of interest to consider are what happens when bonding occurs. In orbital theory the bonding of atoms is a result of the interactions with the orbitals. The second item to consider the hybridization of orbitals. In organic when multiple orbitals on the SAME atom need to interact the combine together to form new orbitals of equal energy to react. This process is know as hybridization and will be explained more in the next podcast.
