Podcastppt2
- 1.
- 2. Chapter 2 Structureand Properties of Organic Molecules
- 3. We like topicture atoms as that of a solar system.
- 4. Electron properties Butthe solar system representation does not accurately define our understanding today. Electrons have always been thought of as particles since their discovery in 1890’s. But in 1923-1924 it was also suggested that electrons also have wave like properties by Louis de Broglie. This is the concept of "wave-particle duality" due to De Broglie. In 1926 Schrodinger was able to show that electrons in a hydrogen atom could be described by a wave function, Ψ . (Not in book)
- 5. Waves In 1923Louis de Broglie a French physicist and Nobel laureate Suggested that electrons are better explained by treating them as waves instead of particles. There are two kinds of waves one must appreciate Traveling waves Standing waves
- 6.
- 7. Standing waves Theones we are interested in Standing waves vibrate at a fixed location Example would be The air rushing in a pipe organ creating a vibrating air column The guitar/violin string The electron in a atomic orbital can be though of as a standing wave HMMMM!!!!
- 8. Guitar Orbital Theory
- 9. Guitar Orbital Theoryof an electron As per our book consider the orbital (a 3-D structure) a guitar string. If you pluck a guitar string at its middle this will result in a standing wave. All of the vibration is displaced upward for a bit them downward for an equal amount of time. A 1s orbital can be described like a guitar string except is it 3D.
- 10. Wave function (thes orbital) The orbital can be described by its wave function Ψ a mathematical depiction of shape and waves as it is vibrating (Fig 2-2) Wave is positive and negative (THIS DOESN’T DESIGNATE CHARGE) it is just telling us where the wave is to rest position. 1s orbital is represented by a circle with the nucleus in the center
- 11. Wave function (thep orbital) Now if you take your guitar and place your finger on a cord and pluck what happens? Your finger keeps the midpoint of the string from moving. This displacement at the midpoint is ALWAYS zero We designate this as a node With a node we notice the string is vibrating out of phase with one another I.E. when one is up the other is down
- 12. Wave function (thep orbital) This vibration on fig 2-3 would be the first harmonic of the guitar string. The first harmonic is = to the P orbital as shown on fig 2-4 is a 2p orbital with its two lobes and a node/nodal plane As seen the signs should be opposite for the respective “lobes”
- 13. Orbitals To summarizethe book Electrons are often described as being in orbitals around an atom that are mathematical "constructs" based on the wave function, Ψ , that describes the motion of an electron. An orbital is, more correctly, a mathematical function that describes the region of high probability in 3D space, around a nucleus, where an electron may be found. Orbitals are commonly represented by the boundary surfaces that encloses the region where there is a 90-95 % probability of finding the electron. In organic chemistry one needs to be most familiar with the s- and p-type orbitals.
- 14. LCAO Atomic orbitalscan combine and overlap to give more complex standing waves. Addition and subtraction of there wave functions gives new wave functions of new orbitals This process is called linear combination of atomic orbitals (LCAO) The number of new orbitals generated always equals the number of orbitals we started with. When orbitals on different atoms interact, they produce molecular orbitals (MO’s) that lead to bonding (or antibonding). When orbitals on the same atom interact, they give hybrid atomic orbitals that give define geometries. This process is known as hybridization Now we can begin to appreciate bonding in organic
- 15.
Editor's Notes
- #3 I hope chapter 1 wasn’t too bad. Now lets look at the structure and Properties of Organic molecules.
- #4 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.
- #5 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.
- #6 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.
- #7 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.
- #8 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.
- #9 So lets look at a guitar string to explain molecular orbitals. I call this guitar orbital theory.
- #10 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.
- #11 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.
- #12 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.
- #13 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.
- #14 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.
- #15 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.