Notes Chapter 2

Chem 243

In this chapter dipole moments are introduced and their relationship to physical properties is examined. This is an immportant concept to learn. A survey of major functional groups is presented to begin putting organic structures into some kind of order. Infrared spectroscopy is introduced to show how the interaction of these functional groups with electromagnetic radiation can depend on structure.

This chapter begins with the construction of larger molecules from carbon and hydrogen. You should get used to the idea that the elements are building blocks and the properties of the molecules are dependent on the properties of the atoms. The concepts of electronegativity and molecular orbitals, including hybridization, are the most important things you should learn from this chapter.

Note the geometries of alkanes, alkenes and alkynes and aromatic compounds and how they are derived from the various hybrid orbitals. Initially, hydrocarbons are considered iand since hydrogen and carbon have similar electronegativity values, there is little charge separation (uneven electron distribution) in alkanes. Looking at the electrons in the pi-orbitals of alkenes, alkynes and aromatic compounds, there is a bit more uneven charge distribution, but these are still usually not very polar molecules. Knowing where the electrons are spending most of their time will be important throughout the course, so consider these concepts to be tools that you will be using in organic chemistry.

When the electronegativities of two bonded atoms are not similar, the electrons tend to be more concentrated at the more electronegative atom. This makes that atom more negative than the one to which it is bonded. WE consider it to have a partial negative charge, while the other atom has a partial positive charge. We can also say that one atom is electron rich while the other is electron deficient. This is an extremely important concept since chemical reactions occur by the transfer of electrons between orbitals. Remembering your high school physics, electrons will tend to go toward electron deficient atoms and electron rich atoms will tend to be attracted to electron deficient atoms. This is the beginning of the acid-base concepts which can be used to describe a large portion of the organic reactions we will study. Positive goes to negative and negative goes to positive,what could be easier! Now that you understand this, you can do most of organic chemistry, but you still have to attend class. The trick is to be able to identify the positive and negative regions in molecules when there are competing forces, so you can't get off that easily. Though the molecules can look complex and it seems like the only way to survive organic chemistry is to memorize everything, there are these fundamental concepts which help clarify the subject, no matter how large the molecules are.

Now what happens when a molecule has one or more polar bonds? Each bond dipole can be expressed as a vector and the dipole moment of the entire molecule is the vector sum of the bond dipoles. Again this is just an application of high school physics.

If a molecule has a positive charge center and a negative charge center these will each attract centers of the opposite charges. So molecules will then be able to attract each other and thus effect bulk properties like melting and boiling points.

Functional Groups

Next, this chapter introduces a large number of functional groups. A functional group is a grouping of atoms which can be part of a larger molecule. This is how organic chemistry is simplified. In general, a functional group will have the same properties regardless of the molecule in which it resides. (This will not be the case when there are other functional groups present and the forces are acting together, but that's for later.) Most of the functional groups we will encounter are introduced here so that we can begin their study immediately and note the similarities and differences between them. As you look through the text, you will see chapters for each of the functional groups. They are grouped this way to simplify their study and reduce the need for useless memorization. Look at the atoms forming each functional group and consider what kind of molecular dipole moment should be present. Is it strong or weak? This gives you an insight into the properties of the molecule.

Infrared Spectroscopy

Besides their effect on bulk properties, dipole moments can be detected directly by their interaction with electromagnetic radiation, specifically, in the infrared region. As will be described in class, the vibrating electric dipole component of infrared radiation happens to lie in the same frequency range of most molecular vibrations, so we can relate these vibrations by their frequencies to particular functional groups. This is a convenient way to identify the presence or absence of many functional groups in an unknown chemical sampleYou probably see some of this mentioned on various CSI TV programs, but they usually oversimplify the matter. Infrared spectroscopy can actually identify a molecule, unambiguously, using the "fingerprint" region of the spectrum, but you have to have the spectrum of the identical compound available for comparison and both spectra have to be recorded under nearly identical conditions. But for our purposes, we can use IR spectra to tell us what functional groups are present or absent in a particular sample and we will be using this technique throughout the course. Don't worry about memorizing the characteristic frequencies of the various functional groups, you will always be given tables with that information. Learn how to identify various characteristics of the spectra. Peaks can be narrow or broad. Broadening can be a result of hydrogen bonding as we will see later. The peaks of functional groups are not always in exactly the same position (same frequency) but other parts of the molecule can cause them to shift in one direction or the other. You should concentrate on these kinds of effects as the course continues. iR will be useful in solving a number of problems, but it has some limitations. Later we will study other spectroscopic and spectrometric methods which, in combination, can provide a more complete picture of the structure of an unknown molecule.

Infrared spectra depend on bond dipoles, bond strengths and atomic masses. So now we are looking at a number of physical constants which we can use to give us structural information about our molecule.

A more important concept here is the use of information from various sources to answer questions. Organic chemistry here becomes a means of teaching another problem solving technique, other than mathematics. Here we begin to sort through available information and try to decide which compounds will actually give rise to these data. If there is a contradiction, then the compound being considered is probably not correct. If you look at these problems as puzzles or mysteries to be solved, they can be much more interesting than just textbook exercises. The problem solving skills you develop in this course can, of course, be applied in other disciplines. This is where memorization becomes less important and logical thinking predominates.