01:160:307 Lecture Notes - Lecture 1: Organometallic Chemistry, Preferred Number, Electron Configuration
1 May 2018
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Introduction
A. What is Organic Chemistry and why do we spend at least a whole year
studying it? Think about all the different substances you encounter every
day. Almost every single one is an organic substance. Look around.
1. Organic chemistry is often defined as the chemistry of compounds of
carbon, [oversimplified all organic compounds contain carbon, but not all
compounds that contain carbon are organic.] To some extent definitions such
as these are arbitrary or historical. We shall use this one: Organic
compounds all have at least one C-H bond, except for a very few that have
only a C-C bond or a C-X bond(X conventionally means halogen). So
compounds such as sodium bicarbonate or calcium carbonate are not defined
as organic compounds. But compounds such as carbon tetrachloride CCl4 (4
–Cl bonds) or oxalic acid, C2O4H2 (1 C-C bond) are.
Most of the organic compounds we will consider in this course consist of
only nonmetals. There is, however, a large important branch of organic
called organometallic chemistry, in which the organic compounds also
contain at least one metal atom. We shall briefly look at some
organometallic compound because of their importance in the synthesis of
other organic compounds.
2. To some laypersons, organic chemistry has a different connotation,
related to “organism” or life. This connotation is mostly historical and
comes from the fact that virtually every chemical of life is organic, one
reason why Organic Chemistry is required for majors in the life sciences.
3. Historical: The Vital Force Theory, which postulated that any substance
from a living thing had a special, almost magical ingredient called Vital
Force, was abandoned in the 19th century. The first in a series of key
experiments that led to its abandonment was the preparation of urea from
apparent inorganic sources. This urea was indistinguishable from urea from
animals, as far as Wohler (who did the experiment) could tell. But the
difficulty of proving that something doesn’t exist is the reason that it took
many more experiments and many years before everyone was finally
convinced that there was no magical, mysterious substance associated with
living things. Therefore all samples of a given pure organic substance are
the same regardless of their origin. A pure synthetic vitamin is the same as a
pure natural one.
4. Most known chemical compounds are organic. There are more than 100
million chemical substances known, most of them organic. That number is
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increasing every day. (50 years ago there were fewer than 1 million) In
addition there are over 66 million genetic sequences known. Technically,
each one is a different organic compound.
B. What’s so special about Carbon? Why are there so many compounds of
Carbon? Review the material on the Periodic Table, the electronic
configurations of atoms, and covalent bond formation in your Gen Chem
book.
1. Position in the Periodic Table. In the middle of the second row, so it’s
small (can get close to other atoms and form multiple bonds) and in the
middle of the EN scale. Thus C is good at forming covalent bonds.
2. Has 4 valence electrons, so it can form 4 covalent bonds by electron
sharing with things that bring a total of 4 electrons. The 4 electrons from C
plus the 4 electrons from the atoms to which the C bonds result in a noble
gas electronic configuration around the C. When an atom in a compound is
associated with 8 valence electrons we say that the atom has a completed
octet.
3. The valence shell electron configuration of carbon is 2s22p2. You may
have learned about “promotion” in the past to explain the formation of four
bonds by C. It is an unnecessary and inaccurate complication. Just remember
carbon has 4 valence electrons and therefore forms 4 bonds, to complete its
octet. Any energy cost of changes in orbitals is more than offset by the
formation of bonds and octet completion. Remember bond formation is
energetically favorable.
But, why are there so many compounds of carbon?
a. Carbon atoms can form chains. Most compounds of carbon contain
such chains, which are two or more carbon atoms linked together [One of
the first skills you must master is to be able to draw these chains and
understand how they are put together.]
b. We need to consider two issues, stability and reactivity, to explain
why so many compounds exist that have these carbon chains.
c. Look at bond energies. The C-C (bond energy = 347 kJ/mol) is a
strong bond. That speaks to stability. The bond is strong because of good
orbital overlap, as we shall see.
d. But could C-C bonds form something better by chemical reaction?
This is a question about reactivity. The most common reagent on Earth is
O2. But the C-O bond energy is 358 kJ/mol, which is not very different
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from the C-C bond energy. So C-C bonds are relatively unreactive under
ordinary conditions.
e. We also observe that C-C bonds are not weakened when the C atoms
form chains or when other atoms are attached to the C atoms that are part
of chains.
Review (Gen Chem)
A. Valence Shell Electronic Configurations
1. Shells and Subshells, Atomic Orbitals
a. Aufbau and electronic configuration
b. Valence shell is the key. In nonmetals the valence shell is made up of
the orbitals with the highest value of n. The orbitals in the valence shell
of a nonmetal are s orbitals and p orbitals.
2. Review and learn the electronic configurations of these important
elements: C, H, N, O, X (halogen), and (less important P and S). They are
all nonmetals and they are all found in organic compounds. Organic cpds
with other nonmetals are rare. Thus in this course we can focus on just a
few nonmetals, know their electronic configurations, relative
electronegativities, and other relevant atomic properties and not have to
worry about a large number of other elements.
B. Covalent Bonds
1. Electron pair sharing. When two atoms share one, two, or three pairs
of electrons we have a covalent bond.
2. The formation of covalent bonds is energetically favorable. This
very important principle operates under the conditions found on the
surface of our planet. (Note when we compare energies, lower energy
is better.) This principle has broad applications in Organic Chemistry.
There are a number of different ways that we can explain it depending
on how we choose to describe bonding.
One very simple explanation is based on electrostatics. The electron is
negative and the nucleus is positive. So there is a favorable interaction
between the electrons of an atom and its nucleus. If the atom now
shares one of its electrons with another atom, the electron interacts
favorably with two nuclei. This advantage of electron sharing has
limitations based on distance and electron properties, however. For
example, the two nuclei can’t be too close to each other or their
positive charges will repel each other.
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