01:160:307 Study Guide - Fall 2018, Comprehensive Midterm Notes - Chemical Bond, Carbon, Electronegativity
12 Oct 2018
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01:160:307
MIDTERM EXAM
STUDY GUIDE
Fall 2018
1
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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