Chemistry Basics

Molecules may be represented in two dimensions by a structural diagram:

 

Ethylene

 

Bromo-Chloro-Fluoro-Iodo-Methane

While this representation allows for the derivation of the three dimensional structure of the left molecule, the right molecule requires further description.

Molecules may be represented in three dimensions by a ball and stick configuration like this. This allows examination of the molecule's configuration, in this case an sp3 hybridized central carbon with 4 different atoms attached:

X axis

Y axis

Z axis

Another method used to describe atoms and their bonding is through the location of the electrons in their outermost shell. Because there is no exact location of where the electron may be found, probability distributions (commonly called orbitals) based on quantum mechanical calculations are sketched showing where the electron is most likely to be found. A few of these representations are presented here to illustrate this approach. The first three types of orbitals for individual atoms are:

 

 

s

 

p

 

d

The p and d orbitals actually change according to how many electrons are in the outer shell. A full p orbital is customarily indicated as shown below:

When two or more atoms are bounded to form a molecule, the orbitals from each atom may be combined into hybrid atomic orbitals, which are useful in describing molecular structural properties such as geometry. A few examples are illustrated below. The hybrid sp combination yields the linear geometry of CO2; sp2 describes the trigonal planar geometry of BF3; and the sp3 combination fits the familiar tetrahedral geometry of CH4.

 

 

sp

 

sp2

 

sp3

Hybrid atomic orbitals are also useful in describing double bond formation. We illustrate this for the case of ethylene, which will be used in the polymer synthesis section as an example of free radical formation in connection with addition polymerization. After the sp2 orbitals with the hydrogen are occupied, two electrons in carbon 2s states interact to form a sigma bond along the C-C centerline and the single electron remaining in each of the unhybridized 2p orbitals interact to form a pi bond (a "combined" p orbital appearance). For a more complete discussion of these topics see any recent college chemistry text.

Chirality

The ability of carbon to form four bonds allows for the possibility of two configurations using the same molecular formula. Particularly, a configuration about the central carbon and its mirror image is possible. The mirror image is not superimposible upon the original configuration; thus it represents a new form of the molecule:

Original molecule

Mirror image

Such molecules are said to be chiral, and the central atom known as the chiral center. One of the configurations about the chiral center is designated S and the other R. The pair of molecule configurations due to chirality are known as enantiomers. Enantiomers have identical chemical properties except towards optically active reagents, and identical physical properties except for the direction of rotation of the plane of polarized light. A mixture of equal amounts of both enantiomers is known as a racemic mixture and is not optically active. It is possible for a molecule to contain two chiral centers and thus four possible configurations may result. This may result in chiral molecules which are not mirror images of each other. Such molecules are known as diastereomers, and have similar chemical properties but may have vastly different physical properties.

Geometric Isomers

 

 

cis

 

trans

I

 

II

Examination of the two molecules above reveals they share the same molecular formula (BrCHCHBr) as well as the location of the double bond. Thus they are isomers; both represent 2-ethylene but their physical properties differ. The difference between the two isomers is hinted by their two dimensional representation but becomes more clear when the examined in three dimensions.

The difference lies in the way the atoms are oriented in space. They are not mirror images of each other, and thus they are termed diastereomers. However, because the rotation about the carbon carbon double bond is hindered (since it would break the pi bond), they are classified as geometric isomers. The two configurations are named by inserting the prefix "cis" (Latin: on this side) for I and "trans" (Latin: across) for II which indicate the location of the bromo groups each on the same or opposite sides of the molecule. Thus, the proper name for I is cis-1,2-dibromo-ethene and II trans-1,2-dibromo-ethene.

Nomenclature

Naming large molecules can be a complex task. The IUPAC Nomenclature Committee has established conventions that eliminate confusion. In general, a polymer with an unspecified number of monomers is named by adding the prefix "poly" to the constitutional repeating unit (CRU). Otherwise, the Greek prefix corresponding to the number of monomers is added to the name of the CRU.

#

Prefix

1

Meth

2

Eth

3

Prop

4

But

5

Pent

6

Hex

7

Hept

8

Oct

9

Non

10

Dec

11

Undec

The name of the CRU is formed by naming each of the subunits. If a polymer contains only one subunit, the prefix "poly" is added to the name of the subunit. For example, if a polymer is formed from a combination of ethylene monomers, its name would be polyethylene. Polymers containing more than one subunit are named based on the largest subunits, and are set off from the prefix "poly" with parentheses.

The naming of the subunits is the most difficult part. The subunits are named by the main constituent of the subunit with smaller molecules added on as prefixes. The best way to demonstrate this nomenclature is to use examples. For example, the monomer:

would be called oxy(1-fluoroethylene). To start, we see that there is an oxygen atom attached to a larger subunit. Therefore, the name will have the prefix oxy-. The rest of the monomer can be seen to be an ethylene molecule with a fluorine atom attached. Since the fluorine atom is attached to the first carbon atom, the name of this part of the monomer is 1-fluoroethylene. Thus the name of the complete monomer is oxy(1-fluoroethylene), and the name of the polymer formed from this monomer would be poly[oxy(1-fluoroethylene)].

For monomers with branched chains, the monomer is named for the longest continuous chain. For example:

The molecule above is named for pentane, since the longest continuous chain is five carbons long. However, there are also two extra methyl groups attached to the 2nd and 3rd carbons in the chain. Therefore, this monomer is called 2,3 dimethylpentene, and the name of the polymer formed from these monomers is called poly(2,3 dimethylpentane). It may be tempting to label this 3,4 dimethylpentane, but convention mandates that you use the name with the smallest numbers possible.

For more resources relating to chemistry and polymers, follow this link.


Intro to Polymers

Virtual Textbook

Polymer Synthesis