Organic Chemistry – A Carbonyl Early Approach (McMichael)

Introduction

Organic chemistry is the chemistry of the element carbon. The compounds formed from carbon and a few other elements (O, N, P, S and H) form the chemical basis for living systems. Most therapeutic drugs are organic compounds. Organic polymers, whether obtained from nature or by synthetic means, are extremely important economic materials. These include plastics, rubber, glues, starch, cotton, and wood, as well as the proteins we must have in our diet.

Why is it called “organic” chemistry?

Historically, when chemists discovered the Law of Definite Proportions at the beginning of the 19th century, it appeared that this law did not apply to the various compounds they had isolated from plant and animal sources. Carbon compounds can be so complex that the ratios of elements in them did not appear to be simple numbers. For example, ordinary table sugar has the molecular formula C H O , not the kind of simple ratio seen with the oxides of copper, Cu O or CuO, for example. Chemists imagined that organic compounds were held together by a mysterious “vital force”.

The beginning of the end of the “vital force” hypothesis is generally considered to be Friedrich Wöhler’s synthesis of urea in 1828. He started with lead cyanate, which is about as “dead” as any chemical can be, and ammonium hydroxide or chloride, also “dead”, which generated ammonium cyanate, NH OCN (empirical formula CH N O) (still “dead”). When he heated the ammonium cyanate, he got urea, H NCONH (molecular formula also CH N O, but atoms arranged differently). Urea is just what the name sounds like, a major ingredient in urine, and was thought at the time to be a purely “organic” chemical. Other syntheses of “organic” compounds from “inorganic” materials soon convinced chemists that organic compounds obeyed the same laws of chemistry as other chemicals.

Structural Principles

First, though, we need to review a few structural characteristics of the carbon atom. These are ideas which were part of your general chemistry courses, but it will help if we briefly restate them.

1. Carbon is tetracovalent. That means that a carbon atom typically makes four bonds to other atoms and that these bonds are covalent–formed by sharing an electron pair between the two atoms joined by the bond. Such arrangements provide eight valence electrons for a carbon atom, so that it’s electronic configuration is like that of the very stable noble gas neon. Similarly, hydrogen forms one covalent bond, oxygen two, and nitrogen three.
2. Carbon can form multiple covalent bonds. That is, a single carbon atom can form a double (to C, O or N) or triple (to C or N) bond to another atom. A double bond would involve two electron pairs between the bonded atoms and a triple bond would involve three electron pairs.
3. Bonds between carbon and atoms other than carbon or hydrogen are polar. That is, in a bond between carbon and oxygen or nitrogen the electrons are closer to the more electronegative element (oxygen or nitrogen) than to the carbon, so the carbon has a slightly positive charge. (Fluorine is the most electronegative element, and the elements close to fluorine in the periodic table are also quite electronegative.)
4. Bonds between one carbon atom and another and between a carbon and a hydrogen are non polar. That is, the electron pair forming the bond is quite evenly shared by the atoms.