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Saturday, 25 June 2011

Organic Chemistry An Introduction

Organic Chemistry

An Introduction

by Anthony Carpi, Ph.D.
To understand life as we know it, we must first understand a little bit of organic chemistry. Organic molecules contain both carbon and hydrogen. Though many organic chemicals also contain other elements, it is the carbon-hydrogen bond that defines them as organic. Organic chemistry defines life. Just as there are millions of different types of living organisms on this planet, there are millions of different organic molecules, each with different chemical and physical properties. There are organic chemicals that make up your hair, your skin, your fingernails, and so on. The diversity of organic chemicals is due to the versatility of the carbon atom. Why is carbon such a special element? Let's look at its chemistry in a little more detail.
Carbon (C) appears in the second row of the periodic table and has four bonding electrons in its valence shell (see our Periodic Table module for more information). Similar to other non-metals, carbon needs eight electrons to satisfy its valence shell. Carbon therefore forms four bonds with other atoms (each bond consisting of one of carbon's electrons and one of the bonding atom's electrons). Every valence electron participates in bonding, thus a carbon atom's bonds will be distributed evenly over the atom's surface. These bonds form a tetrahedron (a pyramid with a spike at the top), as illustrated below:
carbon bonds - Carbon forms 4 bonds
Carbon forms 4 bonds
Organic chemicals get their diversity from the many different ways carbon can bond to other atoms. The simplest organic chemicals, called hydrocarbons, contain only carbon and hydrogen atoms; the simplest hydrocarbon (called methane) contains a single carbon atom bonded to four hydrogen atoms:
carbon-methane - Methane - a carbon atom bonded to 4 hydrogen atoms 
Methane - a carbon atom bonded to 4 hydrogen atoms 
But carbon can bond to other carbon atoms in addition to hydrogen, as illustrated in the molecule ethane below:
carbon-ethane - Ethane - a carbon-carbon bond
Ethane - a carbon-carbon bond
In fact, the uniqueness of carbon comes from the fact that it can bond to itself in many different ways. Carbon atoms can form long chains:
carbon-hexane - Hexane - a 6-carbon chain
Hexane - a 6-carbon chain
branched chains:
carbon-isohexane - Isohexane - a branched-carbon chain
Isohexane - a branched-carbon chain
rings:
carbon-cyclohexane - Cyclohexane - a ringed hydrocarbon
Cyclohexane - a ringed hydrocarbon
There appears to be almost no limit to the number of different structures that carbon can form.  To add to the complexity of organic chemistry, neighboring carbon atoms can form double and triple bonds in addition to single carbon-carbon bonds:
c-ethane c-ethene c-ethyne
Single bonding 
Double bonding
Triple bonding
Keep in mind that each carbon atom forms four bonds. As the number of bonds between any two carbon atoms increases, the number of hydrogen atoms in the molecule decreases (as can be seen in the figures above).

Simple hydrocarbons

The simplest hydrocarbons are those that contain only carbon and hydrogen. These simple hydrocarbons come in three varieties depending on the type of carbon-carbon bonds that occur in the molecule. Alkanes are the first class of simple hydrocarbons and contain only carbon-carbon single bonds. The alkanes are named by combining a prefix that describes the number of carbon atoms in the molecule with the root ending "ane". The names and prefixes for the first ten alkanes are given in the following table.
Carbon
Atoms
Prefix
Alkane
Name
Chemical
Formula
Structural
Formula
1 Meth Methane CH 4 CH4
2 Eth Ethane C2H6 CH3CH3
3 Prop Propane C3H8 CH3CH2CH3
4 But Butane C4H10 CH3CH2CH2CH3
5 Pent Pentane C5H12 CH3CH2CH2CH2CH3
6 Hex Hexane C6H14 ...
7 Hept Heptane C7H16
8 Oct Octane C8H18
9 Non Nonane C9H20
10 Dec Decane C10H22

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