1.1: What is organic chemistry?
- Recognize organic compounds from their formula.
- Understand some logical reason for why nature has selected \(\ce{C}\) and \(\ce{H}\) as the main constituent of organic compounds.
What are organic compounds?
The compounds usually synthesized in living things are called organic compounds. The organic compounds are primarily composed of carbon (\(\ce{C}\)) and hydrogen (\(\ce{H}\)). For example, methane (\(\ce{CH4}\)) produced by decaying plant materials is composed of one \(\ce{C}\) and four \(\ce{H's}\). Often one or more atoms of elements other than \(\ce{C}\) and \(\ce{H}\) are also present in organic compounds, like oxygen (\(\ce{O}\)), nitrogen (\(\ce{N}\)), phosphorous (\(\ce{P}\)), sulfur (\(\ce{S}\)), etc. For example, \(\ce{O}\) atoms are present in glucose (\(\ce{C6H12O6}\)). The atoms other than \(\ce{C}\) and \(\ce{H}\), e.g., \(\ce{O}\) in \(\ce{C6H12O6}\), are called heteroatoms . Figure \(\PageIndex{1}\) illustrates fruits and vegetables composed of organic compounds.
Nature has chosen \(\ce{C}\) and \(\ce{H}\) as primary constituents composing the organic compounds because of several reasons, some of which are the following.
- \(\ce{C}\) is a member of second-row elements in the periodic table that usually make stronger and more stable bonds than the elements in the higher rows.
- \(\ce{C}\) makes four bonds in neutral molecules, which is higher than any other element of the second row can make. For example, \(\ce{C}\) has for bonds in a methane molecule represent as: \(\ce{\scriptsize{H}-\overset{\overset{\Large{H}}|}{\underset{\underset{\Large{H}} |}{C}}\!-H}\), where each line represents a bond.
- \(\ce{C}\) can make chains and rings, e.g., ethane: \(\ce{\scriptsize{H}-\overset{\overset{\Large{H}}|}{\underset{\underset{\Large{H}} |}{C}}-\overset{\overset{\Large{H}}|}{\underset{\underset{\Large{H}} |}{C}}\!-H}\), and a propane: \(\ce{\scriptsize{H}-\overset{\overset{\Large{H}}|}{\underset{\underset{\Large{H}} |}{C}}-\overset{\overset{\Large{H}}|}{\underset{\underset{\Large{H}} |}{C}}-\overset{\overset{\Large{H}}|}{\underset{\underset{\Large{H}} |}{C}}\!-H}\) are chains of two and three \(\ce{C's}\). The ability of \(\ce{C}\) to make a chain of atoms is called catenation .
- A \(\ce{C}\) atom can make bonds with more than two \(\ce{C's}\) resulting in branched compounds that increases the number of compounds possible, e.g. four \(\ce{C's}\) molecule can be in a straight chain as in n-butane: \(\ce{\scriptsize{H}-\overset{\overset{\Large{H}}|}{\underset{\underset{\Large{H}} |}{C}}-\overset{\overset{\Large{H}}|}{\underset{\underset{\Large{H}} |}{C}}-\overset{\overset{\Large{H}}|}{\underset{\underset{\Large{H}} |}{C}}-\overset{\overset{\Large{H}}|}{\underset{\underset{\Large{H}} |}{C}}\!-H}\) or in a branched chain as in isobutane: \(\ce{\scriptsize{H}-\overset{\overset{\Large{H}}|}{\underset{\underset{\Large{H}} |}{C}}-\!\overset{\overset{\Large{H-\overset{\overset{\Large{H}}|}{C}\!-H}}|}{\underset{\underset{\Large{H}} |}{C}}\!-\overset{\overset{\Large{H}}|}{\underset{\underset{\Large{H}} |}{C}}\!-H}\).
- Two \(\ce{C's}\) can make single, double, or triple bonds with each other allowing more variations. For example, ethane (\(\ce{\scriptsize{H}-\overset{\overset{\Large{H}}|}{\underset{\underset{\Large{H}} |}{C}}-\overset{\overset{\Large{H}}|}{\underset{\underset{\Large{H}} |}{C}}\!-H}\)) has all single bonds; ethene ( \(\ce{\scriptsize{H}-{\underset{\underset{\Large{H}} |}{C}}={\underset{\underset{\Large{H}} |}{C}}\!-H}\) ) as double bond; and acetylene ( \(\ce{\scriptsize{H-C≡C-H}}\)) has a triple bond between \(\ce{C's}\).
- \(\ce{H}\) is the lightest monovalent atom that can occupy the valencies of \(\ce{C'}\) not used in \(\ce{C}\) to \(\ce{C}\) bonds.
- \(\ce{H}\) bonded with a strongly electronegative atom like \(\ce{O}\) or \(\ce{N}\) can interact with a \(\ce{O}\) or \(\ce{N}\) atom of a neighboring molecule through hydrogen bonding that plays a vital role in the functioning of organic molecules in living things.
- \(\ce{C}\) or \(\ce{H}\) in an organic compound can be replaced with a heteroatom that tremendously increases the variety of organic compounds available to living things.
Some other elements have the desired characteristics of \(\ce{C}\) and \(\ce{H}\) but are associated with significant disadvantages. For example, silicon (\(\ce{Si}\)), like \(\ce{C}\), makes i) four bonds, ii) straight and branched chains, and iii) a single bond with hydrogen. The drawbacks of \(\ce{Si}\) are i) it is two times heavier than \(\ce{C}\), ii) it makes weaker unstable \(\ce{Si-Si}\), and \(\ce{Si-H}\) bonds compared to \(\ce{C-C}\) and \(\ce{C-H}\) bonds, and iii) its oxidation product is solid silicon dioxide (\(\ce{SiO2}\)) which is insoluble in water and would have been difficult to excrete than the gaseous \(\ce{CO2}\) from the \(\ce{C}\) compounds which are easier to exhale. Similarly, halogens are monovalent like hydrogen, but halogens are significantly heavier than \(\ce{H}\) and make weaker bonds with \(\ce{C}\) than \(\ce{C-H}\) bonds.
Organic chemistry is the study of the properties and reactions of organic compounds.
The following section describes chemical bonding that determines the compound's physical properties and chemical reactivities.