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12.1: The Nature of Organic Molecules

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    Learning Objectives
    • Describe the basic structural properties of simple organic molecules.

    Organic chemistry is the study of the chemistry of carbon-containing compounds. Carbon is singled out because it has a chemical diversity unrivaled by any other chemical element. Its diversity is based on the following:

    • Carbon atoms bond reasonably strongly with other carbon atoms.
    • Carbon atoms bond reasonably strongly with atoms of other elements.
    • Carbon atoms make a large number of covalent bonds (four).

    Curiously, elemental carbon is not particularly abundant. It does not even appear in the list of the most common elements in Earth’s crust. Nevertheless, all living things consist of organic compounds. Most organic chemicals are covalent compounds, which is why we introduce organic chemistry here. By convention, compounds containing carbonate ions and bicarbonate ions, as well as carbon dioxide and carbon monoxide, are not considered part of organic chemistry, even though they contain carbon.

    Structural Properties of Carbon Compounds

    A carbon atom has four valence electrons, it is tetravalent. Carbon can form four covalent bonds, or share electrons with up to four atoms in order to gain a complete octet. The simplest carbon compounds contain only carbon and hydrogen and are called hydrocarbons. Methane, the simplest hydrocarbon, contains a single carbon with four covalently bonded hydrogen atoms. Recalling what you have learned about molecular structures and VSEPR, we know that methane is tetrahedral (four electron groups and no lone pairs).

    Figure \(\PageIndex{1}\): The Tetrahedral Methane Molecule

    Carbon can also form double bonds by sharing four electrons with a neighboring carbon atom or triple bonds by sharing six electrons with a neighboring carbon atom. As shown in Figure \(\PageIndex{2}\) below, carbon with three electron groups attached will be trigonal planar, and carbon with two electron groups attached will be linear.


    Figure \(\PageIndex{2}\): Two carbons can be attached together in single bond, a double bond, or a triple bond. Notice, in each example carbon makes four total bonds. The number of hydrogen atoms in each molecule decreases as the number of carbon–carbon bonds increase.

    Simple hydrocarbon compounds are nonpolar due to the shape and the small electronegativity difference between carbon and hydrogen atoms. When carbon is bonded to a halogen or oxygen atom, the resulting bond is polar. It may be useful to review the section on electronegativity and polarity of bonds and molecules to be able to describe the properties of different organic compounds, specifically how they react and interact with other molecules.

    Comparing Organic and Inorganic Compounds

    Organic compounds, like inorganic compounds, obey all the natural laws. Often there is no clear distinction in the chemical or physical properties among organic and inorganic molecules. Nevertheless, it is useful to compare typical members of each class, as in Table \(\PageIndex{1}\). Keep in mind, however, that there are exceptions to every category in this table.

    Table \(\PageIndex{1}\): Contrasting Properties and Examples of Organic and Inorganic Compounds
    Organic Properties Example: Hexane Inorganic Properties Example: NaCl
    low melting points −95°C high melting points 801°C
    low boiling points 69°C high boiling points 1,413°C
    low solubility in water; high solubility in nonpolar solvents insoluble in water; soluble in gasoline greater solubility in water; low solubility in nonpolar solvents soluble in water; insoluble in gasoline
    flammable highly flammable nonflammable nonflammable
    aqueous solutions do not conduct electricity nonconductive aqueous solutions conduct electricity conductive in aqueous solution
    exhibit covalent bonding covalent bonds exhibit ionic bonding ionic bonds

    This page titled 12.1: The Nature of Organic Molecules is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by Lisa Sharpe Elles.