2.5 Overview of Organic Compounds

Hydroxyl R─O─H

Alcohols contain an ─OH group, which is polar and hydrophilic due to its electronegative O atom. Molecules with many ─OH groups dissolve easily in water.

Aldehydes

Aldehydes have a carbonyl group at the end of the carbon skeleton.

Functional Groups

Also attached to the carbon skeleton are distinctive functional groups, other atoms or molecules bound to the hydrocarbon skeleton. Each type of functional group has a specific arrangement of atoms that confers characteristic chemical properties on the organic molecule attached to it. Table 2.5 lists the most common functional groups of organic molecules and describes some of their properties. Because organic molecules often are big, there are shorthand methods for representing their structural formulas. Figure 2.13 shows two ways to indicate the structure of the sugar glucose, a molecule with a ring-shaped carbon skeleton that has several hydroxyl groups attached.

Amines

Amines have an ─NH2 group, which can act as a base and pick up a hydrogen ion, giving the amino group a positive charge. At the pH of body fluids, most amino groups have a charge of 1+. All amino acids have an amino group at one end.

Carboxyl

Carboxylic acids contain a carboxyl group at the end of the carbon skeleton. All amino acids have a ─COOH group at one end. The negatively charged form predominates at the pH of body cells and is hydrophilic.

Ester

Esters predominate in dietary fats and oils and also occur in our body as triglycerides. Aspirin is an ester of salicylic acid, a pain-relieving molecule found in the bark of the willow tree.

Ketones

Ketones contain a carbonyl group within the carbon skeleton. The carbonyl group is polar and hydrophilic due to its electronegative O atom.

Polymers

Macromolecules are usually polymers (poly = many; mers = parts). A polymer is a large molecule formed by the covalent bonding of many identical or similar small building block molecules called monomers (mono = one).

Hydrocarbon

Many of the carbons are bonded to hydrogen atoms, yielding a hydrocarbon.

Overview of Organic Compounds

Many organic molecules are relatively large and have unique characteristics that allow them to carry out complex functions. Important categories of organic compounds include carbohydrates, lipids, proteins, nucleic acids, and adenosine triphosphate (ATP). Carbon has several properties that make it particularly useful to living organisms. For one thing, it can form bonds with one to thousands of other carbon atoms to produce large molecules that can have many different shapes. Due to this property of carbon, the body can build many different organic compounds, each of which has a unique structure and function. Moreover, the large size of most carbon-containing molecules and the fact that some do not dissolve easily in water make them useful materials for building body structures. Organic compounds are usually held together by covalent bonds. Carbon has four electrons in its outermost (valence) shell. It can bond covalently with a variety of atoms, including other carbon atoms, to form rings and straight or branched chains. Other elements that most often bond with carbon in organic compounds are hydrogen, oxygen, and nitrogen. Sulfur and phosphorus are also present in organic compounds. The other elements listed in Table 2.1 are present in a smaller number of organic compounds.

Isomers

Molecules with the same molecular formula but different structures. For example, the molecular formulas for the sugars glucose and fructose are both C6H12O6. The individual atoms, however, are positioned differently along the carbon skeleton (see Figure 2.15a), giving the sugars different chemical properties.

Phosphate

Phosphates contain a phosphate group, which is very hydrophilic due to the dual negative charges. An important example is adenosine triphosphate (ATP), which transfers chemical energy between organic molecules during chemical reactions.

Macromolecules

Small organic molecules can combine into very large molecules. (macro = large)

Carbon Skeleton

The chain of carbon atoms in an organic molecule.

Sulfhydryl (Thiols) R─S─H

Thiols have an ─SH group, which is polar and hydrophilic due to its electronegative S atom. Certain amino acids (for example, cysteine) contain ─SH groups, which help stabilize the shape of proteins.

Dehydration Synthesis

Usually, the reaction that joins two monomers is a dehydration synthesis. In this type of reaction, a hydrogen atom is removed from one monomer and a hydroxyl group is removed from the other to form a molecule of water (see Figure 2.15a). Macromolecules such as carbohydrates, lipids, proteins, and nucleic acids are assembled in cells via dehydration synthesis reactions.