Summary
Highlights
Carbon is fundamental to life, forming the backbone of biomolecules like proteins, lipids, carbohydrates, DNA, and RNA. Organic chemistry is defined by the study of carbon-containing compounds, particularly those with at least one hydrogen atom. Carbon's ability to form four covalent bonds makes it incredibly versatile, allowing for the creation of complex molecular structures, unlike nitrogen, oxygen, or hydrogen which form fewer bonds.
When carbon forms only single bonds, molecules adopt a tetrahedral (pyramid-like) shape, as seen in methane (CH4) and ethane (C2H6). However, if carbon atoms are joined by a double bond, the molecule becomes flat or planar, like ethene (C2H4), due to the strain on carbon's orbitals. Carbon can also bond with elements other than hydrogen, such as oxygen in CO2 (which is not organic as it lacks hydrogen) or nitrogen as in urea.
Carbon skeletons can vary in length (e.g., ethane, propane, butane), double bond position (e.g., one-butene vs. two-butene), branching (e.g., butane vs. isobutane), and the presence of rings (e.g., cyclohexane, benzene). Molecules composed solely of carbon and hydrogen are called hydrocarbons, which are typically nonpolar and hydrophobic, meaning they do not mix with water, explaining why fats and oils repel water.
Organic chemists use a shorthand for drawing molecules, especially cyclic structures like cyclohexane and benzene, where vertices represent carbon atoms and hydrogens are assumed to fill the remaining valencies to satisfy carbon's four bonds. This shorthand simplifies complex molecular representations and understanding.
Isomers are compounds with the same molecular formula but different structures and properties. Three types are discussed: structural isomers (different covalent arrangements), cis-trans isomers (same covalent bonds but different spatial arrangement, usually around a double bond in a planar molecule), and enantiomers (mirror images that are non-superimposable, like left and right hands), crucial in pharmaceuticals.
Functional groups are specific groups of atoms attached to the carbon backbone of organic molecules that impart special properties, such as polarity, acidity, or basicity. Examples include hydroxyl, carbonyl, carboxyl, amino, sulfhydryl, phosphate, and methyl groups. These groups significantly influence a molecule's chemical reactions and biological activity.
A hydroxyl group (-OH) makes a molecule an alcohol (suffix -ol) and increases its polarity and ability to mix with water. For example, methane becomes methanol. A carbonyl group (C=O) is also polar and can be either an aldehyde (carbonyl at the end of a chain) or a ketone (carbonyl within a chain). Both types contribute to a molecule's polarity and water solubility.
A carboxyl group (COOH) makes a molecule acidic because it can release a proton (H+) into solution, such as in acetic acid. It can be represented in non-ionized or ionized forms. An amino group (-NH2) is basic; it can absorb a proton from solution. Amino acids, the building blocks of proteins, are named for having both an amino group (basic) and a carboxyl group (acidic).
A sulfhydryl group (-SH) makes a molecule a thiol and is polar. Two sulfhydryl groups can form a strong disulfide bond, important for protein structure, like those found in hair. A phosphate group (PO4) is highly negatively charged, making molecules very polar and able to interact with water, vital in energy storage molecules like ATP and in DNA. A methyl group (-CH3) is nonpolar and contributes to a molecule's hydrophobic nature.
Analyzing estrogen and testosterone reveals how different functional groups (hydroxyl, methyl, carbonyl) dictate their properties and biological roles. Estrogen's hydroxyl groups make it hydrophilic, allowing it to mix with blood, while testosterone also features methyl and a ketone carbonyl group. The presence of phosphate groups in ATP (adenosine triphosphate) is crucial for its function as the cell's energy currency.