Chapter 4: Introduction to Organic Compounds

4.1 Representing the Structures of Organic Compounds

Organic compounds are primarily composed of carbon and hydrogen, with possible inclusion of oxygen, nitrogen, sulfur, phosphorus, and other elements. Biomolecules such as proteins, carbohydrates, lipids, and DNA are all organic compounds. Understanding how to represent their structures is fundamental in organic chemistry.

• Molecular Formula: Shows the number of each atom in a molecule (e.g., C2H6).

• Condensed Structural Formula: Displays all atoms but minimizes the number of bonds shown; lone pairs may or may not be included.

• Lewis Structure: Shows all atoms, bonds, and lone pairs, providing complete connectivity.

• Skeletal Structure: Simplifies organic molecules by showing only the carbon skeleton as lines; hydrogen atoms bonded to carbon are implied, while bonds to other atoms are explicitly shown.

Rules for Drawing Skeletal Structures:

• Bonds between carbon atoms are shown as lines.

• Bonds to hydrogen are not shown but are implied.

• Other elements bonded to carbon are shown using their chemical symbol.

• Hydrogens bonded to non-carbon atoms are shown.

• Lone pairs are not depicted.

Steps for Drawing Skeletal Structures:

1. Determine the number of carbons connected end to end.

2. Draw the carbon skeleton.

3. Add bonds to noncarbon atoms.

4.2 Alkanes: The Simplest Organic Compounds

Alkanes are the simplest organic compounds, consisting only of single-bonded carbon and hydrogen atoms. They are classified as saturated hydrocarbons, meaning each carbon atom is bonded to the maximum number of hydrogen atoms.

• Hydrocarbon: Compound made entirely of hydrogen and carbon.

• Saturated: Contains only single bonds between carbon atoms.

• General Formula for Alkanes: , where n is the number of carbon atoms.

• Straight-chain Alkanes: Continuous, unbranched chains of carbon atoms.

• Cycloalkanes: Ring-form alkanes; named by adding the prefix "cyclo" to the alkane name (e.g., cyclopropane).

Properties of Alkanes:

• Alkanes are nonpolar due to similar electronegativities of carbon and hydrogen.

• Nonpolar nature affects their solubility in water (water is polar).

• Alkanes are used as fuel sources and undergo combustion to produce carbon dioxide and water.

Combustion Reaction of Alkanes:

4.3 Families of Organic Compounds—Functional Groups

Organic compounds are classified into families based on their functional groups, which are specific groups of atoms bonded in a particular way. Functional groups determine the chemical properties and reactivity of organic molecules.

• Heteroatoms: Elements other than carbon and hydrogen present in organic compounds.

• Functional Group: The reactive part of an organic molecule; often denoted by "R" for the rest of the molecule.

Common Functional Groups:

• Alkanes: Only single bonds (hydrocarbons).

• Alkenes: Contain carbon–carbon double bonds; unsaturated hydrocarbons.

• Alkynes: Contain carbon–carbon triple bonds; unsaturated hydrocarbons.

• Aromatics: Cyclic compounds with alternating double bonds (e.g., benzene); unusually stable due to resonance.

• Carbonyl Group: Carbon double-bonded to oxygen; found in several families.

Examples and Applications:

• Terpenes (alkenes) are precursors to cholesterol, testosterone, and estrogen.

• Polycyclic aromatic hydrocarbons (PAHs) are found in tobacco smoke and some plastics; some are carcinogenic.

• Pharmaceuticals often contain specific functional groups.

• Lipids (fatty acids) are hydrocarbons; saturated fatty acids have no double bonds, monounsaturated have one, polyunsaturated have two or more.

Dietary Importance: Fats are essential for insulation and protection; FDA recommends most fat intake from mono- and polyunsaturated fatty acids.

4.4 Nomenclature of Simple Alkanes

Naming organic compounds follows systematic rules established by the International Union of Pure and Applied Chemistry (IUPAC). Alkanes can be straight-chain, branched-chain, or cyclic (cycloalkanes).

• Branched-Chain Alkanes: Not all carbon atoms are in a single continuous chain.

• Three Parts of Alkane Names: Prefix (number of carbons), infix (type of bonds), suffix (functional group).

Steps for Naming Branched-Chain Alkanes:

1. Find the longest continuous chain (parent chain) and name it.

2. Identify groups bonded to the main chain (substituents, usually alkyl groups).

3. Number the parent chain starting at the end nearest a substituent.

4. Assign numbers to substituents and list them alphabetically; use Greek prefixes (di-, tri-, tetra-) for multiples.

Haloalkanes: Alkanes with halogen substituents (fluoro, chloro, bromo, iodo); named using the same rules as branched-chain alkanes.

Cycloalkanes: The ring is the parent name if it has more carbons than any substituent. Number the ring to give the lowest possible combination of numbers to substituents.

4.5 Isomerism in Organic Compounds

Isomerism refers to compounds with the same molecular formula but different arrangements of atoms. Isomers can be structural, conformational, or stereoisomers.

• Structural Isomers: Same molecular formula, different connectivity of atoms.

• Conformational Isomers (Conformers): Same connectivity, different spatial arrangement due to rotation about single bonds.

• Stereoisomers: Same molecular formula and connectivity, different spatial arrangement.

Cis–Trans Isomerism:

• Occurs in cycloalkanes and alkenes due to restricted rotation.

• Cis Isomer: Similar groups on the same side.

• Trans Isomer: Similar groups on opposite sides.

• Double bonds are rigid and do not allow free rotation.

Chiral Molecules and Enantiomers:

• Chiral: Objects or molecules that are nonsuperimposable mirror images (e.g., hands, shoes).

• Enantiomers: Pairs of chiral molecules that are mirror images.

• Chiral center: Tetrahedral carbon bonded to four different groups.

• Biological receptors are "handed"; only one enantiomer may be biologically active.

• Example: Thalidomide—one enantiomer is therapeutic, the other is teratogenic.

Steps to Identify Chiral Carbons:

1. Locate tetrahedral carbons.

2. Check if four groups attached are different.

3. Assign chiral centers (often marked with an asterisk).

Chapter 4 Study Guide

• Convert between Lewis, condensed, and skeletal structures.

• Characterize simple alkanes and define saturated/unsaturated hydrocarbons.

• Name the first ten straight-chain alkanes.

• Compare molecular formulas for straight-chain alkanes and cycloalkanes.

• Draw and name branched-chain alkanes, haloalkanes, and cycloalkanes using IUPAC rules.

• Identify and characterize functional groups in organic molecules.

• Distinguish structural, conformational, and stereoisomers (cis/trans, chiral centers).