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 other elements such as oxygen, nitrogen, sulfur, and phosphorus. Biomolecules like proteins, carbohydrates, lipids, and DNA are all examples of organic compounds. Understanding how to represent these compounds is fundamental in organic chemistry.

• Molecular Formula: Shows the number of each type of 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: Illustrates all atoms, bonds, and lone pairs, providing complete connectivity.

• Skeletal Structure: Uses lines to represent bonds between carbon atoms; hydrogen atoms bonded to carbon are implied, while bonds to other atoms are explicitly shown.

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 saturated hydrocarbons, meaning they contain only single bonds between carbon atoms and are fully saturated with hydrogen. They are nonpolar compounds due to the similar electronegativities of carbon and hydrogen.

• Straight-Chain Alkanes: Continuous, unbranched chains of carbon atoms. The names of alkanes are based on the number of carbon atoms (e.g., methane, ethane, propane, butane).

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

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

• Combustion: Alkanes react with oxygen to produce carbon dioxide and water.

Example: The molecular formula C4H8 corresponds to cyclobutane or butene; C5H12 is pentane; C7H16 is heptane; C6H12 is cyclohexane or hexene.

4.3 Families of Organic Compounds—Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Organic compounds are classified into families based on their functional groups.

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

• Hydrocarbon Families: Alkanes, alkenes (contain double bonds), alkynes (contain triple bonds), and aromatics (contain benzene rings).

• Carbonyl Group: A carbon double-bonded to oxygen, found in several families.

• R Group: Used to represent the "rest" of the molecule, simplifying structural representations.

Unsaturated Hydrocarbons:

• Alkenes: Contain carbon–carbon double bonds; more reactive than alkanes.

• Alkynes: Contain carbon–carbon triple bonds; even more reactive than alkenes.

• Aromatics: Cyclic compounds with resonance-stabilized double bonds (e.g., benzene).

Fatty Acids: Long-chain hydrocarbons with a carboxylic acid group. Saturated fatty acids have no double bonds; monounsaturated have one; polyunsaturated have two or more.

Dietary Importance: Fats serve as insulators and protect internal organs. The FDA recommends that most fat intake should come from mono- and polyunsaturated fatty acids.

4.4 Nomenclature of Simple Alkanes

Naming organic compounds follows rules set by the International Union of Pure and Applied Chemistry (IUPAC) to ensure each compound has a unique name. Branched-chain alkanes and haloalkanes are named systematically.

• Parent Chain: The longest continuous chain of carbon atoms.

• Substituents: Groups attached to the main chain but not part of it; named as alkyl groups (e.g., methyl, ethyl).

• Numbering: Carbons are numbered from the end nearest a substituent; substituents are listed in alphabetical order.

• Haloalkanes: Alkanes with halogen substituents (fluoro, chloro, bromo, iodo).

• Cycloalkanes: The ring is the parent chain; substituents are numbered to give the lowest possible combination.

4.5 Isomerism in Organic Compounds

Isomers are molecules with the same molecular formula but different arrangements of atoms. Isomerism is a key concept in organic chemistry, affecting both physical and chemical properties.

• Structural Isomers: Different connectivity of atoms.

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

• Stereoisomers: Same connectivity, different spatial arrangement; includes cis-trans isomers and enantiomers.

• Cis-Trans Isomerism: Occurs in cycloalkanes and alkenes due to restricted rotation; cis (same side), trans (opposite sides).

• Chiral Molecules and Enantiomers: Nonsuperimposable mirror images; contain a chiral center (carbon bonded to four different groups).

Biological Relevance: Only one enantiomer may be biologically active; the other can be inactive or harmful (e.g., thalidomide).

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.

• Identify and characterize functional groups in organic molecules.

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

• Distinguish structural, conformational, and stereoisomers; identify cis/trans isomers and chiral centers.