Introduction to Organic Compounds: Structures, Functional Groups, and Isomerism

Chapter 4: Introduction to Organic Compounds

4.1 Know Your Structures

Organic compounds are primarily composed of carbon and hydrogen, with possible inclusion of other elements such as oxygen, nitrogen, sulfur, and phosphorus. These compounds form the basis of biomolecules essential for life, including proteins, carbohydrates, lipids, and DNA.

• Organic Compounds: Molecules containing carbon atoms bonded to hydrogen, often with other elements.

• Biomolecules: Large organic molecules found in living organisms (e.g., proteins, carbohydrates, lipids, DNA).

4.1 Drawing Chemical Structures

Several conventions are used to represent organic molecules, each providing different levels of detail:

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

• Condensed Structural Formula: Lists all atoms, grouping hydrogens with their attached carbons (e.g., CH3CH3).

• Lewis Structure: Displays all atoms and bonds, including lone pairs.

• Skeletal Structure: Uses lines for carbon-carbon bonds; hydrogens attached to carbons are implied, not shown.

How to Draw Skeletal Structures:

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

2. Draw the bonds between the carbons (the carbon skeleton).

3. Add bonds to noncarbon atoms (heteroatoms).

4.2 Alkanes: The Simplest Organic Compounds

Alkanes are saturated hydrocarbons containing only single bonds between carbon atoms. They are nonpolar and can exist as straight chains or rings (cycloalkanes).

• General Formula: $C_nH_{2n+2}$

• Straight-Chain Alkanes: Unbranched chains of carbon atoms.

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

• 5- and 6-membered rings are most common in nature.

• Alkanes are commonly used as fuels (e.g., methane, propane, octane).

4.3 Functional Groups

Functional groups are specific groups of atoms within molecules that determine the chemical properties and reactivity of organic compounds. Elements other than carbon and hydrogen in organic molecules are called heteroatoms.

• Heteroatoms: Atoms such as nitrogen, oxygen, or sulfur in organic compounds.

• Functional Group: A group of atoms bonded in a particular way, giving the molecule characteristic properties.

• Organic compounds are classified into families based on their functional groups (e.g., alcohols, amines, carboxylic acids).

Alkenes (Unsaturated Hydrocarbons)

• Contain at least one carbon–carbon double bond.

• General formula for simple alkenes: $C_nH_{2n}$

• More reactive than alkanes due to the double bond.

• Undergo addition reactions where the double bond is broken and new atoms are added.

Terpenes

• Compounds containing at least one carbon–carbon double bond and a multiple of five carbons (e.g., 10, 15, 20).

• Examples: β-carotene, D-limonene, α-pinene.

• Serve as precursors for cholesterol, testosterone, and estrogen.

Alkynes

• Contain one or more carbon–carbon triple bonds.

• General formula for simple alkynes: $C_nH_{2n-2}$

• Triple bond is shorter and stronger than a double bond.

• Alkynes are more reactive than alkenes and are rare in nature.

Aromatics

• Compounds with a cyclic structure like benzene.

• Benzene is stabilized by resonance; electrons are delocalized over the ring.

• Resist reactions that would break the aromatic ring.

• Polycyclic aromatic hydrocarbons (PAHs) contain two or more fused benzene rings.

Fatty Acids (Lipids)

• Fatty acids are long-chain carboxylic acids (12–22 carbons).

• Can be saturated (no double bonds), monounsaturated (one double bond), or polyunsaturated (two or more double bonds).

• Most naturally occurring fatty acids have even numbers of carbon atoms.

4.4 Nomenclature of Simple Alkanes

Organic compounds are named systematically using the IUPAC system. Branched-chain alkanes have carbon atoms not all in a single continuous chain.

• The name of an organic compound has three basic parts: prefix (substituents), parent (main chain), and suffix (functional group).

How to Name a Branched-Chain Alkane:

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

2. Identify groups bonded to the main chain but not included in it (substituents, usually alkyl groups).

3. Number the parent chain from the end nearest a substituent.

4. Assign a number to each substituent, list substituents in alphabetical order, and use Greek prefixes (di-, tri-, tetra-) for multiples (ignore these prefixes when alphabetizing).

The Four Simplest Alkyl Substituents

Alkyl Group	Formula
Methyl	-CH3
Ethyl	-C2H5
Propyl	-C3H7
Butyl	-C4H9

4.5 Isomerism in Organic Compounds

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

Structural Isomers

• Same molecular formula, different connectivity of atoms.

• Example: Butane (C4H10) can be n-butane or isobutane.

Conformational Isomers (Conformers)

• Same connectivity, different spatial arrangement due to rotation around single bonds.

• Not different compounds, but different representations of the same molecule.

Cis-Trans Stereoisomers

• Occur due to restricted rotation (e.g., in cycloalkanes or alkenes).

• Cis isomer: Substituents on the same side of the ring or double bond.

• Trans isomer: Substituents on opposite sides.

Cis-Trans Stereoisomers of Alkenes

• Double bonds prevent rotation, leading to distinct cis and trans forms.

Unsaturated Fatty Acids and Omega Designations

• Unsaturated fatty acids often contain cis double bonds.

• Omega (ω) system numbers carbons from the methyl end; the number indicates the position of the first double bond.

• Essential fatty acids (e.g., linoleic and α-linolenic acids) must be obtained from the diet.

Stereoisomers – Enantiomers

• Enantiomers are nonsuperimposable mirror images of each other.

• Example: The two forms of limonene (one smells like oranges, the other like turpentine).

Chiral Centers

• A chiral center is a tetrahedral carbon atom bonded to four different groups.

• Chiral molecules are not superimposable on their mirror images (like left and right hands).

How to Identify Chiral Centers:

1. Locate tetrahedral carbons (four single bonds).

2. Check if all four groups attached are different (consider the entire group, not just the first atom).

3. Mark chiral centers (often with an asterisk).

Chirality in Pharmaceuticals

• Only one enantiomer of a drug may be biologically active; the other can be inactive or harmful.

• Example: Thalidomide—one enantiomer is therapeutic, the other is teratogenic (causes birth defects).

Example: The thalidomide tragedy illustrates the importance of chirality in drug design and safety.

Additional info: The systematic study of organic compounds and their isomerism is foundational for understanding biochemistry, pharmacology, and molecular biology.