Ch 4 Outline Carbon Compounds, Isomerism, and Functional Groups in General Biology

Ch 4

Organic Compounds and Carbon Chemistry

Definition and Scope of Organic Compounds

Organic compounds are traditionally defined as chemical substances containing carbon atoms bonded to hydrogen, often with additional elements such as oxygen, nitrogen, sulfur, and phosphorus. These compounds form the basis of all known life forms.

• Organic compounds include carbohydrates, lipids, proteins, and nucleic acids.

• Some carbon-based substances, such as carbon dioxide (CO2), carbon monoxide (CO), and carbonates, are not considered organic because they lack the typical carbon-hydrogen framework.

Example: Glucose (C6H12O6) is an organic compound, while CO2 is inorganic.

Unique Properties of Carbon

The carbon atom is uniquely suited to form complex molecules due to its ability to make four covalent bonds with other atoms, including other carbon atoms. This property allows for the formation of long chains, branched structures, and rings, which are essential for the diversity of organic molecules.

• Tetravalency: Carbon has four valence electrons, enabling it to form up to four covalent bonds.

• Versatility: Carbon can bond with many elements and form stable, diverse structures.

• Complexity: The ability to form chains and rings leads to a vast array of molecular shapes and functions.

Example: The backbone of DNA is formed by carbon atoms linked in a chain.

Hydrocarbons and Their Variability

Hydrocarbons are organic molecules consisting entirely of carbon and hydrogen. They can vary in length, branching, and the presence of rings.

• Alkanes: Saturated hydrocarbons with single bonds (e.g., methane, ethane).

• Alkenes: Unsaturated hydrocarbons with double bonds (e.g., ethene).

• Alkynes: Unsaturated hydrocarbons with triple bonds (e.g., ethyne).

• Aromatic hydrocarbons: Contain ring structures (e.g., benzene).

Example: Butane (C4H10) can exist as a straight chain or a branched isomer (isobutane).

Sources of Inorganic Carbon

One of the most important sources of inorganic carbon for synthesizing organic molecules is carbon dioxide (CO2). Plants use CO2 during photosynthesis to produce organic compounds.

• Photosynthesis: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

Structural Diversity of Carbon Skeletons

How Carbon Skeletons Vary in Structure

Carbon skeletons can vary in several ways, contributing to the diversity of organic molecules.

• Length: Carbon chains can be short or long.

• Branching: Chains may be unbranched or branched.

• Double Bond Position: Double bonds can be located at different positions along the chain.

• Ring Structures: Carbon atoms can form rings of various sizes.

Example: Pentane (C5H12) can be straight-chained or branched.

Bond-Line Structure Notation

Bond-line (skeletal) structures are simplified representations of organic molecules, showing the carbon backbone as lines and omitting hydrogen atoms bonded to carbon for clarity.

• Each vertex or end of a line represents a carbon atom.

• Hydrogen atoms attached to carbon are not shown explicitly.

Example: The bond-line structure for 2-methylpropane is a three-carbon chain with a branch at the middle carbon.

Isomerism in Organic Molecules

Definition of Isomers

Isomers are compounds with the same molecular formula but different structures and properties.

• Structural isomers: Differ in the covalent arrangement of atoms.

• Cis-trans isomers (geometric isomers): Differ in spatial arrangement around a double bond.

• Enantiomers: Mirror-image isomers that are non-superimposable.

Types of Isomers

• Structural Isomers: Same formula, different connectivity. Example: Butane and isobutane (C4H10).

• Cis-Trans Isomers: Occur due to restricted rotation around double bonds. Example: Cis-2-butene vs. trans-2-butene.

• Enantiomers: Chiral molecules with non-superimposable mirror images. Example: L- and D- forms of amino acids.

Functional Groups in Organic Molecules

Definition and Importance

Functional groups are specific groups of atoms within molecules that confer characteristic chemical properties and reactivity. They play a crucial role in the structure and function of biological molecules.

• Hydroxyl group (-OH): Found in alcohols; makes molecules polar.

• Carbonyl group (C=O): Found in aldehydes and ketones; reactive in many biochemical reactions.

• Carboxyl group (-COOH): Found in acids; acts as an acid by donating H+.

• Amino group (-NH2): Found in amines and amino acids; acts as a base.

• Sulfhydryl group (-SH): Found in thiols; important in protein structure.

• Phosphate group (-PO4): Found in nucleotides; involved in energy transfer.

• Methyl group (-CH3): Nonpolar; affects gene expression.

Functional Groups Table

The following table summarizes major functional groups, their properties, and examples:

Chemical Group	Properties	Examples
Hydroxyl (-OH)	Polar, forms hydrogen bonds, increases solubility	Ethanol
Carbonyl (C=O)	Reactive, found in sugars	Acetone, Propanal
Carboxyl (-COOH)	Acidic, can donate H+	Acetic acid
Amino (-NH2)	Basic, can accept H+	Glycine
Sulfhydryl (-SH)	Forms disulfide bonds, stabilizes proteins	Cysteine
Phosphate (-PO4)	Contributes negative charge, energy transfer	Glycerol phosphate
Methyl (-CH3)	Nonpolar, affects gene expression	5-Methylcytosine

Additional info: The table above is reconstructed from the provided image and standard biology knowledge. Properties and examples are inferred for completeness.