Carbon links form the backbone (skeleton) of organic molecules; the length and shape of carbon chains vary widely.
Examples of carbon chain lengths:
Ethane: C<em>2H</em>6
Propane: C<em>3H</em>8
Some chains have 4, 5, 6, up to 20, up to 60 carbons.
Chains can be linear or branched; branching can create shapes like an upside-down T (e.g., 2-methylpropane).
Chains can contain double bonds in addition to single bonds; some chains contain two double bonds, and the position of a double bond can vary, changing the shape.
Rings can form, such as cyclohexane: C<em>6H</em>12, and aromatic rings with alternating double bonds like benzene: C<em>6H</em>6.
Rings can fuse together, creating more complex four-ring structures (e.g., the steroid hormones estradiol and testosterone).
The difference between molecules like estradiol and testosterone often lies in their functional groups (the specific groups attached to the carbon skeleton). These functional groups determine their unique functions.
Skeletal structures (or skeletal formulas) are simplified representations of molecules, omitting most hydrogens and focusing on the carbon skeleton.
Skeletal structures: rules and practice
Four basic rules for drawing skeletal structures (skeletal formulas):
Rule 1: Each terminal end and the apex of a triangle (or vertex in a line drawing) represent a carbon.
Rule 2: Hydrogens are not drawn, but carbons are assumed to have four bonds; thus each carbon has enough hydrogens to satisfy four bonds.
Rule 3: If a functional group is present, it must be written out (not just implied by the skeleton).
Rule 4: In rings, every apex represents a carbon.
Example: Ethanol
Skeletal form uses a simple arrangement (terminal end and apex) to represent the two carbons; hydrogens are implicit, with each carbon making four bonds.
The functional group hydroxyl (–OH) is written out explicitly in the skeletal representation of ethanol.
Practice (skeletal drawings):
Ethane: two carbons; a single line representing the bond between them; each terminal end is a carbon.
Propane: three carbons; triangular arrangement where the two terminal carbons are connected to the middle carbon.
Butene: four carbons with a double bond placed between carbons 2 and 3 in the zigzag; indicate the double bond visually.
Butane: four carbons in a zigzag with no double bonds.
Branched hydrocarbon: show a branch coming off the main chain (example given shows a carbon pointing upward to indicate a branch).
Visual shorthand: zigzag lines and ring shapes communicate the skeleton; each vertex or apex = a carbon; hydrogens are implied.
Looking ahead: large molecules like fats will be discussed later (chapter 5); space-filling models show a different perspective (glycerol backbone in red, hydrocarbon chains in black/gray).
Hydrocarbons and fats: composition, structure, and energy
Hydrocarbons are organic molecules consisting only of carbon and hydrogen.
Many organic molecules, including fats, contain hydrocarbon components.
A fat molecule (lipid) is described as having:
A glycerol backbone (red in the illustration).
Hydrocarbon chains (often shown in black/gray in the space-filling model).
A fatty-acid component (blue in the illustration is described as the fatty acid).
Fats are high-energy molecules because their hydrocarbon chains store a lot of chemical energy; these chains can release energy when broken or oxidized.
In the illustrated fat molecule, the hydrocarbon chains can be cleaved two carbons at a time to release energy stored in the fat.
Distribution in the body:
Fat molecules are stored in adipose cells (see the diagram showing adipose tissue).
When energy is plentiful, adipose cells can divide to store more fat.
Evolutionarily, storing fat allowed humans to endure periods of food scarcity (hunters and gatherers). Fat storage serves as a long-term energy reserve.
Weight management implications:
When there is an excess of fat storage, adipose cells fill with fat and can divide to store more.
If fat intake is restricted, fat cells may deflate, but they do not disappear; they persist, ready to store fat again when energy intake increases.
Consequently, losing weight can be difficult because fat cells can persist and readily reaccumulate fat when energy intake rises again.
A practical takeaway mentioned is to avoid overfeeding, especially in childhood, since early overfeeding can lead to more fat cells that are harder to reduce later.
Models and perspectives:
The presentation uses a simplified skeletal model and a space-filling model to illustrate fat structure and location in adipose tissue.
The discussion emphasizes both the chemical energy provided by hydrocarbon chains and the biological implications of fat storage in humans.
Connections, implications, and takeaways
Concept connections:
Builds on the idea that organic molecules are built from carbon skeletons connected to hydrogen (and other atoms via functional groups).
Introduces skeletal formulas as a practical shorthand used by chemists due to large numbers of atoms in many molecules.
Sets up later chapters to discuss functional groups in more detail and to examine other large organic molecules beyond fats.
Real-world relevance:
Understanding carbon skeletons helps explain why fats store so much energy and how dietary fat relates to body fat storage.
The discussion of adipose tissue and fat storage has practical implications for dieting, weight management, and public health discussions about fat intake.
Ethical and practical implications:
Balancing energy intake and expenditure is key to managing body fat; this has implications for nutrition guidance, healthcare costs, and lifestyle choices.
The material notes that fat storage is an evolutionary advantage but can pose challenges in modern environments with abundant food.
Illustrative notes:
The skeletal structure rules help students rapidly interpret or draw molecular structures, especially for molecules with long carbon chains or rings.
The functional-group concept is introduced via the hydroxyl group (–OH) example, foreshadowing more complex chemistry in later chapters.
Formulas and notations used in this material:
Ethane: C<em>2H</em>6
Propane: C<em>3H</em>8
Butene: C<em>4H</em>8 (with a double bond)
Cyclohexane: C<em>6H</em>12
Benzene: C<em>6H</em>6
Summary takeaway:
Carbon skeletons form diverse organic molecules through varying lengths, shapes, rings, and functional groups.
Skeletal formulas offer a concise depiction, with explicit functional groups, while maintaining implied hydrogen counts.
Fats illustrate how hydrocarbon chains store energy and how body fat storage has both biological and behavioral implications for health and lifestyle.