Organic Chemistry Lecture Notes: Octet, Bonding, Saturated Hydrocarbons, and Isomers

Octet Rule and Stability

Stability is discussed in terms of having eight electrons around an atom (the octet rule). The speaker highlights that when there are eight electrons, stability is achieved.
For carbon and related atoms, achieving an octet is a driving principle for structure and bonding.

Orbitals, Hybridization, and Bond Angles

The concept of orbitals and hybridization is introduced through statements like “sp3 means …” and references to neutral vs charged states. The idea is that carbon with four sigma bonds uses sp3 hybridization, leading to a tetrahedral geometry.
Bond angle for sp3-hybridized carbon is approximately heta \,\approx\ 109.5^{\circ} (often rounded to 109°) for the bonds around a carbon atom.
For a single carbon with four substituents (e.g., in methane, CH$_4$), the angle between any two C–H bonds is about this value due to tetrahedral geometry.

Charge Distribution, Resonance, and Electronegativity

The discussion includes how negative charge can move (electron pushing) and how that affects which bonds are formed or broken in reactions. This is tied to resonance concepts where electrons delocalize.
When considering where negative or positive charges reside, structures place charges on atoms in a way that is more favorable (e.g., negative charge on more electronegative atoms).
Electronegative comparisons mentioned (in context of where negative charge resides): Oxygen is generally more electronegative than nitrogen; placing negative charge on the more electronegative atom is more stable.
The idea of comparing resonance forms (e.g., which structure is better) is tied to the distribution of charge and the electronegativity of the atoms involved.

Saturated Hydrocarbons and Alkanes

Definition: Saturated hydrocarbons are those with only single bonds (no pi bonds). They are also called alkanes.
Each carbon forms four sigma bonds (sp$^3$ hybridization), satisfying the valence of carbon.
General formula for acyclic alkanes: CnH{2n+2}. For example, a chain with $n$ carbons has 2n+2 hydrogens.
The term “saturated” implies maximum hydrogens for a given carbon skeleton (no multiple bonds).

Nomenclature, Homologous Series, and Examples

The alkane family is a homologous series: each successive member differs by a
-\mathrm{CH_2-}\ unit.
Common names in order: methane, ethane, propane, butane, pentane, hexane, heptane, octane, …
For cyclic hydrocarbons, the name prefix changes: add "cyclo-" (e.g., cyclopropane, cyclohexane).
A cycloalkane obeys a different hydrogen count than the acyclic alkane (e.g., C$n$H${2n}$ for cycloalkanes).

Structural Isomers (Constitutional Isomers) vs. The Same Formula

Structural (constitutional) isomers have the same molecular formula but different connectivity of atoms.
Example discussion: with five carbons, a structure can have one branch (a methyl group) or be linear; both have the same formula but different connectivity.
A five-carbon example: C$5$H${12}$ has multiple constitutional isomers (e.g., n-pentane vs. isopentane/2-methylbutane). The statements emphasize that these isomers have different connectivities but the same formula.

Boiling Points, Intermolecular Forces, and Increasing Carbon Count

Boiling point rises as the number of carbon atoms increases in saturated hydrocarbons.
Reason: larger molecules have greater surface area and stronger London dispersion forces, which makes intermolecular attractions harder to overcome during phase change.
In general, increasing chain length (more CH$_2$ groups) strengthens van der Waals attractions, raising boiling points.

Natural Gas Composition and Methane

Natural gas composition includes methane as a major component: approximately 30% methane in some samples.
Methane is a saturated hydrocarbon (alkane) with formula ext{CH}_4 and is the simplest alkane.
The fact that methane is a major component in natural gas is tied to its gaseous state at room temperature and its relatively simple, small-molecule structure.

Bond Rotation, Conformations, and Newman Projections (Conceptual)

Single bonds in alkanes allow rotation around the C–C axis, leading to different conformations.
Conceptual drawing often involves depicting a carbon with one hydrogen toward you and one hydrogen away, illustrating three-dimensional geometry around a tetrahedral center.
In more complex chains (e.g., butane or larger), rotation about C–C single bonds leads to staggered and eclipsed conformations, with staggered generally being lower in energy.

Rotation Around a Three-Carbon Fragment (General Idea)

The notes reference rotating around a chain segment (e.g., three carbons) to explore different conformations and the resulting spatial arrangement of hydrogens.
This supports understanding of conformational isomerism and how steric effects influence stability and reactivity.

Geometric Considerations and Examples

Methane: ext{CH}_4, tetrahedral with bond angles ~109.5^ ext{o}.
Ethane: ext{C}2 ext{H}6; rotation about the C–C single bond leads to various conformations; no pi bonds, so no restriction on rotation.
Propane: ext{C}3 ext{H}8; similar conformational considerations about C–C rotation apply.
General trend: as carbon count increases, dispersion forces increase, raising boiling points generally.

Quick Reference Formulas and Naming Rules

Alkane family (acyclic saturated hydrocarbons): CnH{2n+2}
Cycloalkanes (cyclic saturated hydrocarbons): CnH{2n}
For cyclic names, prefix with "cyclo-" (e.g., cyclopropane, cyclohexane).
Homologous series: each additional \mathrm{CH_2} unit changes formula by \Delta C = 1, \Delta H = +2, i.e., adds one carbon and two hydrogens.
Structural (constitutional) isomers have the same formula but different connectivity; examples include branched vs unbranched pentane isomers.

Summary of Practical Implications

Octet stability drives bonding patterns and molecular geometry in organic molecules.
Hybridization (sp, sp$^2$, sp$^3$) determines geometry and reactivity: sp$^3$ yields tetrahedral geometry and single bonds; sp$^2$ and sp enable multiple bonds and planarity.
Electronegativity influences charge distribution, resonance stability, and the preferred placement of charges in resonance forms.
Saturated hydrocarbons (alkanes) are relatively unreactive compared to unsaturated hydrocarbons, and their physical properties (boiling points, melting points) scale with molecular size due to dispersion forces.
Structural isomers illustrate how connectivity, not just formula, changes properties and reactivity.
Real-world relevance: natural gas composition and chemical behavior of alkanes underpin industrial processes, fuels, and materials science.

Key Takeaways for Exam Preparation

Remember the octet rule as a stability criterion for main-group elements like carbon.
Know that sp$^3$ hybridization leads to tetrahedral geometry with ~109.5^ ext{o} bond angles.
Be able to differentiate alkanes (saturated, single bonds) from alkenes/alkynes (unsaturated, multiple bonds) and recall the general formulas.
Memorize the alkane naming sequence up to octane at least, and understand how cycloalkanes are named with the cyclo- prefix.
Understand that increasing the carbon chain generally increases boiling point due to stronger dispersion forces, and that branched isomers can have different boiling points.
Recognize constitutional isomers as same formula, different connectivity; use examples like pentane isomers to illustrate.
Be familiar with the concept that charges prefer placement on more electronegative atoms in resonance forms (e.g., O is more electronegative than N).
Have a basic grasp of conformational analysis for single bonds (e.g., staggered vs eclipsed) and how rotation around C–C bonds leads to different conformations.