Intermolecular forces between molecules are much weaker than the forces that hold atoms together (i.e., intramolecular bonds).
A typical carbon–carbon single bond has an energy of about
E_{C-C}^{\text{(single)}} \approx 100\ \text{kcal/mol}.
The intermolecular forces between molecules are weaker, on the order of less than
10\ \text{kcal/mol}. (Even the strongest hydrogen bonds are still much weaker than a typical covalent bond like a C–C single bond.)
Hydrogen bonds represent the strongest of the intermolecular interactions.
When two molecules approach, they can attract or repel one another; initially, most interactions are attractive.
If we keep pushing the molecules closer together, eventually the electron clouds begin to resist and repel one another; this short-range repulsion is tied to the electron clouds.
As two molecules come together, they first experience attraction (due to dispersion, dipole interactions, etc.), and at very short distances electron-electron repulsion becomes significant, leading to a balance that defines a preferred separation.
The concept of Van der Waals radii arises from this balance: beyond a certain proximity, electron clouds increasingly repel, limiting how close molecules can approach.
The First Class of Organic Molecules: Alkanes
These are described as the simplest, least exciting class of organic molecules in this context.
General formula for alkanes:
C{n}H{2n+2}.
They are saturated with hydrogen; you cannot add more hydrogens to saturated alkanes.
This saturation implies that all carbon–carbon bonds are single bonds (no multiple bonds) and there are no additional hydrogens that can be substituted without breaking the saturated framework.
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Real-world relevance and implications (contextual notes):
Alkanes are nonpolar and relatively unreactive under mild conditions.
They serve as fuels and solvents and form the basis for many hydrocarbon structures in organic chemistry.
Their properties (boiling points, melting points, volatility) are influenced by chain length and branching, which relate to van der Waals interactions discussed above.
Connections and Concepts
Foundational idea: strength hierarchy of forces influences physical properties such as boiling/melting points, solubility, and phase behavior.
Van der Waals forces, dipole-induced dipole interactions, and hydrogen bonding collectively determine how molecules interact at a distance and at contact.
Saturation concept (as seen with alkanes) is a foundational principle in organic chemistry that affects reactivity and bonding patterns.
Practical implications include why hydrogen bonding leads to higher boiling points in water compared to hydrocarbons of similar size, and why alkanes are relatively inert but burn readily as fuels.
Quick Reference (key numbers and formulas)
Covalent C–C single bond energy: E_{C-C}^{(single)} \approx 100\ \text{kcal/mol}
Intermolecular forces energy scale (typical): E_{\text{intermolecular}} \lesssim 10\ \text{kcal/mol}
General alkane formula: C{n}H{2n+2}
Conceptual note: van der Waals radii define the distance where electron clouds start to repel, contributing to the short-range repulsion between molecules.
Summary
Intermolecular forces are significantly weaker than the bonds that hold atoms together, yet they govern how molecules interact, condense, and influence macroscopic properties.
Hydrogen bonds are the strongest among these intermolecular interactions, though still weaker than covalent bonds.
Alkanes exemplify saturated hydrocarbons with formula C{n}H{2n+2}, characterized by single bonds and maximal hydrogen content for a given carbon skeleton.