Intermolecular forces: predict physical properties from structure

MCAT content library / Chemistry & physics / Intermolecular forces: predict physical properties from structure

Explain a property difference by the interactions the structures permit and the variables the comparison actually controls.

About 15 minutes, including the worked examples and checks.

Useful foundations

• Molecular geometry: connect bonding, shape, and polarity

On this page

• Concept: separate a molecule from its neighbors
• Pattern: all molecules have dispersion; only some supply other interactions
• Application: distinguish donation from acceptance
• Worked comparison: isolate an interaction in a stated model
• Changed case: solubility needs both sides of the mixture
• Rule: predict a difference, then name its limits
• Check your reasoning
• Continue learning
• Sources

Concept: separate a molecule from its neighbors

Vaporizing a molecular liquid usually separates intact molecules from one another. It does not require breaking every covalent bond inside those molecules. Therefore, a boiling-point comparison concerns intermolecular attractions and phase behavior, not simply which molecule contains the strongest internal bond. Keep the particle being compared and the physical change explicit.¹

Several interactions can operate at once. Naming hydrogen bonding does not switch off dispersion, and having a permanent dipole does not remove the role of size. A useful prediction starts by listing the interactions available to the actual structures and then asks which differences the comparison controls.

Pattern: all molecules have dispersion; only some supply other interactions

London dispersion arises from correlated fluctuations in electron distributions. It occurs between polar and nonpolar particles. More easily distorted electron clouds are more polarizable and generally support stronger dispersion interactions. Size, electron distribution, and the area over which neighboring molecules can approach all matter. Among comparable nonpolar isomers, a compact shape can reduce intermolecular contact relative to an extended shape.¹²

Dipole–dipole interactions involve permanent molecular dipoles. Their energy depends on relative orientation: favorable arrangements bring opposite partial charges closer. The net dipole belongs to the whole molecular geometry, so the presence of a polar bond alone is not enough to establish this contribution. A symmetrical molecule can have polar bonds whose vectors cancel.

For the usual introductory N/O/F hydrogen-bond classification, a donor has a suitably polarized hydrogen bonded to one of those electronegative atoms, and an acceptor supplies an available electron-rich site, commonly a lone pair. An O–H group can donate a hydrogen bond to a neighboring oxygen lone pair. The hydrogen bond is the interaction between the molecules, not the covalent O–H bond itself.¹²

Application: distinguish donation from acceptance

Compare the following neutral structures. The alcohol and ether have the same C₃H₈O formula, but the oxygen is connected differently. That distinction changes which hydrogen-bond roles are available.³

Structure	Usual donor role	Usual acceptor role	Consequences for neighbors
CH₃CH₂CH₂OH	O–H can donate	O lone pairs can accept	Can form alcohol–alcohol hydrogen bonds
CH₃OCH₂CH₃	No O–H or N–H donor	O lone pairs can accept	Can accept from water; lacks its own conventional donor
CH₃CH₂CH₂CH₃	No conventional donor	No conventional acceptor	Dispersion remains present
H₂O	O–H can donate	O lone pairs can accept	Can supply both roles to suitable neighbors

“The ether cannot hydrogen-bond with itself in this conventional donor–acceptor scheme” is different from “the ether cannot hydrogen-bond with water.” Water supplies the donor that the ether lacks. Also check protonation and electron availability before transferring a rule to another functional group. A positively charged ammonium nitrogen has no lone pair to act as a usual acceptor, even though it is still nitrogen.

The alcohol–ether comparison controls molecular formula and molar mass. It does not hold every shape and electronic feature constant, so it is evidence consistent with an interaction explanation rather than a perfect single-variable experiment. A structural comparison becomes stronger when the main alternatives are identified instead of assumed away.

Worked comparison: isolate an interaction in a stated model

The following dataset is an original, simplified simulation exercise, not measured reference data for named chemicals. In pair A, the particle mass, size, shape, baseline dispersion, and permanent-dipole term are identical; A2 adds a directional donor–acceptor interaction. In pair B, particles have the same nonpolar composition and interaction parameters, but their shapes differ. Every boiling temperature is computed at the same external pressure.

Model	Specified difference	Boiling temperature
A1	No added donor–acceptor term	300 K
A2	Added donor–acceptor term	350 K
B1	Extended nonpolar shape	310 K
B2	Compact nonpolar shape	290 K

A2 exceeds A1 by 50 K. Under the stipulated model controls, that contrast identifies the consequence of adding the directional attraction. B1 exceeds B2 by 20 K, illustrating a shape-dependent contact effect without introducing a permanent dipole or hydrogen bonding. These two selected contrasts do not establish that hydrogen bonding is always 2.5 times more important than shape; the effect sizes depend on the specified model and parameters.

A liquid boils when its equilibrium vapor pressure reaches the external pressure. Stronger attractions among otherwise comparable particles generally reduce volatility and raise the temperature required to reach that condition. Comparing boiling temperatures measured at different external pressures would mix the molecular comparison with a change in the boiling criterion.⁴

Changed case: solubility needs both sides of the mixture

A polar site may make favorable contacts with water, but dissolution also rearranges solute–solute and water–water interactions. The balance includes the dispersal of particles, not only the strength of one newly formed attraction. Thus, “can hydrogen-bond” is useful evidence about possible solute–solvent contacts, not a guarantee of unlimited water solubility.⁵

For example, adding a large hydrocarbon region while retaining one OH group increases the nonpolar part of a molecule without adding more hydrogen-bonding sites. Comparing that compound with a much smaller alcohol changes more than mass. Conversely, replacing an OH with an ether linkage removes a donor but can retain an acceptor. These modifications ask different structural questions.

A clean comparison should state temperature, solvent, concentration range, and the charge state of the solute. If changing pH ionizes the compound, strong ion–solvent interactions and acid–base equilibria enter the explanation. That is the bridge to extraction and partitioning, rather than a reason to force an ionic species into a neutral-molecule rule.

Rule: predict a difference, then name its limits

AAMC groups hydrogen bonding, dipole interactions, and dispersion within molecular structure and liquid behavior.⁶ Use those mechanisms to explain a controlled comparison: identify donor and acceptor sites, determine whole-molecule polarity, compare polarizability and contact, and check the physical conditions. Avoid a universal ranking that places every hydrogen-bonding molecule above every nonpolar one. A large, highly polarizable molecule can have substantial dispersion, and a measured property reflects all relevant interactions together.

Check your reasoning

Choose an answer before opening its feedback. Each option explains the reasoning behind that choice.

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Related lessons

• Lipids and membrane properties

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Sources

1. OpenStax Chemistry 2e — Intermolecular Forces

2. OpenStax Organic Chemistry — Noncovalent Interactions between Molecules

3. OpenStax Chemistry 2e — Alcohols and Ethers

4. OpenStax Chemistry 2e — Phase Transitions

5. OpenStax Chemistry 2e — The Dissolution Process

6. AAMC — Content Category 5B