Chemistry · Lesson
Structure, polarity and physical properties
Connect molecular connectivity and symmetry with conditional physical properties. Compare hydrogen-bonding sites and a bounded vapour-pressure fit while keeping formula, polarity, packing and solvent effects distinct.
Back to the Chemistry mapBy the end, you can
- Add stipulated bond-dipole vectors with their directions retained.
- Identify donor/acceptor sites in four selected charge states.
- Invert a vapour-pressure fit only within its temperature range and pressure units.
- Explain why identical formulas and amounts do not establish identical properties.
A net dipole is a vector result
Polar bonds do not guarantee a nonzero net molecular dipole. In an original vector toy, four equal 1.00 D vectors along symmetric tetrahedral directions sum to zero. Two equal vectors at 180° also cancel; two at 120° have resultant 2μcos(60°)=1.00 D. Here D is the debye dipole-moment unit. These values illustrate geometry, not measured dipoles of named molecules.
Real electronic charge redistribution and molecular geometry determine a dipole; assigning independent bond dipoles is an approximation. Net formal charge zero does not imply zero dipole. For an ion, a dipole additionally depends on the chosen origin, so a neutral-molecule polarity shortcut should not be transferred without stating that reference.
Count a structural site rather than imagined permanent bonds
For common selected motifs, an alcohol OH can donate a hydrogen bond and its neutral oxygen can accept. A neutral ether oxygen accepts but has no O–H donor. In ethanamide, the N–H group is a donor and the carbonyl oxygen an acceptor; the delocalized amide-nitrogen lone pair is not an ordinary hydrogen-bond acceptor. Methylammonium has N–H donors but no nitrogen lone-pair acceptor.
The table counts donor sites, acceptor sites and attached donor hydrogens separately. One NH₂ donor atom is one donor site with two hydrogens, not automatically two independent permanent hydrogen bonds. These are structural inferences for the stated forms, not universal bond counts in every solvent. Protonation state, neighboring groups, geometry and environment affect the interactions; amide oxygen can be protonated under sufficiently acidic conditions even though ordinary amide nitrogen is a weak base.
| Specified form | Donor sites | Acceptor sites | Attached donor H |
|---|---|---|---|
| Neutral ethanol, CH₃CH₂OH | 1 | 1 | 1 |
| Neutral dimethyl ether, CH₃OCH₃ | 0 | 1 | 0 |
| Neutral ethanamide, CH₃CONH₂ | 1 | 1 | 2 |
| Methylammonium, CH₃NH₃⁺ | 1 | 0 | 3 |
Exact structural bookkeeping for the four stated ordinary Lewis forms. Counts do not predict the number or strength of actual hydrogen bonds in every environment. Ethanamide’s carbonyl O accepts while its amide N is not an ordinary acceptor; protonated methylammonium has no N lone pair.
Connectivity is only one part of a bulk property
Liquid–vapour equilibrium depends on temperature, pressure and intermolecular interactions; crystalline melting also depends strongly on packing and solid form. Dispersion, electrostatics and hydrogen bonding can all contribute. A familiar homologous-series or branching trend is a bounded comparison, not a universal ranking for every organic structure.
Solubility involves both solute and solvent free energies, concentrations and possible chemical changes such as ionization. Calling a molecule polar does not supply a numerical solubility or guarantee miscibility in every polar solvent. Temperature, pH, salt content and solid form may matter. Choose a specific property and conditions before turning a structural observation into a quantitative prediction.
Check a fit’s units and range before comparing temperatures
The NIST Chemistry WebBook lists Antoine fits in the form log₁₀(P/bar)=A−B/(T/K+C). For the selected ethanol fit, A=5.37229, B=1670.409 and C=−40.191, valid 273–351.70 K. For dimethyl ether, A=4.11475, B=894.669 and C=−30.604, valid 194.93–248.24 K. These are empirical fit parameters from different original datasets, not exact constants or a microscopic theory of boiling.
At the chosen pure-substance equilibrium pressure P=0.800 bar, invert the fit as T/K=B/[A−log₁₀(P/bar)]−C and then check the result against its range. This pressure keeps both examples within their respective fits. The ideal equilibrium boiling comparison equates the pure-substance saturation pressure with the external pressure; it says nothing about mixtures, superheating, fit uncertainty or a pressure-independent boiling point. Do not extrapolate the dimethyl-ether fit to a convenient pressure merely because a formula can be evaluated there.
Equal formula and amount can coexist with different behaviour
Ethanol CH₃CH₂OH and dimethyl ether CH₃OCH₃ both have C₂H₆O and the listed ordinary-composition molar mass 46.0684 g mol⁻¹. Equal stipulated 10.0 g samples therefore contain equal amounts of molecules. Their connectivity, donor capability and equilibrium vapour-pressure behaviour differ.
A formula and mass balance remain useful even when they cannot distinguish isomers. For physical-property interpretation, add the constitution, charge state, spatial arrangement where relevant, phase and measurement conditions. A good explanation combines structural reasoning with actual suitable data, while keeping a model estimate distinct from a measured result.
Worked examples
Add equal vectors with the angle retained
In the original toy, two vectors each have magnitude 1.00 D and angle 120°. Find the resultant magnitude.
- For equal vectors, |μ_total|=2μcos(θ/2).
- At θ=120°, cos(60°)=1/2.
- The magnitude is 2×1.00×1/2=1.00 D.
1.00 D in this symbolic vector toy. Adding scalar magnitudes to get 2.00 D would discard their directions.
Compare pure-substance fit predictions at 0.800 bar
Using only the specified NIST fits, find each equilibrium saturation temperature at P=0.800 bar.
- log₁₀(0.800)≈−0.0969100.
- Ethanol: T=1670.409/[5.37229−(−0.0969100)]−(−40.191)≈345.612 K, or 72.462°C.
- Dimethyl ether: T=894.669/[4.11475−(−0.0969100)]−(−30.604)≈243.031 K, or −30.119°C.
- Check 345.612 K lies within 273–351.70 K and 243.031 K within 194.93–248.24 K. Retained digits audit the arithmetic rather than establish fit uncertainty.
About 345.6 K for ethanol and 243.0 K for dimethyl ether at the selected 0.800 bar, using these empirical fits. The difference is about 102.6 K.
Retain the same amount while changing connectivity
Use the listed molar mass 46.0684 g mol⁻¹ and a stipulated 10.0 g sample of each C₂H₆O isomer.
- For either sample, n = m / M = 10.0 / 46.0684 ≈ 0.2170685 mol.
- Round the amount to the sample’s three significant figures: 0.217 mol.
- The equal formulas and listed molar mass give equal amounts, but the bond graphs differ.
Each sample contains 0.217 mol. Equal amount does not establish equal boiling behaviour or hydrogen-bonding sites.
Try it yourself
Does zero net formal charge require a zero molecular dipole?
Show the answer and reasoning
No.
A neutral molecule can still have a nonuniform charge distribution and a nonzero dipole.
For the stated neutral ethanamide, how many donor sites and attached donor hydrogens are counted?
Show the answer and reasoning
One donor site and two donor hydrogens.
The NH₂ nitrogen is one donor atom carrying two hydrogens; these are not universal counts of actual intermolecular bonds.
May the listed dimethyl-ether Antoine fit be presented as validated at 260 K?
Show the answer and reasoning
No.
Its stated range ends at 248.24 K; numerical evaluation outside that range is extrapolation.
Convert a saturation-temperature prediction of 345.6 K to Celsius at the displayed precision.
Show the answer and reasoning
72.5°C.
Subtract 273.15 and round to one decimal place; the Celsius conversion does not validate the underlying fit.
What to carry forward
- Retain vector directions when reasoning about polarity.
- State charge and protonation forms for donor/acceptor counts.
- A bulk property needs phase and measurement conditions.
- Keep empirical fits inside their validated ranges.
- Formula and amount alone do not distinguish isomer properties.
References
Teaching resources and reference material supporting this lesson’s explanations and examples.
OpenStax, Rice University2 sources
National Institute of Standards and Technology (NIST)2 sources