Predicting and Identifying Boiling Point Trends in Organic Chemistry

Boiling is the process of separating molecules in a liquid. In order for boiling to occur, sufficient energy must be supplied to overcome the intermolecular forces holding the molecules together.
Boiling point (bp) measures the energy required to overcome these forces. Stronger intermolecular forces lead to higher boiling points.
Technically, boiling occurs when the vapor pressure of a liquid is equal to the atmospheric pressure. Lowering atmospheric pressure subsequently lowers the boiling point.
Volatility serves as a qualitative measure; a liquid with a low boiling point is considered highly volatile.
Examples of boiling point extremes include:
- $H_2O$ : $100\,^{\circ}C$ ( $373\,K$ ) due to strong intermolecular forces.
- Argon: $-185\,^{\circ}C$ ( $87\,K$ ) due to weak intermolecular forces.

Trend #1: Strength of Intermolecular Forces. Everything else being equal, boiling point increases based on the types of forces present across four general categories:
- $\text{Ionic Forces (Salts)} > \text{Hydrogen Bonding} > \text{Dipole-Dipole Forces} > \text{London Dispersion Forces}$ .
Trend #2: Molecular Size and Weight. Among molecules with identical functional groups, boiling point increases as the number of carbons increases. This is due to increased surface area as the carbon chain lengthens.
- Example: $C_8H_{18}$ ( $398\,K$ / $125\,^{\circ}C$ ) > $C_6H_{14}$ ( $341\,K$ / $68\,^{\circ}C$ ) > $C_4H_{10}$ ( $272\,K$ / $-1\,^{\circ}C$ ).
- Example (Amines): butylamine ( $351\,K$ / $78\,^{\circ}C$ ) > propylamine ( $322\,K$ / $49\,^{\circ}C$ ) > ethylamine ( $291\,K$ / $18\,^{\circ}C$ ).
Trend #3: Surface Area and Branching. Among molecules with identical functional groups and identical carbon counts, boiling point increases with surface area:
- $\text{Linear (highest surface area)} > \text{Branched} > \text{Highly Branched (lowest surface area)}$ .
- Example ( $C_5H_{12}$ ): Linear ( $310\,K$ / $36\,^{\circ}C$ ) > Branched ( $300\,K$ / $27\,^{\circ}C$ ) > Highly Branched ( $283\,K$ / $10\,^{\circ}C$ ).

Electrostatic Strength: The strength of attraction is governed by Coulomb's equation: $F = k_e \frac{q_1q_2}{r^2}$ . Larger charges ( $q$ ) result in greater attractive forces and higher boiling points.
Ionic Forces (Strongest): Result from attraction between full point charges. They often contain metals like $Na$ , $K$ , $Li$ , or $Mg$ bound to electronegative atoms ( $O$ or $N$ ).
- Examples: $CH_3ONa$ , $t\text{-BuOLi}$ , $KOEt$ .
Hydrogen Bonding: Attraction between large partial charges in highly polarized $O-H$ or $N-H$ bonds.
- Hydrogen bonding with $O-H$ (found in alcohols and carboxylic acids) is stronger than with $N-H$ (found in amines and amides) due to the greater electronegativity of oxygen.
Dipole-Dipole Forces: Attraction between partial charges in bonds involving carbon and electronegative atoms ( $O$ , $N$ , $F$ , $Cl$ , $Br$ , $I$ ). Found in ethers, esters, ketones, aldehydes, and alkyl halides.
London Dispersion Forces (Weakest): Result from instantaneous dipoles caused by temporary fluctuations in electron density. These are present in all molecules but are the dominant force in hydrocarbons.

Question: Rank the following in order of boiling point: $CH_3CH_2CH_2CH_3$ (A), $CH_3CH_2OCH_3$ (B), $CH_3CHOHCH_2CH_3$ (C), $(CH_3)_3C\text{-ONa}$ (D).
Response: $D > C > B > A$ . D is ionic (highest), C has hydrogen bonding, B has dipole-dipole, and A only has London dispersion forces.
Question: How do multiple hydrogen bond sites affect boiling point?
Response: Molecules with more hydrogen bond donors or multiple alcohol/amine groups will exhibit higher boiling points. For example, a molecule with two $O-H$ groups will have a higher boiling point than a similar molecule with only one.
Question: Which isomer of propylbenzene has the highest boiling point?
Response: The molecule with the longest carbon chain and least branching (less sphere-like) will have the highest boiling point. Linear propylbenzene ( $432\,K$ ) > branched isomer ( $425\,K$ ).
Question: What happens when trends compete, such as a molecule with strong forces but low molecular weight vs. a molecule with weak forces but high molecular weight?
Response: When trends operate in opposite directions, boiling points are difficult to predict. Competitive trends such as these are generally avoided in standardized exercises intended to highlight specific trend spotting.