Difficulty: Introductory | Prerequisites: General chemistry (bond polarity, electronegativity), basic organic structure drawing
This topic sits at the intersection of organic structure and physical properties. You already know that molecules attract each other through intermolecular forces; here you learn how the type, strength and geometry of those forces set a compound's melting point. The lab side teaches you a standard purity and identification technique (melting point determination) that you will use throughout organic chemistry. If you can draw hydrogen bonds and recognise molecular symmetry, you have the background you need.
A compound's melting point depends on how strongly its molecules stick together (intermolecular forces) and how neatly they pack (symmetry). Intramolecular hydrogen bonds lower melting points by stealing hydrogen-bonding capacity away from neighbouring molecules, while strong intermolecular hydrogen bonds raise them. Melting point determination is also a quick check for purity: impurities widen and lower the observed range.
Intermolecular forces (IMFs)
Attractive forces that act between separate molecules in a substance. These include London dispersion forces, dipole-dipole interactions and hydrogen bonds. In simple terms, IMFs are the "glue" holding molecules near each other in a solid or liquid.
Intermolecular hydrogen bond
A hydrogen bond that forms between two separate molecules, where an H on one molecule (bonded to N, O or F) is attracted to a lone pair on another molecule's N, O or F. Think of it as a bridge connecting two neighbouring molecules.
Intramolecular hydrogen bond
A hydrogen bond that forms within a single molecule, typically when an H-bond donor and acceptor are close enough on the same structure (e.g. ortho-substituted phenols). In simple terms, the molecule folds in on itself to hydrogen-bond with its own atoms instead of reaching out to neighbours.
Hydrogen bond donor (HBD)
An atom (N, O or F) bonded to a hydrogen that can participate in hydrogen bonding. The donor "offers" its H to a nearby electronegative atom. Think of it as the side of the handshake that brings the hydrogen.
Hydrogen bond acceptor (HBA)
An electronegative atom (N, O or F) that carries a lone pair and can attract a hydrogen from a donor. In simple terms, the acceptor is the lone-pair side of the hydrogen bond.
Melting point (mp)
The temperature at which a solid transitions to a liquid. For pure compounds this is a narrow range (1-2 °C); the sharper the range, the purer the sample.
Melting point range
The span between the temperature at which the first crystal begins to look wet and the temperature at which the last solid disappears. A wide range signals impurities.
Crystal lattice
The regular, repeating three-dimensional arrangement of molecules (or ions) in a solid. Stronger IMFs and better packing produce a more stable lattice and a higher melting point.
Molecular symmetry (in the context of melting points)
How evenly a molecule's shape distributes in space. Symmetric molecules pack more efficiently into the crystal lattice, increasing contact area and attractive forces, which raises the melting point.
Mel-Temp apparatus
A benchtop device used to measure melting points. It heats a sample in a capillary tube at a controlled rate while you observe the phase change through a magnifying window.
Intermolecular H-bonds form between separate molecules. They hold the crystal lattice together and raise the melting point.
Intramolecular H-bonds form within a single molecule. They satisfy the donor and acceptor internally, leaving fewer sites available for intermolecular bonding.
A molecule that "uses up" its H-bonding capacity internally will have weaker attractions to its neighbours, and therefore a lower melting point.
Classic example: 2-nitrophenol (mp ~45 °C) has an intramolecular H-bond between the ortho-OH and the adjacent NO₂. The 4-nitrophenol isomer (mp ~114 °C) cannot form this internal bond, so all its H-bonding is intermolecular, giving it a much higher melting point.
In melting, only intermolecular bonds break. Covalent bonds within molecules stay intact.
The stronger and more numerous the intermolecular forces, the more energy (heat) is needed to pull molecules apart, and the higher the melting point.
Hydrogen bonds are among the strongest IMFs in organic compounds (roughly 10-40 kJ/mol per bond), so the number of available intermolecular H-bond sites is a major driver of melting point differences within a series of similar compounds.
Symmetric molecules pack more tightly and regularly into the crystal lattice.
Better packing means more surface contact between molecules, which increases the total attractive forces.
The result: more energy is needed to disrupt the lattice, so melting point goes up.
Para-substituted compounds (4-position) are typically more symmetric than ortho (2-position) or meta (3-position) isomers, and they tend to have the highest melting points within a positional isomer series.
Purity check: A pure substance melts over a narrow range (1-2 °C). Impurities disrupt the crystal lattice, lowering the onset temperature and widening the range.
Identification: Every pure compound has a characteristic melting point. Comparing your measured value to literature values helps confirm what you have.
Mixed melting point test: If you suspect your unknown is compound X, mix a small amount of known X with your sample and take the melting point. If it stays sharp and unchanged, the two are the same compound. If the range widens and drops, they are different.
Donor (HBD): Look for an H bonded to N, O or F. That H is the one being "donated" to the acceptor's lone pair. Circle these atoms when asked to label HBDs.
Acceptor (HBA): Look for N, O or F atoms carrying lone pairs that can attract a donor's H. Box these atoms when asked to label HBAs.
A single atom can sometimes act as both (e.g. an -OH group: the O is an acceptor via its lone pairs, and also a donor via its bonded H).
In DNA, adenine (A) pairs with thymine (T) through two hydrogen bonds.
The N-H on adenine's exocyclic amino group donates to the C=O on thymine. Here, the amino N-H is the HBD and thymine's carbonyl O is the HBA.
Thymine's N-H donates to a ring nitrogen on adenine. Here, thymine's N-H is the HBD and adenine's ring N (with its lone pair) is the HBA.
Each N and O in the base pair carries lone pairs. When drawing, add two dots for each lone pair on every N and O that is not already using both pairs in bonds.
"Melting breaks covalent bonds." It does not. Melting disrupts intermolecular forces only. The covalent bonds within each molecule remain intact.
"Higher molar mass always means higher melting point." Not within these series. 4-Nitroanisole has a higher molar mass than 4-nitrophenol but a much lower melting point, because replacing -OH with -OCH₃ removes a hydrogen bond donor.
"Intramolecular hydrogen bonds raise the melting point because they are extra bonds." The opposite. Intramolecular H-bonds satisfy bonding internally, reducing intermolecular attractions and lowering the melting point.
"Symmetry affects bond strength." Symmetry does not change the strength of individual IMFs. It affects how efficiently molecules pack, which determines total contact area and aggregate attraction.
⚠️ Be ready to explain why an ortho isomer melts lower than its para isomer, using intramolecular vs intermolecular H-bonding.
⚠️ Know the two reasons for performing a melting point determination: purity assessment and compound identification.
⚠️ Understand that a wide melting range signals impurities, and be able to explain why (disruption of the crystal lattice).
⚠️ Be able to identify HBDs and HBAs in a given structure, including nucleotide base pairs.
⚠️ Expect questions comparing isomers (ortho, meta, para) and asking you to rank melting points with reasoning.
True or false: When a solid melts, its covalent bonds break. (False, only intermolecular forces are overcome.)
Fill in the blank: A pure compound typically melts over a range of ______ °C. (1-2)
True or false: An intramolecular hydrogen bond in 2-nitrophenol increases its melting point relative to 4-nitrophenol. (False, it decreases it.)
Fill in the blank: A hydrogen bond donor is an H atom bonded to ______, ______ or ______. (N, O, F)
True or false: Para-substituted isomers generally have higher melting points than ortho isomers because they are more symmetric. (True.)
Q: What is the difference between an intramolecular and an intermolecular hydrogen bond?
A: An intramolecular hydrogen bond forms within a single molecule (donor and acceptor are on the same structure), while an intermolecular hydrogen bond forms between two separate molecules.
Q: Which type of bond is broken when a substance melts, and why?
A: Intermolecular bonds are broken. Melting provides enough thermal energy to overcome the attractive forces between molecules, allowing them to move from the ordered solid lattice into a disordered liquid. Covalent bonds within molecules remain intact.
Q: 2-Nitrophenol melts at ~45 °C while 4-nitrophenol melts at ~114 °C. Both have the same molecular formula. Explain the difference.
A: In 2-nitrophenol, the -OH and -NO₂ groups are adjacent (ortho), allowing an intramolecular hydrogen bond. This satisfies the H-bonding internally, reducing intermolecular attractions and lowering the melting point. In 4-nitrophenol (para), the groups are too far apart for intramolecular bonding, so all hydrogen bonding is intermolecular, producing a stronger crystal lattice and a higher melting point.
Q: A student measures a melting range of 95-105 °C for a compound whose literature value is 113-114 °C. What does this suggest?
A: The sample is likely impure. Impurities disrupt the crystal lattice, causing the melting point to drop and the range to widen. A 10 °C range (vs the expected 1 °C) is a clear sign of contamination.
Q: How does molecular symmetry affect melting point?
A: Symmetric molecules pack more efficiently into the crystal lattice, maximising contact area and intermolecular attractions. This requires more energy to disrupt, resulting in a higher melting point. Para isomers are typically the most symmetric positional isomers and tend to melt highest.
This material connects directly to spectroscopy and chromatography later in CHM 255. Hydrogen bonding affects not just melting points but also boiling points, solubility and retention times on polar columns. The purity-checking logic here is the same logic behind recrystallisation: a pure compound forms a clean lattice, and disrupting it is the basis of melting point depression.
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