3-Hydroxypropanal in Astrochemistry

• 3-hydroxypropanal is a three-carbon hydroxy aldehyde identified in astrochemical surveys that links high-resolution spectroscopy with grain-surface formation pathways.
• Laboratory synthesis and rotational spectroscopy have established key molecular constants and transitions, enabling its tentative interstellar detection in sources like G+0.693-0.027.
• Abundance studies reveal non-equilibrium isomer distributions, underscoring the influence of kinetics and radical-driven chemistry in complex organic molecule formation.

Searching arXiv for the cited papers on 3-hydroxypropanal, lactaldehyde, and related astrochemical spectroscopy. 3-Hydroxypropanal is a three-carbon hydroxy aldehyde that appears in recent astrochemical literature primarily as HOCH$_2$CH$_2$CHO, with laboratory rotational spectroscopy and a tentative interstellar detection now reported for that species (Fried et al., 21 Aug 2025). In parallel, related work on C$_3$ oxygen-bearing aldehydes has treated the positional isomer 2-hydroxypropanal, also known as lactaldehyde and written CH$_3$CH(OH)CHO, as a comparison species in interstellar surveys (Koucký et al., 2022). Within this literature, 3-hydroxypropanal is significant because it links high-resolution molecular spectroscopy, LTE radiative-transfer analysis, and grain-surface astrochemistry, while also illustrating that interstellar abundances of complex organic molecules do not straightforwardly follow thermodynamic stability (Sanz-Novo et al., 12 Jan 2026).

1. Chemical identity and nomenclature

The recent spectroscopy and astronomy literature identifies 3-hydroxypropanal as HOCH$_2$CH$_2$CHO (Fried et al., 21 Aug 2025). In the G+0.693-0.027 survey of the C$_3$H$_6$O$_2$ isomer family, the same species is written as HO(CH$_2$)$_2$0C(O)H and described as a linear chain aldehyde, an open-chain isomer within that family (Sanz-Novo et al., 12 Jan 2026). The paper also reports a calculated total dipole moment of 3.0 Debye, a property that aids detectability in radio spectroscopy (Sanz-Novo et al., 12 Jan 2026).

A distinct but closely related compound, 2-hydroxypropanal, is explicitly identified elsewhere as lactaldehyde, CH$_2$1CH(OH)CHO, and described as a C$_2$2 oxygen-bearing aldehyde that is the methyl derivative of glycolaldehyde (Koucký et al., 2022). This distinction is important because the observational literature uses both molecules in comparative abundance analyses, but they are not the same isomer.

Within the broader astrochemical context, 3-hydroxypropanal is treated as part of the C$_2$3H$_2$4O$_2$5 isomeric inventory, together with methyl acetate, hydroxyacetone, methoxyacetaldehyde, ethyl formate, propionic acid, glycidol, and lactaldehyde (Sanz-Novo et al., 12 Jan 2026). The available observations suggest that isomer occurrence in the interstellar medium is controlled by formation pathways and kinetics rather than by stability alone. This suggests that structural classification is chemically informative, but insufficient by itself for predicting detectability.

2. Laboratory preparation and rotational spectroscopy

3-Hydroxypropanal was synthesized in the laboratory by a gas-phase modification of the procedure described by Roldán et al. (Fried et al., 21 Aug 2025). The synthesis hydrolyzed 3,3-diethoxy-1-propanol in 1 M H$_2$6SO$_2$7, followed by extraction, drying, solvent removal, and vacuum distillation at 0.1 mbar, collecting the product in a U-tube at $_2$8C; the reported yield was 0.17 g (2.3 mmol, 34%) (Fried et al., 21 Aug 2025). $_2$9H NMR and $_3$0C NMR data confirmed the product identity (Fried et al., 21 Aug 2025).

Its rotational spectrum was measured from 130 to 485 GHz using the Lille FLASH spectrometer and the Prague semiconductor millimeter wave spectrometer (Fried et al., 21 Aug 2025). Because the compound is unstable, the spectra were collected in a continuous flow system, with absorption detected by zero-bias Schottky diodes and frequency uncertainties ranging from 30 to 100 kHz depending on line intensity (Fried et al., 21 Aug 2025). Theoretical conformational analysis identified the most stable conformer, termed “Conformer 1,” and this conformer was reported to predominate because of stabilizing intramolecular hydrogen bonding (Fried et al., 21 Aug 2025).

The spectroscopic analysis fitted about 1194 distinct frequency transitions, or 2342 transitions including blended lines, spanning $_3$1–76 and $_3$2–40 (Fried et al., 21 Aug 2025). The fit used Pickett’s CALPGM/SPCAT suite with the A reduction and $_3$3 (Fried et al., 21 Aug 2025). Selected ground-state constants were reported as

$_3$4

with centrifugal distortion constants including

$_3$5

and

$_3$6

(Fried et al., 21 Aug 2025). The most intense lines are reported to be low-$_3$7 $_3$8-type $_3$9-branch transitions, attributed to $_3$0 D (Fried et al., 21 Aug 2025).

Two perturbed lowest-lying vibrationally excited states, $_3$1 and $_3$2, were also observed and assigned, with Coriolis perturbations treated experimentally and theoretically (Fried et al., 21 Aug 2025). The paper gives the Coriolis coupling expression

$_3$3

and reports partition functions such as $_3$4 and $_3$5 (Fried et al., 21 Aug 2025). These spectroscopic data underwrite subsequent astronomical searches.

3. Interstellar searches and source-by-source observational status

The most extensive dedicated astronomical search for HOCH$_3$6CH$_3$7CHO examined G+0.693-0.027, Sagittarius B2(N), NGC 6334I, IRAS 16293-2422B, and TMC-1 (Fried et al., 21 Aug 2025). A tentative detection was reported only toward the Galactic center molecular cloud G+0.693-0.027 (Fried et al., 21 Aug 2025).

For G+0.693-0.027, the observations used the Yebes 40 m survey from 31 to 50 GHz and the IRAM 30 m survey from 83.2 to 115.41 GHz, 132.28 to 140.39 GHz, and 142 to 173.81 GHz (Fried et al., 21 Aug 2025). The LTE model adopted $_3$8 K, $_3$9 km s$_2$0, and $_2$1 km s$_2$2 (Fried et al., 21 Aug 2025). Multiple transitions were tentatively assigned, including both unblended and blended features, and fitting with SLIM in Madcuba yielded

$_2$3

with an abundance relative to H$_2$4 of $_2$5 (Fried et al., 21 Aug 2025). The later isomer-survey paper reports essentially the same LTE result as $_2$6 cm$_2$7 and $_2$8 for $_2$9 cm$_2$0 (Sanz-Novo et al., 12 Jan 2026). The agreement between these values indicates internal consistency across the two studies.

No detection was reported in the other targeted sources (Fried et al., 21 Aug 2025). The upper limits were:

• Sgr B2(N): $_2$1 cm$_2$2 at $_2$3 K (Fried et al., 21 Aug 2025)
• IRAS 16293-2422B: $_2$4 cm$_2$5 at $_2$6 K (Fried et al., 21 Aug 2025)
• NGC 6334I: $_2$7 cm$_2$8 at $_2$9 K (Fried et al., 21 Aug 2025)
• TMC-1: $_3$0 cm$_3$1 at $_3$2 K (Fried et al., 21 Aug 2025)

The related 2022 ALMA study of 2-hydroxyprop-2-enal also searched for the positional isomer 2-hydroxypropanal in Sgr B2(N1S) using the ReMoCA survey and did not detect it (Koucký et al., 2022). That search used LTE radiative-transfer models generated with Weeds, assuming the same source size, rotational temperature, linewidth, and velocity offset as for acetaldehyde (Koucký et al., 2022). No explicit search or result for 2-hydroxypropanal was described toward IRAS16293 in that study (Koucký et al., 2022).

4. Abundances, ratios, and the C$_3$3H$_3$4O$_3$5 isomeric inventory

In G+0.693-0.027, 3-hydroxypropanal is comparatively minor within the detected C$_3$6H$_3$7O$_3$8 inventory. The isomer survey ranks methyl acetate as the most abundant species at $_3$9, followed by ethyl formate at $_6$0–$_6$1 and hydroxyacetone at $_6$2; lactaldehyde is reported at $_6$3, 3-hydroxypropanal at $_6$4, and methoxyacetaldehyde at $_6$5 (Sanz-Novo et al., 12 Jan 2026). Propionic acid and glycidol were not detected, with upper limits of $_6$6 and $_6$7, respectively (Sanz-Novo et al., 12 Jan 2026).

The same paper states that 3-hydroxypropanal is less abundant than methyl acetate, ethyl formate, and hydroxyacetone by factors of $_6$8–25, is slightly less abundant than lactaldehyde by $_6$9, and is about 2.6 times more abundant than methoxyacetaldehyde (Sanz-Novo et al., 12 Jan 2026). In the dedicated spectroscopy paper, 3-hydroxypropanal is reported to be $_2$0 times less abundant than $_2$1-propanal and $_2$2 times less abundant than acetaldehyde in G+0.693-0.027 (Fried et al., 21 Aug 2025).

For hot-core sources, the principal quantitative constraints come from non-detections. In Sgr B2(N), the dedicated HOCH$_2$3CH$_2$4CHO study states that the molecule is at least 8.5 times less abundant than acetaldehyde (Fried et al., 21 Aug 2025). The 2022 survey of related aldehydes reports that 2-hydroxypropanal in Sgr B2(N1S) has

$_2$5

for source size 2.0", $_2$6 K, linewidth 5.0 km s$_2$7, velocity offset 0.0 km s$_2$8, and vibrational correction 5.68 (Koucký et al., 2022). With acetaldehyde at $_2$9 cm$_2$0, the ratio is

$_2$1

so 2-hydroxypropanal is at least four times less abundant than acetaldehyde in Sgr B2(N1S) (Koucký et al., 2022).

Source or comparison set	3-hydroxypropanal status	Quantitative result
G+0.693-0.027	Tentative detection	$_2$2 cm$_2$3
Sgr B2(N)	Non-detection	$_2$4 cm$_2$5
IRAS 16293-2422B	Non-detection	$_2$6 cm$_2$7
NGC 6334I	Non-detection	$_2$8 cm$_2$9
TMC-1	Non-detection	$_2$00 cm$_2$01

These values show that current detections are highly source-dependent. A plausible implication is that environmental effects, rather than the mere availability of rotational spectroscopy, dominate whether the molecule is observable.

5. Formation chemistry and mechanistic interpretation

The principal formation scenarios advanced for 3-hydroxypropanal are grain-surface routes. The dedicated spectroscopy and detection study cites experiments under astrophysically relevant ice conditions showing formation through radical recombination,

$_2$02

and also proposes

$_2$03

as an alternative radical-recombination channel (Fried et al., 21 Aug 2025). In the same study, 3-hydroxypropanal is also connected to keto-enol tautomerization chemistry in interstellar ices and to a possible O($_2$04D) atom insertion pathway into the terminal CH$_2$05 group of propanal (Fried et al., 21 Aug 2025).

The later C$_2$06H$_2$07O$_2$08 inventory paper generalizes this picture by stating that all detected C$_2$09H$_2$10O$_2$11 isomers are consistent with formation via radical-radical reactions on dust grain surfaces, with CO as the ultimate starting point (Sanz-Novo et al., 12 Jan 2026). It specifically suggests successive hydrogenation of CO as the route to CH$_2$12OH and CH$_2$13CH$_2$14OH, the primary parent species from which the relevant radicals derive (Sanz-Novo et al., 12 Jan 2026). For 3-hydroxypropanal, one pathway is written as

$_2$15

followed by

$_2$16

(Sanz-Novo et al., 12 Jan 2026). The same paper notes laboratory experiments by Wang et al. 2023 and 2024 that demonstrated synthesis of 3-hydroxypropanal under UV irradiation of methanol-acetaldehyde and CO-ethanol ices (Sanz-Novo et al., 12 Jan 2026).

The environmental rationale for detectability in G+0.693-0.027 is likewise grain-centered. Shocks and high cosmic-ray rates are reported to cause sputtering and/or desorption of complex organics from dust grains into the gas phase, making them observable (Sanz-Novo et al., 12 Jan 2026). The dedicated detection paper similarly attributes the chemistry of G+0.693 to a relatively high cosmic ray ionization rate that drives radical production, with non-thermal desorption releasing newly formed complex organic molecules (Fried et al., 21 Aug 2025).

These proposals align with broader modeling of unsaturated hydroxy aldehydes. For 3-hydroxypropenal, a chemically related species, grain-surface radical-radical association followed by tautomerization was identified as the dominant formation route in Nautilus models (Coutens et al., 2022). This suggests that radical association plus post-formation isomerization may be a recurrent motif across hydroxy-aldehyde astrochemistry.

6. Kinetics, reactivity, and astrochemical constraints

Quantum-chemical work on hydrogen atom reactions with unsaturated alcohols and aldehydes provides mechanistic constraints relevant to 3-hydroxypropanal, even though the computed set did not explicitly include HOCH$_2$17CH$_2$18CHO (Zaverkin et al., 2018). The study concludes that hydrogen addition to aldehyde groups is slow, abstraction of the aldehydic hydrogen is among the faster reactions, and quantum tunnelling dominates at the low temperatures of dense interstellar environments (Zaverkin et al., 2018).

For the aldehyde functionality, the study reports that hydrogenation of the C=O group is much slower than addition to unsaturated C–C bonds, with aldehyde C/O addition barriers stretching from approximately 31–41 kJ mol$_2$19 including ZPE (Zaverkin et al., 2018). By contrast, aldehydic hydrogen abstraction has lower activation energies and is kinetically competitive; for propanal at 60 K, the paper reports $_2$20 for abstraction from C1, compared with 3.47 for addition at the carbonyl carbon and 2.01 for addition at the oxygen (Zaverkin et al., 2018). Abstraction of the hydrogen from a primary alcohol group is reported to be much slower at low temperature (Zaverkin et al., 2018).

The study emphasizes that tunnelling rates depend not only on $_2$21 but also on barrier width and shape, with the crossover temperature given by

$_2$22

(Zaverkin et al., 2018). It also stresses that rate constants cannot be predicted solely from reaction class, because attached functional groups can change rates by several orders of magnitude (Zaverkin et al., 2018).

Applied to 3-hydroxypropanal, the paper states that direct hydrogenation of the aldehyde group to yield a diol is expected to be exceedingly slow, whereas abstraction of the aldehydic hydrogen is likely to be rapid if accessible (Zaverkin et al., 2018). Since 3-hydroxypropanal contains both aldehyde and primary alcohol functionalities, this kinetic pattern implies that its destruction or transformation in interstellar ices may be strongly channel-dependent. This suggests that abundance constraints may encode both formation efficiency and selective post-formation reactivity.

7. Significance, limitations, and open issues

The most important current observational result is that 3-hydroxypropanal has a tentative interstellar detection in G+0.693-0.027, but remains undetected in several other chemically rich sources (Fried et al., 21 Aug 2025). The isomer-survey paper explicitly states that the detection of less stable isomers alongside the non-detection of the most stable isomer, propionic acid, shows that observed abundances do not straightforwardly track thermodynamic stability and instead reflect kinetics and astrochemical pathways, particularly surface chemistry (Sanz-Novo et al., 12 Jan 2026).

A central limitation is that the G+0.693-0.027 identification remains tentative. The dedicated study notes that line blending is present, although the best-fitting model including 3-hydroxypropanal explains the observed spectrum (Fried et al., 21 Aug 2025). The isomer-survey paper similarly states that only clean, unblended or minimally blended transitions were used for identification (Sanz-Novo et al., 12 Jan 2026). The need for conservative line selection reflects the persistent problem of spectral confusion in line-rich sources.

Another limitation is nomenclatural and isomeric complexity. The interstellar literature compares HOCH$_2$23CH$_2$24CHO with 2-hydroxypropanal, 3-hydroxypropenal, 2-hydroxyprop-2-enal, propanal, and other C$_2$25 oxygen-bearing species (Koucký et al., 2022, Coutens et al., 2022, Fried et al., 21 Aug 2025). Accurate laboratory spectroscopy is therefore essential not only for detection sensitivity but also for unambiguous assignment. The 2022 survey of related aldehydes explicitly states that accurate spectroscopy and vibrational corrections are necessary for reliable astronomical abundance determinations (Koucký et al., 2022).

Taken together, the current record places 3-hydroxypropanal at the intersection of rotational spectroscopy, LTE line analysis, and grain-surface organic chemistry. The observational evidence supports its presence in at least one Galactic center cloud at low fractional abundance, while the broader pattern of detections and non-detections constrains astrochemical models to reproduce both source dependence and the nonequilibrium distribution of C$_2$26H$_2$27O$_2$28 isomers (Fried et al., 21 Aug 2025, Sanz-Novo et al., 12 Jan 2026).