Chemical Structure of 2-Methyl-2,4-Pentanediol
Molecular Formula, Bonds & Stereochemistry
A complete structural analysis of hexylene glycol (CAS 107-41-5) - from IUPAC name and condensed formula to chirality, hydrogen bonding, and how molecular architecture drives performance.
📋 Table of Contents
- IUPAC Name, CAS Number & Key Identifiers
- Molecular Formula & Molecular Weight
- Condensed Structural Formula Explained
- Full Structural Formula: Bond-by-Bond Analysis
- Stereochemistry: The Chiral Centre at C-4
- Hydrogen Bonding & Intermolecular Interactions
- How Structure Drives Physical Properties
- Structural Comparison: MPD vs Related Diols
- Reactivity: What the Structure Predicts
- Frequently Asked Questions
1 🔬 IUPAC Name, CAS Number & Key Identifiers
Every chemical discussion begins with unambiguous identification. 2-Methyl-2,4-pentanediol carries several names in commerce and literature, all referring to the same compound. Understanding these synonyms is essential when searching technical literature, regulatory databases, and supplier documentation.
| Identifier Type | Value | Notes / Source |
|---|---|---|
| Preferred IUPAC Name | 2-methylpentane-2,4-diol | IUPAC 2013 recommendations (PIN) |
| Common / Trade Name | Hexylene glycol | Most widely used commercial name |
| Abbreviation | MPD | Used in coatings and chemical synthesis literature |
| CAS Registry Number | 107-41-5 | PubChem CID 11593 |
| EINECS Number | 203-489-0 | ECHA substance page |
| INCI Name (cosmetics) | HEXYLENE GLYCOL | International Nomenclature of Cosmetic Ingredients |
| InChI Key | FRJJJAKBRKUSLL-UHFFFAOYSA-N | Standard InChI hashed key |
| Canonical SMILES | CC(O)CC(C)(C)O | Simplified Molecular Input Line Entry System |
| Other synonyms | 2-methyl-2,4-pentanediol; diolane; isol | Older literature and regional trade usage |
2 ⚗️ Molecular Formula & Molecular Weight
The molecular formula of 2-methyl-2,4-pentanediol is C₆H₁₄O₂, placing it in the family of saturated aliphatic diols. Breaking this formula down:
🧮 Formula Breakdown: C₆H₁₄O₂
🔹 C₆ - six carbon atoms forming the backbone (hexane skeleton, hence "hexylene")
🔹 H₁₄ - fourteen hydrogen atoms; consistent with a fully saturated (no double bonds) C₆ diol: CₙH₂ₙ₊₂O₂ → 2(6)+2 = 14 ✔
🔹 O₂ - two oxygen atoms, each part of a hydroxyl (–OH) group at positions 2 and 4
Molecular Weight: 6(12.011) + 14(1.008) + 2(15.999) = 72.066 + 14.112 + 31.998 = 118.17 g/mol
Degree of unsaturation (DoU): (2×6 + 2 − 14) / 2 = 0 - fully saturated, no rings or double bonds
The zero degree of unsaturation confirms that the molecule contains only single bonds - no alkene, alkyne, carbonyl, or ring structures. This fully saturated, open-chain architecture makes MPD chemically stable under most processing conditions, resistant to oxidation, and non-reactive with electrophiles under ambient conditions - all desirable properties for a long-shelf-life industrial solvent.
3 📝 Condensed Structural Formula Explained
The condensed structural formula communicates the connectivity of the molecule without drawing every bond explicitly. For 2-methyl-2,4-pentanediol:
Condensed Structural Formula
(CH₃)₂C(OH)–CH₂–CH(OH)–CH₃
Reading left to right: C2 (tertiary –OH) → CH₂ bridge → C4 (secondary –OH) → terminal CH₃
Reading this formula from left to right reveals the complete connectivity:
🔹 (CH₃)₂C(OH)–: Carbon-2 carries two methyl groups AND one hydroxyl group, making it a tertiary carbon bearing a tertiary alcohol. This is the "2-methyl" and "2-ol" part of the name.
🔹 –CH₂–: Carbon-3 is a simple methylene (–CH₂–) bridge connecting C2 and C4. No substituents other than two hydrogens.
🔹 –CH(OH)–: Carbon-4 bears one hydrogen and one hydroxyl group, making it a secondary carbon with a secondary alcohol - and a chiral centre.
🔹 –CH₃: Carbon-5 is a terminal methyl group. (Note: carbon-6 does not exist in this structure - the "pentane" backbone has only 5 carbons; the sixth carbon is the methyl branch at C-2.)
💡 Common misconception: Students sometimes count six carbons in the name "hexylene glycol" and expect a straight C₆ chain. In reality, the "hex" refers to the total carbon count (5 backbone + 1 methyl branch = 6), not a straight six-carbon chain. The parent chain is pentane (5 carbons), with a methyl substituent at position 2.
4 🔗 Full Structural Formula: Bond-by-Bond Analysis
The full structural formula assigns each atom its correct valence and connectivity. The carbon skeleton and all attached groups are described below as a text-based structural representation:
// Structural Diagram (text representation)
CH₃ OH
| |
CH₃ - C - CH₂ - CH - CH₃
|
OH
Carbon positions: C1(CH₃) - C2 - C3(CH₂) - C4 - C5(CH₃)
Branch: C6(CH₃) attached to C2 | –OH at C2 (tertiary) & C4 (secondary)
Breaking this down carbon by carbon:
| Carbon | Hybridisation | Substituents | Alcohol Type | Key Feature |
|---|---|---|---|---|
| C-1 | sp³ | 3H, 1C (bonded to C-2) | - | Terminal methyl group |
| C-2 | sp³ | –OH, CH₃ (×2 via C-1 & C-6), –CH₂– (to C-3) | Tertiary alcohol | Quaternary C bearing –OH; no H on carbon |
| C-3 | sp³ | 2H, C-2, C-4 | - | Methylene bridge; no functional group |
| C-4 | sp³ | –OH, 1H, C-3, C-5 | Secondary alcohol | ⭐ Chiral centre (R/S) |
| C-5 | sp³ | 3H, 1C (bonded to C-4) | - | Terminal methyl group |
| C-6 | sp³ | 3H, 1C (bonded to C-2) | - | Methyl branch at C-2 (the "2-methyl") |
For interactive 3D visualisation of this structure, PubChem provides a free 3D Conformer viewer: PubChem CID 11593 - 3D Conformer →
5 🧬 Stereochemistry: The Chiral Centre at C-4
Carbon-4 of 2-methyl-2,4-pentanediol bears four different substituents: –OH, –H, –CH₂C(CH₃)₂OH (the C3–C2 chain), and –CH₃ (C5). This satisfies the definition of a chiral centre (stereogenic centre), meaning that C-4 can exist in two non-superimposable mirror image configurations: (R) and (S).
🔴 (R)-2-Methyl-2,4-pentanediol
The (R) enantiomer has the –OH at C-4 oriented according to the CIP R-configuration rule. Slightly different physical and biological properties compared to (S) form.
🔵 (S)-2-Methyl-2,4-pentanediol
The (S) enantiomer is the mirror image. In solution, (R) and (S) forms rotate plane-polarised light in equal and opposite directions.
💡 What Does This Mean in Practice?
All commercial production of 2-methyl-2,4-pentanediol via the aldol condensation–hydrogenation route yields a racemic mixture (approximately equal R and S) rather than a pure enantiomer. The racemate is designated (±)-2-methyl-2,4-pentanediol or simply written without stereodescriptor. For industrial applications - solvent, coalescent, hydraulic fluid - the racemate is functionally equivalent to either pure enantiomer. For certain asymmetric catalysis or biochemical research applications, enantiopure (R)-MPD is available from specialist suppliers at significantly higher cost.
The NIST WebBook entry for CAS 107-41-5 provides optical rotation data and additional stereochemical reference information: NIST WebBook - CAS 107-41-5 →
6 🔗 Hydrogen Bonding & Intermolecular Interactions
The two hydroxyl groups in MPD are strong hydrogen bond donors and acceptors, making intermolecular hydrogen bonding the dominant force governing MPD's physical properties. However, the geometry of these groups - specifically the 1,3-diol arrangement (–OH at C-2 and C-4, separated by one –CH₂– group) - creates a unique possibility for intramolecular hydrogen bonding.
🔬 Intramolecular H-Bond: The Six-Membered Ring
The 1,3-diol geometry of MPD allows the C-2 hydroxyl to form a six-membered pseudo-ring through intramolecular hydrogen bonding with the C-4 hydroxyl oxygen: O(C2)–H···O(C4). This intramolecular interaction:
✅ Reduces the effective hydrogen bond donor capacity for intermolecular associations
✅ Lowers viscosity compared to diols of similar MW that cannot form intramolecular H-bonds
✅ Contributes to MPD's lower hygroscopicity relative to glycerin and ethylene glycol
✅ Explains the solubility of MPD in non-polar organic solvents - the "self-satisfied" H-bond reduces polarity demand from the medium
This structural feature - intramolecular hydrogen bonding in 1,3-diols - is not unique to MPD but is particularly well-expressed here due to the steric shielding of the tertiary C-2 carbon, which enforces a conformation favourable to ring formation. The result is a molecule with higher effective lipophilicity than its LogP alone suggests, which is why MPD dissolves so readily in both water and organic solvents.
7 📊 How Structure Drives Physical Properties
The structural features identified above - branching at C-2, intramolecular H-bonding, tertiary vs. secondary alcohol geometry - directly explain every key physical property of MPD.
| Physical Property | Value | Structural Explanation |
|---|---|---|
| Boiling point (197–198 °C) | High | Two –OH groups create strong intermolecular H-bonds; high energy needed to vaporise |
| Viscosity (~36 mPa·s) | Moderate | Lower than glycerin (1480 mPa·s) due to intramolecular H-bond reducing intermolecular association |
| Water miscibility (complete) | Full | Two –OH groups provide sufficient H-bonding capacity with water to overcome hydrophobic character of C₆ chain |
| LogP (0.58) | Balanced | Methyl branch and intramolecular H-bond reduce effective polarity; C₆ chain adds hydrophobicity |
| Low hygroscopicity | Moderate | Intramolecular H-bond reduces availability of –OH for water absorption from atmosphere |
| Flash point (88 °C) | Combustible | High boiling point → low vapour pressure → high flash point; not flammable at room temperature |
8 ⚖️ Structural Comparison: MPD vs Related Diols
Placing MPD's structure alongside related diols reveals why it occupies its unique performance niche.
| Compound | –OH Positions | Branching | Intramol. H-bond | Chiral Centres | Viscosity (mPa·s) |
|---|---|---|---|---|---|
| MPD (2-methyl-2,4-pentanediol) | C-2, C-4 (1,3-diol) | Yes (C-2) | ✅ Strong | 1 | ~36 |
| 1,2-Propanediol (PG) | C-1, C-2 (1,2-diol) | No | ⚠️ 5-membered ring | 1 | ~40 |
| 1,6-Hexanediol | C-1, C-6 (1,6-diol) | No | ❌ Too distant | 0 | ~22 (at 25°C, liquid near mp) |
| TMPD (2,2,4-trimethyl-1,3-pentanediol) | C-1, C-3 (1,3-diol) | Yes (heavy) | ✅ Moderate | 2 | Solid at RT |
| Glycerin (1,2,3-propanetriol) | C-1, C-2, C-3 | No | ⚠️ Partial | 1 | ~1480 |
For a full formulation-focused comparison, see our dedicated article: 2-Methyl-2,4-Pentanediol vs Other Glycol Solvents →
9 ⚡ Reactivity: What the Structure Predicts
The structural features of MPD directly govern its chemical reactivity. Understanding this reactivity is important both for predicting performance in applications and for synthesis planning.
⚗️ Esterification
Both –OH groups can be esterified with carboxylic acids or anhydrides. The C-4 secondary –OH is more reactive (less steric hindrance) than the C-2 tertiary –OH (fully hindered by two methyl groups). Selective mono-esterification at C-4 is achievable under mild conditions.
🔗 Polyol for Polyurethane
As a diol, MPD reacts with diisocyanates (MDI, TDI, HDI) to form polyurethane hard segments. The branched geometry reduces crystallinity and improves low-temperature flexibility of the resulting polymer.
🚫 Dehydration (Avoid)
Under strong acid at elevated temperatures, MPD can dehydrate to form hexenol or cyclic ether by-products. This is not a concern under normal storage and processing conditions but should be considered in strongly acidic formulations above 100 °C.
🧪 Oxidation
The secondary C-4 –OH can be oxidised to a ketone (4-methyl-4-hydroxypentanone) by strong oxidants. The tertiary C-2 –OH resists oxidation under most conditions. MPD is stable to atmospheric oxidation under normal storage.
10 ❓ Frequently Asked Questions
Q: What is the condensed structural formula of 2-methyl-2,4-pentanediol?
A: The condensed structural formula is (CH₃)₂C(OH)–CH₂–CH(OH)–CH₃. Reading left to right: a dimethyl-substituted tertiary alcohol at C-2, a methylene bridge (–CH₂–) at C-3, a secondary alcohol at C-4, and a terminal methyl group (C-5). The sixth carbon is the methyl branch at C-2.
Q: Does 2-methyl-2,4-pentanediol have a chiral centre?
A: Yes - carbon-4 is a chiral centre because it bears four different substituents (–OH, –H, –CH₃, and the C-3 chain). Commercial MPD is a racemate of (R) and (S) enantiomers. For standard industrial and cosmetic applications, the racemate is used without separation.
Q: Why is it called "hexylene" glycol if the backbone is only 5 carbons?
A: The "hex" in hexylene glycol refers to the total carbon count of the molecule (6 carbons: 5 in the pentane backbone + 1 methyl branch). The IUPAC name "2-methylpentane-2,4-diol" correctly identifies the 5-carbon parent chain. The older "hexylene" naming counts all carbons regardless of branching.
Q: What is the SMILES notation for 2-methyl-2,4-pentanediol?
A: The canonical SMILES is CC(O)CC(C)(C)O. This translates as: CH₃ (C5) – C(OH)(C4) – CH₂(C3) – C(CH₃)₂(OH)(C2). The non-canonical isomeric SMILES specifying stereochemistry at C-4 would include [C@@H] or [C@H] descriptors for the R or S forms respectively.
Q: Where can I find the 3D crystal structure or conformer data for CAS 107-41-5?
A: The most accessible free resource is PubChem CID 11593, which provides a 3D conformer viewer and downloadable SDF/MOL2 files. The NIST WebBook provides spectral data (IR, MS) for structural confirmation. ChemSpider ID 13839042 also provides alternative structure references.
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