C₄H₅NO₂ – Organic Formula, Isomeric Classes, and Representative Compounds
*Encyclopedic Entry (non‑specific to a single substance)*
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1. Introduction
The empirical formula **C₄H₅NO₂** describes any chemical species that contains exactly four carbon atoms, five hydrogen atoms, one nitrogen atom and two oxygen atoms. In organic chemistry this simple stoichiometry can be realized by a variety of structural motifs that differ in connectivity, functional‑group arrangement and stereochemistry. Because the formula contains a heteroatom (nitrogen) together with two oxygen atoms, it is often encountered in nitrile, amide, ester, lactone, imide and related functional groups.
The notation C₄H₅NO₂ follows the IUPAC convention of writing the number of atoms of each element after the symbol, in descending order of atomic number (C > H > N > O). The formula can be written as an empirical (simplest ratio) formula, a molecular formula (if the total number of atoms is known), or as a structural formula when a specific compound is identified. In the absence of a specific name, C₄H₅NO₂ is treated as a *general molecular formula* for a family of compounds.
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2. General Physical–Chemical Characteristics
| Property | Typical Range (for small organic C₄H₅NO₂ isomers) |
|----------|----------------------------------------------|
| **Molecular weight** | 87.09 g mol⁻¹ (C 48 g mol⁻¹ + H 5 g mol⁻¹ + N 14 g mol⁻¹ + O 32 g mol⁻¹) |
| **Molar volume** | 10.1 cm³ mol⁻¹ (approximate for liquids at 25 °C) |
| **Density** | 0.95–1.10 g cm⁻³ (liquid) |
| **Melting point** | –120 °C to –50 °C (depending on structure) |
| **Boiling point** | 90 °C to 150 °C (depending on structure) |
| **Colour** | Colourless to pale yellow (if chromophores present) |
| **Solubility** | Soluble in polar organic solvents (acetone, ethanol, DMF); limited solubility in hexane |
Degree of Unsaturation
The index of hydrogen deficiency (IHD) is calculated as:
\[
\text{IHD} = \frac{2C + 2 + N - H - X}{2}
\]
where X is the number of halogens (zero here). For C₄H₅NO₂, IHD = 3, indicating three rings and/or multiple bonds. This accommodates a variety of structural arrangements (e.g., one ring and two double bonds, three double bonds, or a lactone/ester plus a nitrile).
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3. Possible Structural Motifs
The simplest way to classify all plausible structures for C₄H₅NO₂ is to enumerate the functional‑group families that satisfy the IHD = 3 constraint.
3.1 Nitrile‑Containing Skeletons
| Structure | IUPAC Name | Example (CAS) | Notes |
|-----------|------------|--------------|-------|
| *N‑Methyl‑2‑pyrrolidone* (with a nitrile) | 1‑Cyanomethyl‑3,4‑dihydro‑2‑pyrrolidone | 138‑79‑8 | 4‑Membered lactam bearing a cyanomethyl group; commonly used as a solvent or reagent. |
| *Cyanomethyl acetate* (O‑acylated cyanohydrin) | 2‑Cyanomethyl‑2‑acetoxy‑2‑butanone | 145‑70‑0 | 5‑Membered lactone (α‑hydroxy ester) with a nitrile; used in organic synthesis. |
Both examples satisfy C₄H₅NO₂ and illustrate the IHD constraint: the nitrile provides one triple bond (two unsaturations), while the lactone or lactam ring provides a further unsaturation.
3.2 Lactone/Imide Skeletons
| Structure | IUPAC Name | CAS | Notes |
|-----------|------------|-----|-------|
| *2‑Oxazolidinone* (cyclic carbamate) | 2‑Oxazolidinone | 118‑27‑6 | The minimal lactam skeleton that contains one ring and a nitrile group; frequently used as a protecting group. |
| *Imidate ester* | 1‑(2‑Hydroxyethyl)imidate | 119‑41‑3 | Contains a C=N–O linkage and an ester; a potential intermediate in nucleophilic substitution reactions. |
3.3 Simple Esters and Amides
| Structure | IUPAC Name | CAS | Notes |
|-----------|------------|-----|-------|
| *Acetyl cyanide* | 1‑Cyanocarboxylate | 104‑61‑4 | An acyl cyanide where the cyanide is bound to the carbonyl; highly reactive toward nucleophiles. |
| *Cyanohydrin acetate* | 2‑Cyanohydrin‑1‑acetate | 122‑24‑6 | A cyanohydrin ester, providing a source of cyanide ion upon hydrolysis. |
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4. Synthesis and Reactivity
The small size of C₄H₅NO₂ molecules makes them useful building blocks in organic synthesis, especially in *Cyanide‑based reactions* such as the Hofmann rearrangement and the Biginelli reaction.
Typical synthetic routes include:
- Condensation of aldehydes with cyanamide:
\[
\text{R–CHO} + \text{NH}_2\text{CN} \rightarrow \text{R–CH(NH)}\text{CN}
\]
(followed by esterification or cyclization).
- O‑Acylation of cyanohydrin:
\[
\text{CNCH}_2\text{OH} + \text{Ac}_2\text{O} \rightarrow \text{CNCH}_2\text{OCOCH}_3
\]
yielding a cyanomethyl acetate derivative.
- Hydrolysis of nitriles to amides:
\[
\text{C–(CN)} + \text{H}_2\text{O} \xrightarrow{\text{acid}} \text{C–(CONH)}\text{H}
\]
providing a route to lactams.
These transformations underscore the versatility of C₄H₅NO₂‑type intermediates in the preparation of heterocycles, pharmaceuticals, and polymerization initiators.
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5. Physical and Chemical Properties of Representative Isomers
| Compound | State at 25 °C | Boiling point (°C) | Density (g cm⁻³) | UV Absorption (nm) | IR (cm⁻¹) |
|----------|----------------|--------------------|------------------|--------------------|-----------|
| 2‑Oxazolidinone | Liquid | 104 | 1.00 | 236 (nitrile) | 2250 (C≡N), 1700 (C=O) |
| Cyanomethyl acetate | Liquid | 112 | 1.04 | 236 | 2250 (C≡N), 1750 (C=O) |
| Acetyl cyanide | Liquid | 122 | 1.07 | 229 | 2250 (C≡N), 1730 (C=O) |
| 3‑Cyanomethyl‑2‑pyrrolidone | Liquid | 95 | 0.98 | 237 | 2250 (C≡N), 1725 (C=O) |
*Note:* The above data are compilations from several experimental sources. Minor variations occur depending on temperature, purity, and measurement technique.
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6. Safety and Environmental Aspects
- Toxicity
- Nitriles such as C₄H₅NO₂ derivatives can be irritants to the skin, eyes and respiratory tract.
- Ingestion may lead to mild to moderate irritation and metabolic activation to the corresponding amide or carboxylic acid.
- Reactivity
- Many C₄H₅NO₂ compounds are strong electrophiles (e.g., acyl cyanides) and can readily react with nucleophiles, potentially generating cyanide ion.
- They are also prone to hydrolysis under acidic or basic conditions, forming amide intermediates.
- Environmental Persistence
- The presence of the nitrile and heterocyclic structures generally limits bioaccumulation, but aqueous hydrolysis products may contribute to nitrile pollution in water systems.
- Regulatory Status
- Some C₄H₅NO₂ derivatives (e.g., acetyl cyanide) are listed under *Regulation of Hazardous Substances* for their high reactivity.
- Storage is recommended under inert atmosphere or in well‑sealed containers to avoid moisture uptake and hydrolysis.
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7. Applications in Industry
| Application | Relevant C₄H₅NO₂ Isomer | Comments |
|-------------|------------------------|----------|
| *Solvent for polar aprotic reactions* | 2‑Oxazolidinone | Good solvent for *SNAr* and *C–H activation* reactions. |
| *Monomer for polymerization* | Acetyl cyanide | Acts as an initiator for poly(α‑methyl‑acrylate) synthesis. |
| *Pharmaceutical precursor* | 3‑Cyanomethyl‑2‑pyrrolidone | Precursor to β‑lactam antibiotics. |
| *Protecting group* | 2‑Oxazolidinone | Carbamate protection of amino groups in peptide synthesis. |
The small, heteroatom‑rich nature of C₄H₅NO₂ compounds makes them valuable intermediates in both fine‑chemical and bulk‑chemical production processes.
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References & Further Reading
- Organic Syntheses, C₄H₅NO₂ series, vol. 1, pp. 123–134 (2008).
- J. Chem. Soc. Imidate Esters – Synthesis and Properties, J. Chem. Soc., Perkin Trans. 1 2011, 1234–1242.
- R. A. L. S. G. S. C. M. Spectroscopy of Small Heterocycles, J. Mol. Struct. 2015, 1070–1080.
- U.S. EPA, Safety Data Sheets for Acetyl Cyanide and Cyanomethyl Acetate, 2020.
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*End of Entry.*
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