THIOUREA

Thiourea [(NH2)2CS] is a small organosulfur compound of considerable importance in organic synthesis, coordination chemistry, material science and industrial processes. 
This article provides a detailed, authoritative examination of thiourea: its historical development, synthesis and production methods, physicochemical properties, analytical characterization, reaction chemistry, applications across sectors (including chemical synthesis, textile, pharmaceutical, photographic, rubber vulcanization, and electroplating), environmental fate, toxicology and safety, regulatory status, alternatives, and analytical methods for quality control.
 
Introduction and historical background
Thiourea has been known since the 19th century and was recognized as a sulfur analogue of urea. Early work focused on its basic chemistry and ability to form addition compounds and complexes. Because of its simple structure and versatile reactivity—acting as a nucleophile, a ligand, and a sulfur source—thiourea has found use in a wide variety of chemical processes. 
Historically, thiourea was used in textile dyeing and in photographic fixing; thereafter its role expanded into synthesis and coordination chemistry. More recently, substituted thioureas have been exploited as organocatalysts and as building blocks for heterocyclic compounds.
 
Nomenclature, synonyms and identifiers
IUPAC name: Thiourea (systematic accepted trivial name).
Other names/synonyms: Thiocarbamide; thiourea; carbamidine thio-; urea thio-.
CAS registry number: 62-56-6.
Molecular formula: CH4N2S (often written as (NH2)2CS).
Molar mass: 76.12 g·mol⁻¹.
SMILES: S=C(N)N
InChI (standard): InChI=1S/CH4N2S/c1-3(2)4/h1-2H2
 
Molecular structure and physicochemical properties
Molecular geometry and bonding
Thiourea is isostructural with urea except the carbonyl oxygen is replaced by a sulfur atom. 
The C=S bond is polarizable and has partial double-bond character. 
The molecule is planar around the central thiocarbonyl center with two amino groups which engage in hydrogen bonding—internally and between molecules—giving rise to characteristic solid-state packing and relatively high melting point for its molar mass.
 
Physical properties
Appearance: White crystalline solid (powdered commercial samples are often granular).
Melting point: ~170–172 °C (decomposes on melting in many samples).
Boiling point: Decomposes before boiling; not typically distilled.
Density: ≈1.38 g·cm⁻³ (solid).
Solubility: Highly soluble in water (~1:1 to more, depending on temperature), soluble in polar organic solvents (e.g., ethanol, methanol), sparingly soluble in nonpolar solvents.
Hygroscopicity: Moderately hygroscopic; commercial grades may show slight moisture uptake.
 
Spectroscopic features (summary)
IR: Strong C=S stretching (lower frequency than C=O in urea), N–H stretching bands, NH2 bending and C–N stretching features.
NMR (1H): Amino protons appear as broad signals due to exchange; 13C shows the thiocarbonyl resonance downfield compared with urea carbonyl.
MS: Molecular ion consistent with molar mass; fragmentation patterns include loss of NH2, H2S formation under certain ionization conditions.
 
Thermodynamic and stability data
Thiourea is thermally stable up to its decomposition range; decomposition yields ammonia, hydrogen sulfide and other sulfur‑containing fragments under severe heating. 
It is chemically stable under neutral conditions but can hydrolyze under strongly oxidative or high-temperature acidic/basic conditions to yield urea derivatives, sulfides, or release sulfur-containing species depending on reaction conditions.
 
Production and laboratory synthesis routes
Industrial production
Commercial production of thiourea historically employed the reaction of hydrogen sulfide or sulfurizing agents with urea or from ammonium thiocyanate reduction. 
Typical industrial approaches include:
Urea + H2S route: Urea reacts with hydrogen sulfide under controlled conditions to yield thiourea and water. This method requires safe handling of H2S.
Ammonium thiocyanate rearrangement/reduction: Ammonium thiocyanate can be converted to thiourea under heating or reductive rearrangement conditions.
 
Modern processes favor routes that balance cost, availability of raw materials and safety considerations. Byproduct management (especially H2S containment) and waste minimization are critical in scale-up.
 
Laboratory synthesis
Representative small-scale laboratory methods include:
From ammonium thiocyanate: Heating ammonium thiocyanate under controlled conditions yields thiourea via isomerization; further purification by recrystallization from ethanol/water affords analytically pure material. Example: NH4SCN (heat) → (NH2)2CS.
From urea and sulfurizing reagents: Urea treated with phosphorus pentasulfide (P2S5) or Lawesson’s reagent can be converted into thiourea via thionation of the carbonyl group. 
This method is often used in synthesis when starting from urea derivatives.
 
Purification and quality control
Commercial thiourea is typically purified by recrystallization from water or alcohol–water mixtures. Quality control includes testing for moisture, residual sulfide, heavy metal impurities, and assay by titrimetric or chromatographic methods.
 
Analytical methods and quality control
Key analytical techniques used for identity, purity and impurity profiling:
Melting point determination — an initial quick check.
Fourier-transform infrared spectroscopy (FTIR) — to confirm C=S and N–H functional groups.
NMR spectroscopy (1H and 13C) — to confirm molecular structure; 1H often shows broad NH2 signals.
Elemental analysis (C, H, N, S) — verifies stoichiometry.
High-performance liquid chromatography (HPLC) — for purity and detection of organic impurities.
Gas chromatography (GC) — for volatile impurities; thiourea itself is non-volatile under GC conditions but derivatives or decomposition products may be detected.
Ion chromatography / titration — to determine ionic impurities (e.g., thiocyanate, sulfate) and residual sulfide.
Mass spectrometry (MS) — for molecular weight confirmation and fragmentation pattern analysis.
Analytical method validation follows ICH guidelines when used for pharmaceutical or regulated applications.
 
Reaction chemistry and mechanisms
Thiourea participates in a number of classical and modern organic transformations. Key reaction modes include:
 
Nucleophilic attack via sulfur
The thiocarbonyl sulfur is nucleophilic and can participate in alkylation to give S-alkyl thioureas; S-alkylation followed by rearrangement can be used to introduce varied substituents.
 
Nucleophilic attack via nitrogen
Amino groups allow N‑alkylation or acylation to form N-substituted thioureas. These reactions enable the synthesis of substituted thioureas used as intermediates for heterocycles.
 
Cyclizations to heterocycles
Thiourea is a privileged precursor to numerous heterocycles including thiadiazoles, thiazoles, thiourea‑derived guanidines, and aminothiazoles. Examples:
Synthesis of thiazoles — condensation with α-haloketones leads to thiazole formation via thioamide tautomerism and cyclization.
Formation of 2-aminothiazoles — relevant to medicinal chemistry.
 
Hydrolysis and oxidation
Under strong oxidative conditions, thiourea can be oxidized to urea, carbonyl derivatives, or further to sulfur‑containing oxidation products. 
Hydrolysis under extreme conditions yields ammonia and sulfurous species.
 
Thiourea as sulfur donor
Thiourea serves as a sulfurizing agent for conversion of carbonyls to thiocarbonyls in some synthetic procedures (e.g., the synthesis of thioketones/thiocarbonyl derivatives).
 
Organocatalysis
Recently, thiourea derivatives (especially chiral N,N'-disubstituted thioureas) have been employed as hydrogen-bonding organocatalysts in asymmetric synthesis due to their ability to stabilize transition states and activate electrophiles via dual hydrogen bonding.

SAFETY INFORMATION ABOUT THIOUREA

 
 
 
 
 
First aid measures:
Description of first aid measures:
General advice:
Consult a physician. 
Show this safety data sheet to the doctor in attendance.
Move out of dangerous area:
 
If inhaled:
If breathed in, move person into fresh air. 
If not breathing, give artificial respiration.
Consult a physician.
In case of skin contact:
Take off contaminated clothing and shoes immediately. 
Wash off with soap and plenty of water.
Consult a physician.
 
In case of eye contact:
Rinse thoroughly with plenty of water for at least 15 minutes and consult a physician.
Continue rinsing eyes during transport to hospital.
 
If swallowed:
Do NOT induce vomiting. 
Never give anything by mouth to an unconscious person. 
Rinse mouth with water. 
Consult a physician.
 
Firefighting measures:
Extinguishing media:
Suitable extinguishing media:
Use water spray, alcohol-resistant foam, dry chemical or carbon dioxide.
Special hazards arising from the substance or mixture
Carbon oxides, Nitrogen oxides (NOx), Hydrogen chloride gas
 
Advice for firefighters:
Wear self-contained breathing apparatus for firefighting if necessary.
Accidental release measures:
Personal precautions, protective equipment and emergency procedures
Use personal protective equipment. 
 
Avoid breathing vapours, mist or gas. 
Evacuate personnel to safe areas.
 
Environmental precautions:
Prevent further leakage or spillage if safe to do so.
Do not let product enter drains.
Discharge into the environment must be avoided.
 
Methods and materials for containment and cleaning up:
Soak up with inert absorbent material and dispose of as hazardous waste. 
Keep in suitable, closed containers for disposal.
 
Handling and storage:
Precautions for safe handling:
Avoid inhalation of vapour or mist.
 
Conditions for safe storage, including any incompatibilities:
Keep container tightly closed in a dry and well-ventilated place. 
Containers which are opened must be carefully resealed and kept upright to prevent leakage.
Storage class (TRGS 510): 8A: Combustible, corrosive hazardous materials
 
Exposure controls/personal protection:
Control parameters:
Components with workplace control parameters
Contains no substances with occupational exposure limit values.
Exposure controls:
Appropriate engineering controls:
Handle in accordance with good industrial hygiene and safety practice.
Wash hands before breaks and at the end of workday.
 
Personal protective equipment:
Eye/face protection:
Tightly fitting safety goggles. 
Faceshield (8-inch minimum). 
Use equipment for eye protection tested and approved under appropriate government standards such as NIOSH (US) or EN 166(EU).
 
Skin protection:
Handle with gloves. 
Gloves must be inspected prior to use. 
Use proper glove
removal technique (without touching glove's outer surface) to avoid skin contact with this product. 
Dispose of contaminated gloves after use in accordance with applicable laws and good laboratory practices. 
Wash and dry hands.
 
Full contact:
Material: Nitrile rubber
Minimum layer thickness: 0.11 mm
Break through time: 480 min
Material tested:Dermatril (KCL 740 / Aldrich Z677272, Size M)
Splash contact
Material: Nitrile rubber
Minimum layer thickness: 0.11 mm
Break through time: 480 min
Material tested:Dermatril (KCL 740 / Aldrich Z677272, Size M)
It should not be construed as offering an approval for any specific use scenario.
 
Body Protection:
Complete suit protecting against chemicals, The type of protective equipment must be selected according to the concentration and amount of the dangerous substance at the specific workplace.
Respiratory protection:
Where risk assessment shows air-purifying respirators are appropriate use a fullface respirator with multi-purpose combination (US) or type ABEK (EN 14387) respirator cartridges as a backup to engineering controls. 
 
If the respirator is the sole means of protection, use a full-face supplied air respirator. 
Use respirators and components tested and approved under appropriate government standards such as NIOSH (US) or CEN (EU).
Control of environmental exposure
Prevent further leakage or spillage if safe to do so. 
Do not let product enter drains.
Discharge into the environment must be avoided.
 
Stability and reactivity:
Chemical stability:
Stable under recommended storage conditions.
Incompatible materials:
Strong oxidizing agents:
Hazardous decomposition products:
Hazardous decomposition products formed under fire conditions. 
Carbon oxides, Nitrogen oxides (NOx), Hydrogen chloride gas.
 
Disposal considerations:
Waste treatment methods:
Product:
Offer surplus and non-recyclable solutions to a licensed disposal company. 
Contact a licensed professional waste disposal service to dispose of this material.
Contaminated packaging:
Dispose of as unused product