Camphor

Camphor (/ˈkæmfər/) is a waxy, colorless solid with a strong aroma.[5] It is classified as a terpenoid and a cyclic ketone. It is found in the wood of the camphor laurel (Cinnamomum camphora), a large evergreen tree found in East Asia; and in the kapur tree (Dryobalanops sp.), a tall timber tree from South East Asia. It also occurs in some other related trees in the laurel family, notably Ocotea usambarensis. Rosemary leaves (Rosmarinus officinalis) contain 0.05 to 0.5% camphor,[6] while camphorweed (Heterotheca) contains some 5%.[7] A major source of camphor in Asia is camphor basil (the parent of African blue basil). Camphor can also be synthetically produced from oil of turpentine.

Not to be confused with samphor.
For other uses, see Camphor (disambiguation).
Camphor[1][2]
Structural formula of (R) and (S)-camphor
(+)- and (−)-camphor
Ball and stick model of camphor (both enantiomers).
Names
IUPAC name
1,7,7-Trimethylbicyclo[2.2.1]heptan-2-one
Other names
2-Bornanone; Bornan-2-one; 2-Camphanone; Formosa
Identifiers
3D model (JSmol)
3DMet
1907611
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.000.860
EC Number
  • 200-945-0
83275
KEGG
MeSH Camphor
RTECS number
  • EX1225000
UNII
UN number 2717
  • InChI=1S/C10H16O/c1-9(2)7-4-5-10(9,3)8(11)6-7/h7H,4-6H2,1-3H3 checkY
    Key: DSSYKIVIOFKYAU-UHFFFAOYSA-N checkY
  • InChI=1/C10H16O/c1-9(2)7-4-5-10(9,3)8(11)6-7/h7H,4-6H2,1-3H3
    Key: DSSYKIVIOFKYAU-UHFFFAOYAK
  • CC1(C)C2CCC1(C)C(=O)C2
  • O=C1CC2CCC1(C)C2(C)C
Properties
C10H16O
Molar mass 152.237 g·mol−1
Appearance White, translucent crystals
Odor Fragrant and penetrating
Density 0.992 g·cm−3
Melting point 175–177 °C (347–351 °F; 448–450 K)
Boiling point 209 °C (408 °F; 482 K)
1.2 g·dm−3
Solubility in acetone ~2500 g·dm−3
Solubility in acetic acid ~2000 g·dm−3
Solubility in diethyl ether ~2000 g·dm−3
Solubility in chloroform ~1000 g·dm−3
Solubility in ethanol ~1000 g·dm−3
log P 2.089
Vapor pressure 4 mmHg (at 70 °C)
+44.1°
−103×10−6 cm3/mol
Pharmacology
C01EB02 (WHO)
Hazards
GHS labelling:
GHS02: FlammableGHS07: Exclamation markGHS08: Health hazard
Warning
H228, H302, H332, H371
P210, P240, P241, P260, P261, P264, P270, P271, P280, P301+P312, P304+P312, P304+P340, P309+P311, P312, P330, P370+P378, P405, P501
NFPA 704 (fire diamond)
NFPA 704 four-colored diamond
2
2
0
Flash point 54 °C (129 °F; 327 K)
466 °C (871 °F; 739 K)
Explosive limits 0.6–3.5%[3]
Lethal dose or concentration (LD, LC):
1310 mg/kg (oral, mouse)[4]
800 mg/kg (dog, oral)
2000 mg/kg (rabbit, oral)[4]
400 mg/m3 (mouse, 3 hr)[4]
NIOSH (US health exposure limits):
PEL (Permissible)
 TWA 2 mg/m3[3]
REL (Recommended)
 TWA 2 mg/m3[3]
IDLH (Immediate danger)
 200 mg/m3[3]
Related compounds
Related Ketones
 Fenchone, Thujone
Related compounds
 Camphene, Pinene, Borneol, Isoborneol, Camphorsulfonic acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)
Infobox references

The compound is chiral, existing in two possible enantiomers as shown in the structural diagrams. The structure on the left is the naturally occurring (+)-camphor ((1R,4R)-bornan-2-one), while its mirror image shown on the right is the (−)-camphor ((1S,4S)-bornan-2-one). Camphor has few uses but is of historic significance as a compound that is readily purified from natural sources.

Etymology

The word camphor derived in the 14th century from Old French: camphre, itself from Medieval Latin: camfora, from Arabic: كافور, romanized: kāfūr, perhaps through Sanskrit: कर्पूर, romanized: karpūra, apparently from Austronesian Malay: kapur 'lime' (chalk).[8]

In Old Malay, camphor was called kapur barus, meaning "the chalk of Barus", referring to Barus, an ancient port near modern Sibolga on the western coast of Sumatra.[9] This port traded in camphor extracted from the Borneo camphor trees (Dryobalanops aromatica) that were abundant in the region.[10]

Production

Natural camphor

Camphor has been produced as a forest product for centuries, condensed from the vapor given off by the roasting of wood chips cut from Camphora officinarum, and later by passing steam through the pulverized wood and condensing the vapors.[11] By the early 19th century most camphor tree reserves had been depleted with the remaining large stands in Japan and Taiwan, with Taiwanese production greatly exceeding Japanese. Camphor was one of the primary resources extracted by Taiwan's colonial powers as well as one of the most lucrative. First the Chinese and then the Japanese established monopolies on Taiwanese camphor. In 1868, a British naval force sailed into Anping harbor and the local British representative demanded the end of the Chinese camphor monopoly. After the local imperial representative refused, the British bombarded the town and took the harbor. The "camphor regulations" negotiated between the two sides subsequently saw a brief end to the camphor monopoly.[12]

Synthetic camphor

Camphor is produced from alpha-pinene, which is abundant in the oils of coniferous trees and can be distilled from turpentine produced as a side product of chemical pulping. With acetic acid as the solvent and with catalysis by a strong acid, alpha-pinene is converted to isobornyl acetate. Hydrolysis of this ester gives isoborneol which can be oxidized to give racemic camphor. By contrast, camphor occurs naturally as D-camphor, the (R)-enantiomer.

Reactions

The reactions of camphor have been extensively examined. Some representative transformations include


Camphor can also be reduced to isoborneol using sodium borohydride.

Biochemistry

Biosynthesis

In biosynthesis, camphor is produced from geranyl pyrophosphate, via cyclisation of linaloyl pyrophosphate to bornyl pyrophosphate, followed by hydrolysis to borneol and oxidation to camphor.

Biosynthesis of camphor from geranyl pyrophosphate
Biosynthesis of camphor from geranyl pyrophosphate

Uses

The first significant manmade plastics were low-nitrogen (or "soluble") nitrocellulose (pyroxylin) plastics. In the early decades of the plastics industry, camphor was used in immense quantities[15]: 130  as the plasticizer that creates celluloid from nitrocellulose, in nitrocellulose lacquers and other plastics and lacquers.

Alternative medicine and scent

Camphor has been used for its scent, as an embalming fluid, as topical medication, as a manufacturing chemical, and in religious ceremonies.

Camphor cubes

Camphor has been used as a folk medicine over centuries, probably most commonly as a decongestant.[16] Camphor was used in ancient Sumatra to treat sprains, swellings, and inflammation.[17] Camphor also was used for centuries in traditional Chinese medicine for various purposes.[16] In Europe, camphor was used after the Black Death era.[18]

In the 20th century, camphor was used as an analeptic by injection,[19] and to induce seizures in schizophrenic people in an attempt to treat psychosis.[20]

Camphor has limited use in veterinary medicine by intramuscular injection to treat breathing difficulties in horses.[21]

Topical medication

Camphor is commonly applied as a topical medication as a skin cream or ointment to relieve itching from insect bites, minor skin irritation, or joint pain.[22] It is absorbed in the skin epidermis,[22] where it stimulates nerve endings sensitive to heat and cold, producing a warm sensation when vigorously applied, or a cool sensation when applied gently, indicating its properties as a counterirritant.[16] The action on nerve endings also induces a slight local analgesia.[23]

Respiratory aerosol

Camphor is also used via an aerosol, typically by steam inhalation, sometimes in the form of branded nasal inhaler sticks, to inhibit coughing and relieve upper airway congestion due to the common cold.[24] However, the clinical efficiency of these remedies is challenged.[25]

Other niche uses

Camphor is used by marksmen to blacken the front and rear sights of rifles to prevent the sights from reflecting.[26] This is done by setting light to a small amount of camphor, which burns at a relatively low temperature, and using the soot rising from the flame to deposit a coating on a surface held above it. Historically, this soot blackening was also used to coat Barograph record charts.

Pest deterrent and preservative

Camphor is believed to be toxic to insects and is thus sometimes used as a repellent.[27] Camphor is used as an alternative to mothballs. Camphor crystals are sometimes used to prevent damage to insect collections by other small insects. It is kept in clothes used on special occasions and festivals, and also in cupboard corners as a cockroach repellent. The smoke of camphor crystal or camphor incense sticks can be used as an environmentally-friendly mosquito repellent.[28]

Recent studies have indicated that camphor essential oil can be used as an effective fumigant against red fire ants, as it affects the attacking, climbing, and feeding behavior of major and minor workers.[29]

Camphor is also used as an antimicrobial substance. In embalming, camphor oil was one of the ingredients used by ancient Egyptians for mummification.[30]

Solid camphor releases fumes that form a rust-preventative coating and is therefore stored in tool chests to protect tools against rust.[31]

Perfume

In the ancient Arab world, camphor was a common perfume ingredient.[32] The Chinese referred to the best camphor as "dragon's brain perfume", due to its "pungent and portentous aroma" and "centuries of uncertainty over its provenance and mode of origin".[33]

Culinary uses

One of the earliest known recipes for ice cream dating to the Tang dynasty includes camphor as an ingredient.[34] It was used to flavor leavened bread in ancient Egypt.[35] In ancient and medieval Europe, camphor was used as an ingredient in sweets. It was used in a wide variety of both savory and sweet dishes in medieval Arabic language cookbooks, such as al-Kitab al-Ṭabikh compiled by ibn Sayyār al-Warrāq in the 10th century.[36] It also was used in sweet and savory dishes in the Ni'matnama, according to a book written in the late 15th century for the sultans of Mandu.[37] It is a main constituent of a spice known as "edible camphor" (or kapur), which may be used in traditional South Indian desserts like Payasam and Chakkarai Pongal.[38]

Religious rites

Camphor is widely used in Hindu religious ceremonies. Aarti is performed after placing it on a stand and setting fire to it usually as the last step of puja.[39] Camphor is mentioned in the Quran as being the fragrance of wine given to believers in heaven.[40]

Toxicity

Applied on skin, camphor may cause allergic reactions in some people; when ingested by mouth, camphor cream or ointment is poisonous.[22] In high ingested doses, camphor produces symptoms of irritability, disorientation, lethargy, muscle spasms, vomiting, abdominal cramps, convulsions, and seizures.[41] Lethal doses by ingestion in adults are in the range 50–500 mg/kg (orally). Generally, ingestion of two grams causes serious toxicity and four grams is potentially lethal.[42]

Airborne camphor may be toxic if respired by humans. The Permissible Exposure Limit (PEL) for camphor in ambient air is 2 mg/m3 at exposure time (TWA) not more than 8 hours. 200 mg/m3 is considered a very dangerous concentration (IDLH).[43]

History of synthetic camphor

When its use in the nascent chemical industries (discussed below) greatly increased the volume of demand in the late 19th century, potential for changes in supply and in price followed. In 1911 Robert Kennedy Duncan, an industrial chemist and educator, related that the Imperial Japanese government had recently (1907–1908) tried to monopolize the production of natural camphor as a forest product in Asia but that the monopoly was prevented by the development of the total synthesis alternatives,[15] which began in "purely academic and wholly uncommercial"[15] form with Gustav Komppa's first report:

"... but it sealed the fate of the Japanese monopoly ... For no sooner was it accomplished than it excited the attention of a new army of investigators—the industrial chemists. The patent offices of the world were soon crowded with alleged commercial syntheses of camphor, and of the favored processes companies were formed to exploit them, factories resulted, and in the incredibly short time of two years after its academic synthesis artificial camphor, every whit as good as the natural product, entered the markets of the world ... And yet artificial camphor does not—and cannot—displace the natural product to an extent sufficient to ruin the camphor-growing industry. Its sole present and probable future function is to act as a permanent check to monopolization, to act as a balance-wheel to regulate prices within reasonable limits."[15]: 133–134 

This ongoing check on price growth was confirmed in 1942 in a monograph on DuPont's history, where William S. Dutton said, "Indispensable in the manufacture of pyroxylin plastics, natural camphor imported from Formosa and selling normally for about 50 cents a pound, reached the high price of $3.75 in 1918 [amid the global trade disruption and high explosives demand that World War I created]. The organic chemists at DuPont replied by synthesizing camphor from the turpentine of southern US pine stumps, with the result that the price of industrial camphor sold in carload lots in 1939 was between 32 cents and 35 cents a pound."[44]: 293 

The background of Gustaf Komppa's synthesis was as follows. In the 19th century, it was known that nitric acid oxidizes camphor into camphoric acid. Haller and Blanc published a semisynthesis of camphor from camphoric acid. Although they demonstrated its structure, they were unable to prove it. The first complete total synthesis of camphoric acid was published by Komppa in 1903. Its inputs were diethyl oxalate and 3,3-dimethylpentanoic acid, which reacted by Claisen condensation to yield diketocamphoric acid. Methylation with methyl iodide and a complicated reduction procedure produced camphoric acid. William Perkin published another synthesis a short time later. Previously, some organic compounds (such as urea) had been synthesized in the laboratory as a proof of concept, but camphor was a scarce natural product with a worldwide demand. Komppa realized this, and began industrial production of camphor in Tainionkoski, Finland, in 1907 (with plenty of competition, as Kennedy Duncan reported).

See also

References
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