Everything about Limonene totally explained
| Section2 =
| Section7 =
| SPhrases =
}}
}}
Limonene is a
hydrocarbon, classified as a cyclic
terpene. It is a colourless liquid at room temperatures with an extremely strong smell of
oranges. It takes its name from the
lemon, as the
rind of the lemon, like other
citrus fruits, contains considerable amounts of this chemical compound, which is responsible for much of their smell. Limonene is a
chiral molecule, and as is common with such forms, biological sources produce one
enantiomer: the principal industrial source, citrus fruit, contains
D-limonene ((+)-limonene), which is the (
R)-
enantiomer (
CAS number 5989-27-5,
EINECS number 227-813-5).
Racemic limonene is known as dipentene.
Uses
The major use of d-limonene is as a precursor to
carvone.
As the main odour constituent of
citrus (plant family
Rutaceae),
d-limonene is used in food manufacturing and some medicines, for example, bitter
alkaloids, as a
flavoring, and added to cleaning products such as hand cleansers to give a lemon-orange
fragrance. See:
orange oil. Limonene is common in
cosmetic products.
Limonene is increasingly being used as a
solvent for cleaning purposes, such as the removal of oil from machine parts, as it's produced from a renewable source (citrus oil, as a byproduct of
orange juice manufacture.) It also serves as a paint stripper when applied to painted wood.
As it's combustible, limonene has also been considered as a
biofuel.
In perhaps its most exciting application, limonene is useful for recycling
polystyrene. In the 1990s, researchers at
Sony discovered that limonene dissolves polystyrene, which can be recovered after boiling off the limonene..
Uses of d-limonene
The (
R)-
enantiomer (
CAS number 5989-54-8,
EINECS number 227-815-6) is also used as
botanical insecticide. In contrast to the citrus (orange-lemon) scent (see above),
l-limonene has a piney,
turpentine-like odor.
l-Limonene is used as a fragrance in some cleaning products.
Organic chemistry
Limonene is a relatively stable terpene, which can be distilled without decomposition, although at elevated temperatures it "cracks" to form
isoprene. It oxidises easily in moist air to
carveol and
carvone. Dehydration with sulfur gives to
p-
cymene, hydrogen sulfide, as well as some
sulfides.
Limonene occurs naturally as the (
R)-
enantiomer, but racemises to dipentene at 300 °C. When warmed with
mineral acid, limonene isomerizes to the
conjugated diene α-
terpinene, which can itself easily be oxidised to
p-cymene, an
aromatic hydrocarbon. Evidence for this isomerization includes the formation of
Diels-Alder α-terpinene adducts when limonene is heated with
maleic anhydride.
It is possible to effect reaction at one of the double bonds selectively. Anhydrous
hydrogen chloride reacts preferentially at the disubstituted alkene, whereas
epoxidation with
MCPBA occurs at the trisubstituted alkene. In both cases the second C=C double bond can be made to react if desired.
In another synthetic method
Markovnikov addition of
trifluoroacetic acid followed by
hydrolysis of the acetate gives
terpineol.
The most widely practiced conversion of limonene is to carvone. The three step reaction begins with the regioselective addition of nitrosyl chloride across the trisubstituted double bond. This species is then converted to the
oxime with base, and the hydroxylamine is removed to give the ketone-containing carvone. The final step involves loss of a proton from the cation to form the
alkene.
Safety
Limonene and its oxidation products are skin irritants, and limonene-1,2-oxide (formed by aerial oxidation) is a known skin sensitizer. Most reported cases of irritation have involved long-term industrial exposure to the pure compound,
for example during degreasing or the preparation of paints. However a study of patients presenting
dermatitis showed that 3% were sensitized to limonene.
Although once thought to cause
renal cancer in rats, limonene now is known as a significant
chemopreventive agent with potential value as a dietary anti-cancer tool in humans. There is no evidence for
carcinogenicity or
genotoxicity in humans. The
IARC classifies
d-limonene under Class 3:
not classifiable as to its carcinogenicity to humans.
[
]Notes
Further Information
Get more info on 'Limonene'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://limonene.totallyexplained.com">Limonene Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
