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Poison Hemlock
Poison hemlock (Conium maculatum) contains not only highly toxic amounts of coniine, but also trace amounts of four other similarly poisonous alkaloids. Ingesting less than a tenth of a gram (2.20462x10-4 pounds) of coniine can be fatal for human adults; this is approximately six to eight hemlock leaves. The seeds and roots are also toxic, and in fact are more potent than the leaves. While consuming hemlock is the main threat, poisoning can also arise from skin contact and inhalation.



The presence of hemlock on farmland is an issue for livestock farmers because animals will eat it if they are not well fed or the hemlock is mixed in with pasture grass. Farmers also need to be careful that the hay fed to animals does not contain hemlock. Poison hemlock is most poisonous in the spring when the concentration of y-coniciene (the precursor to other toxins) is at its peak.

Poison hemlock grows quite tall, reaching heights of up to twelve feet. The stalk of hemlock is green with purple spots and completely lacks hair. A biennial plant, hemlock produces leaves at its base the first year but no flowers. In its second year it produces white flowers in umbrella shaped clusters. Hemlock can be confused with the wild carrot plant; however this plant has a hairy stem without purple markings, grows less than three feet tall, and does not have clustered flowers. While the hemlock plant is native to Europe and the Mediterranean region, it has spread to every other continent excluding Antarctica.

The Yellow Pitcher Plant
The yellow pitcher plant (Sarracenia flava) is a carnivorous plant found exclusively in the southeastern United States that produces coniine. The plant uses a mixture of sugar and coniine to simultaneously attract and poison insects, which then fall into a digestive tube. The naming of the plant arises from the shape of the opening to these tubes, which resembles a pitcher. The pitcher shape has a leaf above to prevent rain from diluting the digestive fluids deep in the tubes. These pitchers can grow up to three feet tall. In the spring the plant produces yellow flowers with five petals each.

There are no reports online of human poisoning via the yellow pitcher plant, perhaps because only a small portion of the plant contains coniine, or because it does not contain enough to produce toxicity. It is also not as widespread as hemlock and therefore is less likely to be contacted by humans.

History
The history of coniine is understandably tied to the poison hemlock plant since it was not synthesizable until modern times. While the yellow pitcher plant also contains coniine, it has no traditional uses.

The most famous hemlock poisoning occurred in 399 B.C.E. when the philosopher Socrates drank a liquid infused with hemlock to carry out his death sentence (Socrates had been convicted of impiety toward the gods and corruption of the youth). In fact, hemlock juice was often used to execute criminals in ancient Greece.

Hemlock has had a limited medical use throughout history. The Greeks used it not just as capital punishment, but also as an antispasmodic and treatment for arthritis. Books from the 10th century attest to medical use by the Anglo-Saxons. In the Middle Ages it was believed that hemlock could be used to cure rabies ; in later European times it came to be associated with witchcraft. Native Americans used hemlock extract as arrow poison.

Coniine is the poison used to kill Amyas Crale in Five Little Pigs (published in 1943), also known as Murder in Retrospect, one of Agatha Christie's Hercule Poirot mysteries.

Coniine was first isolated by Giesecke, but the formula was suggested by Blyth and definitely established by Hoffmann.

Chemical Properties
D -(S)-Coniine is a colorless alkaline liquid, with a penetrating odour and a burning taste; has D 0° 0.8626 and D 19° 0.8438, refractive index n23°D 1.4505, and is dextrorotatory, [α]19°D +15.7°. (See comments about the specific rotation below, under "Enantiomers"). L -(R)-Coniine has [α]21°D 15° and in other respects resembles its D -isomer, but the salts have slightly different melting points; the platinichloride has mp. 160 °C (Löffler and Friedrich report 175 °C), the aurichloride mp. 59 °C.

Solubility
Coniine is slightly soluble (1 in 90) in cold water, less so in hot water, so that a clear cold solution becomes turbid when warmed. On the other hand, the base dissolves about 25% of water at room temperature. It mixes with alcohol in all proportions, is readily soluble in ether and most organic solvents. Coniine dissolves in carbon disulfide, forming a complex thiocarbamate.

Crystallization
Coniine solidifies into a soft crystalline mass at −2 °C. It slowly oxidizes in the air. The salts crystallize well and are soluble in water or alcohol. The hydrochloride, B•HCl, crystallizes from water in rhombs, mp. 220 °C, [α]20°D +10.1°; the hydrobromide, in needles, mp. 211 °C, and the D -acid tartrate, B•C4H6O6•2 H2O, in rhombic crystals, mp. 54 °C. The platinichloride, (B•HCl)2•PtCl4•H2O, separates from concentrated solution as an oil, which solidifies to a mass of orange-yellow crystals, mp. 175 °C (dry). The aurichloride, B•HAuCl4, crystallizes on standing, mp. 77 °C. The picrate forms small yellow needles, mp. 75 °C, from hot water. The 2,4-dinitrobenzoyl- and 3,5-dinitrobenzoyl-derivates have mps. 139.0–139.5 °C and 108–9 °C respectively. The precipitate afforded by potassium cadmium iodide solution is crystalline, mp. 118 °C, while that given by nicotine with this reagent is amorphous.

Color Changes
It gives no coloration with sulfuric or nitric acid. Sodium nitroprusside gives a deep red color, which disappears on warming, but reappears on cooling, and is changed to blue or violet by aldehydes.

Enantiomers
Naturally-occurring coniine is present in Conium maculatum as a mixture of the R-(−)- and S-(+)- enantiomers, although the S-enantiomer predominates.

The stereochemical composition of "coniine" is a matter of some importance, since its two enantiomers do not have identical biological properties, and many of the older pharmacological studies on this compound were undoubtedly carried out using the naturally-occurring isomeric mixture.

The common criterion for enantiomeric homogeneity is the specific rotation, [α]D. This value depends on such factors as temperature, solvent and concentration of the analyte, but it is also important to note that a salt such as the hydrochloride of a given enantiomer will not necessarily have the same specific rotation as the same enantiomer of the free base.

Modern values for the specific rotation of the enantiomers of coniine, and their hydrochloride salts are as follows:

S-(+)-coniine (which is identical to D-(+)-coniine, but the "D/L" and "S/R" systems of stereochemical nomenclature are not usually mixed together in the same enantiomer name) has [α]D = +8.4° (c = 4.0, in CHCl3) These authors note that Ladenburg's value of +15° is for a "neat" sample, i.e., one that is undiluted with any solvent.

A similarly high value of +16° for the [α]D of "coniine" is given, without explicit citation of the source, in The Merck Index.

The value of +7.7° (c = 4.0, CHCl3) for synthetic S-(+)-coniine and -7.9° (c = 0.5, CHCl3) for synthetic R-(−)-coniine is given by other chemists

The hydrochloride salt of S-(+)-coniine has [α]D = +4.6° (c = 0.5, in methanol).

The hydrochloride salt of R-(−)-coniine has [α]D = -5.2° (c = 0.5, in methanol).

Many syntheses of coniine have been reported over the last 50 years; one example of a stereoselective synthesis is that of Enders and Tiebes, who cite some of the earlier preparations.