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Physical Data: mp 113.6 °C; bp 185.24 °C; d 4.930 g cm-3; vapor pressure 0.31 mmHg at 25 °C.
Solubility: the solubility of iodine, expressed in g/kg of solvent at 25 °C is: H2O, 0.34; benzene, 164.0; CCl4, 19.2; CHCl3, 49.7; ethyl acetate, 157; ethanol, 271.7; diethyl ether, 337.3; n-hexane, 13.2; toluene, 1875.12 Soluble glacial acetic acid; relatively insol dichloromethane.
Form Supplied in: the natural abundance isotope is 127I. It is a massive bluish-black solid. When sublimed it forms near opaque, doubly refractory orthorhombic crystals that have a metallic luster. Heating iodine generates a violet-colored vapor. Commercially available in >99.5% purity, with bromine and chlorine the primary contaminants. Natural abundance iodine is diatomic, I-I.
Handling, Storage, and Precautions: somewhat corrosive.15 It is stored in a dark bottle or jar, at ambient temperatures. Iodine vapors have a sharp characteristic odor and they are irritating to eyes, skin, and mucous membranes (lachrymatory). Prolonged exposure should be avoided. Ingestion of large quantities can cause abdominal pain, nausea, vomiting, and diarrhea. If 2-3 g of iodine are ingested, death may occur.
Diatomic iodine (I2) is a member of the halogen family that is widely used in organic chemistry. Iodine is less electronegative than the other halogens, and iodides are generally less stable than other halides.16 Oxides of iodine and compounds where iodine is in a positive valence state are much more stable than the other halogens. Iodine forms binary compounds with all elements except sulfur, selenium, and the noble gases, although it does not react directly with carbon, nitrogen, or oxygen.15 Its applications in organic chemistry range from detection of organic molecules in TLC, to addition reactions with unsaturated molecules, to reactions as an electrophilic agent with nucleophilic species. Iodine is used not only as an agent for incorporating an iodine atom but also as an oxidizing agent, a dehydrogenation agent, and as a radiolabel in many biologically important systems.
Iodine reacts with dienes to form a mixture of 1,2-diiodoalkenes and 1,4-diiodoalkenes. When done in the presence of Copper(I) Cyanide, the 1,4-addition product predominates and 1,3-butadiene thus reacts to give an 84% yield of 1,4-dicyano-2-butene (eq 4).22 Allenes react with iodine to give diiodides. When 2,3-pentadiene reacts with iodine in carbon tetrachloride, 2,3-diiodo-3-pentene is formed (eq 5).23 When the reaction is done in methanol, however, 3-methoxy-2-iodo-3-pentene is the product.
There are two very interesting and useful variations of the fundamental addition reactions to alkenes: iodolactonization3 (to form iodolactones) and iodolactamization (to produce iodolactams). When an alkenyl acid reacts with iodine in the presence of a base (such as sodium bicarbonate), the initially formed iodonium ion reacts with the carboxylate anion (generated in situ) to form the iodolactone (eq 6).
Iodine also reacts with cyclopropanes, leading to ring opening and formation of a diiodide.28 The cyclopropane ring in benzocyclopropanes, for example, reacts with iodine to produce the diiodide (eq 12). Cyclopropylcarbinyl systems are opened by iodine, and when a leaving group is available, such as trimethyltin, an alkenyl iodide is formed (eq 13).
Conversion of Alcohols to Iodides.
Alcohols react with iodine and red phosphorus to produce a phosphorus iodide, in situ. Phosphorus iodides have poor shelf lives (they are unstable and decompose under mild conditions) and are prepared immediately prior to use. An example is the conversion of cetyl alcohol to cetyl iodide in 85% yield (eq 14).29 This is the most common method for the conversion of aliphatic alcohols to aliphatic iodides.
Reactions with Ketones, Aldehydes, and Carboxylic Acid Derivatives.
Iodine reacts with ketones as well as with alkenes. The reaction is usually done in the presence of base and proceeds via the enolate anion. This is the fundamental process that occurs in the Lieben iodoform reaction,33 in which a methyl ketone reacts with iodine and sodium hydroxide to give iodoform (CHI3) with oxidative cleavage of the methyl group to produce a carboxylic acid. The H3C-C bond of methyl carbinols [RCH(OH)Me] is also cleaved with this reagent to give the corresponding acid and iodoform. The iodoform reaction constitutes a classical test for the presence of a methyl ketone moiety or a methyl carbinol moiety in an unknown molecule.
Carboxylic acid derivatives can react with iodine without an intermediary enolate anion to produce a-iodocarboxylic acids. a-Iodocarboxylic acid chlorides can also be produced, as when hexanoic acid reacts with iodine and Thionyl Chloride, at 85 °C, to give an 80% yield of 2-iodohexanoyl chloride (eq 22).36 Similarly, butanoic acid reacts with Chlorosulfonic Acid and iodine to give a 94% yield of 2-iodobutanoic acid (eq 23).37 These examples are nothing more than the iodine analog of the Hell-Volhard-Zelinsky reaction.38 The silver salt of pentanoic acid reacts with iodine to produce 1-iodobutane in 67% yield, where decarboxylation occurs under the reaction conditions (eq 24).39 In general, alkyl iodides are formed from silver carboxylates. This is the iodine version of the Hunsdiecker reaction.40 Similar reaction occurs when mercury(II) oxide is added, although the yield is lower.
Iodination of Aromatic and Heteroaromatic Compounds.
Simple aromatic derivatives can be iodinated to generate iodo-substituted aromatic compounds, if activating substituents are present on the aromatic ring. 1,3-Dicyanobenzene, for example, reacts with LDA and iodine to give a 79% yield of 2-iodo-1,3-dicyanobenzene (eq 30).45 In general, unactivated aromatics are less useful since formation of the requisite carbanion is somewhat more difficult.
Conversion of Organoboranes to Iodides.
Substituted alkenes can also be prepared from vinylboranes by reaction with iodine and sodium hydroxide. Reaction of dicyclohexylborane with 1-hexyne gives the vinylborane, and subsequent reaction with basic iodine, in THF, gives a 93:7 cis:trans mixture of 1-cyclohexyl-1-hexene in 85% yield (eq 32).10 When the reaction is done in dichloromethane, a 77:23 cis:trans mixture is produced, but in only 13% yield.10a The poor yield is probably due to the poor solubility of iodine in dichloromethane.
An important reaction of iodine is exchange with an alkyl iodide. The most common method for exchanging an iodide is the Finkelstein reaction,49 which involves treatment of alkyl halides with Sodium Iodide to produce alkyl iodides via an SN2 reaction. Reaction of 1-bromobutane and sodium iodide in dry acetone, for example, gives 1-iodobutane. This exchange also occurs with alkyl iodides. The metal iodides used in this reaction are commercially available, but can be prepared from iodine.
Dimethyl Sulfoxide-Iodine; Iodine-Aluminum(III) Chloride-Copper(II) Chloride; Iodine-Cerium(IV) Ammonium Nitrate; Iodine-Copper(II) Acetate; Iodine-Copper(I) Chloride-Copper(II) Chloride; Iodine-Copper(II) Chloride; Iodine-Nitrogen Tetroxide; Iodine-Potassium Iodate; Iodine-Silver Acetate; Iodine-Silver Benzoate; Iodine-Silver(I) Fluoride; Iodine-Silver Trifluoroacetate; Lead(IV) Acetate-Iodine; Mercury(II) Oxide-Iodine; Thallium(I) Acetate-Iodine; Triphenylphosphine-Iodine.
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