Halogens, elemental alcohols

PHYSICAL PROPERTIES Dense, greenish-yellow diatomic gas clear amber liquid pungent, irritating odor non-metallic halogen element soluble in chlorides, alcohols slightly soluble in cold water MP (-101°C, -149.8°F) BP (-34.9°C, 30.8°F) VD (2.44) VP (4800 mm Hg at 20°C) SG (2.48). 

Chemical Properties. A combination of excellent chemical and mechanical properties at elevated temperatures result in high performance service in the chemical processing industry. Teflon PEA resins have been exposed to a variety of organic and inorganic compounds commonly encountered in chemical service (26). They are not attacked by inorganic acids, bases, halogens, metal salt solutions, organic acids, and anhydrides. Aromatic and ahphatic hydrocarbons, alcohols, aldehydes, ketones, ethers, amines, esters, chlorinated compounds, and other polymer solvents have Httle effect. However, like other perfluorinated polymers,they react with alkah metals and elemental fluorine. 

Halogenation. Normally, 2-halopropane derivatives are prepared from isopropyl alcohol most economically by reaction with the corresponding acid haUde. However, under appropriate conditions, other reagents, eg, phosphoms haUdes and elemental halogen, also react by replacement of the hydroxyl group to give the haUde (46). 

Iodine is the least reactive of the elements in the halogen group 17. Most people associate iodine with the dark-brown color of the tincture of iodine used as an antiseptic for minor skin abrasions and cuts. A tincture is a 50% solution of iodine in alcohol. Although it is still used, iodine is no longer the antibiotic of choice for small skin wounds. Since iodine is a poison that kills bacteria, iodine tablets are often used by campers and others to purify water that is taken from outdoor streams. 

Halohydrins are /J-halogenated alcohols. They can be obtained in H20-containing solvents from alkenes and reagents, which transfer Hal ions. N-Broniosuccinnuide (transfers Br Figures 3.43 and 3.44 as well as 3.47), chloramine-T (transfers Cl Figure 3.46), and elemental iodine (transfers I Figure 3.47) have this ability. Bromonium and chloronium ions react with H20 via an SN2 mechanism. This furnishes the protonated bromo- or chlorohydrins, which are subsequently deprotonated. 

Treatment of an alkene with a strong acid, such as sulfuric acid, that has a relatively nonnucleophilic conjugate base results in the addition of the elements of water (H and OH) to the double bond. This reaction has many similarities to the addition of the halogen acids described in Section 11.2. First H+ adds to produce a carbocation and then water acts as the nucleophile. The reaction follows Markovnikov s rule and the stereochemistry is that expected for a reaction that involves a carbocation—loss of stereochemistry. Some examples are provided in the following equations. Note that the mechanism is the exact reverse of the El mechanism for acid-catalyzed dehydration of alcohols described in Section 10.13. 

Sn2 reactions can occur at elements other than carbon. Common examples in organic chemistry are silicon, phosphorus, sulfur, and the halogens. The formation of the tosylate above by attack of the alcohol on TsCl is an example of an Sn2 reaction at sulfur. Later in this chapter you will see that alcohols attack phosphorus very easily and that we use the reaction between ROH and PBra to make alkyl bromides. Alcohols also react rapidly with Si-CI compounds such as MegSiCI to give silyl ethers by an Sn2 reaction at silicon. You have already seen several examples of silyl ether formation (p. 240, for example), though up to this point we have not discussed the mechanism. Here it is B represents a base such as triethyl amine. 

In their inorganic compounds the three halogens chlorine, bromine and iodine are in this order in regard to their aflSnity for other elements. This seems to hold also in their organic compounds as shown by the replacement of iodine by bromine or chlorine, as given previously, and by the fact that alkyl iodides are the least stable or hemostreactive, while the chlorides are the most stable or the least reactive. This is illustrated by their action upon silver nitrate. Ethyl iodide acts with silver nitrate in alcoholic solution precipitating silver iodide even in the cold. 

The most important derivatives of this mixed type are those obtained by substituting, in alcohols, aldehydes, or acids, some other element or group. This gives us compounds known, in general, as substituted alcoholsj substituted aldehydes, and substituted acids. These three classes of mixed compounds will now be considered. The simplest group of substituted alcohols are the halogen alcohols. Of the halogen alcohols the simplest would be derived from methyl alcohol, e.g.,... 

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