# 14: Organohalogen & Organometallic Compounds

The general term of "organohalogen" refers to compounds with covalent carbon-halogen bonds. Substances such a bromomethane ($$\ce{CH_3Br}$$) and chloroethene ($$\ce{CH_2=CHCl}$$), are examples of organohalogen compounds, whereas others such as the methylammonium chloride salt, which have no carbon-halogen bonds, are not. This chapter is only concerned with compounds that have covalent carbon-halogen bonds.

• 14.1: Prelude to Organohalogen & Organometallic Compounds
There is wide diversity in the nature of organohalogen compounds but we have restricted this chapter to alkyl, cycloalkyl, alkenyl, alkynyl, and aryl halides. Some of the chemistry of the carbon-halogen bonds already will be familiar to you because it involves the addition, substitution, and elimination reactions discussed in previous chapters. We will amplify these reactions and consider nucleophilic substitution by what are called the addition-elimination and elimination-addition mechanisms.
• 14.2: Physical Properties of Organohalogen & Organometallic Compounds
The physical properties of haloalkanes are much as one might expect. Volatility decreases: (a) with increasing molecular weight along a homologous series, (b) with increasing atomic number of the halogen, and (c) with the structure of the alkyl group in the order such that tertiary << secondary << primary for isomeric halides.
• 14.3: Spectroscopic Properties
Organohalogen compounds give rise to strong absorptions in the infrared arising from stretching vibrations of the carbon-halogen bond. The frequency of absorption decreases as the mass of the halogen increases.
• 14.4: Alkyl Halides
The important chemistry of alkyl halides includes the nucleophilic (SN) displacement and elimination (E) reactions. Recall that tertiary alkyl halides normally are reactive in ionization SN1 reactions, whereas primary halides, and to a lesser extent secondary halides, are reactive in SN2 reactions, which occur by a concerted mechanism with inversion of configuration.
• 14.5: Alkenyl and Alkynyl Halides
The most readily available alkenyl halide is chloroethene (vinyl chloride), which can be prepared by a number of routes.
• 14.6: Cycloalkyl Halides
The cycloalkyl halides, except for cyclopropyl halides, have physical and chemical properties that are similar to those of the open-chain secondary halides and can be prepared by the same types of reactions. All the cycloalkyl halides undergo SN2 reactions rather slowly and, with nucleophiles that are reasonably basic, E2 reactions can be expected to predominate.
• 14.7: Aryl Halides
Aryl halides have a halogen directly bonded to a carbon of an aromatic ring.  The simple aryl halides generally are resistant to attack by nucleophiles. However, considerable activation is produced by strongly electron-attracting substituents provided these are located in either the ortho or para positions, or both.
• 14.8: Polyhalogenated Alkanes and Alkenes
Polychlorination of methane yields the di-, tri-, and tetrachloromethanes cheaply and efficiently. These substances have excellent solvent properties for nonpolar and slightly polar substances. Chloroform once was used widely as an inhalation anesthetic. However, it has a deleterious effect on the heart and is oxidized slowly by atmospheric oxygen to highly toxic carbonyl dichloride.
• 14.9: Organometallic Compounds from Organohalogen Compounds
One of the more important reactions of organohalogen compounds is the formation of organometallic compounds by replacement of the halogen by a metal atom. Carbon is positive in carbon-halogen bonds and becomes negative in carbon-metal bonds, and therefore carbon is considered to be reduced in formation of an organometallic compound.
• 14.E: Organohalogen & Organometallic Compounds (Exercises)
These are the homework exercises to accompany Chapter 14 of the Textmap for Basic Principles of Organic Chemistry (Roberts and Caserio).
• 14.10: Properties of Organometallic Compounds
How carbon-metal bonds are formed depends on the metal that is used. Conditions that are suitable for one metal may be wholly unsuited for another. Some organometallic compounds react very sluggishly even toward acids, whereas others react avidly with water, oxygen, carbon dioxide, and almost all solvents but the alkanes themselves. Reactivity increases with increasing polarity of the carbon-metal bond, which is determined by the electropositivity of the metal.
• 14.11: Preparation of Organometallic Compounds
The reaction of a metal with an organic halide is a convenient method for preparation of organometallic compounds of reasonably active metals such as lithium, magnesium, and zinc. Ethers, particularly diethyl ether and oxacyclopentane (tetrahydrofuran), provide inert, slightly polar media in which organometallic compounds usually are soluble. Care is necessary to exclude moisture, oxygen, and carbon dioxide, which would react with the organometallic compound.
• 14.12: Organomagnesium Compounds
For many years the most important organometallic compounds for synthetic purposes have been the organomagnesium halides, or Grignard reagents. They are named after Victor Grignard, who discovered them and developed their use as synthetic reagents, for which he received a Nobel Prize in 1912. As already mentioned, these substances customarily are prepared in dry ether solution from magnesium turnings and an organic halide.
• 14.13: Organomagnesium & Organolithium Compounds in Synthesis
The most important synthetic use of Grignard reagents and organolithium reagents is to form new carbon-carbon bonds by addition to polar multiple bonds, particularly carbonyl bonds. An example is the addition of methyl-magnesium iodide to methanal. With suitable variations of the carbonyl compound, a wide range of compounds can be built up from substances containing fewer carbon atoms per molecule.

Thumbnail: Structure of 1-Chlorobenzene.