Alkyl Halides

FIELDS OF STUDY: Organic Chemistry

ABSTRACT

The characteristic properties and reactions of alkyl halides are discussed. Alkyl halides are an infinite series of organic compounds in which one or more halogen atoms have replaced hydrogen atoms. Their variety and reactivity are due to the electron distribution and physical size of the carbon and halogen atoms. The alkyl halides parallel the series of structures of the alkanes.

The Nature of the Alkyl Halides

The alkyl halides are a series of hydrocarbons, in which at least one hydrogen atom has been replaced by a halogen atom. The halogen atoms in alkyl halides act as a functional group, determining the reactivity of the materials. The geometric arrangement of atomic orbitals on the carbon atom and the halogen atom dictate the alkyl halide’s overall structure. There are four valence electrons in each carbon atom, two in the 2s orbital and one each in two of the three 2p orbitals. By combining the 2s and the three 2p orbitals, the carbon atom is able to form four equivalent sp3 hybrid atomic orbitals, each containing a single electron. The halogen atoms require only one additional electron to form the electron distribution corresponding to the nearest inert gas atom. Thus, halogens readily form a covalent single bond, or sigma (σ) bond, to carbon atoms.

The alkyl halides have two parallel families of acyclic molecular structures and cyclic molecular structures. The carbon atom skeletons of the acyclic alkyl halide series can be expanded infinitely, at least in principle. Halogen atoms can be substituted for hydrogen atoms to any extent in the alkanes (saturated hydrocarbons) to produce the corresponding alkyl halides. The simplest alkyl halide is fluoromethane, CH₃F, having just one carbon atom and three equivalent bonds to as many hydrogen atoms. The one-carbon methane analogs are difluoromethane (CH₂F₂), trifluoromethane or fluoroform (CHF₃), and tetrafluoromethane (CF₄). The corresponding chlorinated analogs CH₃Cl, CH₂Cl₂, CHCl_3, and CCl₄ are all commonly known, as are the brominated and iodinated analogs. The two-carbon ethane has a similarseries of halogenated derivatives. In principle, all of the various hydrocarbons have their halogenated analogs; in practice, however, they begin to destabilize when two or more halogens are present on adjacent carbon atoms. The larger size of the halogen atoms physically strains the molecular structure, and compounds containing two bromine or iodine atoms tend to give up their halogen components, via dehalogenation and dehydrohalogenation reactions, very easily. Alkanes containing several fluorine atoms, and especially the perfluorinated alkanes, are exceptional. The high electronegativity of the fluorine atom makes C−F bonds very stable, while the relatively small size of the fluorine atom does not physically strain the structure of the molecule.

With each additional carbon atom in the hydrocarbon series, the number of possible isomeric forms increases. An isomer is one of two or more chemical species that have the same molecular formula but different molecular structures. The substitution of halogen atoms for hydrogen atoms vastly increases the number of isomeric forms for each chemical formula, beginning with the three-carbon hydrocarbons. Whereas only one isomer of propane (C₃H₈) is known, there are two isomers in which a halogen atom (X) has been substituted for a hydrogen atom, corresponding to the chemical formula C₃H₇X. When two hydrogen atoms have been substituted (C₃H₆X₂), the number of isomers increases to three, then five when three halogens are present (C₃H₅X₃), six when four halogens are present, five when five halogens are present, three for six halogens, two for seven halogens, and one perhalogenated compound. A similar pattern of the number of derivative compounds applies for all of the hydrocarbon structures, in which the highest number of isomers for a particular chemical formula occurs when half of the hydrogen atoms have been substituted by halogen atoms. In the cyclic alkyl halides, the carbon atoms are bonded together in a ring structure, but the same general principles of substituting halogen atoms for hydrogen atoms apply.

The presence of a halogen atom in a hydrocarbon structure also creates an electric dipole in the molecule. The higher electronegativity of the halide atom imparts an "electron-withdrawing effect" so that the electron density in the molecule increases near the halogen atom and decreases elsewhere. This makes possible dipole-dipole interactions that affect both the physical properties and the reactivity of the alkyl halides.

Nomenclature of Alkyl Halides

The alkyl halides are named systematically according to the identity of the parent hydrocarbon from which they have been formed. The hydrocarbon structure is named for the number of carbon atoms with the longest unbranched chain in the molecular structure. The formal name sequence begins with methane, ethane, propane, and butane as common names, and then progresses through names based on the number of carbon atoms: pentane, hexane, heptane, octane, nonane, decane, and so on. For alkenes and alkynes (unsaturated hydrocarbons containing one or more carbon-carbon C=C double bonds or carbon-carbon C≡C triple bonds, respectively), the corresponding -ene or -yne ending of the name is used. When the base chain has been determined, the positions of the substituents (side chains) are assigned so as to have the lowest position numbers in the structure. The position number is inserted into the name of the primary chain immediately before the name of the substituent. Thus, a seven-carbon chain with two chlorine atoms bonded to the second and third carbon atoms from one end would be named 2,3-dichloroheptane, not 5,6-dichloroheptane. When different side chains are present on a hydrocarbon molecule, they are assigned by the priority of their size, again to attain the lowest numbers, and named alphabetically.

Several common names exist for many of the alkyl halides. These continue to be used because they came into common usage before the International Union of Pure and Applied Chemistry (IUPAC) rules of nomenclature were developed. Thus, it is common practice to refer to CHCl₃, CHI₃, and CH₂Cl₂ as chloroform, iodoform, and methylene chloride, respectively, instead of the official IUPAC names trichloromethane, tri-iodomethane, and dichloromethane. The compound CCl₄ is commonly referred to as "carbon tet" (an abbreviation of carbon tetrachloride) instead of tetrachloromethane. Another common usage is to refer to the simpler compounds using an alkyl halide name, such as ethyl bromide, rather than bromoethane.

Organic structures are typically represented by structural formulas, line drawings that depict the angles between the carbon atoms in such a way that every angle vertex and line end represents a single carbon atom. Such line drawings allow chemists to communicate a large amount of chemical information in a very small space. This is a convenient and readily understood representation that makes the nature and identity of the side chains immediately apparent. These line drawings use the elemental symbol of any atom other than carbon and hydrogen in its proper location in the molecular structure. The compound 3,3-dichloro-6-butylundecane, for example, is more readily comprehended when depicted as

than as

Cyclic alkyl halides are named in a similar manner. The basic name reflects the number of carbon atoms in the molecule’s largest ring structure. The first position in the ring structure is assigned to the carbon atom bearing the first substituent of the first carbon atom of a C=C or C≡C bond, and other substituents are assigned so as to attain the lowest set of position numbers. A cyclohexane (C-6) ring with a bromine atom on one carbon atom and an ethyl substituent on another carbon atom two positions farther around the ring would be named 1-bromo-3-ethylcyclohexane. Cyclic alkyl halide structures are as easily drawn as the corresponding hydrocarbon structure.

Formation of Alkyl Halides

Simple alkyl halides can be prepared by a substitution reaction between an alcohol and a hydrogen halide. This normally requires the presence of an inorganic salt, such as zinc (II) dichloride, ZnCl₂. More complex alkyl halides require more specific reactions, however, since substitution in acid proceeds through the formation of a carbonium ion intermediate (an ion containing a positively charged carbon atom), which makes many different outcomes possible depending on the structure of the alcohol molecule. A carbonium ion will always rearrange from its original form to a more stable form, which can involve significant rearrangement of the bonds within the molecular structure. Alternatively, a carbonium ion can eliminate a proton to form an alkene, or it can add a nucleophile (an electron-rich species) that may be present in the reaction mixture.

Other methods of forming alkyl halides normally involve the addition of halogens to alkenes. A standard test for the presence of a C=C bond in a molecule is bromination. A small amount of a solution of bromine (Br₂) in carbon tetrachloride (CCl₄) is added to a solution of the suspected alkene or alkyne (an unsaturated hydrocarbon containing at least one C≡C bond). The intense color of the Br₂/CCl₄ solution quickly disappears as the bromine atoms add to the C=C bond to form the corresponding dibromide compound from the C=C bond. As an electrophile (a species attracted to electrons), the C=C bond is also amenable to the addition of hydrogen halides (HX).

Halogen atoms can also be substituted into hydrocarbons by "radical" mechanisms. A radical is an electrically neutral portion of a molecule with an unpaired electron in a bond-forming orbital. Chlorination by a radical mechanism begins with the chlorine molecule separating into two chlorine atoms. One of the chlorine atoms can "abstract" a hydrogen atom from the molecular structure to form hydrochloric acid (HCl), leaving the rest of the molecule as an alkyl radical. This alkyl radical can then form a bond with the other chlorine atom to produce the alkyl chloride. The process is difficult to control, and products can be unpredictable. Chlorination is the most useful radical process. Bromination and iodination do not proceed sufficiently to be useful in producing alkyl bromides and iodides, while fluorination is so aggressive that it will destroy both the carbon-carbon bonds and the carbon-hydrogen bonds in the molecule.

Reactions of Alkyl Halides

Alkyl halides are highly reactive compounds under the right conditions. An existing alkyl halide can be converted into another halide or a substituted compound through an appropriate substitution reaction. Their most useful and most used application is as alkylating agents in the synthesis of other compounds. They are the key components of both the Friedel-Crafts and Grignard reactions. The Friedel-Crafts alkylation reaction adds the alkyl group of an alkyl halide to an aromatic system, such as the benzene ring. A Lewis acid compound, such as AlCl₃ or FeCl₃, catalyzes the reaction. An activated complex forms from the catalyst and the alkyl halide, which are attracted to the electrons of the aromatic system. This process allows the alkyl group to substitute for a hydrogen atom. For example, the Friedel-Crafts alkylation reaction between benzene and t-butyl chloride (2-chloro-2-methylpropane), with AlCl_3, produces t-butyl benzene (2-methyl-2-phenylpropane), according to the reaction

The Grignard reaction takes place between an alkyl halide and magnesium metal. A magnesium atom becomes interposed between the halogen and carbon atoms to which it was bonded, forming the corresponding alkyl magnesium halide compound. The reaction to form the Grignard reagent must be carried out in "dry" solvent that has been treated to remove any traces of water and is also quite vigorous. Once formed, the Grignard reagent can be applied in a wide variety of reactions that form new bonds between carbon atoms. An addition to a carbonyl group (C=O), for example, can turn an ester (RCOOR') into a ketone (RCOR'), an aldehyde (RCOH) into a secondary alcohol (RR'CHOH), or a ketone (RCOR') into a tertiary alcohol (RR'R"COH). (Note that in the general formulas, R, R', and R" represent alkyl groups.) Grignard reactions are perhaps the most generally useful of all synthesis reactions.

Alkyl halides also react with ammonia (NH₃) to produce amines and with amines to produce more highly alkylated amines. These reactions demand careful control, however, as a mixture of all possible products typically results.

PRINCIPAL TERMS

• dipole: the separation of positive and negative charges within a single molecule due to electron density being relatively high in one part of the molecule and relatively low in another.
• functional group: a specific group of atoms with a characteristic structure and corresponding chemical behavior within a molecule.
• halide: a binary compound consisting of a halogen element (fluorine, chlorine, bromine, iodine, or astatine) bonded to a non-halogen element or organic group; alternatively, an anion of a halogen element.
• hydrocarbon: an organic compound composed solely of carbon and hydrogen atoms.
• substitution reaction: a chemical reaction in which one component of a compound is replaced by a different atom or group of atoms without altering the basic structure of the molecule.

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