Phosphine

FIELDS OF STUDY: Organic Chemistry, Inorganic Chemistry

ABSTRACT

The characteristic properties and reactions of phosphine and its derivatives are discussed. Phosphine derivatives can be primary, secondary, or tertiary and are useful in organic synthesis reactions for the formation of complex molecular structures.

Characteristics and Derivatives of Phosphine

Phosphine is an inorganic compound with the molecular formula PH₃. It belongs to a class of molecules called phosphanes, which, according to the International Union of Pure and Applied Chemistry (IUPAC), are compounds of trivalent phosphorus (that is, phosphorus that can form three single bonds) that have the general formula P_nH_n+2. However, "phosphines" is also the name of a class of organophosphorus compounds derived by replacing one or more of phosphine’s three hydrogen atoms with a hydrocarbon group. Adding to the confusion is the fact that the official IUPAC name of the parent compound, phosphine, is actually phosphane, as this is more analogous to the IUPAC method for naming alkanes.

The phosphorus atom has five electrons in its valence electron shell, meaning that it can accept three electrons in order to achieve a full complement of eight valence electrons. Because it falls in period 3 of the periodic table of the elements, it has two electrons in its 3s orbital and three in its 3p orbitals; in order to minimize electron-electron repulsion between the orbitals, the s and p orbitals can combine, or hybridize, to allow the phosphorus atom to form double or triple carbon-phosphorus bonds. Organophosphorus compounds containing a double carbon-phosphorus bond (C=P) are termed "phosphaalkenes," and those containing a triple carbon-phosphorus bond (C≡P) are termed "phosphaalkynes."

Phosphines can be divided into three categories. The primary phosphines are those in which just one hydrogen atom has been substituted; they are represented generally as R−PH₂, where R is either an alkyl functional group (basic hydrocarbon) or an aryl group (ring-shaped hydrocarbon). In secondary phosphines, two hydrogen atoms have been substituted, as represented by the general formula R₂−PH. Secondary phosphines also form when the phosphorus atom is substituted for a carbon atom in a ring-shaped compound. The tertiary phosphines have had all three hydrogen atoms substituted, giving the general molecular formula R₃−P.

Nomenclature of Phosphines

Because they are not common compounds, the phosphines are named systematically as phosphine derivatives, although the official parent compound name "phosphane" is used instead. The alkyl and aryl groups are named as substituents, or side chains, according to the identity of the parent hydrocarbons from which they were derived, and the appropriate prefixes for those groups are then appended to the "-phosphane" suffix. Thus, a primary phosphine that has been substituted with a methyl group (−CH₃) is called methylphosphane, one substituted with an ethyl group (−C₂H₅) is ethylphosphane, one with a propyl group (−C₃H₇) is propylphosphane, and so on, progressing through the prefixes based on the number of carbon atoms in the substituent: "butyl-" (four), "pentyl-" (five), "hexyl-" (six), et cetera. A secondary or tertiary phosphine substituted with two or three identical functional groups adds a numerical prefix to the substituent prefix, so that a secondary phosphine substituted with two methyl groups is dimethylphosphane, and a tertiary phosphine substituted with three methyl groups is trimethylphosphane. If the substituents are not the all same, the various prefixes are simply listed in alphabetical order. For example, a tertiary phosphine with one benzyl substituent (−CH₂C₆H₅) and two phenyl substituents (−C₆H₅) is named benzyldiphenylphosphane.

If the hydrocarbons substituted for the hydrogen atoms contain their own substituent groups, determining the name of the phosphine can be more difficult. The first step is to establish the name of each substituent carried by the phosphine. A hydrocarbon structure is named for the number of carbon atoms that make up the longest unbranched chain in the molecular structure, as in the case of the prefixes given above, with the positions of substituent groups identified accordingly. After the base chain has been determined, the carbon atoms are numbered in order so that each substituent has the lowest position number possible in the chain, and these position numbers are given immediately before the name of the corresponding substituent. For example, a seven-carbon chain with two amino groups bonded to the second and third carbon atoms would be named 2,3-diaminoheptane if it were a standalone molecule; in this case, as it is a substituent, it takes the prefix form "2,3-diaminoheptyl-." This entire name is then enclosed in parentheses and prefixed by a position number indicating where on the substituent the bond to the phosphorus atom occurs. If the bond is located on the third carbon atom of the 2,3-diaminoheptyl substituent (the same atom as one of the amino groups), the resulting phosphine is named 3-(2,3-diaminoheptyl)phosphane.

The structures of organic compounds are typically represented by 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. Each carbon atom is assumed to be surrounded by the maximum possible number of hydrogen atoms unless otherwise specified, and all other atoms are represented by their elemental symbol. Such line drawings allow chemists to communicate a large amount of chemical information in a small space. For example, both molecular structures below depict 3-(2,3-diaminoheptyl)phosphane, but the image on the right is much more efficient—and, to a trained eye, readily comprehensible—than the one on the left.

Formation of Phosphine and Its Derivatives

Phosphine is a relatively stable, colorless gas with a freezing temperature of −133.5 degrees Celsius (−208.3 degrees Fahrenheit) and a boiling point of −87.7 degrees Celsius (−125.9 degrees Fahrenheit). Depending on its purity, it may have an acrid or garlic-like odor. It can be prepared via the hydrolysis (breaking of chemical bonds due to the presence of water) of a metal phosphide, such as calcium phosphide (CaP). Hydrolysis of calcium phosphide also produces minor quantities of both diphosphane (P₂H₄) and triphosphane (H₂P−PH−PH₂). Another way to produce phosphine is to react elemental phosphorus with activated hydrogen or potassium hydroxide.

Alkyl phosphines can be formed by the reaction of the compound phosphine with an alkyl halide, which is an alkane-derived compound that contains a halogen. A modified Grignard reaction can be used to synthesize aryl phosphines such as triphenylphosphane (P(C₆H₅)₃), a tertiary phosphine substituted with three ring-shaped phenyl groups. A Grignard reaction is one in which a Grignard reagent, formed by the combination of an alkyl or aryl halide with magnesium metal, attacks the carbon-oxygen double bond (C=O) of a carbonyl group, turning it into a single bond by adding another atom or group to the carbon atom. One such reagent is bromo(phenyl)magnesium, also called phenylmagnesium bromide (C₆H₅MgBr), which can react with phosphorus trichloride (PCl₃) to produce triphenylphosphane via a similar mechanism, with the target being the phosphorus atom rather than the carbon atom of a carbonyl group.

Reactions of Phosphines

Phosphines are reactive compounds used for the formation of ylides and phosphonium salts. An ylide is a neutral dipolar molecule consisting of a negatively charged ion, or anion—usually a carbanion, which is a polyatomic anion containing a carbon atom with an unshared pair of electrons—bonded to an atom that is neither carbon nor hydrogen. A phosphonium salt is a type of ionic compound in which the positively charged ion, or cation, is phosphonium (PR₄⁺, where R is either a hydrogen atom or a substituted hydrocarbon).

Phosphonium salt is typically produced by the reaction of a tertiary phosphine, such as the triphenylphosphane produced by a modified Grignard reaction, with an alkyl halide. It can then be combined with a strong base in an appropriate solvent to produce a phosphonium ylide, which features a carbon-phosphorus double bond (C=P). For example, the reaction of triphenylphosphane with bromomethane (CH₃Br), an alkyl halide, produces the salt methyltriphenylphosphonium bromide, which consists of a bromine anion (Br⁻) and the methyltriphenylphosphonium cation ((C₆H₅)₃P–CH₃⁺). When this salt is dissociated in solution and a strong base is added, a hydrogen cation (H⁺) is removed from the methyl group of the methyltriphenylphosphonium, turning it into a methylene group (=CH₂) and forming the neutral compound methylenetriphenylphosphorane ((C₆H₅)₃P=CH₂), an ylide. The double bond between the carbon and the phosphorus is indicated by the "-ene" ending of the "methylene-" prefix.

The ability of the phosphorus atom to form these two extra bonds, in addition to the original three of the phosphine molecule, is due to its position in the periodic table. The fact that phosphorus has five valence electrons, leaving it three short of a full complement of eight, suggests that it is trivalent—that is, able to form a maximum of three chemical bonds—and indeed this is often the case. However, because phosphorus is in period 3 of the periodic table, it has accessible d orbitals, permitting it to undergo chemical processes that are not available to its period 2 counterpart, the nitrogen atom. (Period 3 elements have their valence electrons in the n = 3 electron shell, which is the first one to contain d orbitals.) One such process is the use of the 3d orbitals to form additional bonds, as the phosphorus atom does when forming an ylide, becoming pentavalent (able to form five chemical bonds).

Phosphonium ylides serve as reagents in a type of reaction known as the Wittig reaction. In this reaction, the phosphonium ylide reacts with an aldehyde or ketone, which are organic compounds that contain a carbonyl group, to form an alkene (a compound containing a carbon-carbon double bond) and a phosphine oxide (a phosphine that is double bonded to an oxygen atom via the phosphorus atom).

Reduction of Ylides

Just as phosphines can be used to produce ylides, the reverse is also true. Reaction of the carbon-phosphorus double bond with one molar equivalent of molecular hydrogen (H₂) reduces the ylide to the corresponding phosphine. However, when this type of reduction is too powerful and may affect other functional groups or portions of the ylide molecule, a variety of other reducing agents are available. Reagents such as lithium aluminum hydride and sodium borohydride are more selective about the site of reduction and operate under gentler conditions. Typically, the formation of an ylide and its subsequent reduction to the corresponding phosphine is used in synthetic procedures to increase the selectivity of a reaction that would otherwise produce an unmanageable mixture of products. Often the ylide will undergo a reaction in such a mixture as well, but the ylide product can be recovered cleanly, then reduced to the phosphine and cleaved from the rest of the product.

PRINCIPAL TERMS

• functional group: a specific group of atoms with a characteristic structure and corresponding chemical behavior within a molecule.
• organophosphorus compound: an organic compound containing one or more carbon-phosphorus bonds.
• phosphane: a class of compounds with the general formula P_nH_n+2; also, an alternative name for the compound phosphine (PH₃).
• primary: describes an organic compound in which one of the hydrogen atoms bonded to a central atom is replaced by another atom or group of atoms, called a substituent.
• secondary: describes an organic compound in which two of the hydrogen atoms bonded to a central atom are replaced by other atoms or groups of atoms, called substituents.
• tertiary: describes an organic compound in which three of the hydrogen atoms bonded to a central atom are replaced by other atoms or groups of atoms, called substituents.

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