Molecular Formula
A molecular formula is a concise representation of the composition of a chemical compound, specifying the types and numbers of atoms present in a molecule. For example, the molecular formula for methanol is CH₄O, which indicates one carbon atom, four hydrogen atoms, and one oxygen atom. However, molecular formulas cannot distinguish between isomers—different compounds that share the same molecular formula but have distinct structures. To address this limitation, chemists often use structural formulas, which depict the spatial arrangement of atoms and can differentiate between isomeric forms. This includes semi-structural formulas that provide enough detail to identify specific compounds, such as distinguishing between ethanol and dimethyl ether, both of which have the formula C₂H₆O. Understanding these representations is essential, as the molecular structure significantly influences a compound's properties and reactivity. The ability to accurately communicate chemical information through these formulas is crucial for chemists in their work. Each formula serves as a symbolic language that conveys complex information about the molecular identity and behavior of substances involved in chemical reactions.
Molecular Formula
FIELDS OF STUDY: Organic Chemistry
ABSTRACT
The basic principles of molecular structures and how they are used to communicate chemical information are presented. A chemical formula identifies and numbers each type of atom in a molecule, but it cannot differentiate between isomers. A molecular formula provides some information about the structure of the molecule. A structural formula describes the spatial relationship of the atoms in a molecule and differentiates between isomeric molecules.
Communicating Chemical Information
Through the formation of chemical bonds, atoms combine to form molecules. The identity of any particular molecule is determined by the identity and number of the individual atoms from which it is composed, as well as by their positions relative to each other in the molecule (the molecular structure of the compound). The chemical formula of a compound identifies the kinds and number of atoms that comprise that particular molecule. For example, the simple alcohol methanol consists of one carbon atom, four hydrogen atoms, and one oxygen atom. It has the chemical formula CH4O. The rules of chemical bond formation are such that this is the only compound that can have this chemical formula. Ethanol, next in the alcohol series, is composed of two carbon atoms, six hydrogen atoms, and one oxygen atom. These atoms can be arranged and bonded in different ways. Thus, more than one compound can have the chemical formula C2H6O.
The chemical formula alone is incapable of differentiating between these isomeric forms. To overcome this problem, chemists sometimes use a molecular formula or semi-structural formula to identify whether the formula C2H6O refers to dimethyl ether or to ethanol. The former is written as H3C−O−CH3 and the latter as C2H5−OH, CH3CH2−OH, or Et−OH (in which Et indicates the ethyl group, C2H5). For many compounds, this amount of differentiation is sufficient. For more complex compounds, however, knowing the three-dimensional structure of the molecule is vital, but the number of possible isomers of the same molecular formula mitigates against any such simplistic representation. To overcome this problem, chemists resort to drawing "stick figure" structural formulas that communicate the chemical and structural information about a molecule. These structural formulas chart the electron movements in bond formation and the sequential organization of substituents about the asymmetric centers in optical isomers. Some basic rules have standardized the practice.
A chemical formula is specific to the atoms that make up a corresponding molecular structure. The chemical formula NaCl, for example, identifies sodium chloride (and only sodium chloride). Thus, each chemical formula comprises a specific "word" in a chemical equation and communicates to the chemist that only that specific chemical entity is meant. In the chemical equation
C12H22O11 + 12O2 → 12CO2 + 11H2O
the chemist is informed explicitly that exactly one molecule of C12H22O11 reacts with exactly twelve molecules of oxygen to produce exactly twelve molecules of carbon dioxide and eleven molecules of water. But which of the many possible compounds having that chemical formula is the C12H22O11 that is referred to by this equation? A structural molecular formula would present this information quite clearly to chemists able to read the structural formula.
Nomenclature
The first rule of structural molecular formulas is that they must correspond to the exact name of the compound according to the International Union of Pure and Applied Chemistry (IUPAC) rules of standard nomenclature. If this rule is not followed, the compound being illustrated will have an entirely different chemical identity. The effect is the same as using the wrong words in an oral or written description.
The second rule is that the drawing must show only the bonds between carbon atoms in organic compounds and neither the carbon atoms themselves nor any hydrogen atoms that are bonded to them. The drawing must also include the proper bond angles for the structure, types, and numbers of ions or molecules that surround the particular atom in a compound. Each end of a stick, and each vertex where two sticks connect, is understood to represent a carbon atom with the proper number of hydrogen atoms. Two lines between points indicate a double bond, and three lines a triple bond.
The third rule is that any atoms in the structure other than carbon or hydrogen are represented by their element symbol: O for oxygen, N for nitrogen, Si for silicon, and so on. Thus, cyclopentane, diethylamine, 2-pentanone, and diphenylmethane can be represented respectively as
(Note that the circle inside the hexagons of diphenylmethane indicates the alternating single and double bonds of the benzene ring.) Such structural formulas illustrate the identity of each compound.
Isomers of Organic Compounds
Many different compounds can be represented by the exact same chemical formula. A Lewis structure can do much to identify the particular compound, but it is an inefficient, two-dimensional means of representing a three-dimensional molecule. Only the simplest isomeric relationships, such as ethanol and dimethyl ether, can be differentiated by a Lewis structure. There are many types of isomers, however, and a structural formula is the single best means of identifying them in a chemical equation.
All isomeric forms have one thing in common: they are represented by the same chemical formula, no matter how different the actual compounds are. Structural isomers exist when a basic structure has the same substituents but in different positions on the base structure. Two examples of this include 2-methylpentanol and 3-methylpentanol (both C6H12O) and 1,2-dimethylbenzene, 1,3-dimethylbenzene, and 1,4-dimethylbenzene (all three C8H10). These are represented clearly as
The four bonds to a carbon atom (like those in the pentanol derivatives pictured above) are arranged tetrahedrally. When all four substituents on the four bonds are different, they can be arranged equally in two different ways to form two different isomers. The four substituents are "enantiomers," meaning that the parts are mirror images of each other. A structural molecular formula differentiates between these forms as well, but visualizing the relationship takes some practice.
To visualize an enantiomeric structure, the substituents must be arranged in a sequential order according to certain rules of priority. A C−H bond is given the lowest priority and is accorded the position pointing directly away from the viewer. Substituent priorities are then assigned according to degree of oxidation. In the 2-methylpentanol example, the prioritized order of the substituents is −CH2OH, then −C3H7, and finally −CH3. In one enantiomer, these follow a clockwise sequence, while in the other, their sequence is counterclockwise. These are illustrated in a structural molecular formula by drawing two substituents in their proper relative orientation and using a solid wedge-shaped line to indicate the substituent pointing toward the viewer and a dashed wedge-shaped line for the substituent pointing away from the viewer, as
Careful examination of the two structures will reveal that they cannot be inverted, flipped, or manipulated in any way that makes them identical to each other. The stereochemistry about the "asymmetric center" is clearly represented in this simple way.
This simple representation becomes extremely important for more complex molecules in which the structure effectively extends in three directions, a common feature of biochemically active compounds. A simple tricyclic compound such as tricyclo[3.2.1.02,6]octane is difficult enough to name, let alone keep track of through a word-based identifier. A structural molecular formula, however, makes keeping track of the structure a simple task when the molecule in question can be readily illustrated as
The presence of double bonds and ring structures present the possibility of geometric isomers, due to the restricted motion about the bonds that make up those sites in the molecule. In a simple substituted alkene (an unsaturated hydrocarbon with one or more C=C bonds), substituents can be situated either on the same side of the C=C bond relative to each other or on opposite sides of the C=C bond, as are the two methyl groups in
A ring structure has the same composition, as two substituents can be on either the same side or opposite sides of the ring.
Using Structural Molecular Formulas in Reaction Equations
A structural molecular formula presents information about the identity, structure, and behavior of a specific compound in a clear, understandable manner that, were it written in words, would often require several pages. Molecular formulas are especially valuable in presenting a proposed mechanism that shows the movement of electrons as bonds are reorganized during a chemical reaction. It is not always necessary to use a structural molecular formula, as when stereochemical considerations and other isomers are not important or when the compound is unambiguous based on the chemical formula. A chemist who can read the "hieroglyphics" of structural molecular formulas will know precisely what is being presented in a reaction equation.
PRINCIPAL TERMS
- isomeric: describes chemical species that have the same molecular formula but different molecular structures.
- Lewis structure: a simplified representation of the bonds and unbonded electron pairs associated with the atoms of a molecule.
- molecular formula: a chemical formula that indicates how many atoms of each element are present in one molecule of a substance.
- semi-structural formula: a chemical formula that does not show the complete molecular structure of a compound but gives sufficient information to differentiate it from its isomers.
- stereochemistry: the relative spatial arrangement of atoms and bonds in a molecular structure.
Bibliography
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