Structural formula

The structural formula of a chemical compound is a graphic representation of the molecular structure,

showing how the atoms are arranged. The chemical bonding within the molecule is also shown, either

explicitly or implicitly. Also several other formats are used, as in chemical databases, such

as SMILES, InChI and CML.

Unlike chemical   formulas  or chemical   names , structural formulas provide a representation of the

molecular structure. Chemists nearly always describe a chemical reaction or synthesis using structural

formulas rather than chemical names, because the structural formulas allow the chemist to visualize the

molecules and the changes that occur.

Many chemical compounds exist in different isomeric forms, which have different structures but the same

overall chemical formula. A structural formula indicates the arrangements of atoms in a way that a

chemical formula cannot.

Lewis structures

Representation of molecules by molecular formula

Main article: Lewis structure

Lewis structures (or "Lewis dot structures") are flat graphical formulas that show atom connectivity

and lone pairor unpaired electrons, but not three-dimensional structure. This notation is mostly used for

small molecules. Each line represents the two electrons of a single bond. Two or three parallel lines

between pairs of atoms represent double or triple bonds, respectively. Alternatively, pairs of dots may be

used to represent bonding pairs. In addition, all non-bonded electrons (paired or unpaired) and any formal

charges on atoms are indicated.

The Lewis structure ofwater

Condensed formulas

In early organic-chemistry publications, where use of graphics was strongly limited, a typographic system

arose to describe organic structures in a line of text. Although this system tends to be problematic in

application to cyclic compounds, it remains a convenient way to represent simple structures:

CH3CH2OH (ethanol)

Parentheses are used to indicate multiple identical groups, indicating attachment to the nearest non-

hydrogen atom on the left when appearing within a formula, or to the atom on the right when appearing at

the start of a formula:

(CH3)2CHOH or CH(CH3)2OH (2-propanol)

In all cases, all atoms are shown, including hydrogen atoms.

Skeletal formulas

Main article: Skeletal formula

Skeletal formulas are the standard notation for more complex organic molecules. First used by the

organic chemist Friedrich August Kekulé von Stradonitz the carbon atoms in this type of diagram are

implied to be located at the vertices (corners) and termini of line segments rather than being indicated

with the atomic symbol C. Hydrogen atoms attached to carbon atoms are not indicated: each carbon atom

is understood to be associated with enough hydrogen atoms to give the carbon atom four bonds. The

presence of a positive or negative charge at a carbon atom takes the place of one of the implied hydrogen

atoms. Hydrogen atoms attached to atoms other than carbon must be written explicitly.

Skeletal formula ofisobutanol

Indication of stereochemistry

Several methods exist to picture the three-dimensional arrangement of atoms in a molecule

(stereochemistry).

Stereochemistry in skeletal formulas

Skeletal formula of strychnine. A solid wedged bond seen for example at the nitrogen (N) at top indicates a bond pointing

above-the-plane, while a dashed wedged bond seen for example at the hydrogen (H) at bottom indicates a below-the-plane

bond.

Chirality in skeletal formulas is indicated by the Natta projection method. Solid or dashed wedged bonds

represent bonds pointing above-the-plane or below-the-plane of the paper, respectively.

Unspecified stereochemistry

Fructose, with a bond at the hydroxyl (OH) group upper left of image with unknown or unspecified stereochemistry.

Wavy single bonds represent unknown or unspecified stereochemistry or a mixture of isomers. For

example the diagram to the left shows the fructose molecule with a wavy bond to the HOCH2- group at

the left. In this case the two possible ring structures are in chemical equilibrium with each other and also

with the open-chain structure. The ring continually opens and closes, sometimes closing with one

stereochemistry and sometimes with the other.

Perspective drawings

Newman projection and sawhorse projection

The Newman projection and the sawhorse projection are used to depict specific conformers or to

distinguish vicinalstereochemistry. In both cases, two specific carbon atoms and their connecting bond

are the center of attention. The only difference is a slightly different perspective: the Newman projection

looking straight down the bond of interest, the sawhorse projection looking at the same bond but from a

somewhat oblique vantage point. In the Newman projection, a circle is used to represent a plane

perpendicular to the bond, distinguishing the substituents on the front carbon from the substituents on the

back carbon. In the sawhorse projection, the front carbon is usually on the left and is always slightly

lower:

Newman projection ofbutane

 

sawhorse projection of butane

Cyclohexane conformations

Certain conformations of cyclohexane and other small-ring compounds can be shown using a standard

convention. For example, the standard chair conformation of cyclohexane involves a perspective view

from slightly above the average plane of the carbon atoms and indicates clearly which groups

are axial and which are equatorial. Bonds in front may or may not be highlighted with stronger lines or

wedges.

Chair conformation of beta-D-Glucose

Haworth projection

The Haworth projection is used for cyclic sugars. Axial and equatorial positions are not distinguished;

instead, substituents are positioned directly above or below the ring atom to which they are connected.

Hydrogen substituents are typically omitted.

Haworth projection of beta-D-Glucose

Fischer projection

The Fischer projection is mostly used for linear monosaccharides. At any given carbon center, vertical

bond lines are equivalent to stereochemical hashed markings, directed away from the observer, while

horizontal lines are equivalent to wedges, pointing toward the observer. The projection is totally

unrealistic, as a saccharide would never adopt this multiply eclipsed conformation. Nonetheless, the

Fischer projection is a simple way of depicting multiple sequential stereocenters that does not require or

imply any knowledge of actual conformation:

Fischer projection of   D -Glucose