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Skeletal formula

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Skeletal formula

The skeletal formula of the antidepressant drug escitalopram, featuring skeletal representations of heteroatoms, a triple bond, phenyl groups and stereochemistry

The skeletal formula, sometimes called line-angle formula, of an structural formula that serves as a shorthand representation of its bonding and some details of its molecular geometry. It is represented in two dimensions, as on a page of paper. It employs certain conventions to represent carbon and hydrogen atoms, which are the most common in organic chemistry.

The technique was developed by the organic chemist

  • Drawing organic molecules from chemguide.co.uk

External links

  1. ^ General, Organic, and Biological Chemistry, H. Stephen Stoker 2012
  2. ^ IUPAC Recommendations 1999, Revised Section F: Replacement of Skeletal Atoms
  3. ^  
  4. ^ a b J. Brecher (2006). "Graphical representation of stereochemical configuration (IUPAC Recommendations 2006)" (PDF).  
  5. ^ The Historical Origins of Stereochemical Line and Wedge Symbolism William B. Jensen Journal of Chemical Education 2013 90 (5), 676-677 doi:10.1021/ed200177u

References

Hydrogen bonds are generally denoted by dotted or dashed lines.

Using dashed lines (green) to show hydrogen bonding in acetic acid.

Hydrogen bonds

Alkene stereochemistry

Skeletal formulae can depict cis and trans isomers of alkenes. Wavy single bonds are the standard way to represent unknown or unspecified stereochemistry or a mixture of isomers (as with tetrahedeal stereocenters). A crossed double-bond has been used sometimes; is no longer considered an acceptable style for general use, but may still be required by computer software.[4]

[5] An early use of this notation can be traced back to

  • Solid lines represent bonds in the plane of the paper or screen.
  • Solid wedges represent bonds that point out of the plane of the paper or screen, towards the observer.
  • Hashed wedges or dashed lines (thick or thin) represent bonds that point into the plane of the paper or screen, away from the observer.
  • Wavy lines represent either unknown stereochemistry or a mixture of the two possible stereoisomers at that point.

The relevant chemical bonds can be depicted in several ways:

Stereochemistry is conveniently denoted in skeletal formulae:[4]

Different depictions of chemical bonds in skeletal formulae

Stereochemistry

An alternative style that is more common in academia is the Kekulé structure. Although it could be considered inaccurate as it implies three single bonds and three double bonds (benzene would therefore be cyclohexa-1,3,5-triene), all qualified chemists are fully aware of the delocalization in benzene. Kekulé structures are useful for drawing reaction mechanisms clearly.

Kekulé structure of benzene

delocalization of electrons over the six carbon atoms in the ring, a circle is drawn inside the hexagon of single bonds. This style is very common in introductory organic chemistry texts used in schools.

Benzene ring with circle

Benzene rings

Note: in the gallery above, double bonds have been shown in red and triple bonds in blue. This was added for clarity - multiple bonds are not normally coloured in skeletal formulae.

In more advanced theories of bonding, non-integer values of bond order exist. In these cases, a combination of solid and dashed lines indicate the integer and non-integer parts of the bond order, respectively.

Two atoms can be bonded by sharing more than one pair of electrons. The common bonds to carbon are single, double and triple bonds. Single bonds are most common and are represented by a single, solid line between two atoms in a skeletal formula. Double bonds are denoted by two parallel lines, and triple bonds are shown by three parallel lines.

Multiple bonds

See the article leaving group for further information

Leaving groups

  • Ac for the acetyl group (Ac is also the symbol for the element actinium. However, actinium is almost never encountered in organic chemistry, so the use of Ac to represent the acetyl group never causes confusion)

Functional groups

Aromatic substituents

Alkyl groups

Elements

. A list of pseudoelement symbols is shown below: phenyl group The most widely used symbol is Ph, which represents the [3] There are also symbols that appear to be

Pseudoelement symbols

All atoms that are not carbon or hydrogen are signified by their chemical symbol, for instance Cl for chlorine, O for oxygen, Na for sodium, and so forth. These atoms are commonly known as heteroatoms in the context of organic chemistry.

Explicit heteroatoms

Shown below for comparison are a ball-and-stick model of the actual three-dimensional structure of the ethanol molecule in the gas phase (determined by microwave spectroscopy, left), the Lewis structure (centre) and the skeletal formula (right).

Any hydrogen atoms bonded to non-carbon atoms are drawn explicitly. In ethanol, C2H5OH, for instance, the hydrogen atom bonded to oxygen is denoted by the symbol H, whereas the hydrogen atoms which are bonded to carbon atoms are not shown directly. Lines representing heteroatom-hydrogen bonds are usually omitted for clarity and compactness, so a functional group like the hydroxyl group is most often written −OH instead of −O−H. These bonds are sometimes drawn out in full in order to accentuate their presence when they participate in reaction mechanisms.

NOTE: It doesn't matter which end of the chain you start numbering from, as long as you're consistent when drawing diagrams. The condensed formula or the IUPAC name will confirm the orientation. Some molecules will become familiar regardless of the orientation.

The 3d ball representation of hexane, with carbon (black) and hydrogen (white) shown explicitly.
The skeletal formula of hexane, with carbons number one and three labelled

For example, in the image below, the skeletal formula of hexane is shown. The carbon atom labeled C1 appears to have only one bond, so there must also be three hydrogens bonded to it, in order to make its total number of bonds four. The carbon atom labelled C3 has two bonds to other carbons and is therefore bonded to two hydrogen atoms as well. A ball-and-stick model of the actual molecular structure of hexane, as determined by X-ray crystallography, is shown for comparison, in which carbon atoms are depicted as black balls and hydrogen atoms as white ones.

Implicit carbon and hydrogen atoms

Heteroatoms and other groups of atoms that give rise to relatively high rates of chemical reactivity, or introduce specific and interesting characteristics in the spectra of compounds are called functional groups, as they give the molecule a function. Heteroatoms and functional groups are known collectively as "substituents", as they are considered to be a substitute for the hydrogen atom that would be present in the parent hydrocarbon of the organic compound in question.

The skeleton has hydrogen and/or various substituents bonded to its atoms. Hydrogen is the most common non-carbon atom that is bonded to carbon and, for simplicity, is not explicitly drawn. In addition, carbon atoms are not generally labelled as such directly (i.e. with a "C"), whereas heteroatoms are always explicitly noted as such (i.e. using "N" for nitrogen, "O" for oxygen, etc.)

The skeletal structure of an organic compound is the series of atoms bonded together that form the essential structure of the compound. The skeleton can consist of chains, branches and/or rings of bonded atoms. Skeletal atoms other than carbon or hydrogen are called "heteroatoms".[2]

The Skeleton

Contents

  • The Skeleton 1
  • Implicit carbon and hydrogen atoms 2
  • Explicit heteroatoms 3
  • Pseudoelement symbols 4
    • Elements 4.1
    • Alkyl groups 4.2
    • Aromatic substituents 4.3
    • Functional groups 4.4
    • Leaving groups 4.5
  • Multiple bonds 5
  • Benzene rings 6
  • Stereochemistry 7
  • Hydrogen bonds 8
  • References 9
  • External links 10

[1]

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