How to write and interpret Structural Formulas?
** Organic
chemists use a variety of formats to write structural formulas.
** Structural
Formulas are written by:
(1) Ball-and-stick
model
(2) Electron-dot
formula (lewis structure)
(3) Dash
formula
(4) Condensed
formula
(5) Bond-line
formula
** Examples of
these types of structural formulas are shown in the following Figure using propyl alcohol as
an example:
** An older term
for isomers of this type was structural isomers.
** The
International Union of Pure and Applied Chemistry (IUPAC) now recommends that
use of the term “structural” when applied to constitutional isomers be
abandoned.
** Although
electron-dot formulas account explicitly for all of the valence electrons in a molecule,
they are tedious and time-consuming to write.
** Dash,
condensed, and bond-line formulas are therefore used more often.
** Generally it
is best to draw unshared electron pairs in chemical formulas, though sometimes they
are omitted if we are not considering the chemical properties or reactivity of
a compound. When we write chemical reactions, however, we shall see that it is necessary
to include the unshared electron pairs when they participate in a reaction. It
is a good idea, therefore, to be in the habit of writing unshared electrons
pairs.
** You can read
full subject about Lewis structure on our website
** In the
subject we will talk about the most used structural formulas (Dash formula - Condensed
formula - Bond-line formula)
(1) Dash Structural Formulas
** Dash
structural formulas have lines that show bonding electron pairs, and include
elemental symbols for all of the atoms in a molecule.
** If we look
at the ball-and-stick model for propyl alcohol given in the previous Figure and
compare it with the electron-dot, dash, and condensed formulas in the same Figure
we find that the chain of atoms is straight in those formulas.
** In ball-and-stick
model, which corresponds more accurately to the actual shape of the molecule,
the chain of atoms is not at all straight.
** Also of
importance is this: Atoms joined by single bonds can rotate relatively freely
with respect to one another. This relatively free rotation means that the chain
of atoms in propyl alcohol can assume a variety of arrangements like these:
** It also
means that all of the structural formulas above are equivalent and all
represent propyl alcohol.
** Dash
structural formulas such as these indicate the way in which the atoms are attached
to each other and are not representations of the actual shapes of the molecule.
(Propyl alcohol does not have 90o bond angles.
It has tetrahedral bond angles.)
** Dash
structural formulas show what is called the connectivity of the atoms.
** Constitutional
isomers have different connectivities and, therefore, must have different
structural formulas.
** Constitutional
isomer is different compounds that have the same molecular formula but differ
in the sequence in which their atoms are bonded—that is, their connectivity
** Consider the
compound called isopropyl alcohol, whose formula we might write in a variety of
ways:
** Isopropyl
alcohol is a constitutional isomer of propyl alcohol because its atoms are connected
in a different order and both compounds have the same molecular formula, C3H8O.
In isopropyl alcohol the OH group is attached to the central carbon; in propyl
alcohol it is attached to an end carbon.
** In problems
you will often be asked to write structural formulas for all the isomers that
have a given molecular formula. Do not make the error of writing several
equivalent formulas, like those that we have just shown, mistaking them for different
constitutional isomers.
(2) Condensed Structural Formulas
** Condensed
structural formulas are somewhat faster to write than dash formulas and, when
we become familiar with them, they will impart all the information that is
contained in the dash structure.
** In condensed
formulas all of the hydrogen atoms that are attached to a particular carbon are
usually written immediately after the carbon.
** In fully
condensed formulas, all of the atoms that are attached to the carbon are
usually written immediately after that carbon, listing hydrogens first.
** For example:
** The
condensed formula for isopropyl alcohol can be written in four different ways:
Solved Problem(1):
Write a condensed structural formula for the compound that follows:
Answer:
(3) Bond-Line Formulas
** The most
common type of structural formula used by organic chemists, and the fastest to draw,
is the bond-line formula. (Some chemists call these skeletal formulas.)
** The formula in
the next figure is a bond-line formula
for propyl alcohol.
** The sooner
you master the use of bond-line formulas, the more quickly you will be able to
draw molecules when you take notes and work problems. And, lacking all of the
symbols that are explicitly shown in dash and condensed structural formulas,
bond-line formulas allow you to more quickly interpret molecular connectivity
and compare one molecular formula with another.
How To Draw Bond-Line Formulas
We apply the
following rules when we draw bond-line formulas:
(1) Each line
represents a bond.
(2) Each bend in a
line or terminus of a line represents a carbon atom, unless another group is
shown explicitly.
(3) No Carbons are
written for carbon atoms, except optionally for CH3 groups at the
end of a chain or branch.
(4) No Hydrogens
are shown for hydrogen atoms, unless they are needed to give a three dimensional
perspective, in which case we use dashed or solid wedges (as explained in the
next section).
(5)The number of
hydrogen atoms bonded to each carbon is inferred by assuming that as
many hydrogen
atoms are present as needed to fill the valence shell of the carbon, unless
a charge is
indicated.
(6) When an atom
other than carbon or hydrogen is present, the symbol for that element is
written at the
appropriate location (i.e., in place of a bend or at the terminus of the line
leading to the
atom).
(7) Hydrogen atoms
bonded to atoms other than carbon (e.g., oxygen or nitrogen) are written
explicitly.
** Consider the
following examples of molecules depicted by bond-line formulas.
** Bond-line formulas
are easy to draw for molecules with multiple bonds and for cyclic molecules, as
well. The following are some examples:
Solved Problem(2):Write
the bond-line formula for
Strategy and
Answer:
First, for the
sake of practice, we outline the carbon skeleton, including the OH group as follow:
Then we write
the bond-line formula as:
As you gain
experience you will likely skip the intermediate steps shown above and proceed directly
to writing bond-line formulas.
Three-Dimensional Formulas
** None of the
formulas that we have described so far convey any information about how the
atoms of a molecule are arranged in space.
** Molecules
exist in three dimensions. We can depict three-dimensional geometry in molecules
using bonds represented by dashed wedges, solid wedges, and lines.
(1) A dashed wedge: represents a bond that projects behind the plane of the paper.
(2) A solid wedge: represents a bond that projects out of the plane of the paper.
(3) An ordinary
line: represents a bond that lies in the plane of the paper.
** For example,
the four C-H bonds of methane (CH4) are oriented toward the corners of
a regular tetrahedron, with the carbon in the center and an approximately 1098 angle between
each C-H bond, as was originally postulated by J. H. van’t Hoff and L. A. Le
Bel in 1874.
** The following Figure shows the tetrahedral structure of methane.
** let us
consider some guidelines for representing these bonding patterns in three
dimensions using dashed and solid wedge bonds. In general for carbon atoms that
have only single bonds:
(1) A carbon atom
with four single bonds has tetrahedral geometry and can be drawn with two bonds
in the plane of the paper separated by approximately 1098, one bond
behind the plane using a dashed wedge, and one bond in front of the plane using
a solid wedge.
(2) The dashed
wedge and solid wedge bonds in tetrahedral geometry nearly eclipse each other
when drawn in proper three-dimensional perspective. For carbon atoms with a
double or a triple bond:
(3) A carbon atom
with a double bond has trigonal planar geometry (Section 1.13) and can be
depicted with bonds that are all in the plane of the paper and separated by 1208.
(4) A carbon atom
with a triple bond has linear geometry (Section 1.14) and can be depicted with
its bonds in the plane of the paper and separated by a 1808 angle. Last,
when drawing three-dimensional formulas for molecules:
(5) Draw as many
carbon atoms in the plane of the paper as possible using ordinary lines, then use
dashed or solid wedge bonds for substituent groups or hydrogen atoms that are
needed to show three dimensions.
Some examples
of three-dimensional formulas are shown below:
Solved Problem(3):
Write a bond-line formula for the following compound showing three dimensions
at the carbon bearing the chlorine atom.
Strategy and
Answer:
Then add the chlorine atom at the
appropriate carbon using a three-dimensional representation.
Reference: Organic chemistry / T.W. Graham Solomons , Craig B.Fryhle , Scott A.snyder , / ( eleventh edition) / 2014.
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