Molecule line formula

The resonance interaction of chlorine with the benzene ring can be represented as shown in 13 or 14, and both of these representations have been used in the literature to save space. However, we shall not use the curved-arrow method of 13 since arrows will be used in this book to express the actual movement of electrons in reactions. We will use representations like 14 or else write out the canonical forms. The convention used in dashed-line formulas like 14 is that bonds that are present in all canonical forms are drawn as solid lines, while bonds that are not present in all forms are drawn as dashed lines. In most resonance, a bonds are not involved, and only the n or unshared electrons are put in, in different ways. This means that if we write one canonical form for a molecule, we can then write the others by merely moving n and unshared electrons. 

The alkane series is also called the saturated hydrocarbon series because the molecules of this class have carbon atoms connected by single bonds only, and therefore have the maximum number of hydrogen atoms possible for the number of carbon atoms. These substances may be represented by the general formula C H2,I + 2 and molecules of successive members of the series differ from each other by only a CH2 unit. The line formulas and names of the first 10 members of the series, given in Table 21-2, should be memorized because these names form the basis for naming many other organic compounds. It should be noted that the first parts of the names of the later members listed are the... 

Given a line formula, such as -— (pentane), write the complete structure and clearly show the arrangement of atoms in the molecule. Tell how many hydrogens are attached to each carbon, what the molecular formula is, and what the functional groups are. 

Line formulas for more complicated molecules sometimes give clues as to which atoms are bonded to which others. For example, in compounds containing carbon and hydrogen atoms, perhaps along with atoms of others elements, the atoms bonded to each carbon atom are placed after that atom, as in... 

A variation of the line formula, called a condensed formula, is often used for molecules with branches on a carbon chain. For example, the condensed formula for a compound with a three-carbon chain and a one-carbon branch is... 

In dilute aqueous solution, the amino end of an amino acid molecule acts as a base, and the acid end acts as an acid. Write a line formula for an amino acid in neutral solution after the two ends of the same molecule have reacted with each other. 

A structural formula gives partial or complete information about the way in which the atoms in a molecule are connected and arranged in space. In simple cases, a line formula that is just a sequence of atomic symbols gives structural information provided the reader knows that the formula represents the order of the atoms in the linear structure. 

The simplest representation of a molecule is the line formula. In the line formula we assume that there is a carbon atom at any location where two or more lines intersect. We also assume that there is a carbon at the end of any line and that each carbon in the structure is bonded to the correct number of hydrogen atoms. Compare the structural and line formulas for butane and 2-methylpropane, shown here ... 

Let s now examine our problem of comparing a hashed-wedged line formula with a Fischer projection. The actual problem is how to interconvert hashed-wedged line and Fischer formulas. The key to this is recognizing that Fischer projections are pictures of a molecule in an eclipsed conformation. Very important. So the first step in our comparison is to get the hashed-wedged line formula into an eclipsed conformation. Any 60° rotation will do, such as rotation of the left-hand methyl group up out of the page, toward you ... 

The constitution of a molecule (nmnber of, kind of, and cormectivities of atoms) may be represented by a two-dimensional map in which the interatomic linkages (bonds) are drawn as fines. There are two constitutional isomers that are represented by the molecular formula C2HgO ethanol and dimethyl ether. The differences in cormectivities, which are not evident in the common constitutional inventory C2H O, can be conveyed by t) o-graphical line formulas (CH3CH2OH for ethanol and CH3OCH3 for dimethyl ether), or by structural representations (see Figure 2). As the number and kinds of atoms in substances increase, the number of constitutional isomers increases. 

The structural language of organic chemistry has been developed so that complex molecules can be described in a clear, yet economical way. A molecule as complex as cholesterol can be drawn rapidly in a bond-line formula, while drawing even a condensed formula would require a prohibitive amount of time. 

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. 

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 Fig. 1.3 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. 

Consider the following examples of molecules depicted by bond-line formulas. 

As you become more familiar with organic molecules, you will find bond-line formulas to be very useful tools for representing structures. 

Bond-line formulas are easy to draw for molecules with multiple bonds and for cyclic molecules, as well. The following are some examples. 

Open computer molecular models for dimethyl ether, dimethylacetylene, and OT-l,2-dichloro-l,2-difluoroethene from the 3D Molecular Models section of the book s website. By interpreting the computer molecular model for each one, draw (a) a dash formula, (b) a bond-line formula, and (c) a three-dimensional dashed-wedge formula. Draw the models in whatever perspective is most convenient—generally the perspective in which the most atoms in the chain of a molecule can be in the plane of the paper. 

So far we have mainly considered chiral molecules that contain only one chirality center. Many organic molecules, especially those important in biology, contain more than one chirality center. Cholesterol (Section 23.4B), for example, contains eight chirality centers. (Can you locate them ) We can begin, however, with simpler molecules. Let us consider 2,3-dibromopentane, shown here in a two-dimensional bond-line formula. 2,3-Dibromopentane has two chirality centers ... 

STRATEGY AND ANSWER C2 and C4 are chirality centers in 2-bromO 4-chloropentane. We begin by drawing the carbon chain with as many carbons depicted in the plane of the paper as possible, and in a way that maximizes the symmetry between C2 and C4. In this case, an ordinary zig-zag bond-line formula provides symmetry between C2 and C4. Then we add the bromine and chlorine atoms at 02 and 04, respectively, as well as the hydrogen atoms at these carbons, resulting in formula I. To draw its enantiomer (II), we imagine a mirror and draw a reflection of the molecule. 

The carbon chain in a Fischer projection is always drawn from top to bottom, rather than side to side as is often the case with bond-line formulas. We consider the molecule in a conformation that has eclipsing interactions between the groups at each carbon. 

Bond-line formula (Section 1.7C) A formula that shows the carbon skeleton of a molecule with lines. The number of hydrogen atoms necessary to fulfill each carbon s valence is assumed to be present but not written in. Other atoms (e.g., 0, Cl, N) are written in. 

A structural formula shows the relative placement and connections of atoms in the molecule. It uses symbols for the atoms and either a pair of dots (electron-dot formula) or a line (bond-line formula) to show the elec-H-O-H tron pairs in bonds between the atoms. In water, each H atom is bonded to the O atom, but not to the other H atom. 

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