Skeletal Formula

8 Key excerpts on "Skeletal Formula"

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  eBook - ePub

  A-Level Chemistry's Best Kept Secrets!

  What Top Students Know That You Don't

  • K W David Tan, Leonard Pereira(Authors)
  • 2018(Publication Date)
  • WS EDUCATION

    (Publisher)

  SECTION 3

  ORGANIC CHEMISTRY

  Passage contains an image

  SECTION 3.1A

  INTRODUCTORY — STRUCTURE AND BONDING

  Drawing Skeletal Formula

  A Skeletal Formula is a type of structural formula that serves as a shorthand representation of a molecule’s bonding. In a Skeletal Formula, all the hydrogen atoms are removed from carbon chains, leaving just a carbon skeleton with functional groups attached to it.

  ✓ Comment: Drawing and interpretation of skeletal structures takes time and practice. Although Skeletal Formula may take some getting used to, it is a quick and easy way to represent the structure of a molecule. For examination questions that require candidates to draw all the possible structures based on the given chemical formula, knowledge on Skeletal Formula could prove very useful and save valuable time.

  Example 1

  Draw the Skeletal Formulae of the following compounds.

  (a) CH3 CH2 CH2 CH3

  (b) CH3 CH2 CH2 CH2 Cl

  (c) CH3 CH2 CH2 CH2 OH

  (d) CH3 CH2 CH2 CH2 NH2

  (e) CH3 CH2 CH2 COOH

  (f) CH3 CCCH3

  Solution

  Drawing Structural Formulas from Names

  Step 1: Draw and number the carbon atoms in the parent chain.

  Step 2: Draw the principal functional group.

  Step 3: Draw the substituent(s).

  Step 4: Fill in the remaining hydrogen atoms.

  ✓ Comment: This is a very important skill that all students must have. In some questions on structure elucidation, the names of the compounds are given and candidates are expected to know their structural formulas.

  For a start, here are the names of a few simple organic compounds:

  (a) 2,3-dibromobutane

  (b) 2-methylpentan-1-ol

  (c) 2,2-dimethylbutane

  The general name structure comprises (i) position(s) of particular substituent, (ii) no. of that substituent (mono, di, tri, tetra, penta, etc.), (iii) name of substituent, (iv) name of longest continuous carbon chain bearing the principal functional group (which may or not be the substituent). Consideration must be made for the priority of the functional group (if there is more than one substituent).

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  Communicating Genetics

  Visualizations and Representations

  • Han Yu(Author)
  • 2017(Publication Date)
  • Palgrave Macmillan

    (Publisher)

  As Fig. 7.1 shows, in the line-bond formula, atoms can be visualized as “balls,” a common and intuitive, if inaccurate, metaphorical approach to explaining particles (Baake 2003). The bonds between atoms appear as connecting “lines,” which are also metaphorically intuitive and easy to imagine. Electrons are removed from the depiction, which reduces Fig. 7.1 Line-bond structure of the DNA molecule (Calvin & Calvin 1964, p. 173). Reprinted by permission of American Scientist, magazine of Sigma Xi, The Scientific Research Society 7 The Structural View: 2D Realities and 3D Possibilities 223 visual complexity and helps to focus readers’ attention on atoms and their spatial relationships. Skeletal Formula Though the line-bond formula simplified the Lewis dot structure, it can still, as shown in Fig. 7.1, be visually complex when used to depict genetic research. This is because DNA and gene products, notably pro- teins, are macromolecules that contain large numbers of atoms and bonds—and hence, require elaborate visualization. This is where the Skeletal Formula comes in. In a Skeletal Formula, carbons (C), the most common and essential atom in organic compounds, are omitted. Unless otherwise specified, the end of a line or the junction of two lines is taken to be a carbon. The simple display of therefore, represents three carbons. In addition, hydrogen (H), the atom most likely to bond with car- bon, is omitted when it does bond with carbon. To deduce omitted carbon–hydrogen bonds, one needs to know that a carbon has four valence electrons to form four bonds. Any unspecified bonds within this limit of four are thus omitted carbon–hydrogen bonds. In the middle carbon would have two omitted hydrogen bonds because this carbon already forms two bonds with the two end carbons. Likewise, each end carbon would have three hydrogen bonds because it only has one bond with the middle carbon.

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  Foundations of Chemistry

  An Introductory Course for Science Students

  • Philippa B. Cranwell, Elizabeth M. Page(Authors)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  In more complex situations, chemists often use a Skeletal Formula , where the carbon chain is shown as a zig-zag line. C 2 H 4 O 2 CH 2 O C n H 2n+1 COOH (or C n H 2n O 2 ) CH 3 COOH H C H H C O O H OH O Molecular formula Empirical formula General formula Structural formula Display formula Skeletal Formula Figure 12.1 The different ways that ethanoic acid is represented by each type of formula. Carboxylic acids are a class of mole-cule that will be explored further in Chapter 14 For a refresher, see Chapter 3. The empirical formula represents the simplest whole-number ratio of atoms of each element in a compound. The molecular formula represents the actual number of atoms of each ele-ment in a compound. 404 Core concepts and ideas within organic chemistry Figure 12.2 shows the comparative display and Skeletal Formulae of some organic compounds. Skeletal Formulae are often viewed with horror by students, but once you have become used to them, they are very easy to use. Here are some guidelines to drawing them: 1. Hydrogen atoms that are bonded to a carbon atom are not shown. There are too many, and they clutter the diagram. 2. Chains of carbon atoms are simply drawn as zig-zags in which the point of the zig-zag represents a carbon atom that is also bonded to hydrogen atoms, which are not shown. 3. Double and triple bonds are represented by two or three lines between connecting points, respectively. 4. Heteroatoms (e.g. N, O, P, S) and the halogens (F, Cl, Br, I) are all written using their atomic symbol. If they are bonded to a hydrogen atom, it is also shown. 5. Lone pairs of electrons are not shown on an element unless required. Worked Example 12.1 The display formulae and names for three compounds are given here.

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  The Chemistry of Beer

  The Science in the Suds

  • Roger Barth(Author)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  The skeletal structure (Fig. 5.1d) is a simplification of the Lewis struc- ture in which the symbols for the carbon atoms are omitted. Hydrogen atoms attached to carbon atoms are omitted. All other atoms are shown explicitly. Usually bond angles are represented approximately accurately. The end of FUNCTIONAL GROUPS 107 (a) (b) H H N H O O H C H H C C H H H H H C C C O H C NH 2 HO CH 2 Figure 5.2 (a) Phenylalanine Lewis structure. (b) Phenylalanine modified skeletal. Solid wedge comes toward the reader, dashed wedge goes back. a bond that is not marked as some other atom is understood to be a carbon atom. Each carbon atom must have four bonds, so any carbon with fewer bonds shown is understood to be connected to enough hydrogen atoms to make up four bonds. This convention shows a clear picture of the shape of the molecule and highlights the interesting parts, that is, the groups with atoms that are not carbon or hydrogen (called hetero atoms). Many chemists are not comfortable with the angular zigzags of skeletal structures, so they may prefer hybrid notation, using strict skeletal notation largely for rings, but showing both carbon and hydrogen atoms outside of rings (Fig. 5.2b). We will often put hydrogen atoms near the carbon atoms to which they are bound without showing the bonds. Hydrogen atoms attached to carbon atoms in rings will be omitted except to show certain three-dimensional fea- tures. Understanding the basics of chemical notation is an important part of chemical literacy. 5.2 FUNCTIONAL GROUPS The number of ways to assemble atoms into organic molecules is nearly infi- nite. It is convenient to think about the molecules on the basis of collections of atoms that form parts of the molecules. These collections are called groups or functional groups. Families of compounds are classified by the functional groups they contain. For example a molecule with an −OH group attached to a carbon atom that has no other oxygen attached is an alcohol.

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  Organic Chemistry

  • T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  1.7C 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 Figure 1.3e 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 formulas, all of the atoms that are attached to the carbon are usually written immediately after that carbon, listing hydrogens first. For example, 18 CHAPTER 1 The Basics and work problems. And, lacking all of the symbols that are explicitly shown in dash and con- densed structural formulas, bond-line formulas allow you to more quickly interpret molecular connectivity and compare one molecular formula with another. Bond-line formulas are easy to draw for molecules with multiple bonds and for cyclic molecules, as well. The following are some examples. How To Draw Bond-Line Formulas HELPFUL HINT As you become more famil- iar with organic molecules, you will find bond-line formulas to be very use- ful tools for representing structures. We apply the following rules when we draw bond-line formulas: • Each line represents a bond. • Each bend in a line or terminus of a line represents a carbon atom, unless another group is shown explicitly. • No Cs are written for carbon atoms, except optionally for CH 3 groups at the end of a chain or branch. • No Hs are shown for hydrogen atoms attached to carbon 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). • 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.

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  General, Organic, and Biological Chemistry

  An Integrated Approach

  • Kenneth W. Raymond(Author)
  • 2012(Publication Date)
  • Wiley

    (Publisher)

  In this molecule, C is the central atom. 122 CHAPTER 4 An Introduction to Organic Compounds Many compounds, especially those encountered in organic and biochemistry, are quite large, and drawing their electron dot or Lewis structural formulas can be a time-consuming task. For this reason, chemists have devised more abbreviated methods for representing structure. A condensed structural formula describes the attachment of atoms to one another, without showing all of the bonds. For example, a carbon atom with three attached hydrogen atoms can be written CH 3 and one with two attached hydrogen atoms can be written CH 2 . Several examples of condensed structural formulas are shown in Figure 4.2. In skeletal structures (Figure 4.2), covalent bonds are represented by lines, carbon atoms are not shown, and hydrogen atoms are drawn only when attached to atoms other than carbon. To read a skeletal structure, you assume that a carbon atom appears where lines (bonds) meet and at the end of each line. To simplify matters, nonbonding electrons are sometimes omitted from skeletal and other structural formulas. SAMPLE PROBLEM 4.2 Drawing Lewis structures of polyatomic ions Draw the Lewis structure of NO 2 + . STRATEGY As in NO 2 - , nitrogen is the central atom. When counting available electrons in polyatomic cations, subtract one electron for each positive charge. SOLUTION 1. A nitrogen atom has 5 valence electrons and each oxygen atom has 6. This gives an ini- tial total of 17 available electrons. Because of the positive charge on the ion, 1 electron is subtracted, giving a total of 16 available electrons for NO 2 + . 2. Nitrogen is the central atom, so the initial structure is O N O 3. Having two covalent bonds, the structure above uses only 4 of the available 16 valence electrons. The remaining 12 are added to the oxygen atoms. O N O 4. To give the nitrogen atom an octet, one pair of nonbonding electrons from each oxygen atom is used to make a multiple bond.

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  Chemistry, 5th Edition

  • Allan Blackman, Steven E. Bottle, Siegbert Schmid, Mauro Mocerino, Uta Wille(Authors)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  For example, the molecular formula of water is always written as H 2 O, but this tells us nothing about how the atoms are arranged in the molecule. We cannot tell from the molecular formula alone which of the three possible arrangements of atoms below is the correct one for H 2 O. H H O H O H H H O Structural formulae attempt to show the way in which the atoms in a molecule are covalently bonded together, thereby giving us some structural information. The chemical symbols are still used for each element present, but now the constituent atoms are placed in the order in which they are bonded together and the bonds between neighbouring atoms are represented as lines. A single line represents a single bond, which, as we will see in the chapter on chemical bonding and molecular structure, consists of a pair of electrons. Consider for example the ammonia molecule, NH 3 . We can show the way in which the four atoms are bonded together in this molecule by drawing the structural formula as follows. H N H H From this depiction, it is obvious that the nitrogen atom is bonded to three H atoms by single bonds. It is also common to see ammonia depicted as shown. H H N H This gives us the same structural information as the previous diagram (the N atom is bonded to three H atoms by single bonds), but shows the molecule apparently adopting a slightly different geometry. This illustrates an important point about structural formulae: structural formulae do not necessarily show the correct geometry of a compound, simply because it is difficult to accurately represent three-dimensional molecules in two dimensions. While both of the depictions correctly show all of the bonds in the molecule, neither is strictly correct from a structural point of view as they do not show the actual three-dimensional arrangement of the atoms. Both of them also neglect the fact that there is a pair of electrons on the N atom which does not participate in bonding.

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  Solomons' Organic Chemistry

  • T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
  • 2017(Publication Date)
  • Wiley

    (Publisher)

  18 CHAPTER 1 THE BASICS: Bonding and Molecular Structure 1.7C 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 Fig. 1.3e 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. PRACTICE PROBLEM 1.13 Write a condensed structural formula for the following compound. Consider the following examples of molecules depicted by bond-line formulas. • • HOW TO Draw Bond-Line Formulas CH 3 CHClCH 2 CH 3 Cl Bond-line formula CH CH 2 CH 3 CH 3 Cl = = We apply the following rules when we draw bond-line formulas: • Each line represents a bond. • Each bend in a line or terminus of a line represents a carbon atom, unless another group is shown explicitly. • No Cs are written for carbon atoms, except optionally for CH 3 groups at the end of a chain or branch. • No Hs 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). • 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. • When an atom other than carbon or hydrogen is present, the symbol for that ele- ment is written at the appropriate location (i.e., in place of a bend or at the terminus of the line leading to the atom). • Hydrogen atoms bonded to atoms other than carbon (e.g., oxygen or nitrogen) are written explicitly.

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