Answers to Questions

Introduction to Functional Groups

1. What is the maximum number of covalent bonds each of the following elements will form in a neutral compound with no formal charge on the atom?

   Hydrogen ___1___ Carbon ___4___ Nitrogen ___3___

   Oxygen ___2___ Fluorine ___1___ Boron ___3___

2. The structural formula of a molecule shows all of the atoms in the structure and the order in which they are connected by covalent bonds. Add hydrogen atoms as needed to each atom in the following structural formulas so that each atom has a closed shell electron configuration and zero charge.
   {12026_Answers_Figure_8}
3. Draw a diagram that illustrates the three-dimensional shape of methane, CH₄, the chief component of natural
   {12026_Answers_Figure_11}
   1. Methane has a tetrahedral geometry around the central carbon atom.
   2. The ideal tetrahedral H–C–H bond angle is 109.5°.

Model Activity

1. Build models of ethane, C₂H₆, and propane, C₃H₈, and write out their structural formulas.
   {12026_Answers_Figure_12}
2. Do the C–C single bonds in ethane and propane rotate freely? Explain.

   Yes, the hydrogen atoms on adjacent carbon atoms in ethane can “slide” past each other as the C–C bond turns or rotates.

3. There are two possible structures for butane, C₄H₁₀. Build models of both structures and draw their structural formulas.
   {12026_Answers_Figure_14}
4. The two possible structural formulas for butane are called isomers. Write a general definition of isomers that describes the relationship between the two structures.

   Isomers have the same molecular formula but different structural formulas.

5. Without building models, draw out the possible structural formulas for three isomers of pentane, C₅H₁₂.
   {12026_Answers_Figure_16}
6. Alkanes are hydrocarbons—compounds containing only carbon and hydrogen—in which all of the C–C bonds are single bonds. What is the general formula for an alkane, where n is the number of carbon atoms?

   C_nH_2n+2

7. Alkenes are hydrocarbons that contain at least one C=C double bond in their structure. Build models of ethene (C₂H₄) and propene (C₃H₆) and draw their structural formulas.
   {12026_Answers_Figure_19}
8. Describe the molecular geometry around the C=C double bond in an alkene. What is the H–C–H bond angle in ethene?

   The molecular geometry about the C=C bond is planar—the two carbon atoms and atoms attached to them lie in a plane. The H–C–H bond angle in ethene is 120°.

Properties of Organic Compounds

1. The boiling points of straight-chain alkanes are summarized. a) Classify alkanes as polar or nonpolar compounds. b) Identify the type of intermolecular forces between alkanes. c) Explain the boiling point trend based on the nature and strength of intermolecular forces between molecules.
   {12026_Answers_Table_1}
   1. Alkanes are nonpolar compounds—the difference in electronegativity between that of carbon (2.5) and hydrogen (2.2) is very small. The C–H bonds are therefore nonpolar.
   2. London dispersion forces, also called van der Waals forces, are the principal type of intermolecular forces between nonpolar molecules.
   3. The boiling points of alkanes increase in a regular manner as the size of the hydrocarbon chain (number of carbon atoms) increases. London dispersion forces in general are fairly weak, but increase in strength as the size of molecules increases. This is attributed to the increase in surface area and polarizability as molecules get larger.
2. Oxidation of isopropyl alcohol, C₃H₇OH, gives acetone, C₃H₆O, via the loss of two hydrogen atoms. The boiling point of isopropyl alcohol is 82.4 °C, while that of acetone is 56.2 °C. a) Draw the structures of isopropyl alcohol and acetone. b) Describe the intermolecular forces in each compound and explain the difference in boiling point based on the nature and strength of these forces.
   1. 
      
      {12026_Answers_Figure_21}
   2. Isopropyl alcohol is a polar compound, and the –OH group can form hydrogen bonds between molecules. Acetone is also a polar compound, exhibiting dipole–dipole forces between molecules, but does not form hydrogen bonds. Isopropyl alcohol has a higher boiling point than acetone because hydrogen bonds are stronger than dipole–dipole forces between molecules. It takes more energy (higher temperature) to break the attractive forces between molecules in the liquid state for isopropyl alcohol rather than acetone.
3. Methyl alcohol and ethyl alcohol are miscible with water (infinitely soluble in all proportions). As the number of carbon atoms increases, the water solubility decreases, so that octyl alcohol (8-C atoms) is insoluble in water (<0.1 g per 100 g of water). Explain in terms of a “competition” between the properties of the hydrocarbon chain and the OH group.

   The –OH group in alcohols is highly polar and forms strong hydrogen bonds with water molecules. This factor explains why short-chain alcohols are soluble in water. The hydrocarbon groups (–CH₂–CH₂–) in alcohols, however, are nonpolar, and exhibit only weak interactions between themselves. When any solute dissolves in a solvent, the attractive forces acting between solute particles and those between solvent molecules must be broken and replaced by new attractive forces between the solute and solvent. The solubility of nonpolar hydrocarbons in water is inhibited by the strong attractive forces between water molecules, giving rise to what is often called the “hydrophobic” effect, as if nonpolar hydrocarbons were “afraid” of water. The competition between the hydrophobic effect and hydrogen bonding means that long-chain alcohols are insoluble in water.

4. In 1828, the German chemist Friedrich Wöhler synthesized urea (NH₂CONH₂), an organic compound, from inorganic precursors, ammonium (NH₄⁺) and cyanate (CNO^–) ions. This showed that a “vital force” was not necessary for the synthesis of an organic compound. a) Draw Lewis structures for urea and ammonium and cyanate ions. b) What is the principal “organic” or natural source of urea, and what is its biological role? c) More than 5 million metric tons of urea are produced annually in the United States. What are the main industrial uses of urea?
   1. 
      
      {12026_Answers_Figure_23}
   2. Urea is formed in the body by the metabolism or breakdown of amino acids. It is the principal nitrogen-containing byproduct found in human urine and in the urine of mammals and amphibians. Urea acts as a carrier of nitrogen waste from amino acids and proteins in foods.
   3. Urea is used mainly in nitrogen-release fertilizers and as a raw material for the manufacture of plastics, such as urea–formaldehyde resins (Bakelite), adhesives and plywood.
5. Compounds containing at least one carbon atom attached to four different groups give rise to a special class of isomers, called enantiomers, that are non-superimposable mirror images. a) Build models and complete the following diagrams to show the enantiomers of alanine, an amino acid. b) Explain why enantiometers are non-superimposable and describe why this property of molecules is called “handedness.”
   1. {12026_Answers_Figure_26}
   2. The mirror image molecules or models cannot be superimposed on one another by turning or rotating them. This property of having a non-superimposable mirror image is called “handedness” because our hands (and feet) are examples of non-superimposable mirror images.
      {12026_Answers_Figure_27}
6. The principal allotropes of carbon are diamond, graphite, and fullerenes. a) Describe the structure of each allotrope. b) Relate the properties of these forms of carbon, and their uses, to their structures. Fullerenes have aroused attention because of their potential applications in nanotechnology. Define nanotechnology and describe some uses of carbon in this field.
   1. Diamond is a covalent or network solid in which each carbon atom forms single covalent bonds to four other carbon atoms. The geometry around each carbon atom is tetrahedral and the resulting infinite three-dimensional array of carbon atoms is exceptionally stable, with ideal C–C bond angles and zero strain. Graphite has a planar or layered structure, with each carbon atom sharing valence electrons with three other carbon atoms via single and double bonds. The carbon atoms form planar six-membered rings, which are connected by shared edges. The result is layers of these planar arrays of interconnected six-membered rings. Fullerenes are named for the resemblance of their structures to the geodesic domes devised by the famous architect Buckminster Fuller. Fullerenes are relatively small molecules, compared to diamond and graphite, with 60 or more carbon atoms forming five- and six-membered rings by sharing electrons with three other carbon atoms via a combination of single and double bonds. The rings of carbon atoms fold over and connect with each other to give hollow tubes or spheres, the latter resembling soccer balls in structure. The stable, three-dimensional crystal structure of diamond makes it the hardest known material. The pi electrons in the double bonds in graphite are delocalized via resonance, allowing the electrons to move freely throughout each layer and thus conduct electricity. The layered structure of graphite also gives it a soft and slippery “feel,” making it an excellent lubricant.
   2. Nanotechnology is the study of the properties and uses or application of nanoparticles, solid-phase particles that are on the order of 10^–9 meter (nanometers) in size. Current research is focused on hollow carbon nanotubes (variations of fullerenes) that may be used as conductors or semiconductors for nanoscale wires and electrical components.

Introduction

Explore organic chemistry with this unique, integrated activity booklet set! The five interactive activities, simulations, worksheets and puzzles help students understand the core chemistry principles and concepts and build essential connections to applications of these concepts in their daily lives.

Concepts

• Functional groups
• Organic compounds
• Biological compounds