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6.E: Bonding in Organic Molecules (Exercises)

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  • Exercise 6-1 Write the ground-state configuration for a helium atom with two unpaired electrons \(\left( \ce{He} \: \uparrow \uparrow \right)\) that is in accord with the Pauli principle. Give your reasoning.

    Exercise 6-2 Formulate the electronic configuration of \(\ce{He_2}\) and \(\ce{He_2^+}\) in a manner similar to Figure 6-6(b). The ion \(\ce{He_2^+}\) has been detected spectroscopically; suggest a reason why this ion is more stable than \(\ce{H_2^+}\).

    Exercise 6-3 Write electronic configurations, as in Figure 6-6, for three different excited states of \(\ce{H_2}\), all of which agree with the Pauli principle. Arrange them in order of expected stability. Show your reasoning.

    Exercise 6-4 Indicate the probable hybridization of the \(\sigma\)-bonding orbitals on the atoms labeled with a star for each of the following molecules.

    a. hydrogen cyanide, \(\ce{H} - \overset{*}{\ce{C}} \ce{\equiv N}\)
    b. carbon disulfide, \(\ce{S} \overset{*}{\ce{C}} \ce{=S}\)
    c. methanimine, \(\ce{H_2} \overset{*}{\ce{C}} \ce{=NH}\)
    d. phosphonium ion, \(^\oplus \overset{*}{\ce{P}} \ce{H_4}\)
    e. dichloromethane, \(\overset{*}{\ce{C}} \ce{H_2Cl_2}\)
    f. nitric acid,
    g. \(\ce{H_2} \overset{*}{\ce{C}} \: ^{2+}\)
    h.* \(\ce{H_2} \overset{*}{\ce{C}} \colon\)

    Exercise 6-5 Examine the following structures and predict the most likely geometry, using concepts of orbital hybridization. State whether the molecule should be planar or nonplanar, and list the approximate values expected for the bond angles.

    a. \(\ce{SiCl_4}\)
    b. \(\ce{HCOO}^\ominus\)
    c. \(\ce{CH_3-C \equiv CH}\)
    d. \(\ce{F_2C=C=CF_2}\)
    e. \(\ce{(CH_3)_3O}^\oplus\)

    Exercise 6-6 The \(\ce{P-H}\) bond distance in \(\ce{PH_3}\) is \(1.42 \: \text{Å}\) and the \(\ce{N-H}\) bond distance in \(\ce{NH_3}\) is \(1.01 \: \text{Å}\). Use the bond angle of \(93^\text{o}\) for \(\ce{PH_3}\) and \(107.3^\text{o}\) for \(\ce{NH_3}\) and calculate the distance between the hydrogens for each molecule. Would you expect the repulsion between the hydrogen nuclei in \(\ce{PH_3}\) to be more, or less, than in \(\ce{NH_3}\)?

    Exercise 6-7 Write electron-pair structures including bonding and unshared pairs for each of the following compounds. Predict the preferred shape of the molecule as linear, angular, planar and triangular, tetrahedral, or pyramidal.

    a. \(^\oplus \ce{NO_2}\)
    b. \(\ce{CS_2}\)
    c. \(\ce{O=C=C=O}\)
    d. \(\ce{H_2C=NH}\)
    e. \(\ce{HN=NH}\)
    f. \(\ce{CH_3^-}\)
    g. \(\ce{ClNO}\)
    h. \(\ce{NH_2}\)
    i. \(\ce{BH_4^-}\)

    Exercise 6-8* Use electron-repulsion arguments to explain the following:

    a. The \(\ce{H-N-H}\) bond angle in \(\ce{NH_4^+}\) is larger than in \(\ce{NH_3}\).

    b. The \(\ce{H-N-H}\) bond angle in \(\ce{NH_3}\) \(\left( 107.3^\text{o} \right)\) is larger than the \(\ce{F-N-F}\) bond angles in \(\ce{NF_3}\) \(\left( 102.1^\text{o} \right)\).

    c. The \(\ce{Cl-C-Cl}\) angle in \(\ce{Cl_2C=O}\) (phosgene, \(111.3^\text{o}\)) is less than the \(\ce{H-C-H}\) angle in \(\ce{H_2C=O}\) (methanal, \(118^\text{o}\)).

    d. The \(\ce{H-C-H}\) angle in methanal \(\left( 118^\text{o} \right)\) is greater than the \(\ce{H-C-H}\) angle in ethene \(\left( 116.7^\text{o} \right)\).

    Exercise 6-9 Set up atomic-orbital models to represent the hybrid structures of \(\ce{NO_3^-}\), \(\ce{CO_3^{2-}}\), and \(\ce{N_2O}\).

    Exercise 6-10 Set up an atomic orbital model of each of the following structures with normal values for the bond angles. Evaluate each model for potential resonance (electron delocalization). If resonance appears to you to be possible, draw a set of reasonable valence-bond structures for each hybrid.

    a. \(\ce{CH_2=CH} - \overset{\oplus}{\ce{C}} \ce{H_2}\)
    b. \(\ce{CH_2=CH-CH_2} - \overset{\oplus}{\ce{C}} \ce{H_2}\)
    c. \(\ce{CH_3}- \overset{\oplus}{\ce{C}} \ce{H_2}\)
    d. \(\ce{CH_3CO} \overset{\ominus}{\ce{O}}\)


    Exercise 6-11* Draw valence-bond structures for the phenylmethyl radical, \(\ce{C_6H_5CH_2} \cdot\), and the 4-methylphenyl radical, . Explain why the methyl \(\ce{C-H}\) bonds of methylbenzene (toluene) are weaker than the ring \(\ce{C-H}\) bonds (see Table 4-6).

    Exercise 6-12 Suggest why the molecule \(\ce{Be_2}\) apparently is so unstable that it has not been observed. Explain why \(\ce{Be}\) with an outer-shell electronic configuration of \(\left( 2s \right)^2\) forms \(\ce{BeCl_2}\), whereas \(\ce{He}\) with the configuration \(\left( 1s \right)^2\) does not form \(\ce{HeCl_2}\).

    Exercise 6-13 Indicate the hybridization expected at each carbon in the following:

    a. \(\ce{CH_3CH_2CH_3}\)
    b. \(\ce{CH_3CH=CH_2}\)
    c. \(\ce{HC \equiv C-CH=O}\)
    d. \(\ce{CH_3-CH=O}\)
    e. \(\ce{CH_2=C=CH_2}\)

    Exercise 6-14 Draw atomic-orbital models for each of the following substances. Each drawing should be large and clear with all bonds labeled as either \(\sigma\) or \(\pi\), as shown in the abbreviated formalism of Figures 6-13 and 6-18. Indicate the values expected for the bond angles and whether the molecule or ion should be planar or nonplanar.

    a. \(\ce{BF_3}\)
    b. \(\ce{CH_3} \overset{\oplus}{\ce{N}} \ce{H_3}\)
    c. \(\ce{CH_2=} \overset{\oplus}{\ce{N}} \ce{H_2}\)
    d. \(\ce{CH_2=CH-C \equiv CH}\)

    Exercise 6-15 Write electron-pair structures for each of the following. Include both bonding and nonbonding pairs and predict the preferred shape of the molecule or ion as linear, triangular (planar), angular, tetrahedral, or pyramidal.

    a. \(\ce{CO_2}\)
    b. \(\ce{N \equiv C-O}^\ominus\)
    c. \(\ce{CH_2=C=O}\)
    d. \(\ce{CH_3^+}\)
    e. \(\ce{F_2C=CH_2}\)
    f. \(\ce{CH_3C \equiv N}\)
    g. \(\ce{SiF_4}\)
    h. \(\ce{HCOOH}\)
    i. \(\ce{H_3O}^\oplus\)
    j. \(\ce{CH_3SH}\)
    k.* \(\ce{SO_3}\)

    Exercise 6-16 Draw an atomic-orbital model for each of the compounds listed in Exercise 6-15 that is consistent with the geometry deduced for each.

    Exercise 6-17 Draw an atomic-orbital picture of 1,3-dichloropropadiene, \(\ce{ClCH=C=CHCl}\). Examine the structure carefully and predict how many stereoisomers are possible for this structure. What kind of stereoisomers are these?

    Exercise 6-18 Draw an atomic-orbital picture of 1,4-dichlorobutatriene, \(\ce{ClCH=C=C=CHCl}\). Examine your diagram carefully and predict the number and kind of stereoisomers possible for this structure.

    Exercise 6-19* If methanal, \(\ce{H_2C=O}\), were protonated to give \(\ce{H_2C=} \overset{\oplus}{\ce{O}} \ce{H}\), would you expect the \(\ce{C=} \overset{\oplus}{\ce{O}} \ce{-H}\) angle to be closer to \(180^\text{o}\), \(120^\text{o}\), \(109^\text{o}\), or \(90^\text{o}\)? Explain.

    Exercise 6-20* Draw atomic-orbital models for thiophene and imidazole that are consistent with their being planar compounds with six \(\pi\)-electron systems associated with five atomic nuclei.

    Exercise 6-21* The boron orbitals in diborane, \(\ce{B_2H_6}\), overlap with hydrogen \(1s\) orbitals in such a way to produce a structure having four ordinary \(\ce{B-H}\) bonds, each of which is an electron-pair bond associating two nuclei. The remaining two hydrogens each are bonded to both boron nuclei through an electron-pair bond associated with three atomic nuclei. This type of bond is referred to as a three-center bond.

    a. Would you expect diborane to be planar or nonplanar? Explain, using electron-repulsion arguments.

    b. Make an atomic-orbital diagram for diborane.

    c. Explain why the terminal \(\ce{H-B-H}\) angle is larger than the internal \(\ce{H-B-H}\) angle.

    Exercise 6-22 Inspect each of the following orbital diagrams. The nuclei are represented as filled circles, and all bonding and nonbonding orbitals are labeled. The objective of the question is to identify the compound represented by each diagram, based on the number and type of bonding and nonbonding electrons, the type of orbitals, and the charge (if any) associated with each nucleus.






    • John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."