Chapter 6

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. \(\mathrm{SiCl}_{4}\) b. \(\mathrm{HCOO}^{\ominus}\) c. \(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{CH}\) d. \(\mathrm{F}_{2} \mathrm{C}=\mathrm{C}=\mathrm{CF}_{2}\) e. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{O}^{\oplus}\)

The \(\mathrm{P}-\mathrm{H}\) bond distance in \(\mathrm{PH}_{3}\) is \(1.42 \mathrm{~A}\) and the \(\mathrm{N}-\mathrm{H}\) bond distance in \(\mathrm{NH}_{3}\) is \(1.01 \mathrm{~A}\). Use the bond angle of \(93^{\circ}\) for \(\mathrm{PH}_{3}\) and \(107.3^{\circ}\) for \(\mathrm{NH}_{3}\) and calculate the distance between the hydrogens for each molecule. Would you expect the repulsion between the hydrogen nuclei in \(\mathrm{PH}_{3}\), to be more, or less, than in \(\mathrm{NH}_{3}\) ?

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. \(\oplus \mathrm{NO}_{2}\) b. \(\mathrm{CS}_{2}\) c. \(\mathrm{O}=\mathrm{C}=\mathrm{C}=\mathrm{O}\) d. \(\mathrm{H}_{2} \mathrm{C}=\mathrm{NH}\) e. \(\mathrm{HN}=\mathrm{NH}\) f. \(\mathrm{CH}_{3}\) g. ClNO h. \(\mathrm{NH}_{2}\) i. \(\mathrm{BH}_{4}^{-}\)

Use electron-repulsion arguments to explain the following: a. The \(\mathrm{H}-\mathrm{N}-\mathrm{H}\) bond angle in \(\mathrm{NH}_{4}^{+}\) is larger than in \(\mathrm{NH}_{3}\). b. The \(\mathrm{H}-\mathrm{N}-\mathrm{H}\) bond angle in \(\mathrm{NH}_{3}\left(107.3^{\circ}\right)\) is larger than the \(\mathrm{F}-\mathrm{N}-\mathrm{F}\) bond angles in \(\mathrm{NF}_{3}\left(102.1^{\circ}\right)\). c. The \(\mathrm{Cl}-\mathrm{C}-\mathrm{Cl}\) angle in \(\mathrm{Cl}_{2} \mathrm{C}=\mathrm{O}\) (phosgene, \(111.3^{\circ}\) ) is less than the \(\mathrm{H}-\mathrm{C}-\mathrm{H}\) angle in \(\mathrm{H}_{2} \mathrm{C}=\mathrm{O}\) (methanal, \(118^{\circ}\) ). d. The \(\mathrm{H}-\mathrm{C}-\mathrm{H}\) angle in methanal \(\left(118^{\circ}\right)\) is greater than the \(\mathrm{H}-\mathrm{C}-\mathrm{H}\) angle in ethene \(\left(116.7^{\circ}\right)\).

Suggest why the molecule \(\mathrm{Be}_{2}\) apparently is so unstable that it has not been observed. Explain why Be with an outer-shell electronic configuration of \((2 s)^{2}\) forms \(\mathrm{BeCl}_{2}\), whereas He with the configuration \((1 s)^{2}\) does not form \(\mathrm{HeCl}_{2}\).

Indicate the hybridization expected at each carbon in the following: a. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) b. \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}\) c. \(\mathrm{HC} \equiv \mathrm{C}-\mathrm{CH}=\mathrm{O}\) d. \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{O}\) e. \(\mathrm{CH}_{2}=\mathrm{C}=\mathrm{CH}_{2}\)

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. \(\mathrm{CO}_{2}\) b. \(\mathrm{N} \equiv \mathrm{C}-\mathrm{O}^{\ominus}\) c. \(\mathrm{CH}_{2}=\mathrm{C}=\mathrm{O}\) d. \(\mathrm{CH}_{3}^{+}\) e. \(\mathrm{F}_{2} \mathrm{C}=\mathrm{CH}_{2}\) f. \(\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{N}\) g. \(\mathrm{SiF}_{4}\) h. \(\mathrm{HCOOH}\) i. \(\mathrm{H}_{3} \mathrm{O}^{\ominus}\) j. \(\mathrm{CH}_{3} \mathrm{SH}\) \(\mathbf{k} . * \mathrm{SO}_{3}\)

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

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

If methanal, \(\mathrm{H}_{2} \mathrm{C}=\mathrm{O}\), were protonated to give \(\mathrm{H}_{2} \mathrm{C}=\mathrm{OH}\), would you expect the \(\mathrm{C}=\mathrm{O}-\mathrm{H}\) angle to be closer to \(180^{\circ}, 120^{\circ}, 109^{\circ}\), or \(90^{\circ}\) ? Explain.

The boron orbitals in diborane, \(\mathrm{B}_{2} \mathrm{H}_{6}\), overlap with hydrogen \(1 s\) orbitals in such a way to produce a structure having four ordinary \(\mathrm{B}-\mathrm{H}\) 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 \(\mathrm{H}-\mathrm{B}-\mathrm{H}\) angle is larger than the internal \(\mathrm{H}-\mathrm{B}-\mathrm{H}\) angle.