[試題] 93下化學系普通化學甲第一次期中考(陳振中命題)

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課程名稱︰普通化學甲下  班次01
課程性質︰化學系必修
課程教師︰牟中原  陳振中
開課系所︰化學系
考試時間︰2005.03.25

試題 :

出題老師  陳振中

題目中有些圖無法畫出

有檔案提供

http://www.ch.ntu.edu.tw/~ccchan/Lecture_notes/GenChem/midterm2_ans.pdf


國立台灣大學 九十三學年度 第二學期
化學系 普通化學甲 期中考 2005年3月25日

Planck’s constant       6.6 × 10^-34 J s
Electron mass            9.1 × 10^-31 Kg
Light speed              3.0 × 10^8 m/s
Typical C-C bond length  1.54 A
Typical C=C bond length  1.34 A


Section A (80 %)

1. (a) Assign formal charges to the atoms in each of the following resonance
       forms and determine which species is the major resonance contributor:

       CH3-CH-C≡N <-> CH3-CH=C=N

   (b) Indicate the molecular geometry of the carbon in each of the following
       molecules:
                +             ..
             H－C－H        H－C－H
                |              |
                H              H


2. Consider the following molecule




   (a) Assign the Cahn-Ingold-Prelog configuration of the chiral carbon(s).

   (b) Copy the molecule in your answer book and indicate all the sp2 and sp
       hybridized carbons.

   (c) Explain with details whether all the carbon sp2 planes are on the same
       plane.


3. Consider the π electrons and the relevant molecular orbitals of butadiene
   molecule ONLY:


   (a) Draw the HOMO and LUMO orbitals.

   (b) What is the bond order when the molecule is in its ground state?

   (c) We have learnt that the energy expression of a particle in a 1-D box is
                    n^2 H^2
     given by  En= ────  Estimate the longest wavelength for the electronic
                    8m L^2
       absorption spectrum of butadiene.

   (d) If the molecule now absorbs a photon of wavelength equal to the answer
      in the previous part, explain the changes of all the carbon bond lengths.

4. In MO theory the concepts of “base” and “radical” are quite different
   from one another.

   a) What kind of MO makes a molecule a base? Illustrate with a single example
      that shows it a reactive base, and identify the factor.

   b) What kind of MO makes a molecule a radical?

   c) By drawing two orbital energy diagrams, explain why radicals react with
      radicals, but bases do not react with bases.


Section B (20 %)

1. The peptide bond in a protein is not very reactive compared with amines
   (R-NH2) and carboxylic acids (R-COOH). This reduction of reactivity makes
   proteins resistant to reaction. As a simple analogy, explain in the language
   of MO theory, why the reactivity of the following molecule is lower than NH3
   and HCOH?

2. The structure of CH5+ is important in our understanding of many reaction
   mechanisms and it has been considered as a nonclassical carbocation
   (carbon cation) because it cannot be described by a single Lewis structure.
   The following two structures are the most stable structures obtained from
   QM calculations. Note that the lines do not necessarily represent a bond!

   (a) Identify the symmetry elements of structures A and B.

   (b) The following are two occupied MOs calculated for A, identify the
       constituting atomic orbitals in each MO. Which one is the HOMO?
       Is the HOMO a bonding, anti-bonding or non-bonding orbital?

   (c) The following are two unoccupied MOs calculated for A, identify the
       constituting atomic orbitals in each MO. Which one is the LUMO? Is
       the LUMO a bonding, anti-bonding or non-bonding orbital?

   (d) Although CH5+ cannot be described by a traditional Lewis structure.
       Can you draw a "modified" Lewis structure to describe the bonding of
       this molecule?


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