Chem2O6 - 1997/98

Problem Set #1 ANSWERS September 18, 1997

1. Write the following condensed formulas as Lewis structures:

(i) (CH3)2CHNO2     (ii) CH3CNO     (iii) CH3SO3H     (iv) CHOCO2H     (v) CH3CHCHCOCl


2. (a) Which of the following species would you expect boron trifluoride (BF3) to form a complex with?

(CH3)2CHCH2CH3    CH3CH2OCH3    AlCl3    CH3NHCH3    CH3F

(b) Draw the Lewis structure of the complex.

(c) Which complex would you expect to be the most stable? Why?


(a) CH3CH2OCH3, CH3NHCH3, and CH3F because these three compounds have atoms with lone pairs of electrons. 2-Methylbutane has no lone pairs, while AlCl3 is electron deficient. In fact, it is itself a Lewis acid like BF3 and exists as the dimer (Al2Cl6) rather than the monomer.


(c) The complex with dimethylamine should be the more stable because N is less electronegative than O and F, and is better able to donate its lone pair of electrons to the Lewis acid. The actual order of stability would be N > O > F.

3. Write resonance structures for the following, noting any structures which might be minor contributors.

(i) [CH3CHOH]+     (ii) [CH3CO2H2]+     (iii) [CH3CHNH2]+     (iv) [CH2SO2CH3]-


4. Give line structures for:

(i) isopropylcyclobutane
(ii) citric acid
(iii) 3-pentanone
(iv) 2-butanol


5. Complete the following reactions, indicating (where asked) the structure of any intermediates. Where appropriate, give the stereochemistry of the products.

Parts (a) & (b)


(a) This is an SN2 substitution reaction. The nucleophile attacks the electrophilic carbon of the alkyl halide from the rear, causing loss of the leaving group and inversion of configuration:

(b) Electrophilic addition of Br2 to an alkene. The initial step involves addition of "Br+" to yield the more stable carbenium ion intermediate. The intermediate reacts with Br- from the least hindered side, for overall trans-addition across the double bond.

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18sep97; wjl