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

Answer:


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?

Answer:

(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.

(b)

(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]-

Answer:


4. Give line structures for:

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

Answer:


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

Parts (a) & (b)

Answer:

(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