Chem2O6 - 1997/98

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.

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.

18sep97; wjl