Chemistry 25a
Practice second hour exam M. Walker
YOUR NAME________________________________________________
150 points total. Please answer directly onto the exam paper. You may use non‑programmable calculators and molecular models.
1. (50 points) (a) Draw the structure of the S isomer of the chiral molecule, 2‑bromobutane (the non‑specific structure shown below).
(b) Describe how you could prepare pure (S)-2-bromobutane (as one pure enantiomer) starting from butane.
(c) In your synthesis shown in (b), explain briefly why 2-bromobutane is preferred as a product over 1‑bromobutane.
(d) If pure (S)-2-bromobutane is heated with bromine in the presence of light, the major products are stereoisomers of 2,3-dibromobutane. Draw all of the possible stereoisomers which could be formed in this reaction.
2. (20 points) Draw a 3-D structure for the CH3- carbanion. Is the carbon here sp, sp2, or sp3? What is the angle between the C-H bonds? Is this ion electrophilic or nucleophilic?
3. (40 points) Describe the relationship between the following pairs of isomers as enantiomers, structural isomers, geometric isomers, resonance forms or identical.
4. (40 points) The reaction
is essentially irreversible at -70oC but it is reversible at 100oC. It can give rise to either F (which arises from intermediate D) or G (which arises from intermediate E) as a product. With either product, the reaction is exothermic, and the second step is rate determining. Product F predominates at -70oC but at 100oC, G is the major product.
(a) Draw a reaction coordinate diagram which is appropriate for the information given.
(b) Is the reaction first order or second order, in terms of the starting materials A and B?
(c) Which product is more stable, F or G?
(d) Using your diagram to assist you, explain how it is possible for different products to be major, depending on the temperature. [Note: entropy is not to be considered an issue here.]
Go to the practice exam #2 answer key
Back to the exam page
Back to the Chem 25a page