Exercise 14: - Reactions of alkenes and alkynes

  1. In carbon tetrachloride solution, bromine adds to (E)-3-hexene to yield meso 3,4-dibromohexane.  Propose a mechanism that explains this behavior.

 

  1. Myrcene a fragrant component found in bayberry wax, has the formula C10H16 and is known not to contain any triple bond.  On catalytic hydrogenation, myrcene is converted to 2,6-dimethyloctane.  Ozonolysis of  myrcene followed by treatment with zinc and water yields 2 mol of  formaldehyde, HCHO, 1 mol of acetone (CH3COCH3) and a third compound with formula C5H6O3.

 

a.    How many units of unsaturation are present in myrcene?

 

b.      What is the structure of myrcene?

 

  1. At the beginning of the biogenesis of squalene isopentenyl pyrophosphate, CH2=C(CH3)CH2CH2OPP, is enzymatically isomerized to dimethylallyl pyrophosphate, (CH3)2C=CHCH2OPP.  These two compounds then react together to yield geranyl pyrophosphate,  (CH3)2C=CHCH2CH2(CH3)C=CHCH2OPP.  Assuming that the weakly basic pyrophosphate anion is, like the protonated hydroxyl group, a good leaving group, R-OPP ® R+ + OPP- suggest a mechanism by which geranyl pyrophosphate might be formed.

 

  1. In methanol solution, bromine adds to ethene to yield not only 1,2-dibromoethane but also Br-CH2-CH2­-O-CH3 .  Write a mechanism that explains this behavior.

 

  1. Compound “A”, C10H18O, reacts with dilute H2SO4 at 250C to yield a mixture of two alkenes, C10H16.  The major product, “B”, gives cyclopentanone,  as the sole product on ozonolysis:

 

 

a.    What is the structure of “A”?

 

b.    What is the structure of “B”?

 

  1. Propose a mechanism for the following reaction:

 

 

  1. Starting from 1-methylcyclohexene, propose a synthesis for each of the following:

 

a.    1-methylcyclohexanol

 

b.    2-methylcyclohexanol

 

c.    1-bromo-1-methylcyclohexane

 

d.    1-bromo-2-methylcyclohexane

 

  1. Muscalure is the sex pheromone of the common house fly.  On the basis of the following synthesis, give the structure of muscalure:

                       

n-C13H27CºCH  +  n-BuLi  ®  “A” (C15H27Li)

“A”  +  n-C8H17Br  ®  “B” (C23H44) 

“B”  +  H2, Lindlar catalyst  ®  muscalure (C23H46)

  1. Hydrocarbon “A”, C9H12, absorbs three equivalents of H2 on catalytic hydrogenation.  “A” forms two isomeric ketones on treatment with aqueous H2SO4/mercuric ion.  Oxidation of “A” with KMnO4 gives a mixture of ethanoic acid, CH3CO2H, and acid CH(CH2CO2H)3.  What is the structure of “A”?
  2. Compound “A”, C9H12, absorbs three equivalents of H2 on catalytic hydrogenation.  Ozonolysis gives cyclohexanone and other products.  “A” reacts with NaNH2 in liquid ammonia followed by CH3I to give compound “B”, C10H14.  What is the structure of “A”?