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SolutionsDienes and Alkynes

1. Isolated, conjugated or cumulated?

Classify these dienes:

  1. CH2=CH–CH=CH–CH3
  2. CH2=C=CH–CH3
  3. CH2=CH–CH2–CH2–CH=CH2
  4. 1,3-Cyclohexadiene
  5. 1,4-Cyclohexadiene
  1. Conjugated (1,3-pentadiene).
  2. Cumulated: it is an allene (1,2-butadiene).
  3. Isolated (1,5-hexadiene): the double bonds are separated by two CH2 groups.
  4. Conjugated.
  5. Isolated: the double bonds are separated by one CH2 group.

Double bonds are conjugated when a single bond separates them; cumulated, when they share a carbon; isolated, when one or more sp3 carbons separate them.

2. Isomeric dienes and alkynes

1,3-Butadiene and 1-butyne have the same molecular formula, C4H6. How would you tell them apart with a simple chemical reagent?

With an ammoniacal AgNO3 solution: 1-butyne, a terminal alkyne, gives a precipitate of the silver alkynide; butadiene does not react.

A strong base (NaNH2) can also be used: only the terminal alkyne has an acidic hydrogen (pKa ≈ 25).

3. Degrees of unsaturation

How many degrees of unsaturation does C5H6 have? Propose a structure containing a conjugated diene and another containing a triple bond.

(2·5 + 2 − 6)/2 = 3. For example, cyclopentadiene (a ring and two conjugated double bonds) or pent-1-en-3-yne, CH2=CH–C≡C–CH3 (one double and one triple bond).

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