Organic Chemistry

is a gas at room temperature, so it is kept in the liquid state by using a dry ice/acetone

mixture

The first step in the mechanism of this reaction is transfer of the sorbital electron

from sodium (or lithium) to an spcarbon to form a radical anion—a species with a

negative charge and an unpaired electron. (Recall that sodium and lithium have a

strong tendency to lose the single electron in their outer-shell sorbital; Section 1.3.)

The radical anion is such a strong base that it can remove a proton from ammonia. This

results in the formation of a vinylic radical—the unpaired electron is on a vinylic car-

bon. Another single-electron transfer from sodium (or lithium) to the vinylic radical

forms a vinylic anion. The vinylic anion abstracts a proton from another molecule of

ammonia. The product is the trans alkene.

The vinylic anion can have either the cis or the trans configuration. The cis and

trans configurations are in equilibrium, but the equilibrium favors the more stable

trans configuration because in this configuration the bulky alkyl groups are as far from

each other as possible.

PROBLEM 15

What alkyne would you start with and what reagents would you use if you wanted to

synthesize:

a. pentane? b.cis-2-butene? c.trans-2-pentene? d. 1-hexene?

6.9 Acidity of a Hydrogen Bonded

to an spHybridized Carbon

Carbon forms nonpolar covalent bonds with hydrogen because carbon and hydrogen,

having similar electronegativities, share their bonding electrons almost equally. How-

ever, not all carbon atoms have the same electronegativity. An sphybridized carbon is

more electronegative than an sp^2 hybridized carbon, which is more electronegative

trans vinylic anion

more stable

CC

CH 3

cis vinylic anion

less stable

C

CH 3 CH 3

H

− C −

CH 3

H

CH 3 C

2-butyne

CCH (^3) NH
3 (liq)
Na or Li
H
H
trans-2-butene
− 78 ̊C CH 3
CH 3
CC
(bp=- 78 °C).
250 CHAPTER 6 Reactions of Alkynes • Introduction to Multistep Synthesis
Tutorial:
Synthesis of trans alkenes
using (liq)Na>NH 3
C
a radical anion
C
H NH 2
CH 3 C
a vinylic radical
CH 3 C
a vinylic anion a trans alkene
CH 3 CCCH 3 Na CH 3 CH 3

• Na+


• −NH 2 + Na+ + −NH 2


• 
  

  −−Na H NH 2
  CH 3
  CH 3
  CC
  H
  H
  H
  CH 3
  C
  H
  CH 3
  C
  3-D Molecules:
  Trans and cis vinylic anions
  BRUI06-238_262r4 24-03-2003 11:47 AM Page 250