Organic Chemistry

hydrogen bonded to an spcarbon of a terminal alkyne to form a carbanion called an

acetylide ion, because the amide ion is a stronger base than the acetylide ion.

If hydroxide ion were used to remove a hydrogen bonded to an spcarbon, the reaction

would strongly favor the reactants because hydroxide ion is a much weaker base than

the acetylide ion that would be formed.

The amide ion cannot remove a hydrogen bonded to an or an carbon. Only

a hydrogen bonded to an spcarbon is sufficiently acidic to be removed by the amide

ion. Consequently, a hydrogen bonded to an spcarbon sometimes is referred to as an

“acidic”hydrogen. The “acidic”property of terminal alkynes is one way their reactiv-

ity differs from that of alkenes. Be careful not to misinterpret what is meant when we

say that a hydrogen bonded to an spcarbon is “acidic.”It is more acidic than most

other carbon-bound hydrogens but it is much less acidic than a hydrogen of a water

molecule, and water is only a very weakly acidic compound (pKa=15.7).

sp^2 sp^3

acetylide anion

stronger base

hydroxide anion

weaker base stronger acid

+ HO− RC C− + H 2 O

weaker acid

RC CH

acetylide ion

weaker base

amide ion

stronger base weaker acid

+ −NH 2 RC C− + NH 3

stronger acid

RC CH

252 CHAPTER 6 Reactions of Alkynes • Introduction to Multistep Synthesis

SODIUM AMIDE AND SODIUM

Take care not to confuse sodium amide

with sodium (Na) in liquid ammonia. Sodium amide

(Na+ - NH 2 )

is the strong base used to remove a proton from a terminal alkyne.

Sodium is used as a source of electrons in the reduction of an

internal alkyne to a trans alkene (Section 6.8).

PROBLEM 16

Any base whose conjugate acid has a greater than _________ can remove a proton

from a terminal alkyne to form an acetylide ion (in a reaction that favors products).

PROBLEM 17

Which carbocation in each of the following pairs is more stable?

a. or b. or

PROBLEM 18

Explain why sodium amide cannot be used to form a carbanion from an alkane in a reac-

tion that favors products?

PROBLEM-SOLVING STRATEGY

a. List the following compounds in order of decreasing acidity:

To compare the acidities of a group of compounds, first look at how they differ. These

three compounds differ in the hybridization of the nitrogen to which the acidic hydro-

gen is attached. Now, recall what you know about hybridization and acidity. You know

that hybridization of an atom affects its electronegativity (spis more electronegative

than and is more electronegative than ); and you know that the more elec-

tronegative the atom to which a hydrogen is attached, the more acidic the hydrogen.

Now you can answer the question.

sp^2 , sp^2 sp^3

CH 3 C‚N

+

CH 3 CH“N H

+

CH 3 CH 2 N H 2

+

H 3

HC‚C

+

H 2 C“C

+

H 2 C“C H

+

CH 3 C H

+

H 2

pKa

To remove a proton from an acid in a

reaction that favors products, the base

that removes the proton must be

stronger than the base that is formed.

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