Organic Chemistry

b. Draw the conjugate bases and list them in order of decreasing basicity.

First remove a proton from each acid to get the structures of the conjugate bases, and

then recall that the stronger the acid, the weaker its conjugate base.

Now continue on to Problem 19.

PROBLEM 19

List the following species in order of decreasing basicity:

6.10 Synthesis Using Acetylide Ions

Reactions that form carbon–carbon bonds are important in the synthesis of organic

compounds because without such reactions, we could not convert molecules with

small carbon skeletons into molecules with larger carbon skeletons. Instead, the prod-

uct of a reaction would always have the same number of carbons as the starting

material.

One reaction that forms a carbon–carbon bond is the reaction of an acetylide ion

with an alkyl halide. Only primary alkyl halides or methyl halides should be used in

this reaction.

The mechanism of this reaction is well understood. Bromine is more electronega-

tive than carbon, and as a result, the electrons in the bond are not shared equal-

ly by the two atoms. This means that the bond is polar, with a partial positive

charge on carbon and a partial negative charge on bromine. The negatively charged

acetylide ion (a nucleophile) is attracted to the partially positively charged carbon (an

electrophile) of the alkyl halide. As the electrons of the acetylide ion approach the car-

bon to form the new carbon–carbon bond, they push out the bromine and its bonding

electrons because carbon can bond to no more than four atoms at a time. This is an ex-

ample of an alkylation reaction. An alkylation reactionattaches an alkyl group to the

starting material.

The mechanism of this and similar reactions will be discussed in greater detail in

Chapter 10. At that time we will also see why the reaction works best with primary

alkyl halides and methyl halides.

Simply by choosing an alkyl halide of the appropriate structure, terminal alkynes

can be converted into internal alkynes of any desired chain length.

1-pentyne 4-decyne

1. NaNH 2
   CH 3 CH 2 CH 2 C CH 2. CH 3 CH 2 CH 2 CH 2 CH 2 Cl CH 3 CH 2 CH 2 C CCH 2 CH 2 CH 2 CH 2 CH 3

CH 3 CH 2 CCHC− + 3 CH 2 CH 2 Br CH 3 CH 2 CBrCCH 2 CH 2 CH 3 + −

δ+δ−

C¬Br

C¬Br

3-heptyne

CH 3 CH 2 CCHC− + 3 CH 2 CH 2 Br CH 3 CH 2 CBrCCH 2 CH 2 CH 3 + −

CH 3 CH 2 CH CH CH 3 CH 2 CC CH 3 CH 2 CH 2 CH 2

CH 3 CH 2 OF CH 3 CC NH 2

a.

b. − − − −

−−−

relative basicities CH 3 CH 2 NH 2 > CH 3 CH NH > CH 3 C N

relative acidities CH 3 C NH >

+

CH 3 CH NH 2 > CH 3 CH 2 NH 3

++

Section 6.10 Synthesis Using Acetylide Ions 253

3-D Molecules:

Anion of 1-butyne;

1-bromopropane

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