Summary of Reactions 259

Summary of Reactions

1. Electrophilic addition reactions
   a.Addition of hydrogen halides ( is the electrophile; Section 6.5)

b.Addition of hydrogen bromide in the presence of a peroxide ( is the electrophile; Section 6.5)

c.Addition of halogens (Section 6.5)

d.Addition of water/hydroboration–oxidation (Sections 6.6 and 6.7)

1. BH 3 /THF


2. HO−, H 2 O 2 , H 2 O

RC CR′ RCCH 2 R′ + RCH 2 CR′

OO

H 2 O, H 2 SO 4

HgSO 4

RC CH

RC CH 2 RCCH 3

OH O

1. disiamylborane


2. HO−, H 2 O 2 , H 2 O RCH CH RCH^2 CH

OH O

H 2 O, H 2 SO 4

or

an internal

alkyne

a terminal

alkyne

Cl

Cl Cl

Cl Cl

Cl

Cl 2

CH 2 Cl 2

Cl 2

RC CH RC CH CH 2 Cl 2 RC CH

Br Br

Br Br

Br

Br 2

CH 2 Cl 2

Br 2

RC CCH 3 RC CCH (^3) CH 2 Cl 2 RC CCH 3
Br
peroxide
RC CH + HBr RCH CHBr
Br–
HX excess HX
RC CH RC CH 2 RC
X X
HX HF, HCl, HBr, HI
X
CH 3

H+
has on the addition of HBr to an alkene—it reverses the
order of addition because the peroxide causes to be-
come the electrophile.
When an alkyne undergoes the acid-catalyzed addition of
water, the product of the reaction is an enol. The enol imme-
diately rearranges to a ketone. A ketoneis a compound that
has two alkyl groups bonded to a carbonyl group.
An aldehydeis a compound that has at least one hydrogen
bonded to a carbonyl group. The ketone and enol are called
keto–enol tautomers; they differ in the location of a double
bond and a hydrogen. Interconversion of the tautomers is
called tautomerization. The keto tautomer predominates at
equilibrium. Terminal alkynes add water if mercuric ion is
added to the acidic mixture. In hydroboration–oxidation,
is not the electrophile, is the nucleophile. Consequently,
mercuric-ion-catalyzed addition of water to a terminal
alkyne produces a ketone, whereas hydroboration–oxidation
of a terminal alkyne produces an aldehyde.
Hydrogen adds to an alkyne in the presence of a metal
catalyst (Pd, Pt, or Ni) to form an alkane. Addition of hy-
drogen to an internal alkyne in the presence of Lindlar cat-
H≠-
H+
(C“O)
Br–
alyst forms a cis alkene. Sodium in liquid ammonia con-
verts an internal alkyne to a trans alkene.
Electronegativity decreases with decreasing percentage of
scharacter in the orbital. Thus the electronegativities of car-
bon atoms decrease in the order: Ethyne is,
therefore, a stronger acid than ethene, and ethene is a stronger
acid than ethane. An amide ion can remove a hydrogen bond-
ed to an spcarbon of a terminal alkyne because it is a stronger
base than the acetylide ionthat is formed. The acetylide ion
can undergo an alkylation reaction with a methyl halide or a
primary alkyl halide to form an internal alkyne. An alkylation
reactionattaches an alkyl group to the starting material.
Designing a synthesis by working backward is called
retrosynthetic analysis. Chemists use open arrows to in-
dicate they are working backward. The reagents needed to
carry out each step are not included until the reaction is
written in the forward direction.
Most ethyne produced commercially is for the synthe-
sis of monomers used in the synthesis of polymers.
Polymersare large molecules that are made by linking
together many small molecules called monomers.
sp 7 sp^27 sp^3.
Tutorial:
Common terms in the
reactions of alkynes
BRUI06-238_262r4 24-03-2003 11:47 AM Page 259