Alkyne Addition Reactions (CH 9)

Elimination

Reagent:

1)NaNH2(excess)

2)H2O

Elimination

Reagent:

1)NaNH2(excess)

2)H2O

Hydrohalogenation (2 eq)

Reagent: HX (excess)

Hydrohalogenation (1 eq)

Reagent: HX (1 eq)

Acid-catalyzed hydration

Reagent:

HgSO4

H2SO4, H2O

Hydroboration-Oxidation

Reagent:

1) R2BH

2)H2O2, NaOH

Halogenation (1 eq)

Reagent:

X2 (1 eq)

CCl4

Halogenation (2 eq)

Reagent:

X2 (excess)

CCl4

Ozonolysis

Reagent:

1) O3

2) H2O

Alkylation

Reagent:

1)NaNH2

2)RX

Dissolving Metal Reduction

Reagent:

Na

NH3 (l)

Hydrogenation

Reagent:

H2

Pt/Pd

Hydrogenation with a Poisoned Catalyst

Reagent:

H2

Lindlar’s Catalyst

Chapter 9 Splurge

SECTION 9.1

• A triple bond is comprised of three separate bonds: one σ bond and two π bonds.

• Alkynes exhibit linear geometry and can function as bases or as nucleophiles.

SECTION 9.2

• Alkynes are named much like alkanes, with the following additional rules:

• The suffix “ane” is replaced with “yne.”

• The parent is the longest chain that includes the C≡C bond.

• The triple bond should receive the lowest number possible.

• The position of the triple bond is indicated with a single locant placed either before the parent or the suffix.

• Monosubstituted acetylenes are terminal alkynes, while disubstituted acetylenes are internal alkynes.

SECTION 9.3

• The conjugate base of acetylene, called an acetylide ion, is relatively stabilized because the lone pair occupies an sp-hybridized orbital.

• The conjugate base of a terminal alkyne is called an alkynide ion, which can only be formed with a sufficiently strong base, such as NaNH₂.

SECTION 9.4

• Alkynes can be prepared from either geminal or vicinal dihalides via two successive E2 reactions.

SECTION 9.5

• Catalytic hydrogenation of an alkyne yields an alkane.

• Catalytic hydrogenation in the presence of a poisoned catalyst (Lindlar’s catalyst or Ni₂B) yields a cis alkene.

• A dissolving metal reduction will convert an internal alkyne into a trans alkene. The reaction involves an intermediate radical anion and employs fishhook arrows, which indicate the movement of only one electron.

SECTION 9.6

• Alkynes react with HX via a Markovnikov addition.

• One possible mechanism for the hydrohalogenation of alkynes involves a vinylic carbocation, while another possible mechanism is termolecular.

• Addition of HX to alkynes probably occurs through a variety of mechanistic pathways all of which are occurring at the same time and competing with each other.

• Treatment of a terminal alkyne with HBr and peroxides gives an anti-Markovnikov addition of HBr.

SECTION 9.7

• Acid-catalyzed hydration of alkynes is catalyzed by mercuric sulfate (HgSO₄) to produce an enol that cannot be isolated because it is rapidly converted into a ketone.

• Enols and ketones are tautomers, which are constitutional isomers that rapidly interconvert via the migration of a proton.

• The interconversion between an enol and a ketone is called keto-enol tautomerization and is catalyzed by trace amounts of acid or base.

• Hydroboration-oxidation of a terminal alkyne proceeds via an anti-Markovnikov addition to produce an enol that is rapidly converted into an aldehyde via tautomerization.

• In basic conditions, tautomerization proceeds via a resonance-stabilized anion called an enolate ion.

SECTION 9.8

• Alkynes can undergo halogenation to form a tetrahalide.

SECTION 9.9

• When treated with ozone followed by water, internal alkynes undergo oxidative cleavage to produce carboxylic acids.

• When a terminal alkyne undergoes oxidative cleavage, the terminal side is converted into carbon dioxide.

SECTION 9.10

• Alkynide ions undergo alkylation when treated with an alkyl halide (methyl or primary).

• Acetylene possesses two terminal protons and can undergo two separate alkylations.

SECTION 9.11

• An alkene can be converted into an alkyne via bromination followed by elimination with excess NaNH₂.