Chapter 9: Alkynes

Question 1

Elimination Reactions

Alkynes

Answer 1

Reagents

1. xs NaNH₂ (sodium amide) & NH₃
2. H₂O

Mechanism

Alkynes can be prepared from alkyl dihalides through two successive E2 reactions; a geminal dihalide or a vicinal dihalide may be used

Question 2

Catalytic Hydrogenation

Alkynes

Answer 2

Reagents

H₂ & Pt, Pd, or Ni

or

H₂ & poisoned catalyst
Lindlar’s catalyst or Ni₂B (nickel boride)

Mechanism

Catalytic hydrogenation in the presence of Pt, Pd, or Ni will lead to an alkane

Use of a poisoned catalyst will convert an alkyne to a cis alkene; a trans alkene will NOT be produced

Stereospecificity

Hydrogen is added to the same face of the alkyne; explains the preference for the cis product

Question 3

Dissolving Metal Reduction

Alkyne

Answer 3

Reagents

Na & NH3

Mechanism

An internal alkyne is hydrogenated and converted to a trans alkene

Reaction can NOT be used on a terminal alkyne; must use catalytic hydrogenation in the presence of a poisoned catalyst

Question 4

Catalytic Hydrogenation vs. Dissolving Metal Reduction

Answer 4

An alkane can be produced from an alkyne that is treated with H₂ in the presence of a metal catalyst such as Pt, Pd, or Ni

A cis alkene can be produced from an alkyne that is treated with H₂ in the presence of a poisoned catalyst such as Lindlar’s catalyst or Ni₂B (nickel boride)

A trans alkene can be produced from an alkyne that is treated with sodium metal in liquid ammonia

Question 5

Markovnikov Hydrohalogenation

Alkynes

Answer 5

Reagents

HX (hydrogen halide)
HCl, HBr, or HI

Mechanism

When an alkyne is treated with one equivalent of HX, the halogen will be installed at the more substituted position

When an alkyne is treated with excess HX, two successive addition reactions occur producing a geminal dihalide at the more substituted position

Question 6

Anti-Markovnikov Hydrohalogenation

Alkynes

Answer 6

Reagents

HBr & ROOR (organic peroxide)
ONLY occurs with HBr

Mechanism

Bromine is installed at the less substituted position; produces a mixture of E and Z isomers

Question 7

Interconversion of dihalides and alkynes

Answer 7

Alkynes can be converted to dihalides via hydrohalogenation with excess HX

Dihalides can be converted to alkynes via elimination reactions with a strong base

Question 8

Acid-Catalyzed Hydration

Alkynes

Answer 8

Reagents

H₂SO₄/H₂O & HgSO₄ (mercuric sulfate)

Mechanism

Produces a ketone via the Markovnikov addition of a water molecule across an alkyne

Produces an enol which is rapidly converted to a ketone via keto-enol tautomerization

Question 9

Hydroboration-Oxidation

Alkynes

Answer 9

Reagents

1. R₂BH
   Disiamylborane or 9-BBN
2. H₂O₂ & NaOH

Mechanism

Produces an aldehyde via anti-Markovnikov addition

The hydroxyl group is installed at the less substituted position producing an enol which is rapidly converted into an aldehyde via tautomerization

Question 10

Controlling the regioselectivity of alkyne hydration

Answer 10

Acid-catalyzed hydration of a terminal alkyne produces a methyl ketone

Hydroboration-oxidation of a terminal alkyne produces an aldehyde

Question 11

Halogenation

Alkynes

Answer 11

Reagents

X₂ & CCl₄ (carbon tetrachloride)
ONLY Br₂ or Cl₂

Mechanism

In an excess of X₂, two equivalents of the halogen will add across BOTH π bonds to form a tetrahalide

If only one equivalent of X₂ is used the reaction will proceed via an anti addition, producing the E isomer as the major product

Question 12

Ozonlysis

Alkyne

Answer 12

Reagents

1. O₃
2. H₂O

Mechanism

When treated with ozone followed by water, an internal alkyne will undergo oxidative cleavage to produce TWO carboxylic acids

When a terminal alkyne undergoes oxidative cleavage, the terminal carbon is converted into carbon dioxide while the other carbon still becomes a carboxylic acid

Question 13

Alkylation of Terminal Alkynes

Answer 13

Reagents

1. NaNH₂ (sodium amide)
2. RX

Mechanism

The terminal alkyne is deprotonated by a strong base, creating an alkynide ion that can function as a nucleophile when treated with an alkyl halide