Alkynes

Nomenclature Physical Properties Synthesis Reactions

Alkynes

Contain at least one carbon-carbon triple bond (C≡C).

Also called acetylenes.

Properties of C≡C Bonds

sigma (σ) bond pi (π) bonds

C≡C consists of a σ bond and two π bonds.

C≡C is shorter than a C=C bond.

Properties of C≡C Bonds

The pi bonds force the four atoms involved to be linear.

The e- density of the pi bonds is a cylinder surrounding the two C atoms.

Alkynes

The pi bonds are relatively easy to break, which makes C≡C a functional group.

The pi bonds block nucleophilic attack.

Nomenclature of Alkynes When the chain contains more than

three C atoms, use a number to give the location of the triple bond.

Terminal alkynes have one H on the triple-bonded C. Internal alkynes do not.

CH3C≡CCH2CH3

2-pentyne

pent-2-yne

internal alkyne

CH3C≡CH

propyne

terminal alkyne

IR of Alkynes

1-octyne

a terminal alkyne

IR of Alkynes

4-octyne

an internal alkyne

Nomenclature of Alkynes Apply the same rules you learned for the

alkanes. Use the root name of the longest chain

containing the triple bond, but change -ane to -yne.

In 8th edition of Wade, alkenes and alkynes are given equal priority, but “-en” is first alphabetically, so The numbering starts at the end closer

to the alkene, and the order of naming is “en-yne.”

Nomenclature of Alkynes

Name the following:

(E)-3-methylhept-2-en-4-yne

2,2-dimethylhex-3-yne

Nomenclature of Alkynes

Alkynes as substituents are called alkynyl groups.

C CH ethynylbenzene

Uses and Synthesis of Acetylene

Main use is as a fuel for oxyacetylene welding. It is one of the cheapest organic

elements.

Synthesized from coal: 3C + CaO CaC2 +CO

CaC2+2H2O HC≡CH + Ca(OH)2

natural gas: 2CH4 HC≡CH + 3H2

driven by ∆S, high T, and 0.01s heating time

Stability of Acetylene

Acetylene (HC≡CH) is thermodynamically unstable: HC≡CH(g) 2C(s) + H2(g) This can happen to the compressed

gas. produces a very hot flame when

burned in pure oxygen

Physical Properties of Alkynes

Similar to alkanes and alkenes of comparable molecular weight. nonpolar virtually insoluble in water soluble in organic solvents

Physical Properties of Alkynes

Terminal alkynes (-C≡C-H) have an acetylenic H that is more acidic than the H’s on other hydrocarbons due to the greater s character of the sp bond. The greater s character makes the ≡C-H bond more polar and the acetylenic H more acidic. pKa of terminal acetylenes ≈25

pKa of alkanes ≈ 50

pKa of NH3 is 35 (so -:NH2 reacts with -C≡C-H)

pKa of alcohols ≈ 16 (and alkoxides don’t)

Formation of Acetylide Ions

From the reaction of a terminal alkyne with sodium amide CH3CH2C≡CH + NaNH2

CH3CH2C≡C:- Na+ + :NH3

CH3C≡CCH3 + NaNH2 NR

Acetylide ions are strong nucleophiles.

OH- and RO- are not strong enough to remove the terminal H.

Synthesis of Alkynes

from acetylides (lengthens the C skeleton) an excellent way to make a more

complex alkyne alkylation of an acetylide addition to carbonyl groups

by elimination reactions

Synthesis of Alkynes from acetylides (lengthens the C

skeleton) - an SN2 reaction alkylation of an acetylide

if the halide is 2°, there will also be the elimination product…which would be what?

Synthesis of Alkynes

Predict the product:

CH3C CH

1. NaNH2

2. CH3CH2CH2Br

Synthesis of Alkynes

from acetylides (lengthens the C skeleton) addition to carbonyl groups

the acetylide is the nucleophile addition to aldehydes gives 2° alcohols addition to ketones gives 3°alcohols

Synthesis of Alkynes

from acetylide addition to a carbonyl group

an alkoxide ion

a 3° alcohol

Synthesis of Alkynes

Predict the product:

CH3C CH

1. NaNH2

2.

O

H

3. H3O+

Synthesis of Alkynes by elimination reactions

A vicinal dihalide or a geminal dihalide can undergo a double dehydrohalogenation to form the alkyne

This requires STRONGLY BASIC conditions, and many compounds can’t “take it.” KOH in a sealed tube heated to 200°C NaNH2, 150°C

Dehydrohalogenation with KOH

The heated base is so strong that the triple bond can migrate along the carbon chain to form the more stable internal alkyne.

terminal alkyne, will rearrange

internal alkyne

Dehydrohalogenation with NaNH2

NaNH2 is even stronger than fused KOH. It is so strong that it traps the terminal alkyne as the sodium salt, and no rearrangement occurs.

terminal alkyne, will not rearrange

major component

Synthesis of Alkynes

Predict the product:

Br Br

1.

2.

NaNH2/ 150°C

H2O

Br Br

1.

2.

KOH (fused)

H2O

200 Co

Reactions of Alkynes

Additions reduction to alkanes reduction to alkenes addition of halogens addition of HX addition of water

Markovnikov anti-Markovnikov

Oxidations to α–diketones cleavage

Nucleophilic attack on electrophiles covered in

synthesis

Reactions of Alkynes

Reduction to alkanes by hydrogen. second π bond energy = 226 kJ first π bond energy = 264 kJ Alkynes can undergo double additions

These typically go all the way to the alkane.

R-C≡C-R’ + 2H2(g) RCH2CH2R’Pt, Pd, or Ni

Reactions of Alkynes Reduction to cis-alkenes the syn addition of hydrogen can be stopped at

the alkene stage if a poisoned catalyst is used Ni2B Lindlar’s catalyst: Pd/BaSO4 poisoned with

quinoline (in CH3OH to dissolve C≡C)

Reactions of Alkynes Reduction to trans-alkenes Na/NH3(l) required

Na + NH3(l) e- •NH3 + Na+

The H’s come from NH3. The solvated e- leads to a vinyl radical, which is

more stable in the trans geometry.

Reactions of Alkynes Addition of halogens

One mole X2, syn or anti addition leading to cis- or trans-alkenes

Two moles X2 leads to the tetrahalides

Reactions of Alkynes Addition of HX

Reaction proceeds through a vinyl cation intermediate then a carbocation intermediate, so the addition is Markovnikov.

If a peroxide is used with HBr, the anti-Markovnikov product will be formed. Why?

Reactions of Alkynes Markovnikov addition of water

HgSO4/H2SO4 catalyst product is a ketone, not an alcohol

Keto-enol conversion Can occur in acid or base, but the

mechanisms are different. The mechanism shown is the conversion in

acid.

Reactions of Alkynes anti-Markovnikov addition of water

hindered dialkylborane needed product is an aldehyde, not an alcohol

Reactions of Alkynes Permanganate oxidations

nearly neutral conditions needed product is an α-diketone, not a diol terminal alkynes give keto acids

Reactions of Alkynes Permanganate oxidations

warm, basic conditions cause cleavage products are salts of carboxylic acids (A second step, acidification, is needed

to produce the acids themselves.)

Reactions of Alkynes Ozonolysis followed by hydrolysis

Reactions of Alkynes Predict the products

CH3CHCH2C CH

CH3H2

Pd/BaSO4/quinoline

CH3CHCH2C CH

CH3 HBr (xs)

CH3CHCH2C CH

CH3NaNH2

acetone

1.

2.

Reactions of Alkynes Predict the products

CH3CHCH2C CH

CH3Br2(xs)

CH3CHCH2C CH

CH3HgSO4/H2SO4

H2O

H2O2, OH-CH3CHCH2C CH

CH3

2.

1. Sia2BH