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
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