1. Structure, Reactivity and Synthesis
Topic 4: Alkynes:
Structure, Reactivity and Synthesis
Alkynes are hydrocarbons that contain a carbon-carbon triple bond, which is the strongest and shortest type of carbon-carbon bond that exists. Many of the reactions of alkynes are similar in nature to those of alkenes, but there are important differences too. In this unit, we will examine the structure and reactivity of alkynes as well as how many of the reactions you have learned about thus far are used in the important process of organic synthesis.

2. I. Naming Alkynes
* Alkynes follow the general set of naming rules we saw for alkanes and alkenes, with the name of the parent chain ending in –yne.
Find the parent hydrocarbon. Find the longest carbon chain that contains
the triple bond. Name accordingly using –yne as the suffix.
Number the carbon atoms in the chain. Begin at the end nearest the triple
bond. If the triple bond is equidistant from the two end points, begin at the
end nearest the first branch point.
3. Write the full name.
Number the substituents according to their positions on the chain, and arrange
alphabetically, just as for alkanes.
Indicate the position of the triple bond by giving the number of the first carbon
where it begins.
If more than one triple bond exists, use numbers to indicate the position of
each and use the suffixes –diyne, -triyne, etc.

3. Compounds containing both double and triple bonds are called enynes.
Numbering of an enyne chain begins nearer the first multiple bond,
whether double or triple. A number is used to indicate the positions of
both the double and triple bond. Where a tie exists, the double bond
receives the lower number.

4. II. Preparation of Alkynes: Elimination Reactions
* Alkynes can be prepared by elimination of X2 from a 1,2-dihalide by treatment with excess strong base or by elimination of HX from a vinylic halide by treatment with strong base.
ELIMINATION OF X2 FROM 1,2-DIHALIDES

5. ELIMINATION OF HX FROM VINYLIC HALIDES
Refers to a substituent directly attached to a double bond

6. III. Reactions of Alkynes: Addition of HX and X2
Based on electronic similarity between alkynes and alkenes, you may expect the chemical reactivity of the two functional groups to be similar. While there are indeed many similarities, significant differences also exist.

7. Br Br H CH3C ≡ CH CH3C = CH CH3C ─ C─H H Br H
ELECTROPHILIC ADDITION OF HX
Reaction can be stopped after addition of 1 equivalent of HX
Follows Markovnikov’s Rule (X adds to more sub. carbon)
trans stereochemistry of H and X typically occurs
Addition of excess HX forms the dihalide Br Br H
HBr
HBr
CH3C ≡ CH
CH3C = CH
CH3C ─ C─H H Br H
add’n of 1 eq. of
HBr
add’n of excess
HBr yields dihalide

8. ELECTROPHILIC ADDITION OF X2
Reaction can be stopped after addition of 1 equivalent of X2 to give the di-substituted product
Excess addition of X2 yields the tetra-substituted product
Trans stereochemistry results
CH3CH2 Br
C = C H
Br2
Br2
CH3CH2C ≡ CH
CH3CH2CBr2CHBr2

9. * Although the mechanism for the addition of HX to an alkyne mirrors
Reaction Mechanism:
The reaction mechanism of electrophilic addition of HX to an alkyne is similar to that of an alkene. It occurs in two steps with a vinylic carbocation intermediate forming.
Vinylic carbocation
intermediate
* Although the mechanism for the addition of HX to an alkyne mirrors
that of the mechanism for the addition of HX to an alkene, most other
alkyne additions occur through more complex mechanistic pathways.

10. IV. Hydration of Alkynes: Addition of H2O
* Like alkenes, alkynes can be hydration via two different methods. Addition of H2O in the presence of acid yields the Markovnikov product while indirect addition of H2O via hydroboration yields the anti-Markovnikov product. Unlike in the case of alkenes however, the product of hydration is not exactly an alcohol. This is due to the existence of what is known as tautomerisation.

11. less stable than its keto counterpart
ENOL:
a vinylic alcohol (ene + ol) (-OH is directly attached to a double bonded carbon)
less stable than its keto counterpart
TAUTOMERS:
describes constitutional isomers that interconvert rapidly
in keto-enol tautomerism, the keto tautomer is favored
(*see example above)
*Note: tautomerisation is not the same as resonance. In resonance,
atoms do not move, only electrons do. In tautomerisation, atoms switch
places. Also, resonance is a concept of how molecules exist: they
do not actually switch back and forth between resonance structures. In
tautomerisation, molecules actually do shift back and forth between
isomeric structures.

12. A. Acid- Catalyzed Hydration of Alkynes: Markovnikov Product
Reaction of an alkyne w/ H2O in the presence of acid to form an enol at more substituted carbon (Markonikov product)
Enol undergoes tautomerisation to give the favored ketone product
* See board for mechanism

13. Reaction Mechanism:

14. *Note: This reaction is most useful when applied to a terminal
alkyne because only one product forms. When the reaction
occurs with an internal alkyne, a mixture of ketone products
form.

15. B. Hydroboration of Alkynes: anti- Markovnikov Product
Reaction of an alkyne w/ BH3 in the presence of H2O2/ OH─ to form an enol at less substituted carbon (Markonikov product)
Enol undergoes tautomerisation to give the favored keto product
* See board for mechanism
*Again, this reaction is most useful when applied to a terminal
alkyne because only one product, in this case an aldehyde,
forms. When the reaction occurs with an internal alkyne, a
mixture of products form.

16. Reaction Mechanism:

17. V. Reduction of Alkynes
* Alkynes are easily reduced, which means that they form an increase in bonds to hydrogen, by addition of H2 over a metal catalyst. As a result, alkynes can be reduced to an alkene or further to an alkane. Depending on what reagents are used in the reduction, either product can be selected for.

18. A. Complete Reduction: Alkyne to Alkane
Reduction to the alkane occurs when the alkyne is reacted w/
H2 in the presence of palladium on carbon (Pd/C) as a catalyst

19. B. Incomplete Reduction: Alkyne to Alkene
PATH 1: Formation of Cis Alkenes
Incomplete reduction to the alkene occurs when the alkyne is
reacted with H2 in the presence the less active Lindlar catalyst
The use of H2/ Lindlar catalyst produces CIS alkenes.
PATH 2: Formation of Trans Alkenes
Incomplete reduction to the alkene also occurs when the alkyne
is reacted w/ Na or Li metal in liquid ammonia (NH3).
The use of Na or Li/ NH3 produces TRANS alkenes.

20. VI. Oxidative Cleavage of Alkynes
* Alkynes, like alkenes, can by cleaved by reaction with a powerful oxidizing agent like ozone. A triple bond is generally less reactive that a double bond however, and yields of cleavage products are sometimes low. Like alkenes, the products of alkyne cleavage produce carbonyls, but as part of a different functional group.

21. Oxidative Cleavage of Alkynes:
The products of cleavage of an internal alkyne (R─C≡C─R’) are
two carboxylic acids
The products of cleavage of an external alkyne (R─C≡C─H) are a
carboxylic acid and CO2

22. Acidity of Simple Hydrocarbons
VII. Alkyne Acidity: Formation of Acetylide Anions
* According to the Brønsted- Lowry definition, an acid is any
substance that donates H+. Although we usually think of
oxyacids (H2SO4, H3PO4) or halogen acids (HCl, HBr) in this
context, any compound containing a hydrogen atom can be an
acid under the right circumstances.
The most striking difference between alkynes and alkenes and
alkanes is that terminal alkynes are weakly acidic.
Acidity of Simple Hydrocarbons
Type
Example Ka
pKa
Alkyne
HC ≡ CH
10─25 25
Alkene
H2C= CH2
10─44 44
Alkane
H3C− CH3
10─60 60
Stronger acid
Weaker acid

23. When a terminal alkyne is treated with a strong base, such as sodium
amide (Na+NH2─), the terminal hydrogen is removed and an
ACETYLIDE ANION is formed:
The presence of a negative charge and an unshared electron pair
on carbon makes an acetylide anion extremely ________________.
nucleophilic

24. ALKYLATION REACTION:
Substitution reaction in which a new alkyl group becomes attached to the starting alkyne
The nucleophilic acetylide anion attacks a positively
polarized carbon atom in a haloalkane
- This a BIG deal a new carbon – carbon bond is formed!

25. The reaction conditions for acetylide anion formation and
alkylation are often shown together as a two step reaction process.

26. Acetylide anion alkylation is limited to primary alkyl bromides
and iodides.
In addition to their reactivity as nucleophiles, acetylide anions are
sufficiently strong bases that they can cause dehydrohalogenation
(loss of H + halogen) instead of substitution when they react with
secondary and tertiary alkyl halides.