Updated: Sep 27, 2023
Chapter 7: Alkynes
Table of contents of main topics:
Nomenclature of Alkynes
Acidity of 1-Alkynes
Preparation of Alkynes
Electrophilic Addition to Alkynes
Hydration of Alkynes to Aldehydes and Ketones
Reduction of Alkynes
Summary of Reactions
Alkynes are unsaturated hydrocarbons with at least one carbon carbon triple bond. The bond angle for a carbon that has a triple bond is 180˚C.
Nomenclature of Alkynes
1. Find the longest continuous carbon chain (can be vertical, horizontal, etc) containing the triple bond. The name of the parent chain will start with a prefix signifying the number of carbons in the chain and end with "yne". Start counting from the end closest to the triple bond to give it the lowest number.
2. Name the substituents same as in alkane naming.
Acidity of 1-Alkynes
Hydrogen on terminal alkyne (triple bond at the end of the chain) is acidic and can be removed by a base. Bases used are : NaH, LDA or NaNH2.
Preparation of Alkynes
What do we need in our starting material? Two leaving groups such as two halogens. The leaving groups need to be either on the same carbon (geminal dihalide) or on neighboring carbons (vicinal dihalide).
The reagents used for elimination are strongly basic such as KOH ( potassium hydroxide) or NaNH2 (sodium amide).
1. Preparation of alkynes from alkenes
The reagent for this reaction is 1.Br2/CH2Cl2 2. 2NaNH2
Product: Turn double bond into a triple bond
The first reaction adds two bromines instead of the double bond. The second reaction uses two equivalents of NaNH2 base to do double elimination and make a triple bond.
2. Acetylide ion with primary or methyl alkyl halides
In this reaction, an acetylide ion reactions with primary or methyl alkyl halides via an SN2 reaction to add the alkyl chain. This reaction is very useful because it is a way to add more carbons to a triple bond, which is often encountered in synthesis problems.
For example, if you start with a three carbon terminal alkyne and end up with a longer chain alkyne, this was probably the reaction to elongate the chain.
Electrophilic Addition to Alkynes
3. Addition of bromine and chlorine to alkynes
The reagent for this reaction is 1mol Br2 or Cl2
Product: Turn triple bond into a double bond and add two bromines or chlorines, one on each carbon on the double bond.
If the reagent is 2 moles of Br2 or Cl2
Product: Turn triple bond into single and add four bromines or chlorines, two on each carbon from the triple bond.
4. Addition of hydrogen halides to alkynes
The reagent for this reaction is 2HX where X is Cl, Br or I
Product: Turn triple bond into a single bond and add two halogens, both on the most substituted carbon from the triple bond.
Hydration of Alkynes to Aldehydes and Ketones
5. Hydroboration-Oxidation of alkynes
The reagent for this reaction is 1.BH3 or (Sia)2BH 2. H2O2, NaOH
Product: Turn triple bond into a single bond and add =O to the LEAST submitted carbon from the triple bond. If there is no least substituted carbon, then add to either carbon.
6. Acid-catalyzed hydration of alkynes
The reagent for this reaction is H2O, H2SO4, HgSO4
Product: Turn triple bond into a single bond and add =O to the most substituted carbon from the triple bond.
Reduction of alkynes
7. Catalytic reduction of alkynes
The reagent for this reaction is 2 H2 with Pd, Pt or Ni
Product: Turn triple bond into a single bond by adding four hydrogens one on each carbon from the triple bond.
8. Lindlar catalyst
The reagent for this reaction is H2 with Lindlar's catalyst
Product: Turn triple bond into a CIS double bond.
9. Alkynes with 2Na NH3(l)
The reagent for this reaction is 2 Na NH3(l)
Product: Turn triple bond into a TRANS double bond