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Organic Chemistry by Brown, Iverson, Anslyn, Foote | Mayya's Study Guide | Chapter 7

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.


3-methyl-1-butyne
3-methyl-1-butyne

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.


How to make acetylide ions
How to make acetylide ions

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).

synthesis of alkynes
synthesis of alkynes

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.

acetylide ion reaction with alkyl halide
acetylide ion reaction with alkyl halide

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.

addition of bromine or chlorine to an alkyne
addition of bromine or chlorine to an alkyne


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.

addition of hydrogen halides to alkynes
addition of hydrogen halides to alkynes

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.


Hydroboration-Oxidation of alkynes
Hydroboration-Oxidation of alkynes

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.


Acid- catalyzed hydration of alkynes
Acid- catalyzed hydration of alkynes

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.


catalytic reduction of alkynes
catalytic reduction of alkynes

8. Lindlar catalyst


The reagent for this reaction is H2 with Lindlar's catalyst


Product: Turn triple bond into a CIS double bond.


alkyne with lindlar catalyst
alkyne with lindlar catalyst

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


Alkyne Reaction Cheat Sheet for Organic Chemistry by Brown, Iverson, Anslyn, Foote
Alkyne Reaction Cheat Sheet for Organic Chemistry by Brown, Iverson, Anslyn, Foote

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