Ch 9 (Synthesis and Reaction of alkynes)
Chapter 9 reactions deal with synthesis and reactions of alkynes. For the bare minimum, memorize what reagent is used, and what product is formed. For alkynes, you don’t have to memorize the mechanisms, but again it helps to at least understand it to make it easier to predict the products. I will try to emphasize which reactions are important, and what you should know for each reaction shown below.
Synthesis of Alkynes
Reaction
Example (Mechanism)
Reactants, Reagents, products
Stereochemistry/
Regiochemistry
Important things to note
Synthesis of alkynes from terminal alkynes -A terminal alkyne hydrogen is relatively acidic (pKa ~25), and can become deprotonated using a strong base (NH2-, pKa ~35). The alkyne ion can then react as a good nucleophile in a SN2 reaction with a common electrophile (Must be a primary, unhindered, electrophile!). (You don’t need to specify stereochemistry)
-If reacting with an acetylide, specify 1 equivalent so you only extract 1 hydrogen
Reactant – terminal alkyne (0 or 1 substituent attached)
Reagent – NaNH2, then an electrophile
Product – An substituted alkyne (1 more substituent than started off with)
Stereochemistry – No specific stereochemistry
Regiochemistry – No specific regiochemistry
-Know this reaction! NaNH2 is an important reagent commonly used for formation of an alkyne ion
-This is simply a SN2 reaction, with the nucleophile being the alkyne ion
-A strong base (NaNH2) must be used to deprotonate the alkyne hydrogen
-The electrophile must be primary, unhindered!
Addition of alkynes to carbonyl groups
-Similar to the previous reaction, NaNH2 can extract a hydrogen from a terminal alkyne and form a good nucleophile. The alkyne ion can also react with carbonyl groups (can be formaldehyde, aldehyde, or ketone) , become protonated to form an acetylenic alcohol
The degree of alcohol (1o, 2o, 3o), depends on what carbonyl group is used (formaldehyde = 1o, aldehyde = 2o, ketone = 3o
Reactant- Alkyne
Reagent – NaNH2, then any carbonyl group (formaldehyde, aldehyde, or ketone)
Product - acetylenic alcohol (formaldehyde = 1o, aldehyde = 2o, ketone = 3o alcohol)
Stereochemistry – No specific stereochemistry
Regiochemistry – No specific regiochemistry
-Not as important as other reactions, but still be able to recognize what happens if given the reagents.
-Good for a synthesis question if given an alkyne with an alcohol on the same compound
Synthesis of Alkynes by Elimination
A dihalide (alkane with 2 adjacent halogen substituents) can react with a strong base in very basic conditions to form an alkyne. NaNH2 with heat is used for forming terminal alkynes (alkynes on the very end of the chain).
KOH under very basic and heated condition can be used to eliminate the dihalide to form internal alkynes (alkynes with 2 substituents). Reaction is non-stereospecfic
Reactant- Alkyl dihalide
Reagent – NaNH2 for terminal alkyne, KOH for internal alkyne
Product -
An alkyne (terminal or internal)
Stereochemistry – No specific stereochemistry
Regiochemistry – No specific regiochemistry
-Not a common reaction you see, and most likely see as a test question, but recognize it and what happens just in case
-However, this is a good reaction to know if you need to synthesize an alkyne from an alkene or alkane (alkene can react with Br2, then react with KOH or NaNH2 with heat to form the alkyne)
-A dihalide must be present to be able to do 2 eliminations
Reactions of Alkynes
Reaction
Example (Mechanism)
Reactants, Reagents, products
Stereochemistry/
Regiochemistry
Important things to note
Catalytic hydrogenat-ion to alkanes
-An alkyne can react with 2 equivalents of H2, with a metal catalyst (Pt, Pd or Ni), to hydrogenate the alkyne completely to an alkane. The catalyst is too strong: it will completely hydrogenate from alkyne to alkane, impossible to stop at the alkene
Reactant – Any Alkyne
Reagent – 2 H2, and a metal catalyst (Pt, Pd, Ni)
Product – An
