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In the presence of catalysts such as palladium, platinum, or nickel, alkynes can be hydrogenated to the corresponding alkanes.

Sample reaction

image\Ch 17 sample 17-4.png

Alkynes can be hydrogenated to alkenes by using specific catalysts such as Lindlar palladium. This reaction is stereoselective resulting in syn addition.

Sample reaction

image\Ch 17 sample 17-5.png

An alkyne can be hydrogenated to a trans-alkene by hydrogenating the alkyne with sodium in liquid ammonia.

Sample reaction

image\Ch 17 sample 17-6.png

Acid-Catalyzed Hydration

Alkynes can undergo acid-catalyzed hydration reactions. The product is an enol, which is an unstable intermediate, and is immediately converted to a ketone. The formation of enol is based on Markovnikov addition (hydration).

image\Ch 17 acid-catalyzed.png

Sample reaction

image\Ch 17 sample 17-7.png

Alkynes with Hydrogen Halides

Alkynes can undergo reactions with hydrogen halides. The reaction follows Markovnikov's addition of hydrogen and halogen to the triple bond. When two moles of hydrogen halide gets added to an alkyne, the reaction follows the same Markovnikov's pattern resulting in a geminal dihalide (two halogen atoms attached to the same carbon atom). Study the reactions given below.

Sample reaction

image\Ch 17 sample 17-8.png

image\Ch 17 sam 17-8b.png

Alkynes with Halogens

Alkynes can undergo reactions with halogens forming di- and tetra-halogenated products.
Sample reaction

image\Ch 17 sample 17-9.png


Alkynes can undergo ozonolysis. The products formed are carboxylic acids.

Sample reaction

image\Ch 17 sample 17-10.png

In this reaction, the products formed are butanoic acid and acetic acid. If a terminal alkyne is used, the products are carbon dioxide and a carboxylic acid.
See the example given below.

image\Ch 17 sam 17-10b.png

Reaction Involving the Acidic Hydrogen in an Alkyne

The acidic hydrogen present in terminal alkynes can undergo reactions with Grignard reagents. A sample reaction is shown.

Sample reaction

image\Ch 17 sample 17-11.png

Acetylide with Aldehydes and Ketones
Acetylides are strong bases and are good nucleophiles. So they can add to the carbonyl groups in aldehydes and ketones. The alkoxide ion that is formed during the reaction can be protonated to an alcohol by treating it with aqueous dilute acid.

image\Ch 17 page 232 graph.png

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