Answer Problem 39, parts a–h, using 2-butyne as the starting material instead of propyne.
h. H₂/Lindlar catalyst

Verified step by step guidance

1. Identify the starting material: The problem specifies 2-butyne as the starting material. This is an alkyne with the structure CH3-C≡C-CH3.
2. Understand the reagent: H2 with a Lindlar catalyst is used to selectively hydrogenate alkynes to cis-alkenes. The Lindlar catalyst prevents full hydrogenation to an alkane.
3. Predict the product: When 2-butyne (CH3-C≡C-CH3) reacts with H2/Lindlar catalyst, the triple bond (C≡C) is reduced to a double bond (C=C), and the resulting product is a cis-alkene. The two methyl groups (CH3) on either side of the triple bond will remain in the same relative positions.
4. Draw the structure of the product: The product will be cis-2-butene, with the structure CH3-CH=CH-CH3, where the two hydrogens added during the reaction are on the same side of the double bond.
5. Verify stereochemistry: Ensure that the product is the cis-isomer (Z configuration), as the Lindlar catalyst specifically produces the cis-alkene and not the trans-alkene.

Key Concepts

Alkyne Reduction

Alkynes can be reduced to alkenes or alkanes through various catalytic processes. The reduction of 2-butyne specifically involves the addition of hydrogen (H2) across the triple bond, converting it into a double bond (alkene) or a single bond (alkane) depending on the catalyst used.

Lindlar Catalyst

The Lindlar catalyst is a poisoned palladium catalyst that selectively reduces alkynes to cis-alkenes without further reducing them to alkanes. This catalyst is crucial for controlling the degree of hydrogenation, allowing for the formation of specific isomers in organic synthesis.

Stereochemistry of Alkenes

Stereochemistry refers to the spatial arrangement of atoms in molecules and is particularly important in alkenes, which can exist as cis or trans isomers. The use of the Lindlar catalyst in the reduction of 2-butyne will yield cis-2-butene, highlighting the importance of stereochemical considerations in organic reactions.