Organic Chemistry

Section 6.8 Addition of Hydrogen 249

Hydroboration–oxidation of a terminal

alkyne forms an aldehyde.

Herbert H. M. Lindlarwas born in

Switzerland in 1909 and received a

Ph.D. from the University of Bern.

He worked at Hoffmann-La Roche

and Co. in Basel, Switzerland, and he

authored many patents. His last

patent was a procedure for isolating

the carbohydrate xylose from the

waste produced in paper mills.

is noton the terminal carbon), whereas hydroboration–oxidation of a terminal alkyne

produces an aldehyde(the carbonyl group ison the terminal carbon).

PROBLEM 13

Give the products of (1) mercuric-ion-catalyzed addition of water and (2) hydroboration–

oxidation for the following:

a. 1-butyne b. 2-butyne c. 2-pentyne

PROBLEM 14

There is only one alkyne that forms an aldehyde when it undergoes either acid- or

mercuric-ion-catalyzed addition of water. Identify the alkyne.

6.8 Addition of Hydrogen

Hydrogen adds to an alkyne in the presence of a metal catalyst such as palladium, plat-

inum, or nickel in the same manner that it adds to an alkene (Section 4.11). It is diffi-

cult to stop the reaction at the alkene stage because hydrogen readily adds to alkenes

in the presence of these efficient metal catalysts. The product of the hydrogenation re-

action, therefore, is an alkane.

The reaction can be stopped at the alkene stage if a “poisoned”(partially deactivat-

ed) metal catalyst is used. The most commonly used partially deactivated metal cata-

lyst is Lindlar catalyst, which is prepared by precipitating palladium on calcium

carbonate and treating it with lead(II) acetate and quinoline. This treatment modifies

the surface of the palladium, making it much more effective at catalyzing the addition

of hydrogen to a triple bond than to a double bond.

Because the alkyne sits on the surface of the metal catalyst and the hydrogens are

delivered to the triple bond from the surface of the catalyst, only syn addition of hy-

drogen occurs (Section 5.19). Syn addition of hydrogen to an internal alkyne forms a

cis alkene.

Internal alkynes can be converted into trans alkenesusing sodium (or lithium) in

liquid ammonia. The reaction stops at the alkene stage because sodium (or lithium) re-

acts more rapidly with triple bonds than with double bonds. Ammonia (bp=- 33 °C)

CH 3 CH 2 C H 2

2-pentyne

CCH 3 + catalyst

Lindlar

HH

cis-2-pentene

CH 3 CH 2 CH 3

CC

H 2

CH 3 CH 2 CCHPt/C CH 3 CH 2 CH

H 2

CH (^2) Pt/C CH 3 CH 2 CH 2 CH 3
alkyne alkene alkane
a ketone
an aldehyde
H 2 O, H 2 SO 4
HgSO 4
CH 3 CCH
CH 3 CCH 2 CH 3 CCH 3
OH O

1. disiamylborane
   2. HO−, H 2 O 2 , H 2 O CH^3 CH CH

CH 3 CH 2 CH

OH O

(CH 3 COO−) 2 Pb2+

lead(II) acetate

quinoline

N

Tutorial:

Hydrogenation/Lindlar

catalyst

BRUI06-238_262r4 24-03-2003 11:47 AM Page 249