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[CONTRIBUTIONROM TEE CHEMICAL ABORATORIES TEE UNIVERSITY F

NOTRED A M E ]

A PROPOSED MECHANISM FOR MERCURY CATALYSIS IN

ACETYLENE ADDITION REACTIONS*

G. F. H E N N I O N , R. R. VOGT A N D J. A . N I E U W L A N D t

Received March 27 19

DISCUSSION

The many well-known reactions of acetylene with water, alcohols,

glycols, carboxylic acids, oxy-acids, phenols, etc., which yield acetaldehyde,

acetals, vinyl and ethylidene esters, and the like, are too numerous to

permit of review here. Some corresponding reactions with alkylacetylenes

have recently been described in publications from our 1aboratory.l

These reactions are similar in that they all proceed quite readily when

carried out in an appropriate acid medium in the presence of mercuric

oxide or a suitable mercuric salt. Considerable interest attaches to the

r61es played by the mercury and acid in these reactions. The amounts of

mercuric salt and acid required are so small that these substances must

undoubtedly undergo a definite cycle of reactions which may be repeated

until the mercuric salt is reduced by secondary reactions. Such recurring

reactions may well explain the nature

of

the so-called mercury catalysis

in acetylene chemistry. The mercuric salt must presumably be ionized

since slightly ionizable salts and conditions which depress ionization are,

in general, not suitable.

A small quantity of boron fluoride, when used with an alcohol, forms a

strongly acidic medium. Boron fluoride has been found to be an extremely

effective acid-forming catalyst. Nieuwland, Vogt and Foohey2 showed

* Paper XI11 on the chemis t ry of the a lkylacetylenes and thei r addi t ion com-

pounds; previous paper,

J . Am . Chem. Soc.

68, 892 1936).

t

Editorial note:D r. Nieuwland,

a

member of t h e B o ar d

of

Edi to r s of THIS OURNAL,

died suddenly of a hea r t a t t ack on Jun e 11 th

[Znd. Eng. Chem. News

Ed . , 14, 248

Ju ne 20, 1936)l. His m an y fr iends will welcome this outl ine

of

his views on

a

re-

ac t ion t o t h e knowledge of which he was an ou t s t and ing con t ri bu tor .

1

HENNION , IEUWLANDND COWORKERS,

J . A m . Chem. Soc. a )

66, 1130 1934);

b)

56,

1384 1934);

c ) 66,

1786 1934);

d ) 66,

1802 1934);

e ) 67,

2006 1935);

f) 8,

80

1936).

2

NIEUWLAND,

OGTAND

FOOHEY,

bid., 62,

1021

1930).

159

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160

G .

F. HENNION, R R . VOGT

A N D J. A .

NIEUWLAND

that 0.142 grams of boron fluoride was sufficient to catalyze the conversion

of

250

grams of glycerol to the ethylidene acetal. Later Vaughn3pointed

out that

50 2)

X

l o 4

grams of boron fluoride per milliliter of glycol

is sufficient to catalyze the formation of glycol acetal. In other casesleJ

a

small amount of

a

strong acid, such as trichloroacetic acid, promotes the

catalytic effect of boron fluoride. The lower limit of the quantity of re-

quired mercuric oxide or salt) has not been determined but is known to

be largely dependent upon experimental conditions.

A

great deal of work concerning mercury-acetylene addition compounds

-much of which is questionable or indefinite-has been reported in the

literature. Kutscheroff4 was probably the first to recognize the existence

and catalytic effect of complex mercury-acetylene addition compounds;

nevertheless in many subsequent discussions the r81e of either the mercuric

salt or acid has been unjustifiably ignored.

A

consideration of these catalytic effects should, in the light of present

experimental experience, include the following points.

1.

Both

a

mercuric salt or mercuric oxide which, no doubt, forms the

salt) and

a

strong acid are required. A notable exception to this rule is

the hydration of the acetylenic triple bond. Some acetaldehyde is formed,

for example, by passing acetylene through boiling 20 sulfuric acid.

Certain alkylacetylenes may be converted to ketones5 by dissolving in

strong sulfuric acid followed by dilution with water. Even in these in-

stances, however, the reactions are certainly more advantageously carried

out with the aid of a mercuric salt and

a

more dilute acid.

2. Acetylenic hydrogen cannot be concerned, or is at least unnecessary

for catalysis.

It

has been shown that the dialkylacetylenes form typical

derivatives by addition. For example, methylamylacetylene yields, with

methanol, 3 -dimethoxyo~tane~~.

3. It is logical to believe that the catalytic mercury salt reacts with the

acetylene by addition, being subsequently replaced by the adding molecule.

It

is quite probable that the mercury content of the catalytic intermediate

may depend upon whether acetylene, a monosubstituted acetylene or

a

disubstituted acetylene is used. The authors are of the opinion that re-

placement of acetylenic hydrogen by mercury may be of secondary impor-

tance and not immediately concerned with the catalytic mechanism.

4.

Vinyl ethers add alcohols

t o

form ketals acetals) with extreme ease

when catalyzed by

a

trace of strong acid only.6 It is significant that

mercuric compounds are not required in this step.

VAUQHN,

TOC .

n d .

Acad.

Sci. 4 29

(1933).

4 KUTSCEEROFF,er. 17,

13

1884).

6 BEHAL,

nn . chim.

[6], 6, 270 1888).

KILLIAN, ENNION

ND

NIEUWLAND,

.

A m .

Chem.

SOC . 7

544

1935).

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PROPOSED

MECHANISM

FOR

MERCURY CATALYSIS

3

F

d

6

rl

I

I

e-.

P

161

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162

G.

F.

HENNION R . R.

VOGT

AND

J .

A. NIEUWLAND

5 . The alkylacetylenes react with acetic acid

to

form monoaddition

compounds, e.g. 2-acetoxy-1-heptene from amylacetyleneld. With acety-

lene either mono- or diaddition compounds may be obtained.

6 . The small amount of catalyst needed seems to indicate that its addi-

tion product is extremely reactive and easily decomposed.

7.

I t is logical to believe that the mechanism of this catalysis is,

in its

essential features

independent

o f

the nature

of

the acetylene and the type

of hydroxylated compound being added.

The authors have formulated and here present a mechanism consistent

with these points, plausible, and useful as working tool.

NomencEature.-R and R are independently hydrogen atoms or alkyl

groups originating in the acetylene used. A

represents the acid radical

contained

in

the mercuric salt and or) the acid used. For example when

boron trifluoride and methyl alcohol are employed A becomes -OCHs.

BF,. The existence of the salt Hg OCH3.BF3)2 was established by

OLeary and Wenzke.

B

represents the group attached to the hydroxyl

group of the adding molecule such as hydrogen in the case of water),

alkyl groups in the case of alcohols), or acyl groups in the case of acids).

We are fully aware of the fact that such a mechanism is incapable of

rigid proof; hence none is offered. In fact the extreme reactivity

of

the

intermediate mercury compounds and their susceptibility to reduction

not infrequently to metallic mercury) argues against the possibility of

their isolation and characterization.

It is readily seen that the proposed scheme of reactions is generally

applicable t o the addition of any hydroxy compound t o any type of acet-

ylene. In the event that the acetylene in question is a conjugate enyne

or diyne, where there is a possibility that reagents may add in the

1 , 4

positions, it is only necessary to assume that the catalytic mercuric salt

previously added in that fashion.

SUMMARY

A mechanism for mercury catalysis in acetylene addition reactions is

The proposed scheme

of

reactions is plausible, consistent withffered.

experimental results, and useful as a working tool.

7

OLEARYND WENZKE, bid . , 66, 2118 (1933).