metal-organic compounds - ACS Publications

Preparation from

Lead

of or

Tetraalkyllead

Its

Compounds

Alloys

HYMIN SHAPIRO

Downloaded by CORNELL UNIV on October 10, 2016 | http://pubs.acs.org Publication Date: January 1, 1959 | doi: 10.1021/ba-1959-0023.ch027

Research Laboratories, Ethyl Corp., Detroit, Mich.

Lead is commonly alkylated by one of two basic methods: the reaction of sodium-lead alloy with an alkyl halide, or the reaction of a lead salt with an active organometallic compound. The first method is used for the commercial production of tetraethyllead, a high-tonnage chemical used as an antiknock agent. For this reason, the Ethyl Corp. has conducted research for many years on reactions based on this method. Several modifications of the basic method are outlined for the first time. Descriptions are given of the reaction of active metallic lead with alkyl halides, sulfates, and phosphates both in the presence and absence of organometallic compounds or added metals. Descriptions are also presented of the reactions of various binary and ternary alloys of lead with alkyl halides and sulfates. Factors of practical interest in these reactions are discussed.

H e a v y metals such as lead are most commonly alkylated by one of two basic methods (SI):

In Lowig's method (32), an alkyl halide reacts with an alloy of a heavy metal and an alkali metal, forming the halide of the alkali metal and the alkyl compound of the heavy metal. 4NaPb + 4RC1 -> R Pb + 4NaCl + 3Pb (1) 4

In Buckton's method (4, 36), the halide of a heavy metal reacts with an alkyl compound of a more electropositive metal. 2PbCl + 2R Zn -> R Pb + 2ZnCl + Pb 2

2

4

2

2PbCl + 4RMgCl -> R Pb + 4MgCl + Pb 2

4

2

(2) (3)

Numerous other ways of synthesis have been employed, such as free radical methods, electrolytic reduction reactions, liquid ammonia preparations, and procedures involving quadrivalent lead salts. However, the second method, as modified by Pfeiffer to use the Grignard reagent, has provided both the most practical means of synthesis of organometallic compounds in the laboratory and a means of studying the chemical reactivity of the metals in question. Lowig's method, utilizing monosodium-lead alloy (26), is still used today for the commercial production of tetraethyllead. The sim290

METAL-ORGANIC COMPOUNDS Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

SH API RO—TETRAALKYLLEAD COMPOUNDS

291

p l i c i t y a n d h i g h y i e l d o f t h i s m e t h o d p e r m i t p r o d u c t i o n a t a cost l o w e n o u g h t o a l l o w t h e e c o n o m i c a l use o f t e t r a e t h y l l e a d a s a n a n t i k n o c k agent o n a v e r y l a r g e scale. Lowig's m e t h o d has b e e n s t u d i e d e x t e n s i v e l y , because o f i t s c o m m e r c i a l i m p o r t a n c e . A n u m b e r o f m o d i f i c a t i o n s o f t h i s basic m e t h o d a r e i n d i c a t e d i n t h e p a t e n t l i t e r a t u r e . T h i s p a p e r gives f o r t h e first t i m e a c o m p r e h e n s i v e a c c o u n t o f these methods. V o l u m i n o u s e x p e r i m e n t a l d e t a i l s are u n n e c e s s a r y f o r t h i s p u r p o s e , b u t m a y be g i v e n i n f u t u r e p u b l i c a t i o n s .

Reactions of Metallic Lead


L e a d with A l k y l Halides. B e c a u s e t h e cost o f s o d i u m i s a n i m p o r t a n t e c o n o m i c f a c t o r i n t h e p r o d u c t i o n of t e t r a e t h y l l e a d b y t h e c o n v e n t i o n a l process, a c a r e f u l s t u d y was made of the direct reaction of lead m e t a l w i t h a l k y l halides, using lead itself as t h e r e d u c i n g o r a l k y l a t i n g m e t a l , as i n E q u a t i o n 4. 3 P b + 4 R X -> R P b + 2 P b X 4

(4)

2

F o r a c t i v e m e t a l s s u c h as l i t h i u m , m a g n e s i u m , a l u m i n u m , a n d z i n c , t h i s t y p e o f r e a c t i o n w i t h a l k y l h a l i d e s is w e l l k n o w n . M

+ R X — > [ R M X ] —» R M - h M X

(5)

However, t h e direct a l k y l a t i o n of heavy metals, like lead, is n o t commonly regarded as possible. S u c h a r e a c t i o n f o r l e a d w a s f i r s t a t t e m p t e d w i t h e t h y l i o d i d e b y C a h o u r s (5) i n 1853, b u t he r e p o r t e d o n l y t h e q u a l i t a t i v e f o r m a t i o n o f a s m a l l a m o u n t o f o r g a n o lead compound, w h i c h he d i d not attempt t o identify. I n recent w o r k , l e a d m e t a l has r e a c t e d u n d e r a v a r i e t y o f c o n d i t i o n s w i t h s e v e r a l a l k y l halides, t o f o r m t e t r a a l k y l l e a d compounds i n good y i e l d . C o m m e r c i a l l y c o m m i n u t e d l e a d p o w d e r s a n d flakes, b u t p r e f e r a b l y l e a d f o r m e d a s a residue i n o t h e r organolead reactions (as i n E q u a t i o n 1 ) , was f o u n d suitable f o r a l k y l a t i o n . I t w a s s h o w n t h a t t h e m e t a l m u s t p r e s e n t a l a r g e , c l e a n s u r f a c e , free o f c o m b i n e d o r a d s o r b e d impurities, such as air o r moisture. S u c h active lead gave good yields w i t h m e t h y l b r o m i d e , m e t h y l iodide, a n d e t h y l iodide, b u t d i d not react significantly under u n c a t a lyzed conditions w i t h m e t h y l chloride, e t h y l chloride, a n d ethyl bromide. T h e pre f e r r e d c a t a l y s t s f o r these r e a c t i o n s w e r e s h o w n t o b e i o d i n e a n d i o d i n e - c o n t a i n i n g c o m p o u n d s (85), u s e d i n t h e a m o u n t o f a b o u t 1 a t o m % o f the l e a d . T h e r e a c t i v i t y of t h e a l k y l h a l i d e s t e s t e d decreased i n t h e o r d e r : M e l > E t I > M e B r > M e C I > EtBr > EtCl. G o o d rates o f r e a c t i o n were o b t a i n e d at 100° t o 1 3 0 ° C . I n the r e a c t i o n of e t h y l c h l o r i d e w i t h l e a d residue p r e p a r e d a c c o r d i n g t o E q u a t i o n 1, t e t r a e t h y l l e a d y i e l d s o f a b o u t 6 5 % , b a s e d o n E q u a t i o n 4, were o b t a i n e d i n 5 h o u r s a t 1 3 0 ° C . , u s i n g 1.3 a t o m % i o d i n e as c a t a l y s t . W h e n c o m m e r c i a l l e a d flakes r e a c t e d w i t h e t h y l c h l o r i d e , t h e m a x i m u m y i e l d was of t h e o r d e r of 3 0 % ; w i t h e t h y l i o d i d e , t h e y i e l d i n creased t o 5 0 % . N o t o n l y i s i t possible t o p r e p a r e t e t r a m e t h y l - a n d t e t r a e t h y l l e a d i n t h i s w a y , b u t m i x e d a l k yH e a d c o m p o u n d s c a n b e p r e p a r e d i n a r a n d o m e q u i l i b r i u m m i x t u r e (7) f r o m m i x t u r e s of m e t h y l a n d e t h y l c h l o r i d e s . I t i s also possible t o c o m b i n e t h e r e a c t i o n s o f s o d i u m - l e a d a l l o y a n d l e a d m e t a l i n t o a c o n s e c u t i v e t w o - s t e p process, as i n E q u a t i o n s 1 a n d 4, b y r a i s i n g t h e t e m p e r a t u r e a f t e r t h e r e a c t i o n o f the a l l o y , w i t h o u t s e p a r a t i n g t h e p r o d u c t o f t h i s r e a c t i o n . T h e o v e r - a l l process i s t h e n r e p r e s e n t e d b y Equation 6 : 2 N a P b + 4 E t C l -> E t P b + 2 N a C l + P b C l 4

2

(6)

L e a d with Other Alkylating Agents. L e a d m e t a l , o b t a i n e d a s i n E q u a t i o n 1, h a s also r e a c t e d i n s m a l l - s c a l e e q u i p m e n t w i t h a l k y l s u l f a t e s a n d a l k y l p h o s p h a t e s (27), f o r 2 0 h o u r s , t o g i v e y i e l d s o f t e t r a m e t h y l - o r t e t r a e t h y l l e a d o f 2 0 t o 6 5 % , a t 110° t o 125°C. F o r the catalyst, 3 a t o m % lead iodide was desirable, a l t h o u g h this was n o t required f o r the p r e p a r a t i o n of t e t r a m e t h y l l e a d . T h e yields were based o n the e q u a tions :


292

ADVANCES IN CHEMISTRY SERIES 3 P b + 2 R S 0 -> R P b + 2 P b S 0 2

4

4

(7)

4

9Pb + 4 R P 0 - » 3 R P b + 2Pb (P0 ), 3

4

4

3

(8)

4

L e a d with A l k y l Halides a n d R e d u c i n g M e t a l . A n e x t e n d e d i n v e s t i g a t i o n w a s m a d e o f the a l k y l a t i o n of l e a d m e t a l w i t h a l k y l h a l i d e i n t h e presence o f a r e d u c i n g m e t a l s u c h as m a g n e s i u m . T h i s r e a c t i o n is i n t e r e s t i n g f r o m a c o m m e r c i a l s t a n d p o i n t , i n c o n j u n c t i o n w i t h t h e s o d i u m - l e a d - e t h y l c h l o r i d e r e a c t i o n . I t p r o v i d e s a possible m e a n s o f i n c r e a s i n g t h e y i e l d of t e t r a e t h y l l e a d p e r charge i n p l a n t a u t o c l a v e s (10). T h e reaction can be r u n either separately o r concurrently w i t h the reaction of E q u a t i o n 1, a c c o r d i n g t o t h e f o l l o w i n g , i d e a l i z e d e q u a t i o n s : P b + 4 R C 1 + 2 M g -> R P b + 2 M g C l 4

(9)

2

4 N a P b + 16RC1 + 6 M g - » 4 R P b + 4 N a C l + 6 M g C l


4

2

(10)

T h e a c t i v e l e a d m a y b e p r e p a r e d f o r r e a c t i o n b y v a r i o u s m e a n s , as i n R e a c t i o n 4, a n d t h e m a g n e s i u m i s c o n v e n i e n t l y s u p p l i e d i n t h e f o r m o f c h i p s . A b o u t 10 t o 2 0 % o f a n a l i p h a t i c e t h e r s u c h as d i e t h y l e t h e r , b a s e d o n t h e e t h y l c h l o r i d e p r e s e n t , i s used as c a t a l y s t . A s a r e s u l t , t h e G r i g n a r d reagent is f o r m e d i n s i t u f r o m t h e m a g n e s i u m . I n 2 t o 4 h o u r s a t t h e o p t i m u m t e m p e r a t u r e o f 7 0 ° C , u s i n g a n excess o f e t h y l c h l o r i d e , t e t r a e t h y l l e a d y i e l d s of a b o u t 7 5 % were o b t a i n e d i n s m a l l - s c a l e e q u i p m e n t . O t h e r c a t a l y s t s , s u c h as t e r t i a r y a m i n e s o r a l k y l a m m o n i u m i o d i d e s , m a y b e e m p l o y e d , a l o n g w i t h o t h e r r e d u c i n g m e t a l s s u c h as l i t h i u m ( b u t n o t s o d i u m ) , t o g i v e somewhat poorer yields. O t h e r a l k y l halides ( m e t h y l a n d p r o p y l chlorides, bromides, a n d i o d i d e s ) m a y b e u s e d as t h e a l k y l a t i n g agents f o r t h e l e a d m e t a l , b u t p r e f e r a b l y n o t i n a concurrent reaction w i t h sodium-lead alloy. T h e f o r m a t i o n o f t e t r a a l k y l l e a d is a c c o m p a n i e d i n v a r i a b l y b y t h e f o r m a t i o n o f h e x a a l k y l d i l e a d , w h i c h decomposes s l o w l y t o f o r m a d d i t i o n a l t e t r a a l k y l l e a d a n d l e a d m e t a l . T h e hexaalkyldilead remaining i n the product m a y be converted t o t e t r a a l k y l l e a d b y s u b s e q u e n t t h e r m a l d e c o m p o s i t i o n , o r b y t r e a t m e n t w i t h c a t a l y s t s , s u c h as F i l t r o l (34), a l k y l b r o m i d e s o r i o d i d e s (28), o r a c t i v a t e d c a r b o n (23A), a c c o r d i n g t o : 2 R P b ^ 3 R P b + Pb 6

2

(11)

4

L e a d with A l k y l Halides and Organometallic C o m p o u n d s . I n the t y p e of reac t i o n d e s c r i b e d a b o v e , p r e f o r m e d G r i g n a r d reagent m a y b e u s e d i n l i e u o f t h e m a g n e s i u m c h i p s (13). W i t h r e a c t i o n s u s i n g c h l o r i d e s , t h e r e a c t i o n m a y b e r u n i n a single stage w i t h a s o d i u m - l e a d r e a c t i o n , i f d e s i r e d . T h e e q u a t i o n f o r t h i s p r e p a r a t i o n of t e t r a e t h y l e a d i s : 4 N a P b + l O E t C l + 6 E t M g C l -> 4 E t P b + 6 M g C l 4

2

+ 4NaCl

(12)

I n s t u d y i n g t h i s r e a c t i o n , i t was s h o w n t h a t e t h e r is d e s i r a b l e , b u t n o t necessary, as a c a t a l y s t ( a l t h o u g h a c a t a l y s t of t h i s t y p e i s v i t a l f o r t h e r e a c t i o n w i t h m a g n e s i u m m e t a l ) . T h e r e a c t i o n was c a r r i e d o u t effectively w i t h e t h y l m a g n e s i u m i o d i d e p r e p a r e d i n benzene i n t h e absence o f e t h e r . H e x a e t h y l d i l e a d was n o t f o r m e d u n d e r these noncatalyzed conditions. O t h e r o r g a n o m e t a l l i c c o m p o u n d s , s u c h as e t h y l l i t h i u m , d i e t h y l z i n c , d i e t h y l c a d m i u m , a n d e t h y l c a d m i u m i o d i d e , were effective e t h y l a t i n g agents i n r e a c t i o n s a n a l o g o u s t o E q u a t i o n 12. A t 7 0 ° C . i n s m a l l - s c a l e e q u i p m e n t , y i e l d s were o v e r 9 0 % w i t h t h e l i t h i u m reagent a n d e t h y l c h l o r i d e , b u t t h e z i n c a n d c a d m i u m c o m p o u n d s g a v e g o o d y i e l d s o n l y w h e n e t h y l i o d i d e w a s e m p l o y e d as t h e a l k y l a t i n g agent (14-16). The same t y p e of r e a c t i o n w a s effected u s i n g " s o d i u m - n a p h t h a l e n e c o m p l e x " as t h e r e d u c i n g agent (43). G o o d y i e l d s o f t e t r a a l k y l l e a d were o b t a i n e d b y t h i s m e t h o d w h e n e t h e r o r a m i n e - t y p e c a t a l y s t w a s p r e s e n t , a l t h o u g h free s o d i u m m e t a l a n d a l k y l s o d i u m c o m pounds d i d not yield a n y organolead product. O t h e r k i n d s of a l k y l a t i o n reactions reported i n the l i t e r a t u r e m a y be related t o the a l k y l a t i o n of m e t a l l i c lead w i t h a l k y l halides a n d organometallic compounds.


SH API RO—TETRAALKYLLEAD COMPOUNDS

293

G i l m a n (23) describes a n elegant l a b o r a t o r y m e t h o d f o r t h e p r e p a r a t i o n o f t e t r a a l k y l lead compounds from lead chloride. PbCl

2

+ 3 R L i + R I -> R P b + 2LÎC1 + L i l

(13)

4

R e t r o s p e c t i v e l y , t h i s r e a c t i o n m a y b e classified as t h e d i r e c t a l k y l a t i o n o f a m e t a l as d e s c r i b e d a b o v e . T h e a u t h o r has t e s t e d i t e x p e r i m e n t a l l y , a n d f o u n d t h a t i n t h e r e a c t i o n o f l e a d c h l o r i d e w i t h m e t h y l G r i g n a r d reagent, t h e G i l m a n r e a c t i o n c a n b e c a r r i e d o u t e i t h e r b y h i s p r o c e d u r e , o r i n a t w o - s t e p s y n t h e s i s ; t h e first step b e i n g a c l a s s i c a l G r i g n a r d a l k y l a t i o n , a c c o r d i n g t o P f e i f f e r a n d T r u s k i e r (36) : 2 P b C l + 4 M e M g C l -> M e P b + 4 M g C l 2

4

+ Pb

2

(14)

a n d t h e second, a n a l k y l a t i o n of t h e s e p a r a t e d m e t a l l i c l e a d , i n a c c o r d a n c e discussion a b o v e : P b + 2 M e C l + 2 M e M g C l -> M e P b + 2 M g C l


4

with the (15)

2

T a b l e I summarizes representative y i e l d data o n t h e p r i n c i p a l reactions of lead metal described above.

Table I. Lead Source A Β A Β A

0

Representative Yields of R Pb from Metallic Lead 4

Alkyl Halide EtCl EtCl M e l or EtI EtI Et S0 Me S0 Et P0 EtCl EtCl EtCl EtCl 2

Other Reactant — — — — — — — Mg EtMgCl EtLi Na(CioH )

4

2

3

4

4

8

EtI

Catalyst, Atom % Based on Pb II2 lh 1I 1I 3PbI 3PbI 3PbI 10Et O» 10Et O» — 35C6H N(CH3) , 80CH OCH CH OCH «> 20C H N(CH ) , 95CH OCH CH OCH * 2

2

2

2

2

2

2

5

3

NafCioHs)

6

2

2

6

3

2

3

2

3

2

2

3

R Pb Yield, % 65 30 70 50 55 65 20 75 75 90 4

20 70

Lead source A is metallic lead product of reaction of NaPb with E t C l . R P b yield figure is based on reac tion of metallic lead in subsequent R X reaction, and does not include NaPb-EtCl product. Lead source Β is com mercial Metalead paste. % based on E t C l . « % based on E t I . a

4

b

Reactions of Lead Alloys T h e c o m m e r c i a l success of s o d i u m - l e a d f o r t h e p r o d u c t i o n of t e t r a e t h y l l e a d h a s l e d t o t h e i n v e s t i g a t i o n of n u m e r o u s o t h e r a l l o y s f o r t h i s p u r p o s e . These compositions h a v e c o m p r i s e d e i t h e r b i n a r y o r t e r n a r y a l l o y s of l e a d . T h e b i n a r y a l l o y s h a v e c o n sisted of other compositions w i t h sodium o r have contained other G r o u p I o r G r o u p I I metals. D i m a g n e s i u m - L e a d A l l o y . O n e of the m o s t i n t e r e s t i n g a l l o y s e x p l o r e d is t h e c o m p o s i t i o n d i m a g n e s i u m - l e a d . T h i s a l l o y i s a w e l l - k n o w n i o n i c c o m p o u n d of t h e f l u o r i t e l a t t i c e s t r u c t u r e (24)· G o o d y i e l d s of a l k y l l e a d c o m p o u n d s were o b t a i n e d f r o m t h e alloy according t o : M g P b + 4 R X -> R P b + 2 M g X 2

4

2

(16)

T h e r e a c t i o n m a y b e c a r r i e d o u t w i t h a n a l k y l i o d i d e , i n w h i c h case a c a t a l y s t i s n o t r e q u i r e d (41) o r p r e f e r a b l y , i t m a y b e r u n w i t h a n a l k y l b r o m i d e o r c h l o r i d e (39, 40). W h e n t h e a l k y l a t i n g agent was e t h y l c h l o r i d e , o p t i m u m y i e l d s of 8 5 t o 9 0 % were o b t a i n e d i n 2 h o u r s a t 120°C., i n s m a l l b o m b s , u s i n g 5 e q u i v a l e n t s of e t h y l c h l o r i d e a n d a c a t a l y s t c o m b i n a t i o n of 5 % e t h e r a n d 8 5 % e t h y l i o d i d e , b a s e d o n a l l o y weight. T h i s reaction is s h a r p l y differentiated f r o m the reaction of lead m e t a l w i t h a n a l k y l h a l i d e a n d m a g n e s i u m , because n o h e x a a l k y l d i l e a d i s p r o d u c e d as a b y product.


294

ADVANCES IN CHEMISTRY SERIES

T h e a l k y l chloride reaction was shown t o be operable w i t h a v a r i e t y of catalysts, p r i m a r y a l i p h a t i c ethers a n d a l k y l i o d i d e s b e i n g m o s t u s e f u l . T e t r a a l k y l l e a d c o m p o u n d s f r o m m e t h y l t o i s o p r o p y l a n d η - b u t y l were p r e p a r e d b y t h i s m e a n s . T h e r e a c t i o n of d i m a g n e s i u m - l e a d w i t h e t h e r - c a t a l y z e d a l k y l i o d i d e ( i n the absence o f a l k y l c h l o r i d e ) a p p e a r s t o b e a s i m p l e p r e p a r a t i v e m e t h o d f o r use i n t h e l a b o r a t o r y , once the alloy is available.


Ternary L e a d Alloys. T h e p r e p a r a t i o n o f t e t r a e t h y l l e a d f r o m v a r i o u s t e r n a r y a l l o y s c o n t a i n i n g s o d i u m a n d l e a d h a s b e e n a t t e m p t e d since 1850. A t t h a t t i m e , Lôwig claimed t o have p r e p a r e d tetraethyllead b y the reaction of e t h y l iodide w i t h s o d i u m a n d l e a d a m a l g a m a t e d w i t h excess m e r c u r y . S i n c e t h e n , t h e r e a c t i o n h a s b e e n a t t e m p t e d w i t h s o d i u m - l e a d a l l o y s c o n t a i n i n g s m a l l a m o u n t s of l i t h i u m , p o t a s s i u m , m a g n e s i u m , z i n c , t i n , c o p p e r , a n d s i l v e r . A p p r e c i a b l e a d d i t i o n s of m a g n e s i u m , potassium, a n d zinc have been investigated recently. Some of the most interesting e x p e r i m e n t a l w o r k on the a l k y l a t i o n of lead i n t e r n a r y alloys has been conducted o n systems c o n t a i n i n g lead, s o d i u m , a n d m a g n e s i u m . T h i s t e r n a r y m e t a l s y s t e m is e x c e e d i n g l y c o m p l e x (17, 18), a n d t h e r e f o r e has n o t b e e n f u l l y i n v e s t i g a t e d . T h e a l l o y s t h a t a l k y l a t e best are l o c a t e d o n t h e s o d i u m - l e a d - m a g n e s i u m cross s e c t i o n {11, 12), w h i c h c o n t a i n s a p e r i t e c t i c c o m p o u n d of c o m p o s i t i o n N a M g P b . T h e best of s u c h a l l o y s , r e a c t e d w i t h e t h e r c a t a l y s t a n d excess e t h y l c h l o r i d e a t 8 5 ° C , gave tetraethyllead yields of about 7 5 % , based o n the combination of reducing metals p r e s e n t . T h e r e a c t i o n r e s e m b l e d t h e a l k y l a t i o n o f m e t a l l i c l e a d i n t h e presence of m a g n e s i u m m e t a l , because h e x a e t h y l d i l e a d was f o r m e d as a b y - p r o d u c t . T h e replacement of m a g n e s i u m i n a t e r n a r y alloy b y potassium i s interesting (44)A ternary alloy containing equiatomic proportions of lead and alkali metals, in w h i c h 15 a t o m % o f t h e a l k a l i m e t a l i s p o t a s s i u m , i s c a p a b l e of g i v i n g y i e l d s o f t e t r a ethyllead of over 9 5 % , based on b o t h reducing metals. T h i s compares f a v o r a b l y w i t h the usual y i e l d of about 8 5 % f r o m sodium-lead alloy i n the conventional reaction. T h e p o t a s s i u m n o t o n l y r e a c t s , b u t also increases t h e u t i l i z a t i o n of s o d i u m . T h e r e a c t i o n is m o d i f i e d i n t h a t t h e f o r m a t i o n o f b y - p r o d u c t h y d r o c a r b o n s b y W u r t z - t y p e c o u p l i n g r e a c t i o n s is a l m o s t e l i m i n a t e d . S u c h r e a c t i o n s were u s u a l l y r u n a t 80° t o 1 0 0 ° C . w i t h excess e t h y l c h l o r i d e . A k e t o n e c a t a l y s t (0.2 w e i g h t % acetone b a s e d o n a l l o y w e i g h t ) is v i t a l t o o b t a i n g o o d y i e l d s a t n o r m a l , o p e r a t i n g t e m p e r a t u r e s . T h u s , t h e r e a c t i o n i s different i n m e c h a n i s m f r o m the conventional s o d i u m - l e a d - e t h y l chloride reaction. Nonasodium-Tetralead A l l o y . T h e cost o f r e c y c l i n g l e a d m e t a l i n t h e c o m m e r c i a l s o d i u m - l e a d - e t h y l c h l o r i d e process has a r o u s e d i n t e r e s t i n t h e use o f s o d i u m - r i c h a l l o y s f o r m a n y y e a r s . M o s t of t h e s o d i u m - l e a d a l l o y s , h o w e v e r , are r a t h e r u n r e a c t i v e w i t h e t h y l chloride. T h e sodium-rich alloys between sodium-lead a n d pentasodium-dilead a r e p r o g r e s s i v e l y less r e a c t i v e u n t i l a t t h e c o m p o s i t i o n of p e n t a s o d i u m - d i l e a d r e a c t i o n p r a c t i c a l l y ceases. H o w e v e r , i f t h e a l k y l a t i n g agent i s e t h y l b r o m i d e o r p r e f e r a b l y e t h y l i o d i d e , g o o d y i e l d s m a y b e o b t a i n e d f r o m h i g h e r s o d i u m a l l o y s i n t h e presence o f a m i n e s o r h y d r o x y l c o m p o u n d s , s u c h as p y r i d i n e o r w a t e r (6). T h i s type o f s o - c a l l e d h y d r o u s r e a c t i o n was used c o m m e r c i a l l y f o r a s h o r t t i m e i n t h e 1920's. However, the e c o n o m i c s o f s u c h processes are u n f a v o r a b l e as c o m p a r e d w i t h t h e p r e s e n t s o d i u m - l e a d e t h y l c h l o r i d e process. T h e r e f o r e , t h e a n h y d r o u s r e a c t i o n s o f s o d i u m - r i c h a l l o y s w i t h e t h y l chloride were reinvestigated recently. A t h e r m a l a n a l y s i s of t h e s o d i u m - l e a d phase d i a g r a m d e m o n s t r a t e d t h e existence of a h i t h e r t o u n r e c o g n i z e d o p e n m a x i m u m c o m p o u n d a t t h e c o m p o s i t i o n n o n a s o d i u m t e t r a l e a d (30). C o m p o s i t i o n s a t o r b e l o w t h i s exact l e v e l o f s o d i u m c o n t e n t w e r e r e a c t i v e w i t h e t h y l c h l o r i d e , p r o v i d e d t h a t a c a t a l y s t s u c h as a n ester, a l d e h y d e , o r k e t o n e , w a s p r e s e n t (2). Y i e l d s o f a p p r o x i m a t e l y 7 0 % o n s o d i u m were o b t a i n e d i n a b o u t 2 h o u r s a t 1 0 0 ° C . w h e n a b o u t 0 . 5 % ( o n w e i g h t o f a l l o y ) o f acetone o r e t h y l acetate c a t a l y s t w a s u s e d . A b o v e t h e s o d i u m p e r c e n t a g e c o n t a i n e d i n n o n a s o d i u m tetralead, the alloys became a b r u p t l y unreactive w i t h e t h y l chloride, even when c a t a lyst was present.


295

SHAPIRO—TETRAALKYLLEAD COMPOUNDS

Binary L e a d Alloys with C a l c i u m , Potassium, a n d L i t h i u m . I n a d d i t i o n to s o d i u m a n d m a g n e s i u m , b i n a r y a l l o y s o f l e a d w i t h c a l c i u m (29), p o t a s s i u m , a n d l i t h i u m were i n v e s t i g a t e d w i t h i n t h e p a s t s e v e r a l y e a r s . T h e c o m p o s i t i o n s l i t h i u m - l e a d a n d c a l c i u m - l e a d were r e a c t i v e w i t h e t h y l c h l o r i d e , w h i l e t h e a l l o y p o t a s s i u m - l e a d g a v e y i e l d s o f less t h a n 2 0 % o n p o t a s s i u m u n d e r t h e best c o n d i t i o n s t e s t e d . T h e a l l o y l i t h i u m - l e a d is v e r y e x p e n s i v e f o r c o m m e r c i a l use. U n c a t a l y z e d , t h e a l l o y c a l c i u m - l e a d gave yields of 8 0 % at 7 0 ° C , according t o :


2CaPb + 4EtCl

2CaCl

2

+ Et Pb + Pb 4

(17)

I t is noteworthy that the composition dicalcium-lead, w h i c h is a well-defined i n t e r metallic compound, reacted poorly. M o n o s o d i u m - L e a d A l l o y Reaction with E t h y l C h l o r i d e . M u c h o f t h e r e s e a r c h o n t h e s o d i u m - l e a d - e t h y l c h l o r i d e r e a c t i o n i n recent y e a r s has d e a l t w i t h i t s a c c e l e r a t i o n o r r e t a r d a t i o n . I n the discussion of lead m e t a l reactions, i t was pointed out t h a t t h e s u r f a c e o f t h e l e a d m u s t b e free of c o m b i n e d o r a d s o r b e d i m p u r i t i e s . T h i s i s e q u a l l y t r u e f o r t h e a l l o y r e a c t i o n s , because here also t h e r e i s a heterogeneous r e a c t i o n b e t w e e n s o l i d m e t a l a n d l i q u i d o r gaseous a l k y l h a l i d e . T h e r e a c t i o n v e l o c i t y is t h e r e fore v e r y s u s c e p t i b l e t o s u r f a c e effects. T h e conventional s o d i u m - l e a d - e t h y l chloride reaction is sensitive t o certain i m p u r i t i e s i n t h e e t h y l c h l o r i d e . A s l i t t l e as 0 . 0 0 2 5 % a c e t y l e n e exerts a p o w e r f u l r e t a r d i n g effect. T h i s r e t a r d a t i o n , o r p o i s o n i n g as i t i s c o m m o n l y k n o w n , has b e c o m e so severe i n some a l l o y r e a c t i o n s as t o r e s u l t i n a c o a t i n g o n t h e steel w a l l s o f t h e r e a c t i o n vessel w h i c h r e n d e r s t h e vessel i n o p e r a b l e . T h i s w a l l - r e t a i n e d p o i s o n i s n o t a l w a y s r e m o v e d b y s c r u b b i n g o r s o l v e n t a c t i o n , a n d s o m e t i m e s i t i s necessary t o e t c h t h e steel w i t h a c i d t o restore i t s u s e f u l c o n d i t i o n . I n e x p e r i m e n t a t i o n designed t o increase the p r o d u c t i o n o f t e t r a e t h y l l e a d , i t has b e e n d i s c o v e r e d t h a t t h e r a t e of t h e s o d i u m - l e a d - e t h y l c h l o r i d e r e a c t i o n m a y b e i n creased s i g n i f i c a n t l y b y t h e use o f 0.005 t o 4 % o f a n a c c e l e r a t o r , s u c h as a k e t o n e (25), a l d e h y d e (21), a c e t a l (37), a n h y d r i d e (22), ester (20), o r a m i d e (19). I t is also i m p o r t a n t , c o m m e r c i a l l y , t h a t s u c h a c c e l e r a t o r s h a v e t h e a d d i t i o n a l effect o f o f f s e t t i n g t h e deleterious effect o f m i n o r a m o u n t s o f poisons o f t h e a c e t y l e n e t y p e . T h e r a t e o f t h e s o d i u m - l e a d - e t h y l c h l o r i d e r e a c t i o n is also i n f l u e n c e d s t r o n g l y b y t h e gross s t r u c t u r e a n d s u r f a c e a r e a o f t h e a l l o y . O r d i n a r y m e t h o d s o f c o m m i n u t i o n of t h e a l l o y t o a fine state o f s u b d i v i s i o n r e s u l t i n a decreased r a t e a n d y i e l d . T h i s d e crease i s p r o b a b l y caused b y c o n t a m i n a t i o n of the surface, i n h e r e n t t o t h e u s u a l process o f c o m m i n u t i o n . T h e r a t e c a n be i n c r e a s e d m a r k e d l y , h o w e v e r , b y w e t - g r i n d i n g t h e a l l o y i m m e r s e d i n e t h y l c h l o r i d e a t l o w t e m p e r a t u r e (42). T h u s , t h e a l l o y s u r f a c e c a n be e n l a r g e d , w h i l e i t is p r o t e c t e d u n t i l r a p i d r e a c t i o n is d e s i r e d . T h e r e a c t i v i t y o f t h e a l l o y m a y also b e e n h a n c e d b y d e c r e a s i n g t h e size o f t h e c r y s t a l s . T h i s m a y b e a c c o m p l i s h e d b y q u i c k - c o o l i n g , a s b y d r u m - c a s t i n g (1, 38, 4?, 48). Q u i c k c o o l i n g m a y also b e a c c o m p l i s h e d b y f e e d i n g t h e a l l o y i n the m o l t e n state t o the e t h y l c h l o r i d e (33). M o n o s o d i u m - L e a d A l l o y Reaction with M e t h y l C h l o r i d e . I n t e r e s t h a s b e e n e x pressed o v e r m a n y y e a r s i n t h e use o f m i x e d m e t h y l e t h y l l e a d a l k y l s , s u c h as m e t h y l t r i e t h y l l e a d , as a n t i k n o c k s , o w i n g t o t h e i r g r e a t e r v o l a t i l i t y i n t h e engine m a n i f o l d as c o m p a r e d w i t h t e t r a e t h y l l e a d . T h i s has l e d t o a s t u d y o f m e t h o d s o f m a n u f a c t u r e o f tetramethyllead and the m i x e d m e t h y l - e t h y l lead compounds. I t has b e e n k n o w n t h a t n o n e o f t h e m e t h y l o r e t h y l h a l i d e s , w i t h t h e e x c e p t i o n o f e t h y l chloride, w i l l react a t a n appreciable rate w i t h sodium-lead alloy. Moreover, t h e a d d i t i o n o f one of t h e o t h e r m e t h y l o r e t h y l h a l i d e s t o t h i s t y p e o f e t h y l c h l o r i d e r e a c t i o n t e n d s t o p o i s o n the r e a c t i o n . H o w e v e r , L e w i s a c i d c a t a l y s t s o f the a l u m i n u m h a l i d e t y p e , s u c h as a l u m i n u m c h l o r i d e , p r o m o t e t h e r e a c t i o n (9). O t h e r c a t a l y s t s , s u c h as c o m p o u n d s of b e r y l l i u m , z i n c , b o r o n , p h o s p h o r u s , a n d a r s e n i c , are also effec t i v e , b u t t o a lesser e x t e n t . T h e s e c a t a l y s t s m a d e i t possible t o devise a n e c o n o m i c a l process f o r t h e m a n u f a c t u r e o f t e t r a m e t h y l l e a d o r m i x e d m e t h y l e t h y l l e a d c o m p o u n d s


ADVANCES IN CHEMISTRY SERIES

296

(8) b y m e a n s of a n a l k y l a t i o n w i t h m e t h y l c h l o r i d e o r a m i x t u r e of t h e c o r r e s p o n d i n g a l k y l halides. T h e reaction was usually carried out a t 90° t o 100°C. for 2 t o 4 hours, u s i n g a b o u t 4 a t o m % a l u m i n u m c h l o r i d e o n t h e basis o f s o d i u m , t o g i v e y i e l d s of approximately 8 5 % . S o d i u m - L e a d A l l o y Reactions with D i e t h y l Sulfate. E f f o r t s h a v e been m a d e t o d e v e l o p a c o m m e r c i a l e t h y l s u l f a t e process {46), because d i e t h y l s u l f a t e i s m o r e a v a i l able a n d less e x p e n s i v e as a n e t h y l a t i n g agent t h a n e t h y l c h l o r i d e . I n recent e x p e r i m e n t s , t h e e n t i r e range of s o d i u m - l e a d a l l o y s w a s i n v e s t i g a t e d , t h u s e s t a b l i s h i n g t h e decreasing order of alloy r e a c t i v i t y N a P b > N a P b > N a P b > N a P b . Using s o d i u m - l e a d , y i e l d s o n s o d i u m as h i g h as 7 5 % were o b t a i n e d i n 2 h o u r s a t 120° t o 130°C. i n u n c a t a l y z e d reactions. W i t h higher sodium alloys, a n iodine-containing c a t a l y s t i s r e q u i r e d t o o b t a i n g o o d y i e l d s (45). P e n t a s o d i u m - d i l e a d a l l o y gave 8 0 % y i e l d s o f t e t r a e t h y l l e a d i n 5 h o u r s a t 130° t o 140 ° C , w h e n 1 a t o m % l e a d i o d i d e w a s p r e s e n t , whereas t h e u n c a t a l y z e d r e a c t i o n g a v e o n l y a 1 0 % y i e l d . I n a l l s u c h r e a c t i o n s , h o w e v e r , o n l y one e t h y l g r o u p r e a c t e d , a c c o r d i n g t o :


9

4

5

2

4

4 N a P b + 4 E t S 0 -> E t P b + 4 N a E t S 0 + 3 P b 2

4

4

(18)

4

T h e f a c t t h a t o n l y o n e e t h y l g r o u p is u t i l i z e d i n t h e p r i m a r y r e a c t i o n forces a r e g e n e r a t i o n o r r e c o v e r y o p e r a t i o n . T h e slower r a t e of r e a c t i o n a n d t h e h i g h e r t e m p e r a t u r e s r e q u i r e d are also f a c t o r s w h i c h m a k e the e t h y l s u l f a t e process d i s a d v a n t a g e o u s as c o m p a r e d w i t h t h e e t h y l c h l o r i d e process.

Table II. Composition NaPb

Na Pb Na Pb Na Pb Na Pb Na»Pb Na Pb NaPb NasPb 2

B

4

6

2

4

4

4

2

CaPb Mg Pb 2

NaMgPb (NaK)Pb' a 6 c

Representative Yields of Tetralkyllead from Lead Alloys Alkyl Halide MeCl, MeBr, M e l , EtBr, or E t I EtCl MeCl EtCl EtCl EtCl EtCl EtCl EtCl Et S0 Et S0 Et S0 EtCl EtCl EtCl EtBr EtBr EtI EtI EtCl EtCl EtCl EtCl 2

4

2

4

2

4

Catalyst, % Based on Alloy Wt.

R P b Yield, % 4

— — 4AlCh« —

— — — 0.5C H OOCCH 2

6

3

2

6

3

0.5C H OOCCH — — lPbI * — — 5 E t 0 , 85EtI 2

2

— 15Et 0 2

— 10Et O — 5Et 0 — 0.2CHaCOCH 2

2

3

0 85 85 60 5 0 0 70 0 75 10 80 80 5 85 10 60 75 85 20 75 25 >95

Atom % Al, based on Na in alloy. Atom % I , based on Pb in alloy. Equiatomic proportions of lead and alkali metals, in which 15 atom % of the alkali metal is K . 2

Discussion I t is c l e a r f r o m t h e r e a c t i o n s d e s c r i b e d h e r e i n , t h a t t h e r e is a l a r g e v a r i e t y of possible r e a c t i o n s of l e a d m e t a l o r a l l o y s t o f o r m t e t r a a l k y l l e a d c o m p o u n d s f r o m a l k y l h a l i d e s . T h i s choice encompasses a c o n s i d e r a b l e degree of d i v e r s i t y i n t h e f o r m of t h e m e t a l l i c l e a d , t h e w h o l e g a m u t of r e d u c i n g m e t a l s i n t h e i r m a n y p e r m u t a t i o n s a n d c o m b i n a t i o n s , i n c l u d i n g i n t e r m e t a l l i c c o m p o u n d s of l e a d ; t h e v a r i a t i o n i n t y p e s of a l k y l h a l i d e s , t h e choice o f o r g a n o m e t a l l i c reagents a n d t h e m a n i f o l d possible c a t a l y s t a n d s o l v e n t s y s t e m s . T h e r e a c t i v i t y of t h e different m e t a l - a l k y l h a l i d e s y s t e m s v a r i e s w i d e l y . T h e r e a c t i v i t y e v i d e n t l y depends u p o n : t h e n a t u r e of t h e surface a n d t h e p o r o s i t y of t h e l e a d m e t a l ; t h e l a t t i c e s t r u c t u r e , c r y s t a l size, a n d t h e r m a l h i s t o r y of t h e a l l o y ; t h e a c c e l e r a t o r s o r poisons p r e s e n t i n t h e heterogeneous s y s t e m a n d t h e m o l e c u -



SHAPIRO—TETRAALKYLLEAD COMPOUNDS

297

lar dimensions of the alkyl halide, as well as on the chemical activity of the metal and halide atoms. Many inert systems, such as the system sodium-lead-methyl chloride, respond rapidly to the addition of catalysts. Despite the significant exception of the sodiumlead-methyl chloride system, it appears that methyl compounds react better than ethyl compounds. In general, iodides are superior to bromides and to chlorides as alkylating agents. If iodine were a relatively inexpensive element, it would probably be used for the commercial manufacture of tetraethyllead. The mechanism of the reactions described above is as yet unknown. Unpublished evidence (3) has been obtained that a layer of alkyl halide is adsorbed onto the lead surface, and this is evidently the first step in the reaction. Beyond this, factual infor mation is as yet unavailable. Correspondingly, there is an unpredictably wide varia tion in the yields of the tetraalkyllead product. Patently, in the preparation of tetraethyllead there are many possible ways to reduce the recycling of lead metal, to replace sodium by other metals or to eliminate it, and to obtain more tetraethyllead per autoclave charge per unit time. The reaction of monosodium-lead alloy with ethyl chloride as it is carried out today is unique in its combination of favorable characteristics. Other reactions that have been proposed as replacements carry such liabilities as higher temperature, recovery problems, catalyst problems, lower yield with reducing metal, more expensive halide, etc., factors which detract from their commercial attractiveness. Acknowledgment Several of the author's colleagues at the laboratories of the Ethyl Corp. have co operated in the experimentation reviewed in this paper. To these people thanks are due. Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)

Beste, G. W. (to Ethyl Corp.), U. S. Patent 2,664,605 (Jan. 5, 1954). Beste, G. W., Tanner, H . M., Shapiro, H., Ibid., 2,653,159 (Sept. 22, 1953). Brockway, L., University of Michigan, Ann Arbor, Mich., private communication. Buckton, G. F., Ann. 109, 218 (1859). Cahours, Α., Compt. rend. 36, 1001 (1853). Calingaert, G., Chem. Revs. 2, 43 (1925). Calingaert, G., Beatty, Η. Α., J. Am. Chem. Soc., 61, 2748 (1939). Calingaert, G., Beatty, H . A. (to Ethyl Corp.), U. S. Patent 2,270,108 (Jan. 13, 1942). Ibid., 2,270,109 (Jan. 13, 1942). Calingaert, G., Shapiro, H . (to Ethyl Corp.), Ibid., 2,535,190 (Dec. 26, 1950). Ibid., 2,535,191 (Dec. 26, 1950). Ibid., 2,535,192 (Dec. 26, 1950). Ibid., 2,535,193 (Dec. 26, 1950). Ibid., 2,558,207 (June 26, 1951). Ibid., 2,562,856 (July 31, 1951). Ibid., 2,591,509 (April 1, 1952). Calingaert, G., Shapiro, H., Krohn, I. T., J. Am. Chem. Soc. 68, 520 (1946). Ibid., 70, 270 (1948). Clem, W. J. (to Ε. I. du Pont de Nemours & Co.), U. S. Patent 2,464,399 (March 15, 1949). (20) Clem, W. J., Plunkett, R. J., Ibid., 2,464,398 (March 15, 1949). (21) Ibid., 2,515,821 (July 18, 1950). (22) Clem, W. J., Podolsky, H., Ibid., 2,426,598 (Sept. 2,1947). (23) Gilman, H., Jones, R. G., J. Am. Chem. Soc. 72, 1760 (1950). (23A) Gittins, T. W., Mattison, E. L . (to Ε. I. du Pont de Nemours & Co.), U . S. Patent 2,763,673 (Sept. 18, 1956). (24) Hansen, M., "Der Aufbau der Zweistofflegierungen, Julius Springer, Berlin, 1936. (25) Holbrook, G. E . (to Ε. I. du Pont de Nemours & Co.), U . S. Patent 2,464,397 (March 15, 1949). (26) Kraus, C. Α., Callis, C. C. (to Standard Oil Development Co.), Ibid., 1,612,131 (Dec. 28, 1926). ,,


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(27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40) (41) (42) (43) (44) (45) (46) (47) (48)

ADVANCES IN CHEMISTRY SERIES Krohn, I. T. (to Ethyl Corp.), Ibid., 2,727,053 (Dec. 13, 1955). Krohn, I. T., Shapiro, H., Ibid., 2,555,891 (June 5, 1951). Ibid., 2,594,183 (April 22, 1952). Krohn, I. T., Werner, R. C., Shapiro, H., J. Am. Chem. Soc. 77, 2110 (1955). Leeper, R. W., Summers, L., Gilman, H., Chem. Revs. 54, 101 (1954). Löwig, C., Chem. Zentr. 1852, 575. Madden, H . J. (to Ethyl Corp.), U . S. Patent 2,727,052 (Dec. 13, 1955). McDyer, T. W., Closson, R. D., Ibid., 2,571,987 (Oct. 16, 1951). Pearsall, H . W., Ibid. 2,414,058 (Jan. 7,1947). Pfeiffer, P., Truskier, P., Ber. deut. chem. Ges. 37, 1125 (1904). Plunkett, R. J. (to Ε. I. du Pont de Nemours & Co.), U . S. Patent 2,477,465 (July 26, 1949). Pyk, S. C. (to Associated Ethyl Co., Ltd.), Ibid., 2,561,636 (July 24, 1951). Shapiro, H . (to Ethyl Corp.), Ibid., 2,535,235 (Dec. 26, 1950). Ibid., 2,535,236 (Dec. 26, 1950). Ibid., 2,535,237 (Dec. 26, 1950). Ibid., 2,597,754 (May 20, 1952). Shapiro, H., De Witt, E. G., Ibid., 2,575,323 (Nov. 20, 1951). Ibid., 2,635,106 (April 14, 1953). Shapiro, H., Krohn, I. T., Ibid., 2,688,628 (Sept. 7, 1954). Sullivan, F. W., Chalkley, L. (to Standard Oil Co. of Indiana), Ibid., 1,611,695 (Dec. 21, 1926). Tanner, H . M . (to Ethyl Corp.), Ibid., 2,635,105 (April 14, 1953). Ibid.,2,635,107 (April 14,1953). RECEIVED for review May 10, 1957. Accepted June 1, 1957.


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