RESEARCH
Acetylenics Dodge NH3 A i r c o uncovers a better w a y t o a l k y l a t e sodium a c e t y l î d e : U s e organic diluent, not N H 3
vides a simple system for alkylating sodium acetylide in h i g h yield with alkyl bromides, T. F . Rutledge told the Division of Organic Chemistry. The classic route t o sodium acetyl ide calls for sodium metal or sodium amide to react with acetylene in t h e presence of liquid ammonia. Several years ago, Rutledge. w h o is now with Atlas Powder, set out to improve this reaction, eventually found that xylene could b e used in place of ammonia. After this success, h e tried the same approach with acetylide reactions which previoush had been carried out in ammonia. Rutledge found that so dium acetylide in xylene would easily react with dimethyl sulfate. In fact, an unexpected plus value turned u p : Both alkyl groups reacted completely with the acetylide. I n past work with ammonia, only one group could b e made to take part in the reaction. Rutledge got over 9 0 ' ί conversion of sodium acetylide, with 80 to 85' < go ing to propyne and 10'f ending up as 2-butyne. The next step was to see if an alkyl halide could react in the simple xylene system. The answer was n o . Other common organic diluents didn't work either. But Rutledge felt h e was on the right track, screened a long list of
ACS NATO I NAL MEETN IG
135
A month after Air Reduction announced plans to widen the market for acetylene derivatives (C&EN, March 2 3 , p a g e 2 2 ) , the company says it's o n t h e trail of Λ new way to make mtonoalkyl acetylenes—basic intermedi ates w h i c h can be used to prepare acids, ketones, olefins, and other acetylenics. Kiev point of the process: Use organic diluents in place of hard-to-handle hVniirl ammonia. Monoalkyl acetylenes (1-alkynes) are usually m a d e by letting sodium acetylic3e react with alkyl halides or sulfates, because liquid ammonia has been the only practical diluent for the system, tine reaction is not a good route to commercial acetylene derivatives. Now, trie work at Airco shows that a mixture o>f xylene and dimethyl formamide pro
materials, and wound u p with t h r e e diluents that would work—dimethyl formamide, hexamethyl phosphoramide, and dimethyl acetamide. Dimethyl formamide ( D M F ) was slightly more effective than the others and also t h e least expensive. It was picked for further tests. To evaluate D M F , Rutledge chose the reaction of butyl bromide with sodium acetylide to form 1-hexyne. After a few runs with only fair yields, he hit upon the idea of using D M F mixed with other organics. About 40' < D M F in xylene turned out to b e the best combination. This was a n ideal situation because the sodium acetylide was also prepared in xylene. In general, the DMF-xylene ^xsteiu gives yields equal to or better than liquid ammonia, without the unstable by-products which sometimes turn u p when ammonia is used. Rutledge says that increased solu bility of sodium acetylide is partly re sponsible for the good results in mixed diluents. However, he adds that other factors, such as solvent polarity and solubility of intermediates in the reac tion, must also be involved, and more work is needed to explain completely why the organic system works as well as it does.
Properties Reflect Crystal Difference ACS NATIONAL MEETING Polymer Chemistry
Behind the difference in t h e physical properties of polyethylene and polypropy lene lies a difference in crystal structure. With electron micrographs of single crystals of polyethylene and polypropylene magnified 25,000 times, B. G. Ranby and Ε . Ε . Morehead of American Viscose pointed out this difference to the Divi sion of Polymer Chemistry in a symposium on morphology and motion in high polymers. Linear polyethylene ( t o p ) , they explain, forms lamellar crystals which develop as diamond-shaped single crystals. The thin flake forms a spiral (flake thickness depends mainly on crystallization temperature, ranges from around 110 to about 170 Α.). T h e polymer chains fold back and forth regularly to form the flake; thus, the molecules are oriented at right angles to the plane of the flake. Such a crystal forms easily, Ranby points out, and the flake layers can easily slip and deform. This, he says, probably accounts for the easy deformation of poly ethylene. The crystal of isotactic polypropylene ( b o t t o m ) , on the other h a n d , is quite complex by comparison. It can also b e made to grow as thin flakes, Ranby explains, but it is m o r e irregular in shape than polyethylene and much h a r d e r to form. I n this case, the chains are stiffer, and they crystallize as helixes with three monomer units per turn. Since die folding and packing of polypropylene chains is more restricted a n d difficult than that of polyethylene, its crystal lattice doesn't deform as easily. T h e s e differences have an important bearing on the mechanical properties of the t w o polymers. 44-
C&EN
APRIL
2 0,
1959
