Epoxy Resin Chemistry - American Chemical Society

15 Some Studies on the Preparation of Glycidyl 2-Ethylhexanoate D. A. CORNFORTH, B. G. COWBURN, Κ. M. SMITH, C. W. STEPHENS, and C. G. TILLEY Downloaded by PURDUE UNIVERSITY on May 31, 2013 | http://pubs.acs.org Publication Date: December 3, 1979 | doi: 10.1021/bk-1979-0114.ch015

Anchor Chemical Company Ltd., Clayton, ManchesterM114SR, England The preparation of glycidyl 2-ethylhexanoate has been studied in order to gain an insight into the general mechanism of the reaction of epichlorohydrin with carboxylic acids. The investigation was carried out in two stages. The first stage of the reaction involves the catalysed addition of 2-ethylhexanoic acid to epichlorohydrin to yield 2-hydroxy-3-chloropropyl 2-ethylhexanoate. The reaction is then completed by dehydrochlorination of 2-hydroxy-3-chloropropyl 2-ethyl-hexanoate to yield glycidyl 2-ethylhexanoate (Fig. 1). This compound was selected in order to give intermediates and products of acceptable volatility to allow gas chromatography to be used as the principal analytical tool. Results and Discussion Reaction of Epichlorohydrin with 2-Ethylhexanoic Acid The cetyl trimethy1ammonium bromide catalysed addition of epichlorohydrin to 2-ethylhexanoic acid has been studied at levels of 1, 5 and 7 molar ratios of epichlorohydrin to 2ethylhexanoic acid. The reactions were carried out in toluene in order to facilitate removal of water from the final product (glycidyl 2-ethylhexanoate) by azeotropic distillation. A further advantage in using toluene as solvent is that where glycidyl esters of high molecular weight are being considered the solution viscosities may be kept sufficiently low to effect filtration, i f required, before final distillation. The reaction products were determined by gas chromatography. The rate of consumption of the starting materials was also determined by gas chromatography and other standard analytical techniques.

0-8412-0525-6/79/47-114-211$05.00/0 © 1979 American Chemical Society

In Epoxy Resin Chemistry; Bauer, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

212

Downloaded by PURDUE UNIVERSITY on May 31, 2013 | http://pubs.acs.org Publication Date: December 3, 1979 | doi: 10.1021/bk-1979-0114.ch015

EPOXY RESIN CHEMISTRY

Figure 2.

Reaction of epichlorohydrin with 2-ethylhexanoic acid (1:1 mol ratio)


15.

CORNFORTH ET AL.

Glycidyl 2-Ethylhexanoate

213


1;1 Molar Ratio of E p i c h l o r o h y d r i n to 2-Ethylhexanoic A c i d The concentration-time curve f o r the reactants i s shown i n F i g . 2, and the concentration-time curve f o r the products i s depicted i n F i g . 3. Examination of F i g . 3 shows that as w e l l as the d e s i r e d r e a c t i o n product (the c h l o r o h y d r i n e s t e r ) , there are two other major product components. These were i d e n t i f i e d as 1,3-dichloropropan-2-ol (OL-dichlorohydrin) and g l y c e r o l 1,3-di(2-ethylhexanoate) (hydroxy d i e s t e r ) . The s t r u c t u r e of the hydroxy d i e s t e r was confirmed by i t s independent synthesis from g l y c i d y l 2-ethyl hexanoate and 2-ethyl hexanoic a c i d . The y i e l d of c h l o r o h y d r i n e s t e r was only 50.4%, the remainder of the 2-ethylhexanoic a c i d being converted to the undesirable hydroxy diester. F i g . 4 shows the s t r u c t u r e of the products obtained i n the 1:1 r e a c t i o n . F i g . 5 d e p i c t s the formation of the hydroxy d i e s t e r from g l y c i d y l 2-ethylhexanoate and 2 - e t h y l hexanoic a c i d . A mechanism which i s c o n s i s t e n t with the experimental observations f o r the production of the c h l o r o h y d r i n e s t e r i s given i n F i g . 6. The f i r s t stage of the r e a c t i o n i n v o l v e s i o n i s a t i o n of 2-ethylhexanoic a c i d by the c a t a l y s t c e t y l t r i methy1ammonium bromide. Subsequent attack of the carboxyanion on e p i c h l o r o h y d r i n leads u l t i m a t e l y to the d e s i r e d r e a c t i o n product of the f i r s t stage of the r e a c t i o n . Examination of F i g . 3 shows that the r a t e of formation of O^-dichlorohydrin and the hydroxy d i e s t e r c l o s e l y f o l l o w each other i n d i c a t i n g that t h e i r formation i s interdependent. The by-product formation seems best accommodated by the mechanism shown i n F i g . 7 which involves breakdown of the intermediate chloro-oxy anion by i n t e r n a l n u c l e o p h i l i c displacement, to y i e l d g l y c i d y l 2 - e t h y l hexanoate. The g l y c i d y l 2-ethylhexanoate subsequently r e a c t s with the carboxyanion to y i e l d u l t i m a t e l y the hydroxy d i e s t e r . TheOC-dichlorohydrin may be formed by c h l o r i d e i o n attack on e p i c h l o r o h y d r i n with subsequent p r o t o n a t i o n of the dichloro-oxy anion.1" Further evidence f o r t h i s mechanism i s a l s o provided by the d i r e c t synthesis of the hydroxy d i e s t e r from g l y c i d y l 2ethylhexanoate and 2-ethylhexanoic a c i d ( F i g . 5 ) . I t i s also p o s s i b l e that the c h l o r o h y d r i n may break down thermally to give the undesirable by-products ( F i g . 8). Summary of 1:1 Reaction The r e a c t i o n of e p i c h l o r o h y d r i n with 2-ethylhexanoic a c i d i n a 1:1 molar r a t i o gives r i s e to only 50% of the d e s i r e d product, the c h l o r o h y d r i n e s t e r . The mechanism depicted above i n d i c a t e s that the undesirable hydroxy d i e s t e r i s formed by r e a c t i o n of 2-ethylhexanoic a c i d with g l y c i d y l 2-ethylhexanoate. I t appears therefore that formation o f the hydroxy d i e s t e r competes d i r e c t l y with the c h l o r o h y d r i n e s t e r formation ( F i g . 9 ) .


214



2-5 h

0

J

I

I

I

1

2

3

4

'

5

ι

6

ι

7

I

8

I

9

I

I

10

11

REACTION TIME - HRS

Figure 3.

Reaction of epichlorohydrin with 2-ethylhexanoic acid (1:1 mol ratio; product composition)

(?)

R - C O - O - C H 2 - C H - CH I I OH C£

w

2

2-HYDROXY - 3 CHLOROPROPYL - 2·ETHYLHEXANOATE (

1

^CHLOROHYDRIN^

@

C!-CH -CH-CH -Ci 2

2

OH 1,3 - DICHLOROPROPAN -2 -OL *α -

(3)

R

DICHLOROHYDRIN^cc-DCH

-CO-O-CH2-CH-CH2-O-CO-R OH

GLYCEROL -1,3- Dl -^2 - ETHYLHEXANOATE^ r

HYDROXYDIESTER

Figure 4.

H DE

1

Product structures



CORNFORTH ET AL.

RC0 H +RCO2 C H C H - C H — • R C 0 C H 2

2

2

V

2

2

CHCH C0 R O'H 2

2

R = C H g 0 CH 4

C H 2


Figure 5.

Glycerol-l,3-di-2-ethylhexanoate synthesis

1 R*NBr + CH2-CH-CH2CI

1· • BrCHi-CH-CHiCI + R4N

Br C H - C H -CH2CI + R C0 H 2

• B r C H - CH - C H C I

2

Ο

5

2

θ

+RCcf

2

2

OH

CH -CH-CH CI \ / 0 2

2

+ RCO®

• R C0 - CH - CH -CH CI

J

RCO H[°

2

Z

2

RC0

Figure 6.

2

+

2

R C 0 - C H - CH-CH2CI I OH 2

2

Proposed reaction mechanism Part A

RCO2- C H - C H - C H c T 2

2

/

->RC0 -CH -CH-CH +Cl

%

2

2

2

V

RCO®

ECH CICH CHCH CI I

|R=CH CH CH CH CH (C H,)-J 3

2

2

2

2

2

2

RC0 CH CHCH 0 CR I O RCO2H RC0 H 2

2

2

2

ft

2

R C0 + R C0 CH CH CH 0 CR « 2

2

2

2

OH Figure 7.

2

CICH CHCH CI + RC0 2

2

2

OH

Proposed reaction mechanism Part Β


216


R C 0 C H C H C H Cl I OH 2

2

• R C0

2

C H CH C H + \ /

2

2

HCI

2

0

,

ECH

RC0 H 2


RC0 CH CHCH I OH 2

2

2

0

Figure 8.

2

CR

4-

CICH2 CHCHi CI I OH

By-product formation

C H - C H C H CI 2

2

V - • R C0

R CO2 H

C H CH C H C I I OH

2

2

H

- • R C0 CH 2

R C0

2

CH C H - C H 2

2

2

CH C H C 0 R I OH 2

2

2

V Figure 9.

Competition reaction

CHLOROHYDRIN

REACTION

Figure 10.

TIME - HRS



15.

CORNFORTH

E T A L .


217

I t i s apparent t h e r e f o r e that an increase i n the e p i c h l o r o h y d r i n concentration should promote the formation of the c h l o r o h y d r i n e s t e r , at the expense of the hydroxy d i e s t e r .


5:1 and 7:1 Molar Ratio of E p i c h l o r o h y d r i n to 2-ethylhexanoic a c i d The concentration-time curve f o r the products of the 5:1 r e a c t i o n are shown i n F i g . 10 and that f o r the 7:1 r e a c t i o n depicted i n F i g . 11. As expected the c h l o r o h y d r i n e s t e r i s produced almost e x c l u s i v e l y a t the expense of the hydroxy d i e s t e r , the y i e l d of the l a t t e r compound being reduced to 2.9% i n the 5:1 r e a c t i o n and 1.4% i n the 7:1 r e a c t i o n (ca 50% i n the 1:1 r e a c t i o n ) . Examination of F i g s . 10 and 11 show that the c h l o r o h y d r i n e s t e r formation reaches a maximum a f t e r 3 to 4 hours, t h i s maximum c o i n c i d i n g with the consumption o f a l l the 2—ethylhex— anoic a c i d . I n t e r e s t i n g l y i f the r e a c t i o n i s allowed to continue f o r an excess p e r i o d the concentration of the c h l o r o h y d r i n e s t e r begins to decrease i n the r e a c t i o n mixture. The decrease i n c h l o r o h y d r i n e s t e r concentration i s concomitant with the formation of g l y c i d y l 2-ethylhexanoate and i s accompanied by the formation ofOC-dichlorohydrin. This shows that the c h l o r o h y d r i n i s breaking down (thermally or c a t a l y t i c a l l y ) as depicted i n F i g . 12. At t h i s p o i n t i n the r e a c t i o n a l l the a c i d has been consumed and i t i s t h e r e f o r e not a v a i l a b l e to underto f u r t h e r r e a c t i o n with the epoxy compound to give the hydroxy d i e s t e r as i n the 1:1 r e a c t i o n . At f i r s t s i g h t i t may not seem undesirable to allow the r e a c t i o n to continue, i n order to allow conversion o f the c h l o r o h y d r i n e s t e r to the epoxy compound ( F i g . 12) as the u l t i m a t e aim i s , i n f a c t , to form g l y c i d y l 2-ethylhexanoate by r i n g c l o s u r e of the c h l o r o h y d r i n e s t e r intermediate. However i t was noted that formation of the epoxy compound i s accompanied by formation o f 0 ( ^ d i c h l o r o h y d r i n , and i t was shown i n a separate experiment that f u r t h e r r e a c t i o n of these two compounds could be e f f e c t e d under the r e a c t i o n c o n d i t i o n s , probably g i v i n g r i s e to f u r t h e r undesirable by-products ( F i g . 13). Conclusion The products formed during the r e a c t i o n of e p i c h l o r o h y d r i n with 2-ethylhexanoic a c i d are governed by the r e l a t i v e concentrations of the two r e a c t a n t s . E p i c h l o r o h y d r i n must be present i n excess i n order to achieve a high y i e l d of c h l o r o hydrin e s t e r . In order to optimise the y i e l d of c h l o r o h y d r i n e s t e r the r e a c t i o n must be terminated when a l l the a c i d has been consumed otherwise the r e a c t i o n i s complicated by decomposition and f u r t h e r r e a c t i o n s of t h i s m a t e r i a l .


218



CHLOROHYDRIN

2

3

A

5

6

REACTION TIME - HRS

Figure 11.


R C0 CH 2

C H

2

2

CH C H C I I OH

- C H C H

• R C0

2

2

C I

+

H*

2

C H

2

C H

-

C H

\

2

+

H*

/

+

cf

Ο

+

C I

6

— • C I C H Î

C H C H

2

C I

1

\ / Ο

OH

Figure 12.

Chlorohydrin ester decomposition

R C 0 C H CH - C H + ( θ C H ) CH OH 2

2

2

V Figure 13.

2

INTRACTABLE LIQUID

Further by-product formation


15.

CORNFORTH ET AL.



Stage I I Ring Closure

219

Reaction

In order to study the r i n g c l o s u r e r e a c t i o n i n d e t a i l a masterbatch of the c h l o r o h y d r i n ester was prepared i n 50% toluene s o l u t i o n using a 5:1 molar excess of e p i c h l o r o h y d r i n to 2-ethylhexanoic a c i d . The f i r s t r i n g c l o s u r e process i n v e s t i g a t e d used a technique whereby 50% aqueous sodium hydroxide was added at a constant r a t e under c o n d i t i o n s where the water was removed by azeotropic d i s t i l l a t i o n with the excess e p i c h l o r o h y d r i n and toluene s o l v e n t , g i v i n g a d e h y d r o c h l o r i n a t i o n under e s s e n t i a l l y anhydrous c o n d i t i o n s . F i g . 14 shows the conversion of c h l o r o h y d r i n e s t e r to g l y c i d y l 2-ethylhexanoate using two d i f f e r e n t rates of a d d i t i o n of sodium hydroxide s o l u t i o n (0.25 mol. h r . ~ l and 0.125 mol. h r . ~ " l ) . E x c e l l e n t conversion to g l y c i d y l 2-ethylhexanoate was obtained which may be seen to be r e l a t i v e l y independent of the r a t e of sodium hydroxide a d d i t i o n . However i n order to a t t a i n high y i e l d s (>99%) i t may be seen that 40-50% excess (based on c h l o r o h y d r i n ester) sodium hydroxide was r e q u i r e d . The e p i c h l o r o h y d r i n concentration was a l s o followed during the r e a c t i o n and i t can be seen ( F i g . 14) that t h i s component was a l s o consumed during the r i n g c l o s u r e r e a c t i o n (12-14% conversion). I t can be seen therefore that e p i c h l o r o h y d r i n competes with the c h l o r o h y d r i n e s t e r f o r sodium hydroxide during the d e h y d r o c h l o r i n a t i o n r e a c t i o n . Two competing r e a c t i o n s are e s s e n t i a l l y taking place and indeed i t was found that the excess sodium hydroxide r e q u i r e d was d i r e c t l y a t t r i b u t a b l e to h y d r o l y s i s of e p i c h l o r o h y d r i n . Epichlorohydrin hydrolysis i s h i g h l y undesirable i n the r i n g c l o s u r e r e a c t i o n as i t leads to polymer formation i n the f i n a l product(2,3). The polymeric m a t e r i a l was i s o l a t e d from the r e a c t i o n mixture as a c o l o r l e s s water i n s o l u b l e m a t e r i a l . The polymer was t e n t a t i v e l y assigned the c r o s s - l i n k e d s t r u c t u r e depicted i n F i g . 15 based on mass spectrometric evidence. Interestingly the mass spectrum d i d not show any c h l o r i n e c o n t a i n i n g s t r u c t u r e s i n the polymer showing that sodium hydroxide was i n f a c t h y d r o l y s i n g the c h l o r i n e atom of e p i c h l o r o h y d r i n e i t h e r before or a f t e r p o l y m e r i s a t i o n . In an attempt to overcome e p i c h l o r o h y d r i n h y d r o l y s i s i n the r i n g c l o s u r e r e a c t i o n an a l t e r n a t i v e approach using sodium hydroxide/sodium carbonate s o l u t i o n was sought. A patent(4) i s held by the Dow Chemical Company which describes the use of a s o l u t i o n of an a l k a l i metal hydroxide and an a l k a l i metal carbonate to e f f e c t d e h y d r o c h l o r i n a t i o n . The patent claims that using t h i s technique e p i c h l o r o h y d r i n h y d r o l y s i s i s g r e a t l y reduced. A c c o r d i n g l y t h i s technique was i n v e s t i g a t e d whereby a mixture c o n t a i n i n g sodium carbonate (9.8%), sodium hydroxide (15.7%) and water (74.5%) was s t i r r e d with the c h l o r o h y d r i n e s t e r masterbatch prepared above. The r e a c t i o n was studied i n




220

MOLE RATIO-SODIUM HYDROXIDE TO CHLOROHYDRIN

Figure 14. Conversion of chlorohydrin and epichlorohydrin in the presence of sodium hydroxide under azeotropic conditions: ( ) NaOH added 0.25 mol/hr; ( ) NaOBadded 0.125 mol/hr

CH f 2

CH

Χ-£θ CH

I 2

CH 2

J—F

X=C H,CHC0 4

C H 2

Figure 15.

5

Ο

CH

2

CH

0

CH

2

CH - -

or

H - CH ~ *) ( C H , CHCC0 2

4

2

C H 2

2

CHO -

5

Polymer isolated from product



15.

coRNFORTH ET AL.


221

d e t a i l at 40°C and 75°C using a molar r a t i o of sodium hydroxide to c h l o r o h y d r i n e s t e r of 2:1. The r e a c t i o n s were monitored by gas chromatography. F i g . 16 shows the r a t e of conversion of c h l o r o h y d r i n e s t e r to g l y c i d y l 2-ethylhexanoate and the r a t e of e p i c h l o r o h y d r i n h y d r o l y s i s at both temperatures. I t may be seen that at 75°C e p i c h l o r o h y d r i n h y d r o l y s i s i s i n f a c t the major r e a c t i o n (51.7% conversion) and that only 30% conversion to g l y c i d y l 2-ethylhexanoate i s obtained. The graph c l e a r l y shows that at 75°C the low y i e l d i s not due to consumption of the c h l o r o h y d r i n ester i n an undesirable manner, but i s simply due to removal of sodium hydroxide by r a p i d h y d r o l y s i s of e p i c h l o r o h y d r i n . The r e a c t i o n at 40°C was more encouraging. A f t e r 6.5 hours, at which time a l l the sodium hydroxide had been consumed, a 75% conversion of c h l o r o h y d r i n e s t e r to g l y c i d y l 2-ethylhexanoate was obtained. However, i t can be seen that even at 40°C e p i c h l o r o h y d r i n h y d r o l y s i s i s o c c u r r i n g (15.6% conversion) and a l a r g e amount of sodium hydroxide i s consumed i n t h i s r e a c t i o n . The organic phase from the above r e a c t i o n was removed and r e t r e a t e d with a f u r t h e r p o r t i o n of the sodium hydroxide/ sodium carbonate mixture and the r e a c t i o n continued at 40 C. A high conversion to g l y c i d y l 2-ethylhexanoate was u l t i m a t e l y achieved by t h i s technique (>99%) although by t h i s time the e p i c h l o r o h y d r i n h y d r o l y s i s (24.3%) was considered to be too high f o r t h i s method to be used commercially. In an attempt to reduce the o v e r a l l e p i c h l o r o h y d r i n h y d r o l y s i s the e f f e c t of removal of t h i s component from the r e a c t i o n system was i n v e s t i g a t e d . A sample of the above r e a c t i o n mixture was removed a f t e r the i n i t i a l treatment w i t h sodium hydroxide/sodium carbonate s o l u t i o n and the excess e p i c h l o r o h y d r i n and toluene removed by vacuum d i s t i l l a t i o n . The r e s u l t i n g mixture of c h l o r o h y d r i n e s t e r and g l y c i d y l 2ethylhexanoate was r e s o l v a t e d with toluene and r e t r e a t e d with a f u r t h e r p o r t i o n of sodium carbonate s o l u t i o n at 40 C. However i t was found that no f u r t h e r conversion of the c h l o r o h y d r i n e s t e r to epoxy compound was obtained i n d i c a t i n g that e p i c h l o r o h y d r i n i s required i n the r e a c t i o n system to effect r i n g closure. Indeed, i n a patent h e l d by the Dow Chemical Company(1)it i s observed that e p i c h l o r o h y d r i n (or other s u i t a b l e 1,2epoxide) i s r e q u i r e d i n order to o b t a i n high y i e l d s of g l y c i d y l esters from 2-hydroxy-3-chlorophenyl e s t e r s . A mechanism i n v o l v i n g transepoxidation was proposed ( F i g . 17). No polymer formation was observed i n the above r e a c t i o n ; however, some water s o l u b l e organic components were detected by gas chromatography which were presumably derived from h y d r o l y s i s of e p i c h l o r o h y d r i n . A n a l y s i s of the aqueous residues showed that the sodium carbonate was e s s e n t i a l l y unchanged during the r e a c t i o n , which lead to examination of




222

Figure 16. The % conversion of chlorohydrin and % epichlorohydrin consumed at 40°C and 75° C in sodium hydroxide/sodium carbonate aqueous solution

RCOOCH

2

CHCH I OH

2

CI

R CO 0 CH2 CH - C H

V R —C

Epoxy Resin Chemistry - American Chemical Society

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