Monohydric Alcohols - American Chemical Society

1 Chemistry of Monohydric Alcohols

Downloaded by 80.82.78.170 on January 11, 2017 | http://pubs.acs.org Publication Date: June 15, 1981 | doi: 10.1021/bk-1981-0159.ch001

F. M. BENITEZ Exxon Chemical Company, Specialties Technology Division, P.O. Box 241, Baton Rouge, LA 70821

Alcohols can be regarded as hydroxyl derivatives of hydrocarbons. They can be characterized by the number of hydroxyl groups (monohydric, dihydric, etc.), according to their structure (primary, secondary or tertiary), and by the structure of the hydrocarbon function to which the hydroxyl is attached (aliphatic, cyclic, saturated or unsaturated). This chapter is concerned almost exclusively with the chemistry of saturated aliphatic monohydric alcohols with particular emphasis on the reactions used in the conversion of these alcohols to other useful compounds. Manufacture of many of the alcohols is covered in other chapters. A c i d i t y and B a s i c i t y A l c o h o l s a r e amphoteric and thus can f u n c t i o n both as weak Br^nsted a c i d s and as bases: R-OH + t

R-0

R-OH + HA ^

R0H

+ ZH (ROH a c t i n g as an acid) 2

+

(ROH a c t i n g as a base)

The a c i d i t y of the hydroxyl group can be seen i n the r a p i d proton-deuteron exchange that can take place when a l c o h o l s are d i s s o l v e d i n D^O (Reaction I ) , a l k a l i metals (Reaction I I ) and organometallic reagents (Reaction I I I and I V ) . (I) (ID

(III) (IV)

0097-6156/81/015 9-0001 $05.00/ 0 © 1981 American Chemical Society Wickson; Monohydric Alcohols ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2

MONOHYDRIC ALCOHOLS

The most complete measurements of the a c i d i t y of a l c o h o l s i n water were made some time ago by Long and B a l l i n g e r (1,2_,3) u s i n g c o n d u c t i v i t y methods. The pKa values f o r s u b s t i t u t e d methanols (RCH OH) (2) are a l i n e a r f u n c t i o n of the T a f t a* constants (4,J5; f o r the R s u b s t i t u e n t s , a l l o w i n g the p r e d i c t i o n of the a c t u a l pKa by using the formula: pKa = 15.9 - 1.42 a*. In recent years the question of a c i d i t y and b a s i c i t y has been reopened by the development of techniques to measure them i n the gas phase(6^). The r e s u l t s a v a i l a b l e reemphasize the f a c t that s o l v a t i o n f a c t o r s have a profound i n f l u e n c e on the course of acid-base r e a c t i o n s . Brauman and B l a i r have determined (6) that the a c i d i t y of some s u b s t i t u t e d a l c o h o l s increases as the s i z e and number of s u b s t i t u e n t s i n c r e a s e . This i s e x a c t l y the opposite e f f e c t seen i n s o l u t i o n measurements. The c o n c l u s i o n that must be deduced from t h i s i s that there are two kinds of a c i d i t y that must not be confused: a) an i n t r i n s i c a c i d i t y , which i s best approximated by gas-phase measurements and which r e f l e c t s the p r o p e r t i e s of the ions and the molecules i n i s o l a t i o n , and (b) a p r a c t i c a l l i q u i d phase a c i d i t y i n which s o l v a t i o n e f f e c t s p l a y a very important r o l e . In the i n t e r p r e t a t i o n of s t r u c t u r e a c i d i t y r e l a t i o n s h i p s i n s o l v e n t s , the r e s u l t s w i l l probably be m i s l e a d i n g unless the s t r u c t u r e s being compared are very s i m i l a r .


2

Categories

of Chemical Reactions of

Alcohols

The f o l l o w i n g s e c t i o n s on the chemical r e a c t i o n s of a l c o h o l s has been broken down i n t o f i v e c a t e g o r i e s : (A) N u c l e o p h i l i c r e a c t i o n s of a l c o h o l s , (B) displacement of the hydroxyl group, (C) dehydration of a l c o h o l s , (D) o x i d a t i o n of a l c o h o l s , and (E) a n a l y t i c a l determination of the hydroxyl group. Under each one of these c a t e g o r i e s the d i f f e r e n t types of r e a c t i o n s are organized i n a l o g i c a l manner. Some examples are given, but by no means are a l l the d i f f e r e n t types of a l c o h o l s covered. The reader i s asked to extend the analogies and use the references given to pursue h i s areas of i n t e r e s t . N u c l e o p h i l i c Reactions Any species having an unshared p a i r of e l e c t r o n s may act as a n u c l e o p h i l e , whether i t i s n e u t r a l l i k e an a l c o h o l or negative l i k e the a l k o x i d e i o n . The r a t e of S ^ l r e a c t i o n s i s independent of the s t r u c t u r e and charge of the n u c l e o p h i l e . For S^2 r e a c t i o n s , f a c t o r s l i k e the charge of the n u c l e o p h i l e , i t s degree of s o l v a t i o n and n u c l e o p h i l i c i t y determine the r a t e of the r e a c t i o n (6A). The major trend i n n u c l e o p h i l i c i t y i s to p a r a l l e l base s t r e n g t h . However, sometimes d i f f e r e n c e s between b a s i c i t y and n u c l e o p h i l i c i t y of a species occur because the two are somewhat d i f f e r e n t . B a s i c i t y measures a t t a c k on hydrogen and i t i s

Wickson; Monohydric Alcohols ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

1.

3

Chemistry of Monohydric Alcohols

BENITEZ

thermodynamically c o n t r o l l e d . N u c l e o p h i l i c i t y on the other hand measures a t t a c k on carbon and i t i s k i n e t i c a l l y c o n t r o l l e d . Reactions of the A l k o x i d e Ion. The n u c l e o p h i l i c r e a c t i o n s of the a l k o x i d e i o n (RO") are very s i m i l a r to those of the hydroxide i o n (OH ) w i t h the exception that the l a t t e r has an e x t r a i o n i z a b l e proton which can l e a d to f u r t h e r r e a c t i o n a f t e r the i n i t i a l n u c l e o p h i l i c a t t a c k . The displacement of the bromide i o n from an a l k y l bromide (Reaction V) by an a l k o x i d e has been found to be a f i r s t order r e a c t i o n i n both the r e a c t a n t and s u b s t r a t e ( 7 ) . I t i s i m p l i c i t


RO

0

f

f

+ R -Br

• ROR + B r

0

(V)

i n the r e a c t i o n above that i n v e r s i o n of c o n f i g u r a t i o n w i l l take p l a c e at the a l k y l h a l i d e . The n u c l e o p h i l i c displacement of a h a l i d e by an a l k o x i d e i s commonly known as the Williamson ether s y n t h e s i s and i s s t i l l the best general method f o r the preparat i o n of symmetrical and unsymmetrical ethers (7A). The transformation of c h l o r o h y d r i n s i n t o the corresponding epoxides (Reaction VI) may be regarded as a s p e c i a l case of the W i l l i a m s o n r e a c t i o n . Many epoxides have been made t h i s way

I

I

•C - C -

I

I

CI

OH

I

I

c —

c

V

+ NaCl

(VI)

and the method i s g e n e r a l l y u s e f u l f o r the s y n t h e s i s of f i v e and six-membered r i n g s . There i s a l a r g e amount of evidence f o r an i n t r a m o l e c u l a r mechanism ( 8 ) . When the h a l i d e i s bonded to an a l l y l i c system ( C H ^ C H - C ^ - X ) an a l k o x i d e i o n w i l l r e a c t analogously to the p r e v i o u s l y described S^2 displacement on an a l k y l h a l i d e . The most s i g n i f i c a n t d i f ference i s the r a t e enhancing e f f e c t of the alkene moiety which has been a t t r i b u t e d to a decrease i n the a c t i v a t i o n energy of the r e a c t i o n (9). A second p o s s i b l e mode of r e a c t i o n i s a v a i l a b l e w i t h a l l y l i c h a l i d e s . This mode of displacement i s u s u a l l y c a l l e d S^2 and, i n g e n e r a l , w i l l be promoted r e l a t i v e to the normal displacement when there are s u b s t i t u e n t s on the alpha carbon which tend to i n h i b i t the normal S 2 pathway by i n d u c t i v e or s t e r i c e f f e c t s (Reaction V I I ) . f

N

a

—{

—

( I X )

^ =
RC0 R 4- H 0 (XII) f

2

2

2

E s t e r i f i c a t i o n i s an a c i d c a t a l y z e d r e v e r s i b l e r e a c t i o n which i s known to proceed according to the f o l l o w i n g mechanism: %R 8 © RC-OH + H

OH

n R-C-OH

R I

%

°

H

»

O0H

i * R-C-O^R

1

I H OH

o

1

R-C-OR'

•

OH

II as R-C-OR + H 0 + H 1

2

The best c a t a l y s t s f o r t h i s r e a c t i o n are i n o r g a n i c a c i d s (H SO^, HC1), organic acids ( p - t o l u e n e s u l f o n i c , methanesulfonic) and metal compounds such as t i n and t i t a n i u m d e r i v a t i v e s - e.g. tetraisopropyl titanate. To achieve good y i e l d s of products, not only i s a c a t a l y s t g e n e r a l l y necessary but a l s o the means t o d r i v e the e q u i l i b r i u m to the r i g h t as w r i t t e n i n the r e a c t i o n s above. 2



6

MONOHYDRIC ALCOHOLS

There are many ways of changing the e q u i l i b r i u m , among which are: (a) The a d d i t i o n of an excess of a r e a c t a n t , (b) the removal of the e s t e r or more commonly the water by d i s t i l l a t i o n using an azeotroping agent, and (c) the removal of water by a dehydrating agent. An example i s the commercial p r e p a r a t i o n of e t h y l acetate from an aqueous s o l u t i o n of e t h a n o l , a c e t i c a c i d and s u l f u r i c a c i d . I t happens that the l o w e s t - b o i l i n g l i q u i d i s a ternary mixture of e t h y l acetate (83.2%), ethanol ( 9 % ) , and water (7.8%). In the f i n a l steps of the process the ethanol i s removed by washing w i t h water. Many of the simpler e s t e r s can be made i n t h i s way. The n e c e s s i t y f o r the continuous removal of water can be avoided by operating i n a system composed of an aqueous and a nonaqueous l a y e r . When a mixture of a d i p i c a c i d , methanol, s u l f u r i c a c i d , and ethylene c h l o r i d e i s heated, dimethyl adipate passes i n t o the ethylene c h l o r i d e l a y e r ; the lower l a y e r contains the water (19). E s t e r s can a l s o be made i n s a t i s f a c t o r y y i e l d s by heating an a l c o h o l w i t h the ammonium s a l t of an a c i d under c o n d i t i o n s perm i t t i n g removal of both ammonia and water from the r e a c t i o n mixture. The method i s general and i s e s p e c i a l l y recommended where a c i d c o n d i t i o n s are d e l e t e r i o u s to the r e a c t a n t s . An example i s the s y n t h e s i s of 2-ethylhexyl g l y c o l a t e (20) (Reaction XIII). (Reaction C

H0CH C0 NH o

Z

o

/

Z

4

XIII)

H

2 5

+

^ H 2

5

j X H C H 0 H — • H0CH C0 CH CH o

o

Z C

Z

H

4 9

o

o

Z

Z

+ NH

+

0

v

J

C H A

H0 o

Z

9

In g e n e r a l , the a c i d c a t a l y z e d e s t e r i f i c a t i o n of organic a c i d s can be accomplished e a s i l y w i t h primary or secondary a l k y l or a r y l a l c o h o l s , but t e r t i a r y a l c o h o l s u s u a l l y give carbonium ions which lead to dehydration. The s t r u c t u r e of the a c i d i s a l s o of importance. As a r u l e , the more hindered the a c i d i s alpha to the carbonyl carbon the more d i f f i c u l t e s t e r i f i c a t i o n becomes (20A). Even more f a c i l e than the r e a c t i o n of an a c i d w i t h an a l c o h o l i s the r e a c t i o n of an a l c o h o l w i t h an a c y l h a l i d e (Reaction XIV). 0

I RC-X

+ R'OH

*RC00R

f

+ HX

(XIV)

The r e a c t i o n i s of very wide scope (21), and many f u n c t i o n a l groups do not i n t e r f e r e . A base such as p y r i d i n e i s f r e q u e n t l y added to combine w i t h the HX formed. The a l c o h o l may be primary,


1.

1


BENITEZ

secondary, o r t e r t i a r y a l k y l , or a r y l . E n o l i c e s t e r s can a l s o be prepared by t h i s method, although C - a c y l a t i o n can be a s i d e reaction. When phosgene i s the a c y l h a l i d e , h a l o f o r m i c e s t e r s (22) or carbonates may be obtained (Reaction XV). 0

0

II

II


C l - C - C l + 2R0H

• ROCOR + 2HC1

(XV)

The a l c o h o l y s i s of anhydrides (Reaction XVI) i s s i m i l a r i n scope to the r e a c t i o n of a l c o h o l s w i t h a c y l h a l i d e s . The react i o n i s c a t a l y z e d by general e s t e r i f i c a t i o n c a t a l y s t s , but u s u a l l y they are not needed unless the anhydride i s u n r e a c t i v e or the d i - e s t e r (such as a phthalate) i s the product sought. 0

0

II

II f

1

R-C-O-C-R + R OH

» RCOOR + RCOOH

(XVI)

Reactions w i t h Aldehydes and Ketones. A l c o h o l s may combine a d d i t i v e l y w i t h other carbonyl compounds; such a d d i t i o n compounds are known as hemiacetals or a c e t a l s (Reaction X V I I ) .

0 ,0

H

RC'

OH

0R R'OH

/

f

+ R OH

f

/

1 RCH

• R-CH

\

\ 1

(XVII) 1

OR

OR

When the r e a c t i o n i s c a r r i e d out w i t h a ketone the product i s known as a k e t a l . With low molecular weight unbranched aldehydes and ketones the e q u i l i b r i u m l i e s to the r i g h t . I f i t i s d e s i r e d to make a c e t a l s or k e t a l s of higher molecular weight molecules, the removal of water i s necessary t o d r i v e the e q u i l i b r i u m to the r i g h t (23). Aldehydes and ketones may be converted to ethers by hydrogenation i n an a c i d i c a l c o h o l i c s o l u t i o n (24) c o n t a i n i n g platinum oxide (Reaction X V I I I ) . pto 1

fl

R-C-R + R OH + H

2 f

• R-CH-R + H 0

2

2

H

0

(XVIII)

OR"


MONOHYDRIC ALCOHOLS

8

A d d i t i o n t o Other Unsaturated Molecules, When isocyanates are t r e a t e d w i t h a l c o h o l s , s u b s t i t u t e d methanes or carbamates are prepared (Reaction XIX). f

R-N=C=0 + R OH

» R-NHCOOR'

(XIX)


The r e a c t i o n gives good y i e l d s and i s of wide scope. Cyanic a c i d (HNCO) gives u n s u b s t i t u t e d carbamates. Although the oxygen of the a l c o h o l i s c e r t a i n l y a t t a c k i n g the carbon of the isocyanate, hydrogen bonding complicates the d e t a i l s of the mechanism and the k i n e t i c p i c t u r e (25). In a very s i m i l a r f a s h i o n , a l c o h o l s react w i t h ketenes to g i v e e s t e r s (26) (Reaction XX). 1

^C=C=0 + R'OH

• ^CH-COOR

(XX)

The a d d i t i o n of HC1 to a mixture of an a l c o h o l and a n i t r i l e i n the absence of water leads to the hydrochloride s a l t of the iminoester (27) (Reaction XXI). f

RCrN + R OH + HC1

• R-C=NH ® CI® I OR

(XXI)

2

1

This r e a c t i o n i s known as the Pinner s y n t h e s i s . The s a l t formed may be converted to the f r e e imino e s t e r by treatment w i t h a weak base. I t may a l s o be converted to the corresponding e s t e r by an aqueous a c i d c a t a l y z e d h y d r o l y s i s . The Pinner r e a c t i o n i s of a general nature and i s a p p l i c a b l e to a l i p h a t i c , aromatic and heterocyclic alcohols. A l k o x y l a t i o n . The r e a c t i o n of a l c o h o l s w i t h ethylene oxide gives polymeric products i n which many u n i t s of the ethoxy group are incorporated (Reaction X X I I ) . The r e a c t i o n can be c o n t r o l l e d OH ROH + n CH -CH \^z j z 0 0

0

6

R(0CH CH ) OH Z Zn o

o

(XXII)

by v a r y i n g r e a c t i o n c o n d i t i o n s . Propylene oxide undergoes the same type of r e a c t i o n although not as f a s t due t o the hindrance of the methyl group. F i n a l l y , although the hydroxyl group of most a l c o h o l s can seldom be cleaved by hydrogenation, c e r t a i n a l c o h o l s such as benzyl are s u s c e p t i b l e and o f t e n r e a d i l y undergo r e d u c t i o n (28). The most common c a t a l y s t s are copper chromite and palladium-onc h a r c o a l . M i x t u r e s of A1C1 and L i A l H . have a l s o been used f o r 0


1.

BENITEZ

9


t h i s purpose (29). Though the mechanism of a l c o h o l hydrog e n o l y s i s i s obscure, i n some cases n u c l e o p h i l i c s u b s t i t u t i o n has been demonstrated (30). Displacement of the Hydroxyl Group


A l c o h o l s are among the most e a s i l y obtained reagents of organic chemistry. Because of t h i s , the o v e r a l l conversion of ROH — • RX i s of great importance where X i s t y p i c a l l y a h a l i d e , h y d r i d e , a z i d e , a l k y l or an amine. This s e c t i o n w i l l provide a survey of the importance i n syntheses of r e p l a c i n g a hydroxyl group by other f u n c t i o n a l groups. A l k y l H a l i d e s . The c l a s s i c a l method f o r converting a l c o h o l s to a l k y l i o d i d e s (31) i n v o l v e s heating the a l c o h o l w i t h i o d i n e i n the presence of phosphorus (Reaction X X I I I ) . L i k e other i o d i 6R0H + 2P + 3 I

* 6RI + 2 H P 0

2

3

(XXIII)

3

n a t i o n s using these reagents, the r e a c t i o n proceeds through an intermediate e s t e r which i s decomposed by the i n s i t u generated hydriodic acid. For the p r e p a r a t i o n of bromides and c h l o r i d e s from a l c o h o l s , the corresponding a c i d s , HBr and HC1, are the reagents of choice (32). The mechanism (33) f o r t h i s r e a c t i o n i s b e l i e v e d to i n v o l v e a protonated intermediate (Reaction XXIV) which i s f u r t h e r attacked by the h a l i d e . ROH + HX

• R0%

2

^

» RX + H 0

(XXIV)

2

The observed r e a c t i v i t y gradation f o r t h i s type of r e a c t i o n i s f o r the a c i d : HI > HBr > HC1 > HF, and f o r the a l c o h o l s t e r t i a r y > secondary > primary. Other halogenating agents i n c l u d e P h P C l (34), PBr ( 3 5 ) , A 1 I (36), and many s u l f u r c o n t a i n i n g reagents (37) of which only t h i o n y l bromide and t h i o n y l c h l o r i d e have a t t a i n e d wide a p p l i c a t i o n . The u l t i m a t e choice of the halogenating agent to be used w i l l depend on the stereochemistry d e s i r e d of the f i n a l product (38,39). 2

3

3

Amination. Very few r e a c t i o n s of general scope e x i s t f o r the d i r e c t conversion of a l c o h o l s to amines. Among one of the o l d e s t i s the Bucherer r e a c t i o n which i s used t o convert naphthols (40) and phenols (41) to t h e i r amine d e r i v a t i v e by r e a c t i o n w i t h aqueous sodium b i s u l p h i t e and ammonia (Reaction XXV).


10

MONOHYDRIC ALCOHOLS

NH ROH + NaHS0

^

3

RNH^

+ NaHS0 + H 0 3

(XXV)

2

The R i t t e r r e a c t i o n (42,42A) i s a general method f o r conv e r t i n g a l c o h o l s to amines by r e a c t i o n w i t h a n i t r i l e and a s t r o n g a c i d (Reaction XXVI). In t h i s r e a c t i o n only t e r t i a r y ,

1


ROH + R'CN

• RNHCOR

• RNH

2

+ RC0 H 2

(XXVI)

secondary and b e n z y l i c a l c o h o l s r e a c t as they form the most s t a b l e carbenium i o n s . Of commercial i n t e r e s t i s the d i r e c t r e a c t i o n of a l c o h o l s w i t h ammonia at elevated pressures and temperatures i n the presence of a dehydrating c a t a l y s t such as alumina g e l . This process i s known as ammonolysis and gives a mixture of primary, secondary and t e r t i a r y amines (Reaction XXVII). ROH + NH

¥ RNH + R NH + R N

3

2

2

(XXVII)

D i r e c t displacements of the hydroxyl group by azide are uncommon, but carbonium ions derived from a l c o h o l s are attacked by the azide i o n to give organic azides (43) (Reaction X X V I I I ) .

R C0H + NaN 3

3

• R C N + NaCl 3

3

(XXVIII)

These azides can be f u r t h e r reacted (44) to give an amine as the f i n a l product. T o s y l a t e s . Even though the S^2 r e a c t i o n cannot be performed on a l c o h o l s , the hydroxyl group can be transformed to a good l e a v i n g group by i t s r e a c t i o n w i t h p - t o l u e n e s u l f o n y l c h l o r i d e (p-CH-C^H, S0 C1) . Such a group i s then e a s i l y d i s p l a c e d by a v a r i e t y or n u c l e o p h i l e s i n essence a c h i e v i n g the s u b s t i t u t i o n of the hydroxyl group (44A). By f a r t h i s i s one of the most u s e f u l methods f o r c o n v e r t i n g an a l c o h o l to an alkane, an e s t e r , an amine or any other d e r i v a t i v e of a n u c l e o p h i l e . 2

Dehydration

of A l c o h o l s

The dehydration of a l c o h o l s i s an example of a wide range of e l i m i n a t i o n r e a c t i o n s having the f o l l o w i n g general form (Reaction XXIX). RCH -CRH0H 2

•¥ RCH=CRH +

H0 2

(XXIX)


1.

11


BENITEZ

The dehydration of a l c o h o l s can take p l a c e both i n s o l u t i o n (45) and i n the gas phase (46) . The g e n e r a l r u l e s f o r t h i s type of e l i m i n a t i o n have a l s o been r e c e n t l y reviewed (47). For t h i s reason, i t w i l l not be attempted i n t h i s s e c t i o n to f u l l y explore the area; i n s t e a d , only a b r i e f review w i l l be given. In aqueous a c i d i c s o l u t i o n s of e i t h e r Brjzfasted or Lewis' a c i d s , the dehydration of a l c o h o l s leads to the formation of S a y t z e f f o l e f i n s (48). Dehydration occurs most r e a d i l y i f the a l c o h o l i s t e r t i a r y . For example, the formation of 1,1-diphenylethylene from methyldiphenyl c a r b i n o l (Reaction XXX)


OH

H S0. o

(C H ) -C-CH 6

5

2

¥ (C H ) C=CH

3

6

5

2

+ H 0

2

(XXX)

£

occurs very r a p i d l y j u s t by h e a t i n g the a l c o h o l w i t h d i l u t e s u l f u r i c acid. Isoprene has been prepared by dehydration of 3-methyl-lbutene-3-ol (Reaction XXXI) and butadiene from the dehydration OH (CH ) 3

2

C-CH=CH

» CH =C-CH=CH + H 0

2

2

2

CH

(XXXI)

2

3

of 1,3-butanediol (Reaction XXXII). OH

I

CH -CH-CH -CH -OH 3

2

• CH =CH-CH=CH + 2H 0

2

2

2

(XXXII)

2

The dehydration of 2-pentanol by the s u l f u r i c a c i d method i s of i n t e r e s t s i n c e i t i l l u s t r a t e s the r u l e that b e t a - o l e f i n s forma t i o n i s thermodynamically the favored pathway (Reaction X X X I I I ) . OH

H S0

I

2

CH CH CH -CH-CH 3

2

2

4

• CH CH CH=CH-CH +

3

3

2

ftfl

3

(XXXIII)

A l c o h o l s i n which the beta-hydrogen i s a c t i v a t e d by a double bond undergo dehydration by concentrated a l k a l i n e media to produce dienes (49) (Reaction X X X I V ) . Other products such as ethers are p o s s i b l e when the r e a c t i o n i s done i n the presence of dimethyl s u l f o x i d e (50). CH -OH

2

»CH

OH©

1

r ^ ^ V

(XXXIV) +

H 0 2

3


MONOHYDRIC ALCOHOLS

12

When some s t r a i g h t and branched-chain a l i p h a t i c a l c o h o l s , such as n-propanol, n-butanol and i s o p r o p a n o l , are subjected to h i g h temperatures, dehydrogenation products predominate over dehydration (51). Presumably the e l i m i n a t i o n s take place v i a a six-membered t r a n s i t i o n s t a t e and are c a t a l y z e d by hydrogen h a l i d e s i n the homogeneous phase (52) to produce o l e f i n s . On the other hand, gas phase dehydration over s o l i d c a t a l y s t s i s a v a l u a b l e process f o r the p r e p a r a t i o n of o l e f i n s and e t h e r s . The most s t u d i e d dehydration c a t a l y s t s are the metal oxides (53), but the s e l e c t i v i t y of these c a t a l y s t s i n terms of dehydrat i o n versus dehydrogenation i s not f u l l y understood (54).


O x i d a t i o n of A l c o h o l s Among the many agents a v a i l a b l e f o r the o x i d a t i o n of organic compounds, the ones most commonly used are d e r i v a t i v e s of hexav a l e n t chromium (Cr-VI) or heptavalent manganese (Mn-VII). Chromium t r i o x i d e (CrO-) and sodium dichromate (Na^Cr^O^) are converted to chromium f i l l ) i n the course of o x i d a t i o n s f o r a t o t a l t r a n s f e r of three e l e c t r o n s to each metal atom. With permanganate i n a c i d i c media the manganese ( I I ) i o n i s formed f o r a t o t a l t r a n s f e r of f i v e e l e c t r o n s ; i n n e u t r a l or b a s i c media, manganese d i o x i d e i s formed w i t h a corresponding t r a n s f e r of three e l e c t r o n s . The o x i d a t i o n of a secondary a l c o h o l to a ketone i s u s u a l l y accomplished w i t h a s o l u t i o n of the a l c o h o l and aqueous a c i d i c chromic a c i d i n e i t h e r acetone or a c e t i c a c i d , w i t h a s o l u t i o n of sodium dichromate i n a c e t i c a c i d , or by r e a c t i o n of the a l c o h o l w i t h aqueous a c i d i c chromic a c i d as a heterogeneous system. An example i s the o x i d a t i o n of the s u b s t i t u t e d cyclohexanol below (Reaction XXXV) w i t h sodium dichromate i n s u l f u r i c a c i d (55).

R

R

Na Cr O 2

2

y

(XXXV)

f

The course of t h i s dichromate o x i d a t i o n can be f o l l o w e d s p e c t r o p h o t o m e t r i c a l l y as the yellow-orange a b s o r p t i o n at 350 nm of the chromium VI i s converted to green a b s o r p t i o n of the chromium I I I i o n (56). The probable mechanism of o x i d a t i o n of a l c o h o l s by chromium (VI) s p e c i e s i n v o l v e s the formation of chromate e s t e r s and t h e i r subsequent decomposition to ketones (57). As a r u l e , i n the absence of competing s i d e r e a c t i o n s , the more hindered a l c o h o l s react f a s t e r than the l e s s hindered compounds. I t has a l s o been



1.

BENITEZ

13


found that e l e c t r o n donating s u b s t i t u e n t s a c c e l e r a t e the r a t e of o x i d a t i o n (58). The p r e v i o u s l y described c o n d i t i o n s using dichromate and chromic a c i d are s u f f i c i e n t l y vigorous t o slowly o x i d i z e other r e a c t i v e centers i n the molecule such as e t h e r s , amines, carboncarbon double bonds, and b e n z y l i c and a l l y l i c C-H bonds. To prevent t h i s , a m i l d e r method of o x i d a t i o n can be used, cons i s t i n g of adding an aqueous s o l u t i o n of chromic a c i d (Jones reagent) t o an acetone s o l u t i o n of the a l c o h o l t o be o x i d i z e d (59). Another reagent as m i l d as Jones reagent c o n s i s t s of a chromium t r i o x i d e p y r i d i n e complex. This compound can be used f o r the o x i d a t i o n of a l c o h o l s c o n t a i n i n g a c i d s e n s i t i v e f u n c t i o n s such as a c e t a l s and k e t a l s (60). A convenient and inexpensive procedure f o r the o x i d a t i o n of secondary a l c o h o l s to ketones has been reported (60A) t o i n v o l v e r e a c t i o n of the a l c o h o l w i t h sodium h y p o c h l o r i t e i n a c e t i c a c i d . The y i e l d of the c o r r e sponding ketone i s around 90 percent w i t h many a l c o h o l s . Primary a l c o h o l s react slowly g i v i n g dimeric e s t e r , presumably v i a hemiacetal intermediates. T e r t i a r y a l c o h o l s are r e l a t i v e l y i n e r t to o x i d a t i o n by chromic a c i d ; however, t e r t i a r y 1,2-diols are r a p i d l y cleaved by chromic a c i d provided they are capable of forming a c y c l i c chromate e s t e r (61) (Reaction XXXVI).

Na Cr 0 2

2

?

HC10,

(XXXVI)

Monohydric a l c o h o l s r e a c t r a p i d l y w i t h lead t e t r a a c e t a t e t o form alkoxy lead (IV) intermediates. These intermediates decompose thermally or p h o t o l y t i c a l l y i n a v a r i e t y of ways to produce ketones, e s t e r s , and c y c l i c ethers (62), as can be seen below (Reaction XXXVII). Pb(OCOCH ) 3

R (CH ) -CH-R 1

2

4

4

¥

2

(XXXVII)

R -(CH ) -CH-R, 1

OH

2

4

0-Pb(OCOCH ) 3

0

R CH X

0

II 2

-R o r R -(CH ) -0C-CH 2

4

3

II

or R -(CH ) ~C-R 4

2


14

MONOHYDRIC ALCOHOLS

Both the s t r u c t u r a l f e a t u r e s of the a l c o h o l and the r e a c t i o n cond i t i o n s used are important i n determining which of the decomposit i o n pathways i s f o l l o w e d . I f the lead a l k o x i d e from a primary or secondary a l c o h o l i s formed i n the presence of a donor s o l v e n t , such as p y r i d i n e , o x i d a t i o n to an aldehyde or ketone i s the primary mode of decomposition (63) (Reaction XXXVIII).

CH (CH ) CH -OH

Pb(OCOCH ) i - i - * CH (CH ) CHO Pyridine 25 ° C CH (CH ) CH OCOCH J

5


3

2

3

2

(XXXVIII) 3

When the o x i d a t i o n s are c a r r i e d out i n the presence of calcium carbonate and lead t e t r a a c e t a t e , the major product i s c y c l i c ethers formed through the intermediacy of f r e e r a d i c a l s (64). A n a l y t i c a l Determination of the Hydroxyl Group Some simple " t e s t tube" r e a c t i o n s can be used to determine the presence and/or the type of h y d r o x y l groups i n organic molec u l e s . Although modern spectroscopy has made the knowledge of t h i s type of a n a l y t i c a l chemistry l e s s imperative today, t h i s s e c t i o n w i l l t r y to cover a few of the most important r e a c t i o n s which are s t i l l u s e f u l f o r the f a s t determination of the h y d r o x y l group. (a) Primary and secondary a l c o h o l s w i l l r e a c t w i t h N e s s l e r reagent (65), a mixture of equal volumes of 2N NaOH and potassium mercury ( I I ) i o d i d e s o l u t i o n . Mix a few drops of the substance w i t h 5 mL of the reagent and b o i l the mixture f o r a few minutes. The presence of a primary or secondary a l c o h o l i s detected by the formation of a brownish y e l l o w to gray p r e c i p i t a t e which turns gray on s t a n d i n g . T e r t i a r y a l c o h o l s do not r e a c t w i t h N e s s l e r s reagent. A white p r e c i p i t a t e may be formed on mixing the r e agents, but i t d i s s o l v e s on shaking the t e s t tube. (b) When added to a s o l u t i o n of n-bromosuccinimide (66), primary a l c o h o l s g i v e a s t a b l e c o l o r , secondary a l c o h o l s a f l e e t i n g c o l o r and t e r t i a r y a l c o h o l s no c o l o r . (c) T e r t i a r y a l c o h o l s are dehydrated when b o i l e d w i t h Deniges reagent (67); alkenes, which g i v e a y e l l o w p r e c i p i t a t e , are formed. The r e a c t i o n i s thus of alkenes, not of t e r t i a r y a l c o h o l s . Primary a l c o h o l s do not r e a c t , but some secondary a l c o h o l s r e a c t almost as r e a d i l y as the t e r t i a r y a l c o h o l s . For a complete treatment of simple q u a l i t a t i v e and q u a n t i t a t i v e r e a c t i o n s f o r a l c o h o l s , the reader i s r e f e r r e d to V e i b e l ' s book (69) which deals i n depth w i t h a l a r g e number of a n a l y t i c a l reactions. f

1


1.


BENITEZ

15


Conclusion In the f u t u r e , there i s no doubt that a l c o h o l s w i l l p l a y a major r o l e not only as f u e l components (70,71) but a l s o as feed stocks f o r the syntheses of more complicated organic compounds (72). A great amount of research e f f o r t i s p r e s e n t l y d i r e c t e d to the economic conversion of CO and t o methanol (73), and on homologation of methanol to higher a l c o h o l s (74). Conversion of s y n t h e s i s gas from c o a l to a c e t i c anhydride (75) through the intermediacy of methyl acetate (from methanol) w i l l soon be a commercial r e a l i t y . Dramatic e n t r i e s have been made i n t o the technology of the e i g h t i e s by d i r e c t u t i l i z a t i o n of CO and to produce, f o r example, methanol and p o l y h y d r i c compounds (76) ( R e a c t i o n XXXIX). This unique r e a c t i o n has a t t r a c t e d c o n s i d e r a b l e a t t e n t i o n due to a

CO + H z 2

R

h

C a t a l

y

s t

^

CH^QH + HOCH (CHOH) CH OH 2

X

Bases

2

(XXXIX)

X = 0,1,2

r e l a t i v e l y h i g h s p e c i f i c i t y to ethylene g l y c o l , and to the unusual type of c a t a l y s t s which bears s u p e r f i c i a l r e l a t i o n s h i p to homogeneous c a t a l y t i c systems ( 7 7 ) . Other advances i n c l u d e the c o m m e r c i a l i z a t i o n of a process to make o x a l i c a c i d (78) (Reaction X L ) . C00C,H 2 C H O H + 2C0 + 1/2 0 4

9

2

— Cat

C0 H 2

^2°-*

|

C00C.H 4 9 FT

|

( X L )

C0 H 2 O

I t i s i n t e r e s t i n g that hydrogenation of the intermediate d i b u t y l o x a l a t e would produce the g l y c o l and the s t a r t i n g b u t a n o l . T h i s new area of chemistry i s s t i l l i n i t s i n f a n c y when compared to the other r e a c t i o n s covered i n t h i s chapter, but many e x c i t i n g developments can be a n t i c i p a t e d i n the f u t u r e .


16

MONOHYDRIC ALCOHOLS

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Monohydric Alcohols - American Chemical Society

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