CHAPTER 2
THEORY
In general, e th e rific a tio n is a preparation process of ethers- organic compounds which contain two groups attached to oxygen. Its general formula is ROR, ArOR or ArOAr
where R = a lk y l group Ar = A ryl group
v/ith the presence o f a lk a li reagent, e th e rific a tio n can occur in a system having alcohols, phenols and sugar alcohols (4, 5s 6, 7ร 8).
E th e rific a tio n is fre qu e ntly and commomly obtained by the re action between a lk y l halide (o r.su b s titu te d a lk y l halid e ) w ith sodium alkoxide 03T sodium phenoxide as shown in the fo llo w in g equation (5).
RONa + RX ---t ROR + NaX
With the presence of a lk a li, e th e rific a tio n o f cellulose can evidently occur by the reaction of cellulose w ith e th e rify in g agents such as organic halides, alkene oxides, or o le fin activated by polar substituent groups (1, 7)»
As previously mentioned, cellulose alone does not react w ith e th e rify in g agent. Since there are 3 hydroxyl groups in each glucose u n it w ith in ce llu lo se chain, such hydroxyl groups must be activated by a lk a li treatment to form a lk a li cellulose which is able to undergo fu rth e r e th e rific a tio n (1, 7)«
T h eo retica lly, the a ction of a lk a li on ce llulose or a lk a li
cellulose form ation may proceed in accordance w ith 2 d iffe re n t schemes (4)
(a) Cellulose forms true alcoholates with a lk a li metal which replaces the hydrogen ion as in other monohydric and polyhydric alcohols
RCell(OH)J + N a O H ---■» RCell(OH) 2. ONa + แ20
(๖) A molecular or addition compound is formed in such a way that molecules of a lk a li are added to the hydroxyl groups of cellulose:
RCell(OH) + N a O H --- » RCell(0H)2.0H.Na0H
However, there is , at present, no direct proof that either of these compounds is in fact formed but the composition of a lk a li cellulos depends on several factors liste d b^-low as mentioned by N .I. N ik itin (4)
- concentration of a lk a li
- temperature of the a lk a li treatment - nature of the hydroxide employed
- individual properties of the starting cellulose and
- id en tity of the medium in which the a lk a li has been dissolved According to scheme (a), alcoholates are hydrolysed even in the presence of small amounts of water as pointed out by Rogovin (4, 9)•
In a separate work, J. Chedin and A. Tribot reported that there is almost no etherification in the absence of water (10). This contra
diction was c la rifie d by the la tte r group that water was a solvating agent acting as a solvent and carrier fo r the sodium hydroxide, as an agent to swell and solvate the cellulose so that the etherifying agent penetrates readily, and as a diluent fo r the etherifying agent (1, 10).
N .I, N ik itin also cited out that a lk a li cellulose may be practically made in various reaction media such as aqueous solution, alcoholic solution and other nonaqueous solution (4)- However, the rate of
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hydrolysis of a lk a li compound considerably decreases i f alcoholic so lu tio n is used rathe r than an aqueous so lu tio n (4, 6, 9j l l ) .
As regard to scheme (2), Webber, Ashar and Champetier found that the amount of bound a lk a li is much la rg e r in nonaqueous solutions than in aqueous solu tion (4). They also found th a t the small amounts of water which are added to ce rta in higher alcohol produce the increas
ing in the sorption of NaOH and that the rate of establishment of equilibrium in nonaqueous so lu tio n is exceeding slow. According to
the re su lts obtained from th e ir work, they may ind ica te th a t the composition of a lk a li cellulose is also affected by fa cto rs other than the re la tiv e rate of form ation and hydrolysis of a lk a li compounds (4, 1 2).
Under separate work, G.A. P etropavlovskii and N .I. N ik itin
found th a t the increasing o f temperature increased the reaction rate (1 3)*
They also suggested that the heat involved during m ixing must be con tro lle d tc prevent the loss of degree of polym erization since cellulose
is a high polymer consisting of glucose u n its linked in long chain by polym erization.
The extent of e th e rific a tio n is commomly considered by deter
mining the degree of s u b s titu tio n (บ .ร.) o f the product obtained. In a cellulose d e riva tive , บ.ร. is defined as the average number of hydroxyl groups substituted per anhydroglucose u n it. T h e o re tica lly, the maximum บ.ร. is 3 as regard to the to ta l number o f hydroxyl groups containing in each glucose u n it in the cellulose chain (1 4).
Most of the works reported were conducted by sim ply using single reaction medium. The a dd itio n of organic solvents such as alcohol to
the reaction m ixture was re ce n tly proposed in order to elim inate dough (1 5)»
A ce rta in e th e rific a tio n o f a lk a li ce llulose in which e ith e r chloroacetic acid or sodium chloroacetate is used as e th e rify in g agent is known as carboxymethylation ( l) . Tim ell and Rydholm showed that the s u b s titu tio n can be f ir s t take»’, place a t the 6” p o s itio n fo r it s D.S. approximately up to 0 . 1 5 and then the secondary hydroxyls a t 2“
and 3” positions begin to p a rtic ip a te in reaction (4, 1 7, 18).
The carboxymethylation equation is generally expressed as, Î / RCell(OH)5 + CICHgCOONa + NaOH — --- > RCell(OH)2OCH2COONa
+ NaGl + แ20 ( l) The D.S. of the product and also its s o lu b ility depends upon the reaction conditions used. Yehia Fahmy and O lfa t Mansour pointed out that the products w ith minimum D.s. of about 0 . 4 are water soluble and ones w ith D.s. lower than 0 .4 are almost d ilu te - a lk a li soluble(1 9).
They also suggested that apart from the main reaction o f carboxymethylation (see equation 1), the sodium chloroacetate is hydrolysed to sodium
glycolate as followed ;
ClCH2C00Na + NaOH s H0CH2C00Na + NaGl (2)
This side reaction is induced by enhancing rea ction -variab les which hence adversely influence the rate of the main reaction of e th e r ifi
cation o f ce llu lo se .
Eiji Hayakawa and Yazaemon Marita used water miscible or water immiscible organic solvents to increase the D.s. (
16).
The influence of reaction variables on the D.s. and the alkali
solubility and on the reactivity was studied by R.R. McLauglin and
J.H.E. Herbst in
1950(
20). They found that the efficiency of carboxy-
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m éthylation could be improved by lower reaction temperature, decreasing excess sodium hydroxide and by increasing time.
Considering equation ( l) , Matao Nakamura and K oichiro Watanabe suggested that the expression is also true fo r the fo llo w in g form (2 1) I
RCell(OH) + xClCH^COONa + xNaOH ^ = = = ^ RCell(OH) _x (0CH2C00Na)x + xNaCl + xH20 (3) where X = degree of su b s titu tio n
As we know, the extent o f e th e rific a tio n is commomly considered by determ ining the D .s . of the product obtained. I t is worthwhile to present here w ith such d e ta il.
Determination of degree of s u b s titu tio n a) Determination of moisture content.
In order to obtain the exact weight of the product used fo r ca lcu la tin g the D.S., the moisture content must be f ir s t determined.
The moisture content can be calculated by using the fo llo w in g equation (1 4)ะ
percentage of moisture = (a/b) (1 0 0)
where A = mass loss on drying
B =ะgrams of sample used
In p ra ctice , dry m grams o f product fo r about 5 hours in vacuum dryer. A fte r cooling, fin d the weights of sample before and a fte r drying.
Let m = weight of sample
a = weight of n icke l crucible and it s cover b = v/eight of n icke l cru cib le , cover and
sample a fte r drying
Therefore, percentage o f moisture = |(m + a) - b I (10 0) ( 4 )
๖) Determination of D.s.
The quantity of sodium in the etherification product can be calculated as followed I
from RCell(OH) + xCICHpCOONa + xNaOH "
RCell(OH) x(0CH2C00Na) + xNaCl + xH20 The percentage of sodium is given by the equation
% Na = (25x) (100) (5)
162 + 80x where
RCell(OH) is taken as the formula of the anhydroglucose unit (CgH^02(0H )j). Its molecular weight is 162.
X is degree of substitution (d.s.) which is the average number of hydroxyl groups substituted per anhydroglucose u n it.
RCell(OH) _x(0CH2COONa)x is the etherification product having molecular weight 162 + 80” .
Therefore} X (162) (%Na)
2500 - (80)(%Na) (6)
