Carboxylic Acids in physics

7/28/2019 Carboxylic Acids in physics

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C H A P T E R 1 0Carboxyl ic Acids4-X-Benzoic AcidsHalf Salts of Carboxylic AcidsReferences

170172173

Figures TablesFigure 10-1 175 (166)Figure 10-2 176 (166)Figure 10-3 177 (166)Figure 10-4 178 (167)Figure 10-5 179 (168, 169)Figure 10-6 180 ( 168, 169)Figure 10-7 181 (168, 169)Figure 10-8 182 (168, 169)Figure 10-9 183 (168, 169)Figure 10-10 184 (169)Figure 10-11 185 (169)Figure 10-12 186 (169)Figure 10-13 187 (169)Figure 10-14 188 (169)Figure 10-15 189 (170)Figure 10-16 190 (170)Figure 10-17 191 (170)Figure 10-18 192 (170, 171)Figure 10-19 193

Table 10-1Table 10-2Table 10-3Table 10-4Table 10-4aTable 10-5Table 10-6Table 10-7Table 10-8Table 10-9

194195 (167)196 (167)197 (170)198 (170)199 (171)200 (172)201 (172)202 (172)203 (172)

*Numbers in parentheses indicate in-text page reference.

In the condensed phase or in concentrated solution carboxylic acids exist as a double hydrogenbonded dimer as illustrated here:\ C->/__o... \

o d \9 9 9 O ' - - - C

Structure A

165


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1 6 6 Carboxylic AcidsT h u s , t h e s e d i m e r s h a v e a c e n t e r o f s y m m e t r y , a n d o n l y t h e o u t - o f - p h a s e ( C - - O ) 2 s t r e t c h i n gm o d e i s I R a c ti v e, a n d o n l y t h e i n - p h a s e ( C = O ) 2 s t r e t c h i n g m o d e i s R a m a n a c t i ve . H o w e v e r , i nt h e v a p o r p h a s e a t e l e v a t e d t e m p e r a t u r e s c a r b o x y l i c a c i d s e x i s t i n t h e m o n o m e r i c f o r m( R - C O z H o r ~ b - C O 2 H ) .

T h e v a sy m . ( O H . . . O = C ) 2 m o d e f o r s t r u c t u r e A i s v e r y b r o a d o v e r th e r a n g e 2 5 0 0 - 3 0 0 0 w i t hs u b s i d i a r y m a x i m a , w h i c h a r e d u e t o c o m b i n a t i o n a n d o v e r t o n e s i n F e r m i r e s o n a n c e w i t hv a sy m . ( O H - . - O = C ) 2 . T h e V i p ( C = O ) 2 o c c u r s i n t h e r e g i o n 1 6 2 5 - 1 6 8 7 c m - 1 i n t h e R a m a n , i n -p l a n e ( C - - O H ) 2 b e n d i n t h e r e g i o n 1 3 9 5 - 1 4 4 5 c m - 1 , a n d 7 ( O H ) 2 i n t h e r a n g e 8 7 5 - 9 6 0 c m - 1 .

Ta b le 10 .1 l i s ts IR va po r -pha s e da t a , a nd CC 14 a nd CHC 13 s o lu t i o n -pha s e da t a f o r a l i pha t i cc a r b o x y l i c a c i d s ( 1 , 2 ) . T h e m o n o m e r i c v O H f r e q u e n c y f o r t h e a l i p h a t i c c a r b o x y l i c a c i d o c c u r snea r 3580 , 3534 , a n d 3 51 9c m -1 in the va po r , C C14 , a nd CHC 13 pha s es , r e s pec tive ly . In CC14s o l u t i o n t h e d e c r e a se i n t h e v O H f r eq u e n c y i s d u e t o i n t e r m o l e c u l a r h y d r o g e n b o n d i n g b e t w e e nthe a c id p r o to n a nd the C1 a tom o f CC14 (C O zH . . . C1CC13). The C1 a tom s a r e l e s s ba s i c i nCHC13 tha n they a r e in CC14 ; however , the vO H s t il l o c c u r s a t l ower f r eque nc y in CHC 13 tha n inC C 14 s o l u t i o n . T h i s is a t tr i b u t e d t o t h e f o l lo w i n g d o u b l y h y d r o g e n - b o n d e d c o m p l e x i n C H C 1 3,wh ic h c a us es vOH to s h i ft t o a l ower f r eq uenc y tha n in CC14 s o lu t i o n (2 ).

H I C ~." Cl0/ C ~ - O'

RStructure B

The vC=O f r equenc ies f o r thes e a l ipha t i c c a r boxy l i c a c ids dec r ea s e in f r equenc y in the o r de rf o r v a p o r ( 1 7 8 9 - 1 7 6 9 c m - 1 ) , C C 1 4 ( 1 7 6 7 . 3 - 1 7 5 0 . 3 c m - 1 ) , a n d C H C 1 3 ( 1 7 5 6 . 3 - 1 7 6 9 c m - 1 ) , a n di n t h e o r d e r o f in c r e a s ed b r a n c h i n g o n t h e a - c a r b o n a t o m [ C H 3 to C ( C H 3 ) 3] w i t h i n e a c hphys i c a l pha s e .

F i g u r e 1 0 .1 s h o w s p l o t s o f v C - O f o r a l i p h a ti c c a r b o x y l i c a c id s i n CC14 o l u t i o n a n d i n C H C 13s o l u t i o n v s t h e n u m b e r o f p r o t o n s o n t h e a c id a - c a r b o n a t o m ( 2) . T h e n u m b e r s 1 - 5 o n e a c hc u r ve in F igs . 1 -4 a r e f o r a c e t i c a c id , p r op ion i c a c id , bu ty r i c a c id , i s obu ty r i c a c id , a ndt r ime thy l a c e t i c a c id , re s pec t ive ly . S imi l a r c u r ves wer e ob ta ined p lo t t i ng v C - O vs cr* a ndv C - O v s E s. T h e ~ * v a l u es a r e a m e a s u r e o f t he i n d u c t i v e c o n t r i b u t i o n o f th e a l k y l g r o u pa nd E S s a mea s u r e o f the s t e r i c f a c to r o f the a lky l g r o up (3 ) . Thus , i t a ppea r s tha t bo th in duc t ivea n d s t e r i c f a c t o r s a f f e c t v C = O f r e q u e n c i e s a s w e l l a s t h e i n t e r m o l e c u l a r h y d r o g e n - b o n d e dc omplex fo r med , a s d i s c us s ed p r ev ious ly .

T h e o u t - o f - p h a s e ( C - O ) 2 s t r et c h i n g , V o p ( C = O )2 , m o d e s o c c u r i n t h e r a n g e 1 7 0 2 . 2 -1 7 1 4 . 6 c m - 1 a n d 1 6 9 9 . 6 - 1 7 1 2 . 5 c m - 1 i n CC14a nd CHC 13 s o lu t i on s , r e s pec t ive ly , a nd w i th ineach so lven t the f req uenc i es de c rease as o-* an d Es inc rease . In the case o f Vip (C =O )2, i t is not IRactive.F igur es 10.2 an d 10.3 show, respec tively , p lo ts o f vC --O vs m ole % CHC 13/CC 14 a r id Vop(C =O ) 2 vs mo le % CHC13/CC14 fo r a l i pha t i c c a r b oxy l i c a c ids . In CC14 o l u t i o n v C - - O o c c u r a tthe h ighes t f r equenc y , a nd then dec r ea s e in f r equenc y a s the mo le % CHC13/CC14 i s i nc r ea s edf r om ~ 10 to 100 . On the o the r ha nd , V op(C=O) 2 dec r ea s es l i nea r l y a s the mo le % CHC13/CC14 i sinc r ea s ed f r o m 0 to 100. The es s en ti a l l y l i nea r dec r ea s e in bo th v C - O a nd Vop (C -O ) 2 a f t e r the


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V a r i a b l e s i n D a t a I n t e r p r e t a t i o n 16 7i n i ti a l i n t e r m o l e c u l a r h y d r o g e n b o n d f o r m a t i o n i n t he c a s e o f v C = O i s a t t r i b u t e d t o t h e r e a c t i o nf ie ld o f t h e s o l v e n t s y s t e m . T h e r e a c t i o n f ie l d i s d e f in e d a s f o l lo w s : [ R ] = ( 6 - 1 ) / ( 2 ~ + n 2)wh e re 6 i s t h e d i e l e c t r i c co n s t an t an d n i s t h e r e f r a c tu re i n d ex o f t h e so lv en t sy s t em ( 4 ) . I t h a sb e e n s h o w n t h a t t h e r e is a l i n e a r r e la t i o n s h i p b e t w e e n t h e m o l e % C H C 1 3 /C C 1 4 a n d t h e r e a c t i o nf i e l d ( 3 ) . T h e r e ac t i o n f i e l d i n c re a se s a s t h e m o le % C HC 1 3 /C C 1 4 i n c re a se s an d th i s i s a r e su l t o fa n i n c r e a s i n g d i e l e c t r i c c o n t r i b u t i o n o f t h e s o l v e n t s y s t e m a c c o m p a n i e d b y a d e c r e a s e i n t h ere f r a c t i v e i n d ex o f t h e so lv en t sy s t em . T h e d ec re a se i n t h e V o p ( C - O) 2 f r eq u en c y i n g o in g f ro ms o l u t i o n i n C C 1 4 t o s o l u t i o n i n C H C 1 3 i s a t t r i b u t e d t o t h e f o l l o w i ng i n t e r m o l e c u l a r l y h y d r o g e n -b o n d e d s t r u c t u r e ( 2 ) :

R \C ~ O . 9 - H/ \

C C I 3 ~ H 9 9 . 0 O. . . H~CCl 3\ /H 9 9 9 O-- - -C \ R

Structure C

T h e g rad u a l d ec re a se i n f r eq u en cy i s a t t r i b u t ed t o t h e i n c re a se i n t h e r e ac t i o n f i e l d a s t h e m o le %CHC13/CC14 i s increased .

F ig u re 1 0 .4 sh o w p lo t s o f A(vC=O)/A[Vop ( C = O ) 2 v s m o l e % C H C 1 3 /C C 1 4 . T h e s e p l o t si n d i c a t e t h a t a s t h i s r a t i o i n c re a se s , a s t h e m o le % C HC 1 3 /C C 1 4 i n c re a se s , t h e co n cen t r a t i o n o fm o l e c u l e s e x i s t i n g a s S t r u c tu r e B i n c r e a s e s w h i l e c o m p o u n d s e x i s t i ng a s S t r u c t u r e A d e c r e a s ew i t h a s u b s e q u e n t i n c r e a s e i n S t r u c t u r e C .

I R v a p o r - p h a s e b a n d s i n t h e r a n g e 1 1 0 5 - 1 1 7 8 c m - 1 a r e a s s ig n ed t o v C - C - O a n d I R v a p o r -p h a s e b a n d s i n t h e r a n g e 5 7 1 - 5 8 0 c m - 1 a r e a s s i g n e d to 7 C = O ( 1) . T h e f r e q ue n c y s e p a r a t i o nb e t w e e n t h e s e t w o m o d e s i n t h e v a p o r p h a s e v a r i e s b e t w e e n 5 2 0 a n d 5 9 8 c m - 1 .

T a b le 1 0 . 2 li s ts R a m a n d a t a a n d a s s i g n m e n t s f o r v ( C = O ) 2 a n d v C = C f o r c a r b o x y l i c a c i d s ( 6 ).F o u r o f t h e a c i d s l i s t e d i n T a b l e 1 0 . 2 e x h i b i t a R a m a n b a n d , w h i c h c a n b e a s s i g n e d a s

V o p (C = O ) 2 i n t h e r a n g e 1 7 1 2 - 1 7 3 0 c m - 1 . A p p a r e n t l y t h e s e f o u r h y d r o g e n - b o n d e d c a r b o x y l i ca c i d s d o n o t h a v e a c e n t e r o f s y m m e t r y i n t h e n e a t p h a s e . T h e V i p (C = O ) 2 h a s w e a k t o s t r o n gr e la t i ve R a m a n b a n d i n t e n s i ty a n d o c c u r s i n t h e 1a n g e 1 6 3 0 - 1 6 9 4 c m - . T h e l o w e s t f r e q u e n c y ise x h i b i t e d b y 3 - ( 4 - h y d r o x y - 3 - m e t h o x y p h e n y l ) - 2 - p r o p i o n i c a c i d , a n d t h e h i g h e s t f r e q u e n c y i se x h i b i t e d b y p o l y m e t h a c r y l i c a c i d .

T h e v C = C m o d e o c c u r s a s l o w a s 1 5 5 7 c m - 1 i n t h e c a s e o f t r i c h l o r o a c r y l i c a c i d t o a s h i g h a s1690 c m - 1 i n t h e c a se o f i t a co n i c a c id ( 6 ) .

T ab l e 1 0 .3 l i s t s I R g ro u p f r eq u en cy d a t a f o r a ce t i c a c id an d i t s d e r i v a t i v e s i n t h e v ap o r an dso lu t i o n p h ase s ( 1 ,7 ) . T h e vOH...C1CC13 f r e q u e n c y o c c u r s i n t h e r e g i o n 3 5 3 4 . 3 - 3 5 0 0 . 5 c m - 1 i nC C I 4 s o l u t i o n , a n d f o r vOH...HC1CC12 n t h e r a n g e 3 5 2 1 . 0 - 3 4 7 9 . 6 c m - 1 . T h e l o w e r f r e q u e n c i e si n C H C 1 3 s o l u t i o n a r e a t t r i b u t e d t o th e d o u b l e h y d r o g e n b o n d c o m p l e x e d a s p r e v i o u s l yd i scu ssed h e re . T h e l o wes t vOH f r e q u e n c y i s e x h i b i t e d b y t r i f l u o r o a c e t i c a c i d a n d t h e h i g h e s tvOH f r eq u en cy i s ex h ib i t ed b y t r im e th y l a ce t i c a c id . T h e l o wes t T a f t o r* v a lu e i s f o r t r im e th y l -a ce t i c a c id [ - 0 . 3 0 0 ] an d th e h ig h es t T a ft o r* v a lu e i s f o r t r i fl u o ro a ce t i c a c id [ 2 .7 7 8 ] ( 3 ).Mo reo v e r , t h e h ig h es t p K a v a lu e i s f o r t r im e th y l a ce t i c a c id [ 5 .0 3 ] an d th e l o w es t p K a v a lu e i s fo r


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168 Carboxyl ic Acidst r i f l uo r oa c e t i c a c id [0 . 05 ] (8 ) . Thes e da t a s how tha t a s the OH p r o ton bec omes mor e a c id i c thevOH m o d e d e c r e a s e s i n f r e q u e n c y i n b o t h C C 1 4 a nd CHC 13 s o lu t i ons . M or eove r , i n gene r a l thef r e q u e n c y s e p a r a t i o n b e t w e e n C C 1 4 a n d C H C 1 3 s o l u t i o n b e c o m e s l a r ge r a s t h e O H p r o t o nb e c o m e s m o r e a c i d ic d u e t o t h e f o r m a t i o n o f s t r o n g e r O H . . . C 1 b o n d s .

F igu r e 10 .5 is a s c hema t i c o f the num be r o f pos s ib l e r o t a t i ona l c on fo r m er s f o r the t ri - , d i -, a ndha loa c e t i c a c id monomer s (14 ) . X i s one ha logen a tom, 2X i s two ha logen a toms , 3X i s th r eeh a l o g e n a t o m s . T h e f o l l o w i n g s y m b o l s a r e u s e d : E ( X ) i n d i c a t e s t h a t t h e c a r b o n y l g r o u p i sec l ip s ed by X ; E(H) ind i c a tes tha t the c a r bony l g r oup i s ec l ip s ed by H ; A(X) ind i c a tes tha t X i sa n t i t o the c a r bony l g r oup ; a nd A(H) ind i c a tes tha t H i s a n t i t o the c a r bony l g r oup .

Ta f t cr va lues ha ve been c o r r e l a t ed w i th the vC - -O a nd a bs o r b a nc e va lues f o r a c e ti c a c id a ndi t s de r iva t ives (9 ) . The r e does no t a ppea r t o be a l i nea r r e l a t i ons h ip be tween vC=O a nd Ta f t o ' *va lues . Fo r exam ple, C1 acet ic (v C --O, 1791 vs 1 .05) , C12 acet ic (v C --O, 1784 vs 1 .94) , an d C13a c e t i c (vC=O, 1789 vs 2 . 65 ) in C C 1 4 s o lu t i on . On the o the r h a nd the Vop (C - -O) 2 f r equenc ies f o rthe mono- , d i - , a nd t r i c h lo r oa c e t i c a c ids inc r ea s e in f r equenc y a s the Ta f t a * c ons t a n t s i nc r ea s ein va lue (C1 acet ic ac id , 1737 cm -1 vs 1 .05 ; C12 acet ic ac id , 1744 vs 1 .94; and C13 acet ic ac id ,1752 c m - 1 v s 2 . 65) . The r e fo r e , f a c to r s o the r tha n Ta f t ' s i nduc t ive e f f ec t mus t a f f ec t vC- -O a ndVop ( C - O ) 2. I n t e r m o l e c u l a r h y d r o g e n b o n d i n g b e t w e e n C = O a n d ( C = O ) z , a s d i s c u s s e dp r ev ious ly , i s one f a c to r tha t a f f ec t s thes e s t r e t c h ing modes . The o the r f a c to r ha s been r epo r tedto be the ex i s t enc e o f r o t a t i ona l c on fo r m er s (7 ) .

T h e ~ - h a l o g e n a t e d a c e t i c a c i d s e x i s t i n g i n r o t a t i o n a l f o r m s 1 X - A ( X ) , 2 X - A ( X ) , a n d3 X - A ( X ) X c a n f o r m i n t r a m o l e c u l a r h y d r o g e n b o n d s w i t h t h e a - h a l o a t o m a s d e p i c t e d i nS t r u c t u r e D . T h e i n t r a m o l e c u l a r h y d r o g e n b o n d w o u l d b e e x p e c t e d t o l o w e r t h e v C = Of r e q u e n c y , b e c a u s e t h e C = O b o n d w o u l d b e w e a k e n e d d u e t o t h e i n d u c e d c o n t r i b u t i o n f o rS t r uc tu r e E .

o oI I I _c ~ C ~ o c .c ~ " < oI I I i .X - 9 - H X . . . H

S t r u c t u r e D S t ru c t . u r e E

T h e r e f o r e , t h e p o l a r A ( X ) f o r m s w o u l d b e e x p e c t e d t o e x h i b i t t h e l o w e s t v C = O f r e q u e n c i e sw i t h i n e a c h h a l o s er i es . C o m p o u n d s s u c h a s 2 - m e t h o x y a c e t i c a c i d a n d p y r u v i c a c id i n th e v a p o rpha s e a l s o ex i s t i n s t r uc tu r es s uc h a s (D) a nd (E) . In thes e c a s es the a c id p r o ton i si n t ra m o l e c u l a r l y b o n d i n g t o th e o x y g e n a t o m o f t h e C H 3 0 o r C H 3 C = O g r o u p , r es p ec t iv e ly(1 ,13) .

F i g u r e s 1 0 . 6 - 1 0 . 9 s h o w t h a t i n e i th e r C C 1 4 o r C H C 1 3 s o l u t i o n , t h e v C = O m o d e d e c r e a s e s i nf r e q u e n c y w i t h i n e a c h s e ri e s ( X 3 C O z H , X 2 C H C O O 2 H , X C H 2 C O 2 H , a n d R C O z H ) a s t h e p K ainc rea ses or a s the ac id beco me s less ac id ic (e .g . , see E C1, Br , an d I ) . F igu res 10.1 0-1 0.13 (14)s h o w t h a t i n e i t h er C C 1 4 o r C H C 1 3 s o l u t i o n t h e v C = O o r V op ( C = O ) 2 m o d e i n c r e a se s i nf requ enc y w i th in each of the fo ur ser ies as a* va lue s inc rease (e .g . , see Br 2 , C12, an d F z) . Thus , a sm o r e s i g m a ( ~ r) e l e c t ro n s a r e d o n a t e d t o t h e a c id c a r b o n y l g r o u p , t h e v C = O o r V o p (C - -O ) m o d e


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Variables in Data Interpretation 1 6 9d e c r e a s es i n f r e q u e n c y d u e t o c r e l e c t r o n s b e i n g d o n a t e d t o t h e c a r b o n y l g r o u p , a n d a s m o r e cre l e c t r o n s a r e w i t h d r a w n f r o m t h e a c i d C = O o r ( C = O ) 2 g r o u p s , t h e v C - - O o r V o p ( C = O ) 2in c re a se i n f r eq u en cy . T h i s ag ree s w i th wh a t was d i s cu ssed p rev io u s l y h e re . Ho wev e r , t h eopp osi te o r an e r ra t i c b eha v io r i s no ted in F ig . 10 .6 , Br 3 to Br , C13 to C1 , F 3 to F; F ig . 10 .7 , Br 3 toBr, C13 to C1; Fig. 10.8, Br to Br 3, C1 to C13, F to F3; Figs. 10.9 an d 10.10, B r to Br 3, C1 to C13, F toF3; Fig. 10 .11 , Br to Br 3, C1 to C13; an d Fig . 10 .12 , Br to Br 3. In these cases the vC = O orV o p (C = O ) 2 m o d e d e c r e a s e s i n f r e q u e n c y a s t h e p K a va l u e i s i n c r e a s e d o r t h a t t h e v C = O o rVo p (C - -O) 2 d o es n o t i n c re a se i n f r eq u en cy a s t h e T a f t ~r* v a lu e i n c re a se s . T h i s e r r a t i c b eh av io r i sa t t r i b u t ed t o t h e ex i s t en ce o f r o t a t i o n a l co n f o rm e rs , t h e r e su l t o f r o t a t i o n o f t h e XC H 2 , X2 C H,a n d C X 3 g r o u p s a b o u t t h e C - C - - O b o n d a s d e p i c t e d in F ig . 1 0. 5.

F ig u re 1 0 .1 3 sh o ws p l o t s o f v C - - O f o r t h e h a l o ac e t i c a c id s v s m o le % C HC 1 3 /C C 1 4 so lu t i o n s .I n F ig . 1 0 .13 , two p l o t s a re n o t ed f o r i o d o ace t i c a c id ( 1 4 ) . T h e I R b a n d n ea r 1 7 6 9 c m - 1 i sa s s i g n ed t o t he r o t a t i o n a l c o n f o r m e r I I - E ( I ) a n d t h e I R b a n d n e a r 1 7 3 6 c m - 1 i s a s s i g n e d tor o t a t i o n a l c o n f o r m e r I I - E ( H ) . I n t h e c as e o f c h l o r o a c e t i c a c i d , t w o r o t a t i o n a l c o n f o r m e r s a r en o t e d a t h i g h m o l e % C H C 1 3 / C C 1 4 . T h e l o w e r f r e q u e n c y v C = O b a n d i s a s s i g n e d t o t h er o t a t i o n a l c o n f o r m e r 1 C 1 - E ( H ) a n d t h e h i g h e r f r e q u e n c y IR b a n d t o r o t a t i o n a l c o n f o r m e r1 C 1 - E ( C 1 ) . I n t h e c a se o f f l u o ro ace t i c a c id a t l o w m o le % C HC 1 3 /C C 1 4 so lu t i o n s , t h e l o wf r e q u e n c y IR b a n d i s a s s i g n e d to r o t a t i o n a l c o n f o r m e r 1 F - E ( H ) , t h e h i g h e r f r e q u e n c y IR b a n d i sa s s i g ne d t o r o t a t i o n a l c o n f o r m e r 1 F - E ( F ) . T h e o t h e r 1 - h a lo a c e t i c a c id s ex i s t i n th e f o r m o fr o t a t i o n a l c o n f o r m e r 1 X - E ( X ) . T h e d i h a l o g e n a t e d a c i d v C - - O f r e q u e n c i e s a r e a s s i g n e d t o t h e2 X - A ( X ) r o t a t i o n a l c o n f o r m e r s . D i b r o m o a c e t i c a c i d e x h i b i t s a b a n d i n t h e r e g i o n 1 7 9 7 . 4 -1 7 9 9 . 3 c m - 1 i n g o i n g f r o m CC14so lu t i o n t o C HC 1 3 so lu t i o n , a n d th i s I R b a n d i s a s s i g n ed t or o t a t i o n a l c o n f o r m e r 2 B r - E ( B r ) . T h e t r i h a l o a c e t ic a c i d v ( C = O ) I R b a n d f r e q u e n c i es ( 1 4 ) a r ea s s i g n e d t o r o t a t i o n a l c o n f o r m e r 3 X - A ( X ) .

F ig u re 1 0 .14 sh o ws p l o t s o f Vo p ( C = O) 2 v s m o le % C HC 1 3 /C C 1 4 so lu t i o n s . I n F ig . 1 0 .1 4 , t h ep lo t f o r t r i f l u o ro ace t i c a c id sh o ws th a t Vo p (C - - O) 2 i n c re a se s i n f r eq u e n cy a s t h e m o le %C HC 1 3 /C C 1 4 i n c re a se s ( 1 4 ). T h i s n o t ed e x ce p t i o n i n t h i s se r i e s o f p l o t s i s a t t r i b u t ed t o t h ef o l l o w i n g - - t h e t w o C F 3 g r o u p s i n t h e i n te r m o l e c u l a r l y h y d r o g e n - b o n d e d d i m e r a r e r o t a t i n g t ot h e m o r e p o l a r f o r m a s t h e m o l e % C H C 1 3 /C C 1 4 is in c r e a s e d . T h e m o s t p o l a r f o r m i s w h e r e t h eC F 3 g r o u p s a r e i n r o t a t i o n a l c o n f o r m e r 3 F - E ( F ) . T h e f ie ld e ff ec t o f th e e c l i p s e d F a t o ma p p a r e n t l y o v e r r i d e s t h e f i e l d e f f e c t o f t h e s o l v e n t s y s t e m , a n d t h e i n t e r m o l e c u l a r h y d r o g e nb o n d i n g b e t w e e n C H C 1 3 p r o t o n s a n d t h e c a r b o n y l g r o u p s , b e c a u s e V o p ( C = O ) 2 i n c r e a s e s r a t h e rt h a n d e c r e a s e s i n f r e q u e n c y a s t h e m o l e % C H C 1 3 / C C 1 4 i s i n c r e a s e d . T h e 3 F - E ( F ) r o t a t i o n a lc o n f o r m e r f o r V o p( C = O ) 2 i s r e a s o n a b l e , b e c a u s e t h e O H p r o t o n s i n t h i s c a s e a re i n t r a m o l e c u l a r l yh y d r o g e n - b o n d e d w i t h t h e tw o C = O g r o u p s, w h e r e a s i n th e c a se o f t h e m o n o m e r , t h e 3 X - Ar o t a t i o n a l c o n f o r m e r i s s t a b i l i z e d b y i n t r a m o l e c u l a r h y d r o g e n b o n d i n g v i a ( X - . . H O ) .

I n c o n c l u s i o n , t h e s e h a l o g e n a t e d a c e t i c a c i d s e x i s t a s r o t a t i o n a l i s o m e r s . W i t h i n a s e r i e s t h eh i g h e s t v C = O f r e q u e n c y r e s u l t s f r o m t h e c o n f o r m e r w h e r e t h e h a l o g e n a t o m e c l i p s e s t h ec a r b o n y l o x y g e n a t o m d e n o t e d a s E ( X ). T h e l o w e s t v C = O f r e q u e n c y is w h e r e t h e h a l o g e n a t o mi s a n t i w i t h t h e c a r b o n y l o x y g e n a t o m d e n o t e d a s A (X ) . I n th e a n t i c o n f i g u r a t i o n t h e a c i d p r o t o nb o n d s i n t r a m o l e c u l a r l y w i t h t h e a n ti h a l o g e n a t o m , c a u s i n g a w e a k e n i n g o f t h e C = O b o n d . T h ein d u c t i v e e f f e c t o f t h e h a l o g en a t o m ( s ) i s ( a re ) i n d ep e n d e n t o f m o lecu l a r g eo m e t ry , b u t t h e f ie l de ff ec t i s d e p e n d e n t u p o n m o l e c u l a r g e o m e t r y . T h u s , t h e f i e ld ef fe c t u p o n t h e C = O g r o u p i sr e s p o n s i b l e fo r t he r e l a ti v e ly h i g h v C = O f r e q u e n c y w i t h r o t a t i o n a l c o n f o r m e r s w i t h E ( X )s t r u c t u r e , b u t n o t w i t h A ( X ) s t r u c t u r e ( 1 4 ) .


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170 Carboxylic AcidsI t ha s been found tha t the type o f c a r boxy l i c a c id o f f o r m RCO2H ha d to be iden t i f i ed be fo r e

i ts a p p r o x i m a t e p K a v a l u e c o u l d b e c a l c u l a t e d f r o m v C = O a n d V o p ( C = O ) 2 f r e qu e n c i e s r e c o r d e di n C C 1 4 s o l u t i o n ( 1 0) . T h e v C = O a n d V o p (C = O ) 2 f r e q ue n c i e s r e c o r d e d i n C C 1 4 s o l u t i o n h a v eb e e n r e p o r t e d t o c o r r e l a te i n a l i n e a r m a n n e r w i t h p K a v a l u e s ( 1 1 ) . I t h a s b e e n r e p o r t e d t h a tVop(C- -O) 2 ba nd in tens i t i e s f o r a r om a t i c c a r bo xy l i c a c ids a r e h ighe r tha n thos e f o r a l i pha t i cc a r bo xy l i c a c ids (12 ) . In the va po r -pha s e a t t emp er a tu r e s a bove 150 ~ c a r bo xy l i c a c ids ex i s to n l y i n t h e m o n o m e r i c s t a t e . A t l o w e r t e m p e r a t u r e b o t h t h e m o n o m e r a n d d i m e r c a r b o x y l i c a c i dc a n e x i s t i n e q u i l i b r i u m . W i t h i n c r e a s e i n t e m p e r a t u r e t h e e q u i l i b r i u m s h i f t s t o w a r d t h em o n o m e r i c s p e c i e s ( 1 , 1 3 ) .

4 - X - B E N Z O I C A C I D STa b le 10 . 4 li s ts IR da t a a nd a s s ignm en ts f o r 4 -X-benzo i c a c ids in the va po r , C C14 a n d / o r CHC13s o l u t i o n p h a s e s . T h e O H , C = O , a n d ( C - - O ) f r e q u e n c y r a n g e s a n d a s s i g n m e n t s a r e c o m p a r e d

rangerange

here:

range

vapor, c m - 1 ( 1 ) C C 1 4 , cm 1 (14) CHC13, cm 1 (14)v O H v O H . . . C IC C 1 3 v O H . 9 C1 H CC1 23582-3595 3529-3544 3519-3528vC=O vC=O vC---O. 9 HCC l31758-1768 1735-1751 1719-1744Vop C=O )2 Vop C=O)2 Vop(C=O)2 999 HCCI3)21689-1707 1687-1707

T h i s c o m p a r i s o n s h o w s t h a t t h e v O H , v C = O , a n d v a sy m . ( C = O ) 2 d e c r e a se i n f r e q u e n c yp r og r es s ing in the o r de r va po r , CC 14 , a nd CHC 13 s o lu t i ons . In the c a s e o f Vop (C =O ) 2 , thef r equenc y c ha n ge in go ing f r o m C C14 a nd CHC13 i s s ma l l due to the f a c t tha t i n the d im er f o r me a c h O - H g r o u p is a l re a d y h y d r o g e n b o n d e d t o e a c h o f t h e C = O g r o u p s, a n d th e O - H o x y g e na t o m i s m u c h l e s s b a s i c ; t h e r e f o r e , t h e C H C C 1 3 p r o t o n w i l l n o t f o r m a s t r o n g i n t e r m o l e c u l a rhy d r o gen bo nd a s in the c a s e o f vC =O . . . HC C 13 . S ee Ta b le 10 . 4a fo r the f a c to r s tha t a f fec t theC O 2 H a n d ( C O z ) 2 g r o u p s f o r 4 X - b e n z o i c a c i d s i n C H C 1 3 a n d C C 1 4 s o l u t i o n s .

F igu r e 10 . 15 s hows p lo t s o f v O H f o r 4 -X-benzo i c a c ids vs Ha mmet t ' s r r p va lues . Thes e p lo t ss ho w a r e l a t i o ns h ip w i th the Ha mm et t O-p va lues , bu t m a n y o f the po in t s f al l o ff the l i nea r l i ne .

F igu r e 10 . 16 s hows p lo t s o f vC - -O fo r 4 -X-benzo i c a c ids vs Ha mm et t s O-p va lues . L inea rr e l a t i o n s h i ps a r e a p p a r e n t e x c e p t f o r t he 4 - t e r t -b u t y l a n d 4 - m e t h o x y a n a l o g s f o r C H C 1 3 s o l u t i o n(14) .

F igu r e 10 .17 s ho ws p lo t s o f Vop(C=O) 2 f o r 4 -X-benzo i c a c ids vs H a m me t t s ~rp va lues . T hep lo t s f o r bo th CC14 a nd CHC 13 s o lu t i o ns a r e l i nea r i n th r ee d i f f e r en t s egmen ts (14 ) .

F igu r e 10 . 18 s hows p lo t s o f Vop (C =O ) 2 f o r 4 -X-benzo i c a c ids vs m o le % CHC13/CC14 . T he C1,Br, H , CH3 , t e r t - bu ty l , a nd m eth ox y a na logs a l l dec r ea s e es s en ti a l l y l i nea r l y in go in g f r om CC14 toCHC13 s o lu t i ons . In a dd i t i on , the f r eq uenc y s epa r a t i on be tw een thes e l i nea r p lo t s dec r ea s es a sHa m me t t s Crp va lues inc r ea s e in va lue. Ho wever , i n the c a s e o f the 4 -n i t r o a nd 4 - c y a no a na logsthe Vop(C- -O) 2 fr equenc ies a c tua l l y inc r ea s e in go ing f r om CC l 4 to CHC 13 s o lu t i on .

I n C C 1 4 s o l u t i o n , t h e 4 -n i t r o a n a l o g w o u l d h a v e t h e m o s t a c i d i c O H p r o t o n a n d t h e m o s tb a s ic C - O o x y g e n a t o m s . T h er e f o r e, a c o m p l e x s u c h a s ( F ) w o u l d c a u s e th e C = O b o n d t o b e


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Variables in Data Interpretation 171

x9 c i 3 c c i

9 H ~ c ~ ~ - ~ . . ~ I 9 H / ~II o o

~ , ( ~ 0 " - , o / ~ . . " - c . = c , ,x A . J :H----CC~

Complex F Com plex Gs t r eng thened . Ther e fo r e , t h e V o p (C = O ) 2 m o d e w o u l d v i b r a t e a t a h i g h e r f r e q u e n c y t h a n i n a c a ses u c h a s C o m p l e x G . In a d d i t io n , t h e N O 2 g r o u p o f a r y l NO 2 h a s b e e n s h o w n t o f o r mi n t e r m o l e c u l a r h y d r o g e n b o n d s w i t h C H C 1 3 ( 1 5 ), a n d b e t w e e n C 1 3 C H . . -N C f o r b e n z o n i t r i l e( 1 6) . I n t e r m o l e c u l a r h y d r o g e n b o n d i n g o f th e C H C 1 3 p r o t o n w i t h t h e 4 - N O 2 o r 4 - c y a n o g r o u pwou ld a l s o c a us e a n inc r ea s e o f V o p ( C = O ) 2 i n t h e se ( C l s C - H C 1 C C 1 3 ) . c o m p l e x s o l u t i o n s .M o r e o v e r, i n t e r m o l e c u l a r h y d r o g e n c o m p l e x e s i n v o l v i n g t h e C H 3 C 13 p r o t o n w i t h t h e g s y s t e mo n o n e o r b o t h s i d e s o f t h e p l a n a r p h e n y l g r o u p w o u l d a l s o c a u s e v C - - O t o i n c re a s e i n f r eq u en c y .These Vop(C~O) 2 shi f ts a re 0 .27 an d 0 .65 cm -1 for the NO 2 a n d CH30 ana logs , respec t ively . Fort h e C1, B r, H , C H 3, (C H 3 ) C , a n d C H 3 0 t h e se s h if t s a r e - 1 . 0 1 , - 1 . 1 0 , - 1 . 3 3 , - 2 . 3 4 , - 2 . 2 7 , a n d-2 . 52 , r e s pec t ive ly .

F igu r e 10 . 18 s how s p lo t s o f vC =O fo r 4 -X-benzo i c a c ids vs mo le % CHC13/CC14. In CC14s o lu t i on , the Br a na log o c c u r s a t a h ighe r f r equenc y tha n the C1 a na log , a nd in C HC13 s o lu t i o nt h e C N a n a l o g o c c u r s a t a h i g h e r f r e q u e n c y t h a n t h e NO 2 a na log . Thes e c a s es a r e exc ep t i ons tot he c o r r e l a ti o n o f v C = O v s H a m m e t t O'p va lues .

T h e 4 -t e r t .- b u t y l b e n z o i c e x h i b i t s t w o v C - O b a n d s . T h e l o w e r fr e q u e n c y b a n d f r e q u e n c ie sl i st e d b y t h e c o m p u t e r a r e i n th e r a n g e 1 7 2 1 . 5 5 - 1 7 1 8 . 0 c m - 1 T h e v C = O f o r t h e 1 7 2 1 . 5 5 -1 7 1 8 . 0 c m - 1 I R b a n d i s i n g o o d a g r e e m e n t w i t h t h e v C - - O v s H a m m e t t % v a l ue s . T h e h i g h e rv C = O f r e q u e n c y b a n d f o r 4 - t e r t- b u t y l b e n z o i c i n c r e a se s i n f r e q u e n c y a s t h e m o l e % C H C 1 3 /C C 1 4i s inc r ea s ed wh i l e no ne o f the o the r 4 -X-benzo i c a c ids s how th i s oppo s i t e t r end . A pos s ib l ee x p l a n a t i o n f o r t h i s v C = O f r e q u e n c y b e h a v i o r i s t h a t t h e t e r t - b u t y l g r o u p i s h y p e r c o n j u g a t e d ,a n d t h a t t h e C 1 3 C - H p r o t o n i s h y d r o g e n b o n d e d t o t h e p h e n y l g s y s t e m w h i l e t h e p o s i t i v e l yc h a r g e d t e r ti a r y c a r b o n a t o m i s a s s o c i a te d w i t h t h e C 1 3 C H c h l o r i n e a t o m . S u c h a n i n t e r a c t i o nw o u l d c a u s e t h e v C = O m o d e t o i n c r e a se i n f r e q u e n c y a s o b s e r ve d . B e c a us e 4 -t e r t .- b u t y l b e n z o i ca c id exh ib i t s two v C - O f r equenc ies i t i s a ppa r en t tha t i t ex i s t s a s c lu s te r s i n CHC13 s o lu t i ons .A l l o f the 4 -X-benzo i c a c ids s how b r ea ks in the p lo t s i n the r a nge 1 0 - 30 m o l % CHC13/CC14s o lu t i on , wh ic h ind i c a tes tha t o the r CHC13/CC14 c o mple xes o f (C13C -H. . -C1CC 13) n w i th the 4-X-benzo i c a r e a l s o p r es en t .

Ta b le 10 .5 l i st s IR va po r -pha s e da t a a nd a s s ignm en ts f o r a n th r a n i l i c a c ids. Thes e a c ids ha vethe f o l l ow ing ba s i c s t r uc tu r e : H

I

IOH


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172 Carboxylic AcidsThe vO-H stretching frequencies occur in the range 3484-3494 c m - 1 . The vasym. NH 2 mode

occurs in the range 3518 -35 25cm -1 and vsym. NH 2 in the range 33 82 -339 5c m -1. Thefrequency separation of 123-141 c m - 1 indicates that the N-H proton is intramolecularlyhydrogen bonded to the C=O group as was illustrated here. A weak band in the region3430-3436 c m - 1 is assigned to vOH. . .N . The vC=O mode is in the range 1724-1732 c m - 1 . TheNH 2 bending mode is exhibited in the range 1613- 162 5cm -1. A medium-s trong band in therange 1155-1180 c m - 1 is attributed to O- C= stretch.

N-methyl anthranilic acid and N-phenyl anthranilic acid exhibit vN -H . . .O=C at 3392 and3348 c m - 1 , respectively. The N -H . . . O- -C intramolecular hydrogen bond is stronger in the caseof the N-phenyl analog due to the inductive effect of the phenyl group compared to that for theN-methyl group.

The vC=O...H-N frequencies occur near 1719 c m - 1 for N-methyl and N-phenyl anthranilicacid compared to 1724-1732 cm -1 for the other anthranilic acids studied.

Table 10.6 lists infrared data for acrylic acid and methacrylic acid in 0 to 100mol %CHC13/CC14 solutions. The sym. CH 3 bend ing mode for methacryli c acid increases steadilyfrom 1375.5 to 1377.8cm -1 as the mole % CHC13/CC14 is increased from 0 to 100, while theCH2= wag mode for methacrylic acid increases wag only 0.5 cm-1, from 948.7 to 949.2 cm-1. Inthe case of acrylic acid, the CH2-- bending mode decreases steadily from 1433.1 to 1429.9 c m - 1as the mole % CHC13/CC14 increases from 0 to 100, while the viny l twist mode varies from 983.9to 983.6 to 984 c m - 1 .

Table 10.7 lists Raman data for carboxylic acid salts (6). In some cases vasym. CO 2 is observedin the Raman, and its intensity is always less than the Raman in tensi ty for vsym. CO 2. TheRaman bands in the region 138 3-1 468 cm -1 are assigned to vsym. CO 2.

Frequency assignments for vasym. CO 2 and vsym. CO 2 are summarized in Table 10.8. Studyof this table suggests that the inductive effect causes vasym. CO 2 for the dich loroace tate ion tooccur at higher frequency than the other carboxylate ions.

Table 10.9 lists IR data for carboxylic acid salts. This table demonstrates as well that theinduct ive effect also increases the vasym. CO 2 frequency as it increases from 1585cm -1 forsodium acetate to 1640 c m - 1 for sodium difluoroacetate. It appears that a divalent cation such ascalcium (Ca) lowers the vasym. CO2 frequency compared to a monovalent cation such aslithium (Li) or sodium (Na). Compare lithium and calcium formate (1604 vs 1596cm -1) andsodium and calcium 2-ethylhexanoate (1555 vs 1545 c m - 1 ) while the opposite effect is noted forvsym. CO 2 (1382 vs 1400 cm -1) and (1415 and 1424cm -1) for the formate and 2-ethyl-hexanoates, respectively.

H A L F S A L T S O F C A R B O X Y L I C A C I D SSodium hydrogen diacetate is an example of a transmission anomaly observed within broadabsorption bands of solids. This anomally indicates that there are perturbations betweenoverlapping energy levels in the solid state.

Sodium hydrogen diacetate is classified as a type A acid salt with a centrosymmetric anion:Sodium hydrogen diacetate has 24 molecules per unit cell in its crystal structure (18). The

broad intense absorption is determined by coupling between two or more vibrating(- O. -- H. . . O- ) n groups (19).


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V a r i a b l e s i n D a t a I n t e r p r e t a t i o n 1730/ . "

HsC--C.\ ' ,o - . . H . . . 0N a +

T h e r e ar e t w o C - C s t r e tc h m o d e s i n th e c e n t r o s y m m e t r i c a n i o n , a v s y m . ( C - C ) 2 a n d v a s y m . ,( C - C ) 2 . T h e v a sy m . ( C - C ) 2 m o d e y i el d s a st r o n g R a m a n b a n d a t 92 0 c m - 1 , a nd i t i s th i s modet h a t in t e ra c t s , c a u s i n g t he t r a n s m i s s i o n w i n d o w i n t h e b r o a d b a n d a t 9 2 0 c m - 1 (see Fig. 10.19a).F igu r e 10 . 19b i s tha t f o r the (CD 3) 2 a na log , a nd in th i s c a s e the va s ym. C - C mo de h a s s h i f t ed to8 7 4 c m - 1 e x h i b i t i n g t h e t r a n s m i s s i o n a n o m a l y a t t h i s f r eq u en c y . A n o t h e r a n o m a l y n e a r840 c m - 1 r e s u l t s f r om the CD 3 r oc k ing m od e a t th i s f r equenc y (20 ) .

A p p l i c a t i o n o f t h e p e r t u r b a t i o n t h e o r y r e d i s t ri b u t e s b o t h e n e r g y l e ve l s a n d a b s o r p t i o nintens i ty . Ne ar E ~ the in te rac t ion is grea tes t and the resu l t ing sh i f t o f the levels leads to a fa l li n ene r gy - l eve l dens i ty . The l o s s in a bs o r p t i on in tens i ty in th i s r eg ion a ppea r s a s a ga in in theo the r r eg ions a nd the pe r tu r ba t i on invo lves a l l the E ~ a nd E ~ l evel s . The in ten s i ty m a y ber ed i s t r i bu ted ove r a r e l a t ive ly l a r ge f r equenc y r a nge . Th i s y i e lds w i th in the b r oa dba nd a na r r owr e g i o n o f i n c r e a se d t r a n s m i s s i o n , w i t h s o m e r e g i o n s o f i n c r e a s e d a b s o r p t i o n n e a r b y ; t h e l a t t e rm i g h t n o t b e e a s i l y n o t e d i n t h e s p e c t r u m ( 2 0 ) .

Bec a us e Eva ns wa s the f i r st to exp l a in thes e pec u l i a r e f fec t s tha t a pp ea r s o met im es in the s o l ids t a te IR s pec t r a , i t i s now r e fe r r ed to a s the "Eva ns Tr a ns mis s ion Ho le" o r "Eva ns Ho le . "

R E F E R E N C E S1. Nyquis t , R. A. (1984) . The Interpretation of Vapor-phase Infrared Spectra: Group Frequency D ata. Ph i l ade lph i a : Sad t l e r

Resea r ch Labo r a to r i e s , Div i s ion o f Bio -Rad .2 . Nyquis t , R. A., Clark , T. D., and Streck, R. (1994) . Vib. Spectrosc., 7, 275.3. Taft, R. W. Jr. (1 956). In Steric Effects in Organic Chemistry, M. S . Newman (ed . ) , Chap . 3 , New York : Wi ley .4 . Buck ingham, A. D. (1960) . Can. J. Chem ., 308, 300.5 . Nyquis t , R. A., Putz ig , C. L., and Hasha, D. L. (1989) . Appl. S pectrosc., 1049.6. (1984) . Sadtler Standard Raman Spectra, Ph i l ade lph i a : Sad t l e r Resea r ch Labor a to r i e s , Div i s ion o f Bio -Rad Labor a -

tor ies .7 . Nyquis t , R. A. and Clark , T. D. (1996) . Vib. S pectrosc., 10, 203.8 . Ch r is tense n, J . J . , Hanse n, L. D., and Isat t , R. M. (1976) . Handbook of Proton Ionization and Related-ThermodynamicQuantities, New York : Wi ley In te r s c i ence .9 . B el lanato, J . and Barac el6 , J . R. (1960) . Spectrochim. Acta, 16, 1333.

10. G olde n, J. D. S . (1954) . Spectrochim. Acta, 6, 129.11. St. Fleet, M. (1962). Spectrochim. Acta, 18, 1537.12. Bro oks, C. J . W., Egl in ton, G ., and M ora n, J . F . (1961) . J. Chem . Soc. , 106.13. Welt i , D. (1970) . Infrared Vapour Spectra, Lo n d o n : H e y d e n , i n c o o p e r a t i o n w i t h S a d t l e r Re s e a r c h La b o r a t o r i e s ,

Ph i l ade lph i a , PA.14. Nyquis t , R. A. and Clark , T. D. (1995) . Vib. S pectrosc., 8, 387.15. Nyquis t , R. A. and Set t iner i , S . E. (1990) . Appl. S pectrosc., 44, 1552.16. Ny quis t , R. A. (1990) . Appl. S pectrosc., 44, 1405.


10/39

1 7 4 Carboxylic Acids17. Ny quis t , R. A., Putz ig , C . L., and L eugers , M. Anne (19 97) . Infrared and Ra ma n Spectral Atlas of Inorganic Compoundsand Organic Salts , vol . 1 , p . 72. Academic Press , Boston.18. S plak ma n, J . C. and Mil ls , H. H. (1961) . J. Chem . Soc. , 1164.19. Alber t , N. and Badger , R. M. (1958) .J . Chem. Phys . , 29, 1193.20. Evans, J . C. (1962) . Spectrochim. Acta, 18, 507.


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Carboxylic Acids in physics