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Explanat ion. The reason why the actual order i s di fferent from the expected order may be explained as fol lows :

The basicity ofan amine in aqueous solution d oes not entirely depend upon the electron den sity on the N-atom but also

depends upon the stability of the conjugate acid formed by accepting a proton from the solution.

Th e stability of the

conjugate acid, in turn, depends upon the extent of H-bonding. Obviously, greater the number of H-atoms on the N-atom.

more stable is the conjugate acid. Thus, the conjugate acid of 1 amine is the most stable since it has

three H-atoms

which

can form H-b onds ; the conjugate acid of 2 amine is

less stable

since it has two H-atom s whi le that of 3 amine is the

least

stable

since it has only

one H-atom

which can form H-bond as shown below :

H OH

2

R N H

2

+ H

1

> R N H O H

2

1Amine

H O H

2

(Most stable)

R

\ R \ + / H OH

2

) N H + H

+

n

2

r /

x

h o h

2

2 Amin e (Less stable)

R \ R \

R N + H > R N H O H ,

R R ^

(Least stable)

Thu s, on the basis of stability of the con jugate acids alone, the basic strength of am ines sh ould fo llow the order :

R N H

2

> R

2

N H > R

3

N .

In actual pract ice, these two opposing factors balance each other in case of 2 amine. T his makes 2 amine to be the

strongest. 3Amines are weaker bases than 2 amines since their conjugate acids are less stable than those of 2 am ines

while 1

0

amines are less basic than 2 amines since the electron density on the N-atom is less and he nce the lone pair of

electrons on nitrogen is less easily available for protonation.

0b) Effect of e lectron-withdrawing groups. Electron-wididrawing groups because of their - I -

effect tend to decre ase the electron d ensity on the N-ato m there by mak ing the lone pair of electrons less

readily available for protonation. Thus, the presence of electron-withdrawing groups on the N-atom

decreases the basicity of amines. For example, tris (trifluomethyl) amine is virtually non basic due to

F

3

C CF

3

power fu l - I - e f f e c t o f the th ree CF

3

groups.

+

CF

3

1.10.4. FIEL D EFF EC TS*

We have discussed above that inductive effect operates through bonds. There is another effect which

does not operate through bonds but

occurs directly through space or solvent molecules. This is called

field

effect. Although it is very difficult to separate inductive effect from field effect, it has been done in a

number of cases generally by taking advantage of the fact the field effect depends upon the geometry of the

molecule but the inductive effect depends upon the nature of bonds. For example, in isomers Ia nd II,die

inductive effect of the chlorine atoms on the position of electrons in the COOH group should be the same

since the same bonds intervene. In odier words, acidity of die two acids should be die same.

H w C I

C 1

W

H

Hv A - C I C k A - H

C O O H

C O O H

p K

a

= 6-07 pK

a

= 5-67

* Not included in K.U. and M.D.U. syl labi .

P r o d e e p ' s

O R G A N I C C H E M I S T R Y V O L . I .


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R C O O H + H

2

O ^

N

R C 0 0 - + H

3

0

+

The strength of an acid is usually expressed in terms of i ts dissociat ion or acidi ty constant K

a

.

[ R C O O ] [ H

3

0

+

]

K a =

[ R C O O H ]

0 r

= g K

a

Since K

a

is directly proportional to the concentration of H

3

0

+

, therefore,

higher the value ofK

a

or

lower the value of pK

a

, stronger is the acid.

Alternatively, the value of K

a

depends upon the con centrat ion

of carboxylate anion (R C O O ) wh ich, in turn, depends upon i ts s tabil i ty . In other words, any facto r

which stabil izes the RCO O anion relat ive to RC OO H, should increase the acidi ty while any factor

which destabil izes the RC OO anion relat ive to RCO OH de creases the acidi ty of the carboxylic acid.

In the l igh t o f these arguments , i t i s expected tha t e lec t ron donat ing g roups (EDG) because

of the i r + I - ef f ec t would de s tab i l ize the carbox yla te an ion r e la t ive to carboxyl ic ac id by in te ns i fy ing

the negat ive charge and hence shou ld decrease the ac id i ty o f the carboxyl ic ac id . Conver se ly ,

e l ec t r o n - w i t h d r aw i n g g r o u p s ( E W G ) b ecau s e o f t h e i r - I - e f f ec t w o u l d s t ab i l i z e t h e ca r b o x y l a t e

an ion r e la t ive to carboxyl ic ac id by d isper s ing i t s negat ive charge and hence shou ld increase the

acid i ty o f the carboxyl ic ac id

E WG C t > 0 E D G * C t > 0

0 _ J O j

E W G dispe r ses -v e c harg e by w i thdraw ing EDG in tens if ies the -ve charge by donating e lec trons,

e l ec t ro ns , s t ab i l i ze s the ca rb ox yl a t e ion , des tabi l izes the carboxyla te ion , decreases the ac id

increases the acid s trength strength

In nutshell it may be stated that electron-withdrawing groups increase the acidity while electron-

donating groups decrease the acidity of carboxylic acids.

Let us now briefly discuss the effect of various substituents on the acidity of carboxylic acids.

( / ) Ef f ec t o f e le c t ron -don at in g su bs t i tuen ts alky l g roup s . Let us compare the r e la tive s t r eng th

of formic acid (HCOOH) and acetic acid (CH

3

C O O H )

O O

II II

H C O H C H

3

C - > O - * H

Formic acid

(pK

a

= 3-75) Ace tic acid

{pK

a

= 4-76)

M ethyl grou p presen t in acetic acid d ue to its + I-effec t increas es th e electron-d ensity in the

OH bond. As a result , release of H

+

ions f rom acetic acid wil l be more dif f icult as compared to

formic acid. Hence,formic acid is a stronger acid than acetic acid.

Alternatively, C H

3

grou p bec ause of its + I-effect intensifies the negativ e charge on acetate ion

and thus destabilizes it relative to formate ion. Hence, formic acid is a stronger acid that acetic acid.

C H

3

- ^ - c ^

o

> 0

Acetate ion

STRUCTURE AND BON DING


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(;/ ) Effe ct of electron-w ithdra win g group s.

Let us consider the relative acid strength of acetic

acid and chloroacetic acid.

O O

II II

CH

3

- > C - 0 -> H C 1 ^ < - C H

2

< - C < - H

Acetic acid

(p K

a

= 4-7 6) Chloroac etic acid

(pK.

a

= 2-86)

The chlorine atom present in chloroacetic acid becau se of its -I- eff ec t withdraw s electrons from

the O H bond. As a result, the electron density in the O H bond decrease s. In other wo rds,

the O H bond w eakens and thus the release of H

+

ions from chloroacetic acid is facilitated relative to

acetic acid.

Hence, chloroacetic acid is a stronger acid than acetic acid.

C H

3

C L

> 0 C l - ^ - C H

2

- c f

> 0

N > J > J

C H

3

group donates e lect rons and Cl-a tom wi thdraw s e lect rons and

thu s destabilizes the acetate ion thu s stabilizes the chloroa cetate ion

Alternatively,

Cl-atom because of its

-I-effect

tends to disperse the negative ch arge on chloroac etate

ion while CH

3

group because of its + I-effect tends to intensify the negative charge on acetate ion. As

a result, chloroacetate ion is more stable than acetate ion and hence chloroacetic acid is a stronger acid

that acetic acid.

Important conclusions.

From the above discussion following conclusions can be drawn :

(i) The stronger the + I-effect of the substituent, or greater the number of electron-donating

groups, the weaker is the acid.

For example,

C H

3

C H , - M - C O O H > C H X H , - > C O O H > ' ^ C H CO OH > C H , > ~ C > - C O O H

3 3 2

C H , ^

3

A

C H j - s .

/

t

Ethanoic acid Propanoic acid , , , ,

2 -Methylpropanoic

acid

3

2 , 2 -D i m e t h y l p ro p an o i c

acid

(/;) The stronger the -I-effect of the substituent, strong er is the acid. For example,

0

2

N * - C H

2

< - C O O H > N C < - C H

2

F K= 2 -4 7 )

(p K

a

- 2-57)

CI Br C H

2

< - C OO H > I C H

2

< - C O O H

Chlor oacetic acid Brom oacetic acid Iodoac etic acid

(pK-

a

= 2-86)

(p K

a

= 2-90) (pK

fl

= 3-16)

This is due to the reason that -I-effect decreases in the order :

- N 0

2

> -C N > -F > -C I > -B r > - I .

Further,

largethe number of electron withdrawing substituents, stronger is

the acid. For example,

CI CI

+ I

C I C < C O O H > CI


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However, the acidity of the two acids has been found to be different. This is obviously due to the

field effect which is different for the two isomers. In isomer I, the two chlorine atoms being closer to

the -COOH group, exert greater field effect as compared to that in isomer II. Consequently, isomer II

is a stronger acid than isomer I. This is supported by their pK

a

values. From die above example, it

appears that sometimes field effects are even more pronounced tiian inductive effects.

An example of field effect which occurs through solvent molecules is provided by comparison of

first and second dissociation constants of dicarboxylic acids (Table 1.5).

TABLE 1.5. Dissociation constants of some dicarboxylic acids

Acid

PKJ

K\ K I

Oxalic '

1

1-271 4-266

989

Malonic acid

2-86

5-70 692

Succinic acid 4-21 5-64 26-9

Glutaric acid

4-34 5-27

8-51

Adipic acid 4-41 5-28 7-41

Since these acids contain two carboxyl groups, ionization can occur in two stages :

H 0

2

C ( C H

2

) C 0

2

H + H

2

0 ^

N

H 0

2

C ( C H

2

) C 0

2

+ H

3

0

+

...(/)

H 0

2

C ( C H

2

)

c o

- + H

2

0

v

n

- 0

2

C ( C H

2

)

m

C 0

2

+ H

3

0

+

...(H)

During the first stage of ionization, i.e., equilibrium (/'), the acid can lose a proton from two

positions but the half ion (conjugate base, CBj) can add a proton to only one position. If no other

factors operate, it is reasonable to assume that the acid in equilibrium (/), should be twice as strong as

a monocarboxylic acid,

i.e.,

K' = 2 x K

fl

of RC0

2

H .

During the second stage of ionization,

i.e.,

equilibrium (//), the half ion can lose a proton from only

one

position but the dianion (CB

2

) can add a proton to

two

positions. Therefore, for equilibrium

(if),

K^

= 1/2 K

a

of RC 0

2

H . Thus, equilibrium (/') is satistically four times as favou red as equilibrium (//). Thus,

on the basis of above assumption that no other factors operate, the value of K* is expected to be four

times K^. In practice, the two constants widely differ from the factor 4 (Table 1.5).

To explain the above experimental results, it has been suggested that electrostatic effect of the

negatively charged carboxylate ion, CBj, would require additional energy to separate the proton from

the carboxyl group. In other words, a field effect occurs through the solvent and this increases the ratio

K* / K^ above 4. Further as the chain length increases,

i.e.,

as ^-increases, both the field effect and the

+ I-effect would decrease and so the ratio would approach 4.

| 0 | ELECTROM ERIC EFFECT

Unlike inductive effect, electromeric effect is a temporary effect and operates in unsaturated

compounds only at the demand of the attacking reagent. It involves com plete transfer of %-eleclrons of

a multiple (double or triple) bond to one of the bonded atoms. Consider, for example, the attack of a

nucleophile (i.e. ~CN) to a carbonyl group. In the presence of a nucleophile, the 7i-electrons of the

carbon-oxygen double bond are completely transferred to the mote electronegative oxygen atom :

Nucleophile added -,

+

> c = o * = = = f e . i > c - r O I

Nucleophile removed

* In c l u d ed i n Pa n j a b an d Ja m m u U n i v e rs i t y Sy l l ab i

STRUCTURE AND BONDING


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As a result of this electron displacement, the posi t ive charge on the carbon atom of the carbonyl

group increases and this facilitates the attack of the nucleophile on it.

Such a complete transfer of the

electrons of a multiple bond to one of the bonded atoms (usually m ore electronegative) in the presence

of an attacking reagent is called the

e lec tromer ic e f f ec t

or simply the

E-effect .

I If , how ever, the attacking reagent is remove d without the reaction being allow ed to take plac e, the

molecule reverts back to the original state. Thus, electromeric effect is only temporary in nature.

1.11.1. TYPES OF ELECTROMERIC EFFECT

Like inductive effect , electromeric ef fect is also of two types, i.e., + E - e f f ec t an d - E - e f f ec t .

If the %-electrons are transferred to that atom of the double bond to which the attacking reagent

gets finally attached, the effect is called + E-e ffe ct . For example, the addit ion of acids to alkenes.

> C > C C < ( + E - e f f e c t)

H

I f ,on the other han d, % -electrons are transferred to an atom of the double bond other than the one

to which the attacking reagent gets finally attached, the effect is called

- E - e f f ec t . Fo r exam p l e , th e

addition of cyanide ion to the carbonyl group.

> C * = 0 ^ +

X

C N > C O - ( - E - e f f e c t )

C N

Sig n i f ic anc e o f e lec t ro m er i c e f f ec t . I t may be no ted that so f ar , i t has no t been poss ib le to

dist inguish exper imental ly between I - and E-effects . But the fact that the react ivi ty of a molecule

increases considerably upon the close approach of the reagent def ini tely suggests the involvement of

some electronic ef fect such as electromeric ef fect . In cer tain m olecules both I - and E-effe cts operate

together. Sometimes they assist or reinforce each other and sometimes they oppose, Whenever they

oppose each other, it is always the E-effect which dominates over the I-effect.

1.11.2. COMPARISON O F INDUCTIVE AN D ELECTROMERIC EFFECTS

INDUCTIVE EFFEC T

ELECTROMERIC EFFECT

1. I t operates only in saturated compounds

containing at least one polar cr-bond.

2. It is a permanent effect and involves the mere

displacement of a-electrons.

3. It does not require any outside attacking reagent

for its operation.

4. The displaced e lectron pair does not leave its

molecular orbital but due to inductive effect,

there occurs a slight distortion in the shape of

the molecular orbital of the polar a-bond.

5. Due to inductive effect only a partial separation

of positive and negative charges occurs but ions :

are not formed.

1. It occurs in unsaturated compounds containing

at least one double or a triple bond which may-

or may not be polar in nature.

2. It is a t emporary e f fec t and involves the

cleavage of a multiple bond with complete

transfer of a shared pair of electrons to one of

the bonded atoms.

1 3. It takes place only in the presence of an attacking

reagent.

4. The e lec t ron pai r which gets t r ans fer r ed

completely during electromeric effect leaves its

original molecular orbital and gets transferred

to a new molecular orbital.

5 . Due to electrom eric ef fe ct , there occ urs a

complete transfer of electrons from the reagent

to the substrate or

vice-versa

and hence ions

are always formed.

mmmmmmwm * mmMm& x-.mmss m

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