The electron donating groups are those which donate the electrons with respect to the carbon atom. These groups are called +I groups and this effect is called +I effect.
Electron Withdrawing and Electron Donating Groups – Inductive Effect
The inductive effect is the displacement (partial shifting) of the sigma electron towards a more electronegative atom of the sigma bond. Due to this effect, partial charges are developed. Start with a general example:
C-C-C-X
- The electronegativity of X is greater than C
X is more electronegative so the electrons are displaced toward the more electronegative atom X. so X gets a partial negative charge and C gets a partial positive charge.
C-C-C-Y
- The electronegativity of Y is less than C
Carbon gets a partial negative charge and the Y atom gets a partial positive charge because C is a more electronegative atom than Y so the electrons are displaced toward the more electronegative atom C. And this is called the inductive effect. (This effect is not in pi-bond)
Electron Withdrawing Groups
If a group (like X) attracts the electrons with respect to carbon is called an electron withdrawing group.
C-C-C𝛿+– X𝛿−
The electron withdrawing groups are also called -I groups and this effect is called the -I effect.
Order of -I groups
–+NF3 > –+NR3 > –+NH3 > -NO2 > -C≡N > -COH > -COR > -COOH > -F > Cl > Br > I > -OR > -OH > -C≡CH > NH2 ≈ ph > -CH = CH2 > H
Electron Donating Groups
if a group (like y) donates the electrons with respect to the carbon atoms is called electron donating group.
C-C-C𝛿-– X𝛿+
The inductive effect travels and decreases with distance. It means that this effect is not stopped on the first carbon. This effect travels up to three carbons and then vanishes.
C3𝛿+-C2𝛿+-C1𝛿+– X𝛿−
Order of +I groups
CH2– > NH– > O– > COO– > (CH3)3C+ > (CH3)2HC+ > CH3CH2+
What is the importance of inductive effect in Organic chemistry?
Let us take the question of stability,
Which of the following is more stable?
(a) F-CH2-COO–
(b) Cl-CH2-COO–
(c) Br-CH2-COO–
(d) I-CH2-COO–
The charged species are unstable as compared to the neutral species. The longer the charge remains in place, the more unstable the specie. Due to the more electronegativity of fluorine, it attracts electron more and more electron is distributed so it is more stable. So the order of stability is (a) > (b) > (c) > (d).
With the help of stability, we also find the acidic order. Let a weak acid that is partially dissociated into its ions, HA.
HA ⇌ H+ + A– (conjugate base)
A good acid is one whose conjugate base is stable. The more stable the conjugate base, the better the acid.
Which of the following is the best acid?
(a) F-CH2-COO–
(b) Cl-CH2-COO–
(c) Br-CH2-COO–
(d) I-CH2-COO–
The order of the -I effect is F > Cl > Br > I. As we discussed above, the more stable the conjugate base, best the acid. So due to the -I effect F-CH2-COO- is more stable. The order will be F-CH2-COO- > Cl-CH2-COO- > Br-CH2-COO- > I-CH2-COO-. So we can say that “the more the -I effect, the more will be the acidic nature” (not 100% true)
Which is more acidic?
(a) CH3-COO–
(b) Cl-CH2-COO–
(c) Cl2-CH-COO–
(d) Cl3-C-COO–
The number of chlorine atoms in (d) is more than in (c), (b), and (a). There is three, two, and one -I group in (d), (c), and (b) respectively. The more the -I group, the more will be the acidic character. So the acidic order will be Cl3-C-COO– > Cl2-CH-COO– > Cl-CH2-COO– > CH3-COO–.
Which is more acidic?
(a) CH3-COOH
(b) Cl-CH2-COOH
(c) Cl-CH2-CH2-COOH
“The more the -I group, the more the stability, and more will be the acidic character”
In (a) there is no -I group, and in (b) and (c) only one -I group (Cl). But as we know that the -I effect is decrease with distance (the more the distance, the lower the -I effect). So the order will be Cl-CH2-COOH > Cl-CH2-CH2-COOH > CH3-COOH.
Arrange in the acidic order.
(a) (CH3)3C-OH
(b) (CH3)2CH-OH
(c) (CH3)CH2-OH
(d) CH3-OH
CH3-OH > (CH3)CH2-OH > (CH3)2CH-OH > (CH3)3C-OH
Reason:
In (a), three methyl groups (+I group) are attached which donate more electrons to O- than (b), (c), and (d). In (CH3)3C-OH, when H+ is removed the conjugate base is formed, and (CH3)3C-O- is unstable. To get stability, the charge on O- should be distributed but CH3 also shows the +I effect and donate the electrons to O-. So that is highly unstable. But in the case of CH3-OH, when H+ is removed, CH3-O- has only one methyl group and donates electrons to O- so it is also unstable but comparatively lower than (CH3)3C-OH. The lower the +I effect, the more the stability, and the more the acidic character or vice versa.
Result conclude from inductive effect:
- The more the -I effect, the more the acidic character.
- The more the +I effect, the lesser the acidic character or the more the basic character.
Arrange in the acidic order.
(a) (CH3)2 CH-COO–
(b) Cl – CH2 – COO–
(c) CH3-COO–
- (CH3)2 CH-COO– and CH3-COO–gives +I effect. ((a) gives more +I effect than (c))
- Cl – CH2 – COO– gives -I effect.
The acidic order will be Cl – CH2 – COO– > CH3-COO– > (CH3)2 CH-COO–.
Note:
A 100% sure method to find the acidic nature is to find the Ka value of acid.
HA ⇌ H+ + A–
Ka = [H+] [A–] / [HA]
The more the Ka value, the more the acidic character is or vice versa.
Practice Questions
Arrange in the acidic order.
(a) CCl3-COOH
(b) HCF2-COOH
(c) C I3-COOH
Arrange in the acidic order.
(a) F-CH2-COOH
(b) Cl-CH2-COOH
(c) Br-CH2-COOH
(d) I2-CH2-COOH
