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The -I effect is shown by :

Last updated date: 16th Jul 2024
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The group which has electron withdrawing group nature shows -I effect. The presence of electronegative elements in a molecule make it an electron-withdrawing group and such a group shows -I effect.

Complete step by step answer:
First, let’s understand that -I effect and then we will see among the options given to us.
-I is the Electron withdrawing inductive effect. For that first, we should know what the inductive effect is.
Inductive effect is the phenomenon of displacement of sigma electrons towards highly electronegative atoms. This results in generation of two polar ends - one which has positive charge and other with negative charge.
This effect is distance dependent and is seen in sigma bonds only.
Inductive effect is divided into two types –
Electron withdrawing inductive effect (-I) – When the electronegative atoms are joined by a chain of other atoms, normally carbon; then it is electron withdrawing inductive effect. Example- halogens, carboxylic groups etc.
Electron releasing inductive effect (+I) – Some groups are less electron withdrawing than hydrogen. Such groups are called electron releasing groups. When such groups are attached with a Carbon chain; they show electrons releasing inductive effects. Example- alkyl groups.
The oxygen atom in the -COOH group is highly electronegative. Thus, this atom pulls electrons of sigma bond towards itself attributing to electron withdrawing nature of the group. So, now we can say that the -COOH group is an electron-withdrawing in nature. Thus, it will show -I effect.
Now, for option b.) which is a - $C{H_3}$ group. Here, the carbon is more electronegative than hydrogen. So, it will attract the shared pair of electrons towards itself. While the carbon is less electronegative to most of the other elements in the periodic table. Thus, it can easily donate its electrons and will act as an electron releasing in nature.
Now, if we see option c.) and d.) which are -$C{H_2}C{H_3}$ and -$CH{R_2}$. These are also categorised under alkyl groups. Thus, these will also show electron releasing effects like the methyl group in option b.).

Thus, the correct answer is the option A


-It must be noted that the alkyl groups are attached to the atom which is more electronegative in nature which generally is the case. Example- in alcohols, alkyl group is attached with oxygen atom while in alkyl halide, it is attached with halide atoms. These oxygen and halides are all more electronegative than carbon.
-But in some cases, say RMgX, the alkyl group shows electronegative effect because it is attached to Mg which is highly electron donating in nature. So, the group attached must be taken care of. In general, due to low electronegativity of carbon, it is taken as electron releasing in nature.