Question

Arrange the following species in the decreasing order of $-$ I effect.

Answer 1

The above three species contain a carbonyl group $(C = O)$ attached to it. First one is an aldehyde group, second is a ketone group and the third one is an acyl chloride. To find the order of inductive ($-$ I) effect, we must know the electron withdrawing nature of the molecules attached to the carbonyl carbon.

Complete answer: Carbonyl group consists of a $s{p^2}$ hybridised carbon with trigonal planar geometry and this functional group is common to several classes of organic compounds like aldehydes, ketones, carboxylic acids, esters etc. The carbonyl carbon is doubly bonded to an oxygen atom. Due to the electronegativity difference between carbon and oxygen, bond polarisation takes place with the electron density shifting towards oxygen making it partially negative. Carbon thus becomes partially positive and acts as an electrophile.
Inductive effect refers to the polarisation of a $\sigma$ bond due to presence of electron withdrawing or electron donating groups. It arises due to the difference in electronegativity between the atoms.
Strength of $-$ I effect depends on the electron withdrawing nature of groups attached. The decreasing order of $-$ I effect in the given species is:

Chlorine is a highly electronegative atom. It pulls the electron density towards it through the sigma bond, making the carbonyl carbon more electropositive and increases its electrophilic nature. Therefore, acyl chlorides have comparatively higher $-$ I effect than the other two. Alkyl groups (R) are less withdrawing than hydrogen. They are considered as electron donating groups that exhibit positive inductive effect. They push the electron density away from them and make the carbonyl carbon less electrophilic.

Note:
Inductive effect is a permanent effect that arises due to the polarization of sigma bonds between atoms. A permanent displacement of sigma electrons takes place towards the more electronegative atom or group. But, it is a weaker effect compared to hyper conjugation, resonance effects etc. It fades away with distance. The magnitude of $\pm I$ becomes less significant as the distance from the electron donating or withdrawing group increases.