Why does CH​​​​​3 show +I effect and behave as an electron donor?


The Inductive effect is a measure of how much a given group either donates or attracts electron density.  You can most easily understand this by looking at the acidity of various substituted carboxylic acids.  

First, you have to remember that anything that pulls electrons away from the carboxylic acid group has a negative inductive effect and anything that donates electrons to a carboxylic acid group will have a positive inductive effect.  

Second you have to remember that anything that pulls electron density from a carboxylic acid makes the acid stronger.  In other words--when the carboxylic acid group is attached to a group that removes electron density it is making that carboxylic acid oxygen-hydrogen bond that much more polarized (toward oxygen) and making it easier for the hydrogen to ionize.  By the same token, anything that makes the electron density greater in the carboxylic acid group will make the acid weaker by making the oxygen-hydrogen bond less polar.

So let's look at some carboxylic acids and their acidity (pKa; remember, the stronger the acid, the lower the pKa):

Acid                  pKa
HCOOH            3.75
CH3COOH        4.76
CH3CH2COOH  4.87

As you can see, alkyl groups, compared to hydrogen, greatly decrease the acidity of the carboxylic acid group and thus are donating electron density to the O--H bond in the acid.

Why this happens is actually pretty straight-forward.  the electronegativity of carbon is lower than nitrogen or oxygen.  That is, the carbon-oxygen bond is polar with increased electron density on the oxygen end.  From this one would predict that chloroacetic acid (ClCH2COOH) would have a lower pKa than the 4.76 shown above for acetic acid.  The pKa for chloroacetic acid is 2.86. 

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