Question

Why does the partial charge on the acidic hydrogen increase in the series \(\mathrm{CH}_{3} \mathrm{COOH}, \mathrm{CH}_{2} \mathrm{ClCOOH}, \mathrm{CHCl}_{2} \mathrm{COOH}, \mathrm{CCl}_{3} \mathrm{COOH} ?\)

Short answer

The partial charge on the acidic hydrogen increases in the series CH3COOH, CH2ClCOOH, CHCl2COOH, CCl3COOH due to the increasing number of highly electronegative chlorine atoms. These atoms exert a strong pull on electron density, enriching the positive charge on the acidic hydrogen.

Step-by-step solution

Understanding the Concept of Electronegativity

Electronegativity is a measure of how strongly atoms attract the electrons in a bond. Chlorine (Cl) is more electronegative than both carbon (C) and hydrogen (H), meaning it attracts more electron density towards itself. This will pull electron density from the OH group in the carboxylic acid, leading to a more positively charged hydrogen.

Analysis of CH3COOH

In the case of CH3COOH, there are no electronegative atoms attached to the carbon that's directly attached to the carboxyl group. Therefore, there is no electron pulling from the carboxylic acid side. Consequently, the positive charge on the acidic hydrogen (in the OH group) remains relatively low.

Analysis of CH2ClCOOH

In CH2ClCOOH, one of the hydrogens in the methyl group is replaced by a chlorine atom. Due to the chlorine's high electronegativity, it pulls electrons towards itself, causing a partial negative charge on it and a partial positive charge on the carbon atom. This extra positive charge on the carbon atom results in a denser positive charge on the acidic hydrogen.

Analysis of CHCl2COOH and CCl3COOH

The same logic applies for CHCl2COOH and CCl3COOH in which two and three hydrogens are replaced by chlorines, respectively. As the number of chlorine atoms increases, the electron pulling effect builds up and hence, pulls even more electron density away from the carboxylic group increasing the positive charge on the acidic hydrogen.

Key concepts

Electronegativity

Electronegativity is a fundamental concept in chemistry, particularly when discussing the behavior of molecules and how atoms within them interact. It refers to the ability of an atom to attract shared electrons in a chemical bond. Generally, atoms with higher electronegativity will pull more electron density towards themselves, making the bond polarized.

For example, chlorine (Cl) has a higher electronegativity compared to carbon (C) and hydrogen (H). When chlorine is bonded to a carbon atom that is also bonded to a hydrogen atom, the chlorine atom will attract more of the shared electron density, leaving the hydrogen atom with a partial positive charge. This is due to hydrogen's lower electronegativity which makes it less effective in competing for the shared electrons.

When we consider how electronegativity affects molecular structure and charge distribution, we can see that it plays a crucial role in determining the physical and chemical properties of compounds. An understanding of these principles is essential for comprehending more complex chemical phenomena such as acidity and reactivity.

Carboxylic Acids

Carboxylic acids are a class of organic compounds that contain a carboxyl group (\r\(-COOH)\r\). This functional group consists of a carbon atom double-bonded to an oxygen atom (\r\(C=O)\r\) and single-bonded to an OH group.\r

The acidic hydrogen in question is the one attached to the oxygen atom of the OH group. In the case of acetic acid (\r\(CH_3COOH)\r\), this hydrogen is prone to dissociate, creating a charge separation as it leaves behind an electron pair with the oxygen. This process is known as ionization and is what gives the carboxylic acids their acidic nature.

However, the tendency for this hydrogen to dissociate, and hence the acidity of the carboxylic acid, can be affected by what is attached to the carbon chain of the molecule. In simple terms, the more electronegative atoms (like chlorine) attached to the neighboring carbon atoms, the more polarized the \r\(OH\r\) bond becomes, and consequently, the more acidic the molecule is, as the hydrogen ion (\r\(H^+\r\)) is more easily released.

Inductive Effect

The inductive effect is a concept used to describe the transmission of charge through a chain of atoms in a molecule, which can ultimately influence the molecule’s reactivity and properties. An electronegative atom, such as chlorine, when attached to a hydrocarbon chain, can pull electron density towards itself through the sigma bonds (\r\(\sigma\r\)-bonds) - the single bonds holding the atoms together.

In the context of carboxylic acids, such as those in the exercise (\r\(CH_3COOH, CH_2ClCOOH, CHCl_2COOH\text{, and }CCl_3COOH\r\)), the presence of electronegative chlorine atoms introduces an inductive effect that starts from the chlorine and moves through the carbon atoms towards the carboxyl group. As more chlorine atoms are added, the cumulative inductive effect increases, drawing more electron density away from the acidic hydrogen.

This effect is responsible for the observed increase in the partial positive charge on the hydrogen of the carboxylic acid's \r\(OH\r\) group as you go from \r\(CH_3COOH\r\) to \r\(CCl_3COOH\r\). Each additional chlorine atom enhances the inductive effect, increasing the hydrogen's acidity due to the greater positive character. This information not only helps us understand acidity trends but is also crucial for grasping reactivity patterns in various organic compounds.