Question

Which is most acidic (a) Oc1ccccc1 (b) Oc1cccc(Cl)c1 (c) Oc1ccccc1Cl (d) Oc1ccc(Cl)cc1

Short answer

The most acidic compound is (b) 4-chlorophenol.

Step-by-step solution

Identify the Functional Groups

Look at each compound and identify the functional groups present. Compound (a) is phenol; (b) is 4-chlorophenol; (c) is a 2-chlorophenol; and (d) is 3-chlorophenol. Each has a hydroxyl group (-OH) attached to a benzene ring, but compounds (b), (c), and (d) also have a chlorine atom attached to different positions on the benzene ring.

Recognize Chlorine's Effect on Acidity

Consider how chlorine affects acidity. Chlorine is an electron-withdrawing group through both inductive and resonance effects. These characteristics generally enhance the acidity of phenols by stabilizing the conjugate base, the phenoxide ion.

Analyze the Position of Chlorine

Determine the effect of chlorine based on its position. In compound (b), chlorine is para to the hydroxyl group, in (c) it is ortho, and in (d) it is meta. The inductive effect is stronger when chlorine is ortho or para compared to meta due to proximity and resonance stabilization.

Compare Acidities Based on Inductive and Resonance Effects

Rank the acidities by considering both inductive and resonance effects. The para position (b) allows for strong resonance stabilization and thus greater acidity compared to the ortho (c) and meta (d) positions. Therefore, compound (b) will be the most acidic.

Key concepts

Functional Groups

Functional groups are specific groups of atoms within molecules that have characteristic properties. In organic chemistry, they play a crucial role in determining the molecule's chemical reactions. A functional group can contain atoms of carbon, hydrogen, nitrogen, oxygen, sulfur, or halogens such as chlorine.
For the compounds in the original exercise, each possesses a hydroxyl group (-OH) attached to a benzene ring, making them derivatives of phenol. The presence of this hydroxyl group classifies these compounds as phenols, which are known for their slightly acidic nature. However, when additional groups, such as halogens, are attached to the benzene ring, they modify the properties of the compound, including its acidity.
Understanding functional groups helps to predict how molecules will behave in chemical reactions, which is essential when assessing compound acidity or the effect of substituents like chlorine.

Chlorine's Effect on Acidity

Chlorine is a halogen and exhibits a significant impact on the acidity of compounds with which it is associated. When chlorine is present in organic molecules, it acts as an electron-withdrawing group. This means that it pulls electron density away from other parts of the molecule, including functional groups like the hydroxyl group.
In the phenol derivatives mentioned, the chlorine atom stabilizes the conjugate base, which is the phenoxide ion, through its electron-withdrawing effect. It does this via both inductive and resonance effects, which ultimately enhance the compound's acidity. The greater the ability of the substituent to stabilize the negative charge on the conjugate base, the more acidic the compound becomes.
Therefore, the presence of chlorine in phenol derivatives increases acidity compared to phenol itself, making the position of chlorine crucial to the level of acidity.

Inductive and Resonance Effects

Inductive and resonance effects are fundamental concepts in understanding the behavior of molecules that contain electronegative atoms like chlorine. The inductive effect involves the transmission of charge through a chain of atoms in a molecule, which leads to the polarization of bonds. Electron-withdrawing groups, such as chlorine, exert a negative inductive effect when they pull electron density towards themselves and away from less electronegative atoms.
The resonance effect involves the delocalization of electrons across a molecule, enhancing stability. In the case of 4-chlorophenol (compound b), chlorine can stabilize the phenoxide ion through resonance, where electrons are shared between atoms and the entire structure contributes to the overall stability of the system.
The position of chlorine relative to the functional group (ortho, meta, or para) influences which of these effects predominates. Para position typically allows for both inductive and resonance contributions, maximizing the stability and therefore the acidity of the molecule.

Phenol Derivatives

Phenol derivatives are compounds derived from phenol, which is a benzene ring bonded to a hydroxyl group. These derivatives are created by substituting one or more hydrogen atoms on the benzene ring with different functional groups, such as halogens. The structure of phenol derivatives influences many properties, including its acidity.
In the exercise, each compound is a variation of phenol with chlorine atoms attached at different positions on the ring. These positions—ortho, meta, and para—are critical in determining the compound's overall acidity as they affect how well the chlorine can withdraw electrons through inductive and resonance effects.
Understanding these derivatives' behavior helps to predict their reactivity and physical properties, such as acidity. 4-chlorophenol, where chlorine is para to the hydroxyl group, typically shows stronger acidity compared to its ortho and meta counterparts due to optimal resonance stabilization. This illustrates how slight changes in structure can lead to significant differences in chemical properties.