Question

4-Bromobicyclo \([2,2,2]\) octane-1-carboxylic acid is more acidic than 5 -bromopentanoic acid. Explain this observation by considering the transmission of the electrical effect of the C-Br dipole.

Short answer

4-bromobicyclo[2,2,2]octane-1-carboxylic acid is more acidic than 5-bromopentanoic acid because the C-Br dipole in 4-bromobicyclo[2,2,2]octane-1-carboxylic acid can transmit its electrical effect directly onto the carboxylic acid group through the ring system, thereby stabilizing the carboxylate anion formed upon deprotonation. In contrast, the C-Br dipole in 5-bromopentanoic acid is less effective at transmitting its electrical effect to the carboxylic acid group due to the presence of an alkyl chain. This results in a less stable carboxylate anion and lower acidity.

Step-by-step solution

Draw the structures of both compounds and their anions

First, let's draw the molecular structures of 4-bromobicyclo[2,2,2]octane-1-carboxylic acid, 5-bromopentanoic acid, and their corresponding anions, which form after losing a proton from the carboxylic acid group. This will help us better understand their geometries and the orientations of the C-Br dipoles. 4-Bromobicyclo[2,2,2]octane-1-carboxylic acid: Structure: \[\begin{array}{c} \text{COOH}\\ | \\ \text{Br-C}_\text{3}\text{H}_\text{6}-\text{C}_\text{3}\text{H}_\text{6}-\text{C}_\text{3}\text{H}_\text{6} \end{array} \] Anion: \[\begin{array}{c} \text{COO}^-\\ | \\ \text{Br-C}_\text{3}\text{H}_\text{6}-\text{C}_\text{3}\text{H}_\text{6}-\text{C}_\text{3}\text{H}_\text{6} \end{array} \] 5-Bromopentanoic acid: Structure: \[\text{CH}_\text{3}-\text{CH}_\text{2}-\text{CH}_\text{2}-\text{CH}(Br)-\text{CH}_\text{2}-\text{COOH}\] Anion: \[\text{CH}_\text{3}-\text{CH}_\text{2}-\text{CH}_\text{2}-\text{CH}(Br)-\text{CH}_\text{2}-\text{COO}^-\]

Analyze the transmission of the electrical effect of the C-Br dipole

Now that we have the structures, we can analyze the electrical effect of the C-Br dipole, remembering that the bromine atom is more electronegative than carbon, so the C-Br bond is polarized with a partial positive charge on carbon and a partial negative charge on bromine. In 4-bromobicyclo[2,2,2]octane-1-carboxylic acid, the C-Br dipole is oriented in such a way that it can transmit its electrical effect directly onto the carboxylic acid group through the ring system. The presence of bromine atom helps to stabilize the negative charge on the carboxylate anion when deprotonation occurs, making the compound more acidic. On the other hand, in 5-bromopentanoic acid, the C-Br dipole is located farther away from the carboxylic acid group and the transmission of its electrical effect is less efficient due to the presence of an alkyl chain. This makes the carboxylate anion less stable when deprotonation occurs, resulting in a weaker acid.

Draw conclusions

Based on our analysis, we can conclude that 4-bromobicyclo[2,2,2]octane-1-carboxylic acid is more acidic than 5-bromopentanoic acid, because the C-Br dipole in the former is better at transmitting its electrical effect to the carboxylic acid group via the ring system, thus stabilizing the carboxylate anion formed upon deprotonation and increasing acidity.

Key concepts

Carboxylic Acids

Carboxylic acids are organic compounds containing a carboxyl group (\(-\text{COOH}\)). This functional group is pivotal in determining their chemical behavior, including their acidity. The acidity is a result of the ability to donate a proton (\( ext{H}^+\)) from the carboxyl group, leaving behind a carboxylate anion (\(- ext{COO}^-\)).
- The strength of a carboxylic acid is influenced by several factors, such as electronegativity and the stability of the resulting anion.- A more stable carboxylate anion corresponds to a stronger acid because the ion is less likely to re-accept the proton.- In the context of the given exercise, the structural differences between 4-bromobicyclo[2,2,2]octane-1-carboxylic acid and 5-bromopentanoic acid play a crucial role in their relative acidic strength.

Dipole Effect

The dipole effect is an important concept when discussing the acidity of carboxylic acids. It refers to the polarization of a bond due to differences in electronegativity between two atoms. This effect is significant in the C-Br bond in the mentioned compounds.
- Bromine is more electronegative than carbon, leading to a polarized C-Br bond. - This polarization generates a partial positive charge on carbon and a partial negative charge on bromine.
In the case of 4-bromobicyclo[2,2,2]octane-1-carboxylic acid, this dipole is efficiently aligned to influence the carboxylic group because of the ring system's structure which allows a direct transmission of this effect. This enhances the stability of the carboxylate anion post-deprotonation, resulting in greater acidity compared to 5-bromopentanoic acid. The latter suffers from a less effective dipole transmission due to its linear structure.

Anion Stability

The stability of the anion formed after deprotonation is a major determinant of acidic strength. In carboxylic acids, the stability of the carboxylate anion greatly impacts how easily the acid can donate a proton.
- Anions are generally stabilized by having electron-withdrawing groups nearby, which can delocalize the charge. - In 4-bromobicyclo[2,2,2]octane-1-carboxylic acid, the bromine atom acts as an electron-withdrawing group, stabilizing the carboxylate anion.
By contrast, in 5-bromopentanoic acid, the electron-withdrawing effect of bromine is attenuated by the alkyl chain, resulting in a less stable anion and therefore a weaker acid. Stability is enhanced by the position and nature of substituent groups, which in effect influence the level of acidity.

Bicyclic Compounds

Bicyclic compounds contain two interconnected rings, which can influence the properties of the molecule, including acidity. The structure of 4-bromobicyclo[2,2,2]octane-1-carboxylic acid is an interesting example where the bicyclic nature allows for effective transmission of dipole effects.
- The unique bicyclic framework provides rigidity and a specific spatial orientation that can enhance dipole alignment with the carboxyl group. - This orientation permits direct interaction between the dipole moment and the carboxylic acid group.
Compared to the linear chain of 5-bromopentanoic acid, the bicyclic structure facilitates greater anion stability, thus increasing acidity. The strategy of using bicyclic systems in molecular design can be an effective way to modify the chemical behavior of compounds.

Electronegativity

Electronegativity plays a critical role in the acidity of compounds. It's a measure of how strongly atoms attract bonding electrons. In this context, the electronegativity of the bromine atom influences the molecules’ behavior.
- Bromine's relatively high electronegativity compared with carbon creates a strong dipole in the C-Br bond. - This dipole affects the overall stability of the carboxylate ion in the corresponding acids.
In 4-bromobicyclo[2,2,2]octane-1-carboxylic acid, bromine’s high electronegativity contributes to an effective stabilization of the carboxylate anion, increasing acidity. By contrasting with 5-bromopentanoic acid, the difference in electronegative effects due to molecular structure becomes apparent, emphasizing the importance of molecular design in acidic strength. Electronegativity guides the strength and directionality of dipoles within molecular bonds, directly impacting chemical properties.