Chapter 20 Flashcards
(9 cards)
Why do carboxylic acids tend to have increased boiling points when compared to their alcoholic analogue?
“Carboxylic acids can form two hydrogen-bonding interactions, allowing molecules to associate with each other in pairs. These hydrogen-bonding interactions explain the relatively high boiling points of carboxylic acids. For example, compare the boiling points of acetic acid and ethanol. Acetic acid has a higher boiling point as a result of stronger intermolecular forces.”
Klein, D. (2017). Carboxylic Acids and Their Derivatives. In S. Bruno (Ed.), Organic Chemistry (3rd ed., pp. 898–953). essay, John Wiley and Sons, Inc.
What makes the acidic proton of a carboyxlic acid so much more acidic than the proton of an alcoholic analogue?
The conjugate base of a carboxylic acid is stabilized by resonance becuase of the conjugation between the carbonyl oxygen and sp3</sub> oxygen.
“In the conjugate base of acetic acid, the negative charge is delocalized over two oxygen atoms, and it is therefore more stable than the conjugate base of ethanol. The delocalized nature of the charge can be seen in an electrostatic potential map of the acetate ion.”
Klein, D. (2017). Carboxylic Acids and Their Derivatives. In S. Bruno (Ed.), Organic Chemistry (3rd ed., pp. 898–953). essay, John Wiley and Sons, Inc.
Which of the folowing molecules is more acidic?
para-hydroxyacetophenone is more acidic because the conjugate base is more stabilized by resonance than the conjugate base of meta-Hydroxyacetophenone. This resonance stabilization can be illustrated by mapping out the different resonance structres for both para and meta substituted acetophenone. The more resonance structures that can be drawn for a given molecule, the more stable. More resonance structures for a given conjugate base indicates that there is more charge distribution and the negative charge for the conjugate base is more delocalized throughtout the conjugated pi-system.
It is important to note that ortho-hydroxyacetophenone gives us the same numberr of valid resonance structures as the para substituted compound. Generally, we will be able to write more resonance structures for ortho/para substituted aromatic systems, leading to increased charge distrubution and overall stability.
How is the pka affected by a highly electronegative heteroatom substituent on the α, β, and(or) γ position of a carboxylic acid?
Why is the pka affected this way?
An electronegative substituent on the α, β, and(or) γ position of a carboxylic acid can significantly decrease the pka (increase the acidity), by decreasing the elctron density of the carboxylate and thus stabilizing the conjugate base of the carboxylic acid. The effect is most pronounced when the electron-withdrawing group is located at the α position. As the distance between the chlorine atom and the carboxylic acid group increases, the effect of the chlorine atom is diminished.
Klein, D. (2017). Carboxylic Acids and Their Derivatives. In S. Bruno (Ed.), Organic Chemistry (3rd ed., pp. 898–953). essay, John Wiley and Sons, Inc.
How will an electron withdrawing (EWG) or electron donating (EDG) substituent on the para position of benzoic acid affect its pka?
Where Z = EWG or EDG
Klein, D. (2017). Carboxylic Acids and Their Derivatives. In S. Bruno (Ed.), Organic Chemistry (3rd ed., pp. 898–953). essay, John Wiley and Sons, Inc.
Similar to the substituent effect at the α, β, and γ position of nonaromatic carboxylic acid; a EWG or EDG substituent on benzoic acid can also affect its acidity. Where an EWG will decrease the substituted benzoic acid’s pka and an EDG will increase the pka.
Klein, D. (2017). Carboxylic Acids and Their Derivatives. In S. Bruno (Ed.), Organic Chemistry (3rd ed., pp. 898–953). essay, John Wiley and Sons, Inc.
In the following reaction, a carboxylic acid is reduced to an alcohol. The mechanism for the reduction of a carboxylic acid by LiAlH4 involves an initial formation of the corresponding aldehyde, can this intermediate product be isolated is using a 1:1 equivalent of LiAlH4 to substrate?
No, because of the reactivity of aluminum hydride, a reaction with a 1:1 molar equivalent of LiAlH4 to substrate will produce a mixture containing the substrate, aldehyde, and alcohol.
An aldehyde is produced as an intermediate in the reduction of a acyl chloride by LiAlH4, however; the aldehyde cannot be isolated due to the reactivity of the hydride. How can this reaction be modified so that the aldehyde can be isolated?
Adding alkoxy ligands on the aluminum Lewis acid atom will allow for the aldehyde to be isolated. The substitued (tBuO)3AlH will react quickly with the acyl chloride but will not react with the resulting aldehyde, allowing for it to be isolated.
Klein, D. (2017). Carboxylic Acids and Their Derivatives. In S. Bruno (Ed.), Organic Chemistry (3rd ed., pp. 898–953). essay, John Wiley and Sons, Inc.
Propose a viable mechanism for the reduction of acetic acid using lithium aluminum hydride.
What are the reaction conditions for the following transformation?
