Carboxylic Acids Flashcards
1
Q
Carboxylic Acid
A
- Contains both a carbonyl group and a hydroxyl group, bonded to the same carbon.
- With three bonds to oxygen atoms, this is one of the most oxidized functional groups encountered in o-chem.
- Always terminal groups.
2
Q
Carboxylic Acid Nomenclature
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- Carboxylic acids are named by adding the suffix -oic acid to the parent root when the carboxylic acid is the highest-priority functional group.
3
Q
Common Names of Carboxylic Acids
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4
Q
Cyclic Carboxylic Acids & Salts Nomenclature
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- Cyclic carboxylic acids are named by listing the cycloalkane with the suffix carboxylic acid.
- Salts of carboxylic acids are named beginning with the cation, followed by the name of the acid with the ending -oate replacing -oic acid.
4
Q
Dicarboxylic Acids
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- Have a carboxylic acid group on each end othe molecule.
- Suffix is -dioic acid.
5
Q
Hydrogen Bonding
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- Carboxylic acids are polar because they contain a carbonyl group and can also form hydrogen bonds because they contain a hydrogen bonded to a very electronegative atom (in this case, the hydroxyl oxygen).
- Carboxylic acids display particularly strong intermolecular attractions because both the hydroxyl oxygen and carbonyl oxygen can participate in hydrogen bonding.
- As a result, carboxylic acids tend to form dimers: pairs of molecules connected by two hydrogen bonds.
- Multiple hydrogen bonds elevate the boiling and melting points of carboxylic acids past those of corresponding alcohols.
- Boiling points also increase with increasing molecular weight.
6
Q
Acidity
A
- The hydroxyl hydrogen of a carboxylic acid is quite acidic. This results in a negative charge that remains after the hydrogen is removed and resonance stabilization occurs between both of the electronegative oxygen atoms.
- Delocalization of the negative charge results in a very stable carboxylate anion.
- The more stable the conjugate base is, the easier it is for the proton to leave, and thus, the stronger the acid. Carboxylic acids are relatively acidic.
7
Q
Acidity Trends - Substituents on carbon atoms near carboxyl group
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- Substituents on carbon atoms near a carboxyl group influence anion stability and therefore affect acidity.
- Groups like NO2 or halides are electron-withdrawing and increase acidity.
- In contrast, NH2 or OCH3 are electron-donating groups that destabilize the negative charge, decreasing the acidity of the compound.
- The closer the substituent groups are to the carboxyl group, the greater the effect will be.
8
Q
Acidity Trends - Dicarboxylic Acids
A
- In dicarboxylic acids, each -COOH group influences the other.
- Carboxylic acids are electron-withdrawing due to the electronegative oxygen atoms they contain.
- The net result is that dicarboxylic acids are more acidic than the analogous monocarboxylic acids.
- However, when one proton is removed form the molecule, the carboxylate anion is formed, resulting in an immediate decrease in the acidity of the remaining carboxylic acid. If the second group were deprotonated, it would create a doubly charged species with two negative charges repelling each other.
- Due to this instability, the second proton is actually less acidic than the analogous proton of a monocarboxylic acid.
9
Q
β-dicarboxylic Acids
A
- Dicarboxylic acids in which each carboxylic acid is positioned on the β-carbon of the other; there are two carboxylic acids separated by a single carbon.
- These compounds are notable for the high acidity of the α-hydrogens located on the carbon between the two carboxyl groups. Loss of this acidic hydrogen atom produces a carbanion, which is stabilized by the electron-withdrawing effect of both carboxyl groups.
10
Q
1,3-Dicarbonyl Compounds
A
- This also applies to the alpha-hydrogens in a β-diketone, β-ketoacids, β-dialdehydes and other molecules that share the 1,3-dicarbonyl structure.
11
Q
Synthesis of Carboxylic Acids
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- Carboxylic acids can be prepared via oxidation of aldehydes and primary alcohols. The oxidant is often a dichromate salt, chromium trioxide or potassium permanganate (KMnO4).
12
Q
Nucleophilic Acyl Substitution
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- Many of the reactions in which carboxylic acids (and their derivatives) participate proceed via a single mechanism: nucleophilic acyl substitution.
- The difference from nucleophilic addition to an aldehyde or ketone focuses on the existence of a leaving group in carboxylic acids and their derivatives.
- In this case, after opening the carbonyl via nucleophilic attack and forming a tetrahedral intermediate, the carbonyl can reform, thereby kicking off the leaving group.
- In these reactions, the nucleophilic molecule replaces the leaving group of an acyl derivative.
- These reactions are favored by a good leaving group - weak bases (conjugate of strong acids) make good leaving groups.
- They are also favored in either acidic or basic conditions, which can alter the reactivity of the electrophile and nucleophile.
13
Q
Acyl Derivative
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- Encompass all molecules with a carboxylic-derived carbonyl, including carboxylic acids, amides, esters, anhydrides and others.
14
Q
Amides
A
- Carboxylic acids can be converted into amides if the incoming nucleophile is ammonia (NH3) or an amine.
- This can be carried out in either an acidic or basic solution to drive the reaction forward.
- Amides are named by replacing the -oic acid suffix with -amide in the name of the parent carboxylic acid. Any alkyl groups on the nitrogen are placed at the beginning of the name with the prefix N-.
15
Q
Resonance of Amides
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- Amides exist in a resonance state where delocalization of electrons occurs between the oxygen and nitrogen atoms.
18
Q
Esterification
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- A condensation reaction with water as a side product.
- In acidic solutions, the carbonyl oxygen can be protonated, which enhances the polarity of the bond, thereby placing additional positive charge on the carbonyl carbon and increasing its susceptibility to nucleophilic attack.
- This condensation reaction occurs most rapidly with primary alcohols.
19
Q
Lactones
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- Esters that are cyclic.
- Named by replacing -oic acid with -lactone.
20
Q
Anhydrides
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- Can be formed by the condensation of two carboxylic acids.
- Named by replacing the acid at the end of the name of the parent carboxylic acid with anhydride, whether cyclic or linear.
21
Q
Reduction
A
- Carboxylic acids can be reduced to primary alcohols by the use of LiAlH4.
- Aldehyde intermediates may be formed in the course of this reaction, but they will be reduced to the alcohol.
- The reaction occurs by nucleophilic addition of hydride to the carbonyl group.
- Gentler reducing agents like NaBH4 are not strong enough to reduce carboxylic acids.
22
Q
Decarboxylation
A
- Describes the complete loss of the carbonyl group as carbon dioxide.
- Common way of getting rid of a carbon from the parent chain.
- 1,3-dicarboxylic acids and other β-keto acids may spontaneously decarboxylate when heated.
- Under these conditions, the carboxyl group is lost and replaced with hydrogen.
- Because both the electrophile and nucleophile are in the same molecule, the reaction proceeds through a six-membered ring in its transition state.
- The enol that is initially formed from the destruction of the ring tautomerizes to the more stable keto form.
