What Are Carboxylic Acids?

An organic compound composed of a hydrocarbon group and a carboxyl group is called a carboxylic acid. The boiling point of saturated monocarboxylic acids is even higher than that of alcohols of similar relative molecular mass. For example, the relative molecular mass of formic acid and ethanol is the same, but the boiling point of ethanol is 78.5 ° C, and formic acid is 100.7 ° C.

Chinese name: carboxylic acid
Foreign name: Carboxylic Acid

Meaning: An organic acid composed of a hydrocarbon group and a carboxyl group.
Structure: RCOOH
Functional group: COOH

Introduction to carboxylic acids

Introduction of carboxylic acids

A carboxylic acid is an organic acid composed of a hydrocarbon group and a carboxyl group.

Carboxylic acid structure

Structural formula of monocarboxylic acid Carboxylic acid ^[1] (RCOOH) (Carboxylic Acid) is the most important type of organic acid. A class of compounds with the general formula RCOOH or R (COOH) n, functional group: -COOH. X-ray diffraction proved that the bond length of carbonyl group in formic acid is 123pm longer than the normal carbonyl group of 122pm; the bond length of C-O is 131pm smaller than that of C-0 in alcohol; 143pm; It's 127pm.

Classification of carboxylic acids

In the general formula RCOOH, R is an aliphatic hydrocarbon group or an aromatic hydrocarbon group, and is called an aliphatic (family) acid or an aromatic (family) acid, respectively. It can be divided into monobasic acid, dibasic acid and polybasic acid according to the number of carboxyl groups. It can also be divided into saturated acids and unsaturated acids.

It is acidic and reacts with alkali to form salts. It generally reacts with phosphorus trichloride to form acid chloride; dehydrates with phosphorus pentoxide to form acid anhydride; reacts with alcohol to form ester under acid catalysis; reacts with ammonia to form amide; uses lithium aluminum tetrahydride (LiAlH4) to reduce to alcohol. It can be prepared by oxidation of the side chains of alcohols, aldehydes, unsaturated hydrocarbons, aromatic hydrocarbons, etc., or by nitrile hydrolysis, or by the reaction of Grignard reagents and dry ice. Saponification with fats or waxes derived from animals and plants can obtain straight-chain fatty (family) acids of 6 to 18 carbon atoms.

Carboxylic acid nomenclature

example 1 Saturated fatty acids are named with the longest carbon chain including the carboxyl carbon atom as the main chain. The number of carbon atoms in the main chain is called an acid, and the number is started from the carboxyl carbon atom. When naming unsaturated fatty acids, the main chain should be the longest carbon chain including the carboxyl carbon atoms and the carbon atoms of each carbon-carbon heavy bond, starting from the carboxyl carbon atom and specifying the position of the heavy bond.

Example 2 The dibasic acid is named with the longest carbon chain including two carboxyl carbon atoms as the main chain, and the number of carbon atoms in the main chain is called "a certain diacid". ^[2]

Example 3 When a carboxylic acid directly connected to an alicyclic ring is named, a suffix such as "carboxylic acid or dicarboxylic acid" can be added after the name of the alicyclic hydrocarbon; the carboxylic acid named on the alicyclic ring is the name of the alicyclic hydrocarbon Linked to the name of the fatty acid. In addition, whether the carboxyl group is directly connected to the alicyclic ring or the alicyclic side chain, the alicyclic ring can be named as a substituent.

Example 4 Aromatic acids can be named as aromatic substituents of fatty acids.

Physical properties of carboxylic acid

carboxylic acid Among the saturated monocarboxylic acids, formic acid, acetic acid, and propionic acid have strong acidity and irritation. Containing 4-9 C atoms, it has a putrid stench and is an oily liquid. Contains more than 10 C are paraffin-like solids, with low volatility and no odor.

This is due to the presence of hydrogen bonds between formic acid molecules. According to methods such as electron diffraction, lower acids are present as dimers even in steam due to the presence of hydrogen bonds. The intermolecular hydrogen bond energy of formic acid is 30KJ / mol, while the intermolecular hydrogen bond of ethanol is 25KJ / mol.

The melting point of a linear saturated monocarboxylic acid changes in a zigzag manner with the increase in the number of C atoms in the molecule. The melting point of an acid with an even number of C atoms is higher than the melting point of an adjacent two acid with an odd number C atoms. In the chain, the terminal methyl groups and carboxyl groups are on both sides of the chain, and in the odd C atom chain, the former has higher symmetry on the same side of the C chain, which can make the carboxylic acid crystal lattice more closely arranged. , They have a large attraction between them, and a higher melting point.

The carboxyl group is a hydrophilic group and can form hydrogen bonds with water, so lower carboxylic acids can be miscible with water at any ratio. As the relative molecular mass increases, the hydrophobic group (hydrocarbon group) becomes larger and larger, and the solubility in water becomes more and more. small.

X-ray studies of long-chain fatty acids have proved that the C chains in these molecules are arranged in a zigzag pattern, and the carboxyl groups between the two molecules are associated by hydrogen bonding. The associated bimolecules are arranged layer by layer regularly. In the middle of the layer are carboxyl groups that are associated with each other, and the gravity is very strong. The layers and the layers are adjacent to each other with a weakly attractive hydrocarbon group, which is easy to slide with each other. This is why the higher fatty acids have lubricity.

Carboxylic acid properties

Chemical description of carboxylic acids

In the carboxylic acid molecule, the carboxyl carbon atom forms three bonds with the hydrocarbyl group and two oxygen atoms in sp2 hybrid orbit. These three bonds are on the same plane, and the remaining p electron forms an bond with the oxygen atom. , Constitutes a bond of C = O in a carboxyl group, but the oxygen on the -OH portion of the carboxyl group has a pair of unshared electrons, which can form a p- conjugated system with a bond. Due to the p- conjugation, the electron cloud on the oxygen atom on the -OH group moves to the carbonyl group, and the electron cloud between OH is closer to the oxygen atom, which makes the polarity of the OH bond stronger and facilitates the dissociation of the H atom. So carboxylic acids are more acidic than alcohols. When H is dissociated from the carboxylic acid, the p- conjugate is more complete, the bond length is averaged, and the negative charge on the -COOCO group is no longer concentrated on one oxygen atom, but is evenly distributed on two oxygen atoms.

Carboxylic acid reaction type

(1) Carboxylic acid is a weak acid, which can react with alkali to form salt and water. Such as: CH3COOH + NaOH CH3COONa + H2O

(2) Substitution reaction of OH on the carboxyl group. Such as:

Esterification reaction: R-COOH + ROH RCOOR + H2O

Acid halide reaction: 3RCOOH + PCl ₃ 3RCOCl + H ₃ PO ₃

Anhydride-forming reaction: RCOOH + RCOOH (heating) R-COOCO-R + H2O

Amide reaction: CH3COOH + NH3 CH3COONH4;

CH ₃ COONH ₄ (Heating) CH ₃ CONH ₂ + H ₂ O

Reaction with metal: 2CH3COOH + 2Na 2CH3COONa + H2

2CH3COOH + Mg (CH3COO) 2Mg + H2

(3) Decarboxylation reaction: Except formic acid, homologues of acetic acid are not easy to remove carboxyl groups (loss of CO2), but decarboxylation reactions can also occur under special conditions, such as the strong heat of anhydrous sodium acetate and soda lime Methane generation: CH3COONa + NaOH (hot melt) CH4 + Na2CO3 (CaO as catalyst)

HOOC-COOH (heating) HCOOH + CO2

Note: Decarboxylation reactions are an important class of reactions that shorten the carbon chain.

(4) Reduction reaction

RCOOH (LiAlH4) RCH2OH

Nomenclature of carboxylic acids

Early discovered carboxylic acids are often named after the source. For example, formic acid was originally produced by distilling red ants, called formic acid; acetic acid was originally obtained from vinegar, called acetic acid; butyric acid had the smell of rancid milk, called butyric acid; hexanoic acid, caprylic acid, and capric acid were also called It is linoleic acid, linoleic acid, and arabinic acid, because they are all present in goat fat; benzoic acid is present in benzoin gum, which is called benzoic acid. ^[3]

Generally, simple carboxylic acids are named according to common nomenclature. The longest carbon chain containing a carboxyl group is selected as the main chain, and the position of the substituent is started from the carbon atom adjacent to the carboxyl group. The Greek letters a, , , , etc. are sequentially designated. Aromatic acids are named as derivatives of benzoic acid; more complex carboxylic acids are named according to international nomenclature. The longest carbon chain containing a carboxyl group is selected as the main chain. position.

The aliphatic dicarboxylic acid is named by taking the longest carbon chain containing two carboxyl groups in the molecule as the main chain, and adding the name and position of the substituent. Lower fatty acids C1 to C3 are liquid, have a pungent odor, and are soluble in water. The intermediate fatty acids C4 to C10 are also liquids, have an unpleasant odor, and are partially soluble in water. Higher fatty acids are waxy solids, tasteless and insoluble in water. Both dibasic fatty acids and aromatic acids are crystalline solids. Aromatic acids are less soluble in water and can be recrystallized from water. Except for higher homologues, saturated dicarboxylic acids are easily soluble in water and ethanol. The boiling point of a carboxylic acid is higher than that of an alcohol having a similar molecular weight.

The melting points of the linear saturated monocarboxylic acids and dicarboxylic acids increase with the number of carbon atoms, and the melting points of the carboxylic acids with even carbon atoms are higher than those of the adjacent two carboxylic acids with odd carbon atoms. The most prominent property of carboxylic acids is acidity, which is a weak acid, which is stronger than carbonic acid. Carboxylic acids can form salts with metal oxides or metal hydroxides. Alkali metal salts of carboxylic acids have greater solubility in water than the corresponding carboxylic acids. Lower and intermediate fatty acid alkali metal salts can dissolve in water. Higher fatty acid alkali metal salts can form colloidal solutions in water. Soap is long-chain fatty acid sodium.

Lower fatty acids are important chemical raw materials. For example, pure acetic acid can be used to make artificial fibers, plastics, flavors, and drugs. Higher fatty acids are the foundation of the oil industry. Dicarboxylic acids are widely used in the fiber and plastics industries. Some aromatic acids, such as benzoic acid ^[4] , salicylic acid, etc., have many important industrial uses.

Important derivatives of carboxylic acids

Carboxylic acid is a very important type of chemical substance, and many other common chemical substances can be derived, including: acid halides, acid anhydrides, esters, and amides. These types of carboxylic acid derivatives have their own characteristics and have important applications in the chemical industry.

Carboxylic acid substituted carboxylic acid

A compound in which a hydrogen atom on a hydrocarbon group in a carboxylic acid molecule is replaced by another atom or group having a functional group property is called a substituted carboxylic acid. According to the difference of the substituted functional group, it can be divided into halo acids, hydroxy acids, carbonyl acids, and amino acids. Many hydroxy acids and carbonyl acids are intermediate products of biological metabolism; some hydroxy acids also have therapeutic value for certain diseases; amino acids are the building blocks of proteins.