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learning goals
- Describe the structure and properties of carboxylic acids and esters.
- Name common carboxylic acids and esters.
The vinegar smell is caused by the presence of acetic acid, a carboxylic acid, in vinegar. The smell of ripe bananas and many other fruits is due to the presence of esters, compounds that can be produced by reacting a carboxylic acid with an alcohol. Because esters do not have hydrogen bonds between molecules, they have lower vapor pressures than the alcohols and carboxylic acids from which they are derived.
Bothcarboxylic acidsEEstercontain a carbonyl group with a second oxygen atom single bonded to the carbon atom in the carbonyl group. In a carboxylic acid, the second oxygen atom also bonds with a hydrogen atom. In an ester, the second oxygen atom is bonded to another carbon atom. The names of carboxylic acids and esters contain prefixes that indicate the length of the carbon chains in the molecules and are derived using naming rules similar to those for inorganic acids and salts (see these examples):
The functional groups of an acid and an ester are shown in red in these formulas.
carboxylic acids
Carboxylic acids are common in nature, often in combination with alcohols or other functional groups, as in fats, oils and waxes. They are components of many foods, medicines, and household products (Figure \(\PageIndex{1}\)). Not surprisingly, many of them are better known by common names based on Latin and Greek words that describe their source.
The carboxyl group contains the \(\ce{C=O}\) of the carbonyl group, where the carbon atom is also bonded to a hydroxyl group \(\left( \ce{-OH} \right)\). Acarboxylic acidIt is an organic compound that contains the carboxyl functional group. The general formula of a carboxylic acid can be abbreviated as \(\ce{R-COOH}\). The carbon atom of the carboxyl group can be bonded to a hydrogen atom or to a carbon chain. Naming a carboxylic acid is as follows: Name the starting compound by finding the longest continuous chain containing the carboxyl group. change it-eam End of the Alcannams to-oic acid.
Carboxylic acids are weak acids, meaning they are not 100% ionized in water. In general, only about 1% of the molecules of a carboxylic acid dissolved in water are ionized at any one time. The remaining molecules are not dissociated in solution.
We produce carboxylic acids by oxidizing aldehydes or alcohols whose -OH functional group is on the carbon atom at the end of the chain of carbon atoms in the alcohol:
The simplest carboxylic acid, formic acid (HCOOH, formic acid), was first obtained by distillation of ants (lat.Resopal, meaning "ant"). The stings of some ants inject formic acid, and the stings of wasps and bees contain formic acid (as well as other toxic materials).
The next higher homologue is acetic acid (CH3COOH, acetic acid) produced by fermenting cider and honey in the presence of oxygen. This fermentation produces vinegar, a solution containing 4-10% acetic acid and a number of other compounds that contribute to its flavor. Acetic acid is probably the best known weak acid used in educational and industrial chemistry labs.
Pure acetic acid freezes at 16.6°C, just below normal room temperature. In the poorly heated laboratories of the late 19th and early 20th centuries in northern North America and Europe, acetic acid often “froze” on the storage shelf. Because of this, pure acetic acid (sometimes called concentrated acetic acid) became knownglacial acetic acid, a name that survives to this day.
The third homologue, propionic acid (CH3CH2COOH, propionic acid) is rarely encountered in everyday life. The fourth homologue, butyric acid (CH3CH2CH2COOH) is one of the worst smelling substances imaginable. It is found in rancid butter and is one of the components of body odor. By detecting extremely small amounts of these and other chemicals, hounds can track down fugitives.
Many carboxylic acids occur naturally in plants and animals. Citrus fruits such as oranges and lemons contain citric acid (Figure \(\PageIndex{4}\)). Ethanoic and citric acids are commonly added to foods to give them a sour taste.
Benzoic, propanoic and sorbic acids are used as food preservatives for their ability to kill microorganisms that can cause spoilage. Methane and acetic acid are widely used in industry as raw materials for the manufacture of paints, adhesives and coatings.
Ester: The sweet smell of RCOOR'
AEsteris an organic compound derived from a carboxylic acid in which the hydrogen atom of the hydroxyl group has been replaced by an alkyl group. The structure is the product of a carboxylic acid (the \(\ce{R}\) part) and an alcohol (the \(\ce{R'}\) part). The general formula for an ester is shown below.
The group \(\ce{R}\) can be a hydrogen or a carbon chain. The \(\ce{R'}\) group must be a carbon chain, as a hydrogen atom would make the molecule a carboxylic acid.
Esters are formed by the reaction of acids with alcohols. For example, the ethyl acetate ester, CH3CO2CH2CH3, is formed in the reaction of acetic acid with ethanol:
Chemistry is everywhere: esters, aromas and flavors
Esters are very interesting compounds, in part because many have very pleasant odors and tastes. (Remember, never taste anything in the chemistry lab!) Many esters are naturally occurring and contribute to the smell of flowers and the taste of fruit. Other esters are industrially synthesized and added to foods to improve their smell or taste; It is likely that if you eat a product that contains artificial flavors in its ingredients, those flavors are esters. Here are some esters and their uses thanks to their smell, taste or both:
| Ester | It tastes/smells like | Ester | It tastes/smells like |
|---|---|---|---|
| Allylhexanoat | Pineapple | isobutilformiato | raspberry |
| Benzylacetato | Pera | Isobutilacetato | Pera |
| butyl butanoate | Pineapple | Methylphenylacetate | Honig |
| Ethyl butanoate | Banana | not the caprilate | orange |
| Ethylhexanoato | Pineapple | Pentilacetato | Litter |
| Ethylheptanoat | Damascus | Propiletanoato | Pera |
| Ethylpentanoato | Litter | Propylisobutyrate | Rum |
Major natural esters include fats (such as lard, tallow, and butter) and oils (such as flaxseed, cottonseed, and olive oil), the esters of the trihydroxyalcohol glycerol, C3H5(OH)3, with large carboxylic acids such as palmitic acid, CH3(CH2)14CO2H, stearic acid, CH3(CH2)16CO2H, and oleic acid, \(\mathrm{CH_3(CH_2)_7CH=CH(CH_2)_7CO_2H}\). Oleic acid is an unsaturated acid; contains a double bond \(\mathrm{C=C}\). Palmitic and stearic acids are saturated acids that do not contain double or triple bonds.
observation
Vegetable fats and oils are esters of long-chain fatty acids and glycerin. Phosphoric acid esters are extremely important for life.
Esters are common solvents. Ethyl acetate is used to extract organic solutes from aqueous solutions - for example, to remove caffeine from coffee. It is also used to remove nail polish and paint. Cellulose nitrate is dissolved in ethyl acetate and butyl acetate to form lakes. As the paint “dries” the solvent evaporates, leaving a thin film on the surface. High boiling point esters are used as plasticizers (plasticizers) for brittle plastics.
Summary
- A carboxylic acid is an organic compound that contains the carboxyl functional group.
- The general formula of a carboxylic acid can be abbreviated as \(\ce{R-COOH}\).
- Many carboxylic acids are used in the food and beverage industry as flavorings and/or as preservatives.
- An ester has an OR group attached to the carbon atom of a carbonyl group.
- Vegetable fats and oils are esters of long-chain fatty acids and glycerin.
- Esters are common in nature and often have pleasant odors and are often responsible for the characteristic aromas of fruits and flowers.
Credits and Attributions
- Libretext: The Fundamentals of GOB Chemistry (Ball et al.)
- TextMap: Beginning Chemistry (Ball et al.)
- OpenSTAX