Carboxylic Acids

Submitted by jwmoore on Sat, 01/15/2011 - 11:23

Serving wine usually involves a rather elaborate ceremony in which the host tastes the wine before pouring it for the guests. One reason for this is the possibility that the wine may have been spoiled by exposure to air.

Certain bacterial enzymes are capable of converting ethanol to ethanoic acidIn Arrhenius theory, a substance that produces hydrogen ions (hydronium ions) in aqueous solution. In Bronsted-Lowry theory, a hydrogen-ion (proton) donor. In Lewis theory, a species that accepts a pair of electrons to form a covalent bond. (acetic acid) when oxygen is present:

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The same reaction occurs when cider changes into vinegar, which contains 4 to 5 percent acetic acid. Acetic acid gives vinegar its sour taste and pungent odor and can do the same thing to wine.

Acetic acid, CH3COOH, is an example of the class of compounds called carboxylic acids, each of which contains one or more carboxylThe functional group consisting of a carbon atom bonded to a hydroxyl group and doubly bonded to an oxygen atom; found in carboxylic acids: -C(=O)OH. groups, COOH. The general formula of a carboxylic acid is RCOOH. Some other examples are

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Formic acid (the name comes from Latin word formica meaning “ant“) is present in ants and bees and is responsible for the burning pain of their bites and stings. Butyric acid, a component of rancid butter and Limburger cheese, has a vile odor. Adipic acid is an example of a dicarboxylic acid—it has two functional groups—and is used to make nylon.

Since the carboxyl group contains a highly polarDescribes a molecule that has separated, equal positive and negative charges that consitute a positive and a negative pole; such a molecule tends to assume certain orientations more than others in an electric field. Image:C-Odouble bond.jpg as well as an OH group, hydrogen bonding is extensive among molecules of the carboxylic acids. Pure acetic acid is called glacial acetic acid because its melting pointThe temperature at which a solid becomes a liquid. Also called freezing point. of 16.6°C is high enough that it can freeze in a cold laboratory. As you can see from the table below, acetic acid boils at a higher temperatureA physical property that indicates whether one object can transfer thermal energy to another object. than any other organic substance whose molecules are of comparable size and have but one functional group. It is also quite thick and syrupy because of extensive hydrogen bonding.

Boiling Points of Some Organic Compounds Whose Molecules Contain 32 or 34 Electrons.

Name Projection Formula Type of Compound Boiling Point in degrees C
Isobutane Branched Alkane -10.2
n-Butane Normal Alkane -0.5
Methyl ethyl ether Ether 10.8
Methyl Formate Ester 31.5
Propanal Aldehyde 48.8
Acetone Ketone 56.2
2-Propanol Alcohol 82.4
1-Propanol Alcohol 82.4
Acetic Acid Carboxylic acid 117.9
Ethylene Glycol Dialcohohl (two OH groups) 198

Below is a Jmol model of acetic acid. In the general menu to the left, click on partial charges. Each atom in the molecule will be assigned a partial charge. It is clear that the oxygen atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. are sharing electrons unequally and causing other parts of the molecule to gain a partial positive charge in the carboxyl carbon and hydrogen. Further, this induces a partial negative charge on the methyl carbon, leading to positive charges on the methyl hydrogen atoms.

An even better way to view the electron distribution is with the Molecular Electrostatic Potential (MEP) Surface options. One can look at "MEP on isopotential surface", which show surfaces where electrostatic potential is the same, but the most informative option here is the "MEP on Van der Waals Surface" radio button. This shows the potential along the van der Waals surface of the molecule. The closer to red on the color spectrum, the more negative the potential at that surface is, the closer to blue, the more positive. One can see that both oxygen atoms are centers of partial negative charge, while the acidic hydrogen atom has a substantial partial positive charge, and the methyl group is also has a partial positive charge. One more way to look at the molecule, is to use the "MEP on a plane" button. Choose the XY plane, and then click "Set Plane Equation." This will show the electrostatic potential along the axis of symmetry for the molecule. While two hydrogen atoms on the methyl group are out of the plane, this view still allows one to see how partial charge is distributed along the backbone of the molecule in a way the van der Waals surface does not. From this modeling of the acetic acid molecule, hopefully it is becoming clear how the macroscopic properties we discussed arise.

Acetic acid is synthesized commercially according to the reaction shown above, but silver is used as a catalystA substance that increases the rate of a chemical reaction but that undergoes no net change during the reaction. instead of bacterial enzymes. It is also prepared by reading air with propane separated from natural gasA state of matter in which a substance occupies the full volume of its container and changes shape to match the shape of the container. In a gas the distance between particles is much greater than the diameters of the particles themselves; hence the distances between particles can change as necessary so that the matter uniformly occupies its container.. The liquidA state of matter in which the atomic-scale particles remain close together but are able to change their positions so that the matter takes the shape of its container acetaldehyde obtained in this reaction is then combined with oxygen in the presence of manganese(II) acetate to make acetic acid. About half the acetic acid produced in the United States goes into cellulose acetate from which acetate fibers are made.