Aldehydes and Ketones

Submitted by ChemPRIME Staff on Thu, 12/16/2010 - 13:06

Drinking methanol is harmful, not because of the CH3OH molecules themselves, but rather because the human body converts these molecules into methanal (formaldehyde) molecules by combination with oxygen:

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Formaldehyde, H2CO, is very reactive—in the pure state it can combine explosively with itself, forming much larger molecules. Consequently it is prepared commercially as a water solutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture., formalin, which contains about 35 to 40 percent H2CO. Formalin is made by combining methanol with air at 550°C over a silver or copper catalystA substance that increases the rate of a chemical reaction but that undergoes no net change during the reaction.. It is used as a preservative for biological specimens, in embalming fluids, and as a disinfectant and insecticide—not a very good substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. to introduce into your body. The biggest commercial use of formaldehyde is manufacture of Bakelite, melamine, and other plastics.

The Image:C-Odouble bond.jpg functional groupA set of atoms in a molecule that confer on the molecule a characteristic set of physical and chemical properties. found in formaldehyde is called a carbonylThe functional group consisting of a carbon atom doubly bonded to an oxygen atom; found in aldehydes and ketones. group. Two classes of compounds may be distinguished on the basis of the location of the carbonyl group. In aldehydes it is at the end of a carbon chain and has at least one hydrogen attached. In ketones the carbonyl group is attached to two carbon 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.. Some examples are

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Chemical tests may be performed to determine the presence of a carbonyl group. In the video below, a solution of 2,4-dinitrophenylhydrazine is added to test tubes containing 2-propanol, an alcohol; 2-propanone (acetone), a ketone; and propionic 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. a carboxylic acid. 2,4-dinitrophenylhydrazine only reacts with the carbonyl group of 2-propanone, forming an orange precipitate.

The reaction that occurs is:

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Another test can distinguish between aldehydes and ketones. Here is a video of the Silver Mirror Tollens Test for Aldehydes:

Tollens reagent is an aqueousDescribing a solution in which the solvent is water. solution of silver nitrate, sodium hydroxide, and a little ammonia. (In the video the ammonia comes from reaction of ammonium ions in ammonium nitrate and hydroxide ions from sodium hydroxide to form water and NH3.) If an aldehyde, in this case, glucose, is added to the solution, the Ag+ is reduced by the aldehyde, and the aldehyde is oxidized into a carboxylic acid. This produces silver metalAn element characterized by a glossy surface, high thermal and electrical conductivity, malleability, and ductility., which coats the flask and creates the mirror. A similar reaction does not occur for ketones, so only aldehydes produce the silver mirror. The equation for the reaction in the video is:

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The endings al and one signify aldehyde and ketone, respectively. The general formula for an aldehyde is Image:Aldehyde.jpg, while for a ketone it is Image:Ketone.jpg . Note that every ketone is isomeric with at least one aldehyde. Acetone, for example, has the same molecular formulaThe chemical formula of a substance written using the subscripts that reflect the number of each kind of atom present in a molecule of the substance. For example, the simplest formula for ethane is CH3, but the molecular formula is C2H6 because there are 2 C atoms and 6 H atoms in a molecule of ethane. (C3H6O) as propanal.

Aldehyde and ketone molecules cannot hydrogen bondAn attractive force, either intramolecular or intermolecular, between an electronegative atom and a hydrogen atom attached to another electronegative atom. among themselves for the same reason that ethers cannot—they do not contain hydrogens attached to highly electronegative atoms. The carbonyl group is rather 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., however, since the difference between the electronegativities of carbon (2.5) and oxygen (3.5) is rather large, and there are usually no other dipoles in an aldehyde or ketone molecule to cancel the effect of C==O.

Therefore the boiling points of aldehydes and ketones are intermediateIn chemical kinetics, a species that is formed in an early step in a reaction mechanism and then consumed in a later step; evidence of existence of an intermediate may be important for the interpretation of a rate law. between those of alkanes or ethers on the one hand and alcohols on the other. Acetaldehyde, CH3CH2CHO, boils at 20.8°C midway between propane (–42°C) and ethanol (78.5°C). The boilingThe process of a liquid becoming vapor in which bubbles of vapor form beneath the surface of the liquid; at the boiling temperature the vapor pressure of the liquid equals the pressure of the gas in contact with the liquid. points of propanal and acetone are compared with other organic compounds in the table of the boiling points of comparable organic compounds which shows the same trend.

Of all the aldehydes and ketones, formaldehyde and acetone are of greatest commercial importance. Uses of formaldehyde have already been mentioned. Like other ketones, acetone is mainly useful as a solventThe substance to which a solute is added to make a solution., and you may have used it for this purpose in the laboratory. Acetone and other ketones are somewhat toxic and should not be handled carelessly.