Like all other simple ketones, propanone is a liquid at room temperature. It has a boiling point of 133° F. (56° C). It mixes well with water and like butanone (methyl ethyl ketone), another simple ketone, propanone is an excellent solvent for dissolving organic compounds. Although the carbonyl group in ketones is less reactive than that in aldehydes, it is still prone to react with other chemicals. Like aldehydes, ketones can be converted into alcohols by the addition of hydrogen under suitable conditions. Higher pressures of hydrogen gas are needed for ketones to combine with hydrogen. Acetals, imines, and cya-nohydrins are other useful chemicals formed from ketones and aldehydes.
Acetals
When mixed with an excess of alcohol, aldehydes react to form acetals and hemiacetals, which are useful in chemical synthesis, the for
Organic chemistry: Aldehydes and ketones 83
Formalin,an aqueous solution of ethanal (formaldehyde), has long been used for preserving biological specimens. For this purpose, it is usually preferred to ethanol (alcohol), which dehydrates specimens and distorts their tissues. Formalin has a pungent smell and irritating fumes.
Propanone (acetone) A<
CH3— C-CH3
Keto form (99.99%)
2,4-pentadione
♦l^U-
o
II
C—CH,
CH~C^..s
CHf
Keto form (20%)
mation of more complex compounds from simpler ones. During the reaction, the double bond in the carbonyl group is destroyed by an attacking alcohol molecule. This alcohol molecule replaces the carbonyl group on the aldehyde molecule, forming a hemiacetal. Acetals are formed when hemiacetals are subjected to further attack by alcohol.
The carbonyl group in ketones may be similarly modified to yield hemiketals and ketals. As acetals and ketals broadly share the same chemical properties, they are usually referred to collectively as acetals.
Imines are formed by reacting aldehydes or ketones with amines. Amines are organic compounds obtained from ammonia. Some of them are important in biology. Their formation is a key step in the metabolism of amino acids, the building blocks of proteins. Chemists use cyanohydrins (made by reactions of aldehydes and ketones with hydrogen cyanide) in chemical synthesis.
Keto-enol tautomerism
In chemical synthesis, a useful feature of ketones is that they can exist in two forms, depending on their surroundings. Changing from one form to another is known as tautomerism. This involves various chemical changes. One effect is to transform the ketone into a vinyl alcohol, possessing a carbon-carbon double bond and a hydroxy! grouping. This is known as the enol form. In the keto form, the carbonyl group remains intact. Synthetic chemists can easily make either form, depending on which form (or tautomer) they require for use.
gen atom. This reversibility is called tautomerism. The more complicated diketone 2,4-pentadione, with two carbonyl groups, exists mainly as the enol form (D). The extra strength conferred by its six-membered ring makes it more stable than the keto form (C).
A simple ketonesuch as propanone (acetone) exists almost entirely as the keto form (A). In this form, the oxygen atom is doubly bonded to the carbon atom of the carbonyl group. A tiny proportion changes re-versibly to the enol form (B). In this form, the oxygen atom is joined to a hydro-
Ketones,particularly propanone (acetone) and cyclo-hexanone, find major uses as industrial solvents. Ketones are especially important for cellulose compounds such as those used in paints and lacquers. The formulation called cellulose thinner is used to make cellulose paint thin enough for spraying (left).
CH3— C—CH2
CrL
^j—w—9 •*— {^_J 9—w
0 0
1 II
~CH3
Enol form (80°/
j.)
Methanoic acid
Ethanedioic acid
There are various types of organic acids.All con
tain at least one carboxyl group (the pink squares). The diagram above illustrates the main types: saturated aliphatic, such as methanoic and ethanoic acid (blue boxes); unsaturated aliphatic, such as 2-propenoic acid (purple box); dicarboxylic, such as ethanedioic acid (pale green box); aromatic, such as benzoic acid (darker green box); and poiyfunctional (containing one or more radical groups in addition to the carboxyl group), such as 2-hydroxypropanoic acid (pale brown box).
The venomof ants(seen attacking a wasp, below left) and the bark of willow trees (below right) both contain organic acids. Methanoic acid is in the ants' venom, and salicylic acid in willow bark.
Ethanoic acid
V /°
H —C—c'
H/ OH
2-propenoic acid
Hx 1 #° h —c = c — c
H- I -OH
Benzoic acid
2-hydroxypropanoic acid H — C—C—C
Organic acids
Organic acids are a group of compounds characterized by the carboxyl group, which is itself made up of a carbonyl group and a hydroxyl group. A carbonyl group contains a carbon atom and an oxygen atom, connected by a double bond. A hydroxyl group is a specific arrangement of atoms in which a hydrogen atom is bonded to an oxygen atom. Organic acid compounds are commonly called carboxylic acids.
Organic acids are weak compared to mineral acids. This is because, when in solution (put into a liquid mixture of two or more substances), organic acids dissociate (break up) less completely. They thus produce a lower concentration of hydrogen ions than do mineral acids. It is the hydrogen ions that react with other substances and cause the burning or corroding for which acids are known.
The most common type of carboxylic acid attaches itself to an alkyl group (a compound consisting solely of carbon and hydrogen). Other types attach themselves to a ring system (carbon atoms in the shape of a ring) or to another carboxylic group (becoming dicarboxylic acids). Some types are unattached, forming no bond with another atom or group of atoms. Such carboxylic acids are called unsaturated.