Propanone and simple ketones

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.

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.

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).

^j—w—9 •*— {^_J 9—w

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.

2-propenoic acid

^Hx 1 #° h —c = c — c

H- I -OH

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.

Date: 2015-12-11; view: 1779