Aldehydes and Ketones Chapter 23 Hein * Best * Pattison * Arena Version 1.0 Colleen Kelley Chemistry Department Pima Community College © John Wiley and Sons, Inc.
Chapter Outline 23.1 Structures of Aldehydes and Ketones 23.4 Chemical and Physical Properties of Aldehydes and Ketones 23.5 Common Aldehydes and Ketones 23.6 Condensation Polymers 23.2 Naming Aldehydes and Ketones 23.3 Bonding and Physical Properties
Structures of Aldehydes & Ketones
Both aldehydes and ketones contain a carbonyl ( C=O) group.
The general formula for the saturated homologous series of aldehydes and ketones is: CnH2nO
In a linear expression, the aldehyde group is often written as: CHO
In the linear expression of a ketone, the carbonyl group is written as: CO
Naming Aldehydes & Ketones
IUPAC Rules for Naming Aldehydes To establish the parent name, select the longest continuous chain of carbon atoms that contains the aldehyde group. The carbons of the parent chain are numbered starting with the aldehyde group. Since the aldehyde group is at the beginning (or end) of a chain, it is understood to be number 1.
IUPAC Rules for Naming Aldehydes 3. Form the parent aldehyde name by dropping the –e from the corresponding alkane name and adding the suffix –al. 4. Other groups attached to the parent chain are named and numbered as we have done before.
Naming Aldehydes
Common Names for Aldehydes
Dialdehydes In dialdehydes, the suffix –dial is added to the corresponding hydrocarbon name.
IUPAC Rules for Naming Ketones To establish the parent name, select the longest continuous chain of carbon atoms that contain the ketone group. Form the parent name by dropping the –e from the corresponding alkane name and add the suffix –one.
IUPAC Rules for Naming Ketones 3. If the chain is longer than four carbons, it is numbered so that the carbonyl group has the smallest number possible; this number is prefixed to the parent name of the ketone. 4. Other groups attached to the parent chain are named and numbered as we have done before.
Naming Ketones
Common Names for Ketones
Bonding and Physical Properties
Bonding The carbon atom of the carbonyl group is sp2-hybridized and is joined to three other atoms by sigma bonds. The fourth bond is made by overlapping p electrons of carbon and oxygen to form a pi bond between the carbon and oxygen atoms.
Bonding Because the oxygen atom is considerably more electronegative than carbon, the C=O group is polar. Many of the chemical reactions of aldehydes and ketones are due to this polarity.
Properties Unlike alcohols, aldehydes and ketones cannot hydrogen-bond to themselves, because no hydrogen atom is attached to the oxygen atom of the carbonyl group. Aldehydes and ketones, therefore, have lower boiling points than alcohols of comparable molar mass.
Chemical Properties of Aldehydes & Ketones
Reactions of Aldehydes & Ketones Oxidation aldehydes only Reduction aldehydes and ketones Addition
Oxidation of Aldehydes Aldehydes are easily oxidized to carboxylic acids by a variety of oxidizing agents, including (under some conditions) oxygen of the air.
Tollens test The Tollens test (silver-mirror test) for aldehydes is based on the ability of silver ions to oxidize aldehydes.
Fehling and Benedict Tests Fehling and Benedict solutions contain Cu2+ ions in an alkaline medium. In these tests, the aldehyde group is oxidized to an acid by Cu2+ ions.
Tollens, Fehling & Benedict Tests Because most ketones do not give a positive with Tollens, Fehling, or Benedict solutions, these tests are used to distinguish between aldehydes and ketones.
Biochemical Oxidation of Aldehydes When our cells ‘burn’ carbohydrates, they take advantage of the aldehyde reactivity. The aldehyde is oxidized to a carboxylic acid and is eventually converted to carbon dioxide, which is then exhaled. This stepwise oxidation provides some of the energy necessary to sustain life.
Reduction of Aldehydes & Ketones Aldehydes and ketones are easily reduced to alcohols.
Addition Reactions of Aldehydes & Ketones Common addition reactions: Addition of alcohols hemiacetal, hemiketal, acetal, ketal Addition of hydrogen cyanide (HCN) cyanohydrin Aldol Condensation (self-addition)
Addition of Alcohols Aldehydes react with alcohols in the presence of a trace of acid to form hemiacetals:
Addition of Alcohols In the presence of excess alcohol and strong acid such as dry HCl, aldehydes or hemiacetals react with a second molecule of the alcohol to give an acetal:
Addition of Alcohols to Aldehydes and Ketones
Addition of Hydrogen Cyanide The addition of HCN to aldehydes and ketones forms a class of compounds known as cyanohydrins:
Aldol Condensation (Self-Addition) In a carbonyl compound, the carbon atoms are labeled alpha (), beta (), gamma (), delta (), and so on, according to their positions with respect to the carbonyl group. -C-C-C-C=O The hydrogen atoms attached to the -carbon have the unique ability to be more easily released as protons than other hydrogens within the molecule.
Aldol Condensation (Self-Addition) An aldehyde or ketone that contains -hydrogens may add to itself or to another -hydrogen containing aldehyde or ketone.
Common Aldehydes & Ketones
Formaldehyde (Methanal) Formaldehyde is made from methanol by reaction with oxygen (air) in the presence of a silver or copper catalyst. 2 CH3OH + O2 2H2C=O + 2H2O Formaldehyde is widely used in the synthesis of polymers. Ag heat
Acetaldehyde (Ethanal) Its principal use is as an intermediate in the manufacture of other chemicals, such as acetic acid and 1-butanol.
Acetone and Methyl Ethyl Ketone Acetone is used as a solvent in the manufacture of drugs, chemicals, and explosives. It is also used as a solvent. Methyl ethyl ketone (MEK) is also widely used as a solvent, especially for lacquers.
Condensation Polymers
Phenol-Formaldehyde Polymers (Bakelite) Each formaldehyde molecule reacts with two phenol molecules to eliminate water. The polymer is then formed.
Phenol-Formaldehyde Polymers (Bakelite) Polymers of this type are still used, especially in electrical equipment, because of the insulating and fire-resistant properties.
The End