Outline 16.1 The Carbonyl Group 16.2 Naming Aldehydes and Ketones 16.3 Properties of Aldehydes and Ketones 16.4 Some Common Aldehydes and Ketones 16.5 Oxidation of Aldehydes 16.6 Reduction of Aldehydes and Ketones 16.7 Addition of Alcohols: Hemiacetals and Acetals
Goals 1. What is the carbonyl group? Be able to recognize the carbonyl group and describe its polarity and shape. 2. How are ketones and aldehydes named? Be able to name the simple members of these families and write their structures, given the names. 3. What are the general properties of aldehydes and ketones? Be able to describe such properties as polarity, hydrogen bonding, and water solubility. What are some of the significant occurrences and applications of aldehydes and ketones? Be able to specify where aldehydes and ketones are found, list their major applications, and discuss some important members of each family. What are the results of the oxidation and reduction of aldehydes and ketones? Be able to describe and predict the products of the oxidation and reduction of aldehydes and ketones. What are hemiacetals and acetals, how are they formed, and how do they react? Be able to recognize hemiacetals and acetals, describe the conditions under which they are formed, and predict the products of hemiacetal and acetal formation and acetal hydrolysis.
16.1 The Carbonyl Group Carbonyl compounds are distinguished by the presence of a carbonyl group C O and are classified according to what is bonded to the carbonyl carbon.
16.1 The Carbonyl Group Since oxygen is more electronegative than carbon, carbonyl groups are strongly polarized. The polarity of the carbonyl group gives rise to its reactivity. The bond angles between the three substituents on the carbonyl carbon atom are 120°.
16.1 The Carbonyl Group Aldehydes and ketones have similar properties because their carbonyl groups are bonded to carbon and hydrogen atoms that do not attract electrons strongly. Aldehyde A compound that has a carbonyl group bonded to at least one hydrogen, RCHO. Always ends a carbon chain. Ketone A compound that has a carbonyl group bonded to two carbons in organic groups that can be the same or different, RCOR’. Always within a carbon chain.
16.2 Naming Aldehydes and Ketones The simplest aldehydes are known by their common names, which end in -aldehyde. In the IUPAC system, the final -e of the name of the alkane with the same number of carbons is replaced by -al. When substituents are present, the chain is numbered beginning with the carbonyl carbon.
16.2 Naming Aldehydes and Ketones Most simple ketones are best known by common names that give the names of the two alkyl groups bonded to the carbonyl carbon followed by the word ketone. Ketones are named systematically by replacing the final -e of the corresponding alkane name with -one (pronounced own). The numbering of the alkane chain begins at the end nearest the carbonyl group.
16.2 Naming Aldehydes and Ketones Chemical Warfare Among the Insects Insects have evolved extraordinarily effective means of chemical protection. The millipede Apheloria corrugata protects itself by discharging benzaldehyde cyanohydrin. While cyanohydrin is safe, the decomposition reaction releases deadly hydrogen cyanide gas. The weapon of the bombardier beetle is benzoquinone. When threatened, the bombardier beetle initiates the enzyme-catalyzed oxidation of dihydroxybenzene by hydrogen peroxide. A hot cloud (up to 100 °C) of irritating benzoquinone vapor shoots out of the beetle’s defensive organ with such force that it sounds like a pistol shot.
16.3 Properties of Aldehydes and Ketones The polarity of the carbonyl group makes aldehydes and ketones moderately polar. They boil at a higher temperature than alkanes with similar molecular weights. Individual molecules do not hydrogen-bond with each other, which makes aldehydes and ketones lower boiling than alcohols. In a series, the alkane is lowest boiling, the alcohol is highest boiling, and the aldehyde and ketone fall in between.
16.3 Properties of Aldehydes and Ketones Aldehydes and ketones are soluble in common organic solvents. Those with fewer than five or six carbon atoms are soluble in water because they are able to accept hydrogen bonds. Simple ketones are excellent solvents because they dissolve polar and nonpolar compounds.
16.3 Properties of Aldehydes and Ketones Some naturally-occurring aldehydes and ketones have distinctive odors:
16.3 Properties of Aldehydes and Ketones Aldehydes and ketone molecules are polar due to the presence of the carbonyl group. Since aldehydes and ketones cannot hydrogen-bond with one another, they are lower boiling than alcohols but higher boiling than alkanes because of their polarity. Common aldehydes and ketones are typically liquids. Simple aldehydes and ketones are water-soluble due to hydrogen bonding with water molecules, and ketones are good solvents. Many aldehydes and ketones have distinctive odors. Simple ketones are less toxic than simple aldehydes.
16.4 Some Common Aldehydes and Ketones FORMALDEHYDE (HCHO): TOXIC BUT USEFUL At room temperature, formaldehyde is a colorless gas with a pungent, suffocating odor. Low concentrations in the air (0.1–1.1 ppm) can cause eye, throat, and bronchial irritation, and higher concentrations can trigger asthma attacks. Skin contact can produce dermatitis.
16.4 Some Common Aldehydes and Ketones FORMALDEHYDE (HCHO): TOXIC BUT USEFUL Formaldehyde is formed during incomplete combustion of hydrocarbon fuels and is partly responsible for the irritation caused by smog-laden air. Formaldehyde can cause serious kidney damage, coma, and sometimes death; it is a breakdown product of methyl alcohol, and is one of the reasons that drinking methanol is so toxic.
16.4 Some Common Aldehydes and Ketones FORMALDEHYDE (HCHO): TOXIC BUT USEFUL Formaldehyde is commonly sold as a 37% aqueous solution under the name formalin. It kills viruses, fungi, and bacteria by reaction with amino groups in proteins, allowing for its use in disinfecting and sterilizing equipment. On standing, formaldehyde polymerizes into a solid known as paraformaldehyde.
16.4 Some Common Aldehydes and Ketones FORMALDEHYDE (HCHO): TOXIC BUT USEFUL Formaldehyde is found in polymers such as adhesives for binding plywood, foam insulation for buildings, textile finishes, and hard and durable manufactured objects. The first completely synthetic and commercially successful plastic was a polymer of phenol and formaldehyde known as Bakelite. Urea–formaldehyde polymers are now more widely used than Bakelite. Once the final polymerization of such materials is finished, no further melting and reshaping is possible because of its cross-linked, three-dimensional structure.
16.4 Some Common Aldehydes and Ketones FORMALDEHYDE (HCHO): TOXIC BUT USEFUL Because of concern over the toxicity of formaldehyde from polymeric materials, their use in most household applications is now limited.
16.4 Some Common Aldehydes and Ketones ACETALDEHYDE (CH3CHO) SWEET SMELLING BUT NARCOTIC Acetaldehyde is a sweet-smelling, flammable liquid formed by the oxidation of ethyl alcohol. It is less toxic than formaldehyde, and small amounts are produced in the normal breakdown of carbohydrates. At one time, acetaldehyde was used in the production of acetic acid and acetic anhydride, but it is a general narcotic, and large doses can cause respiratory failure. It is most commonly used for the preparation of polymeric resins, and in the silvering of mirrors.
16.4 Some Common Aldehydes and Ketones ACETONE (CH3COCH3) A SUPER SOLVENT Acetone is one of the most widely used of all organic solvents. It dissolves most organic compounds and is also miscible with water. Acetone is volatile and is a serious fire and explosion hazard when allowed to evaporate in a closed space. No chronic health risk has been associated with casual acetone exposure. When the breakdown of fats and carbohydrates is out of balance, acetone is produced in the liver.
16.5 Oxidation of Aldehydes Alcohols can be oxidized to aldehydes or ketones. Aldehydes can be further oxidized to carboxylic acids. In aldehyde oxidation, the hydrogen bonded to the carbonyl carbon is replaced by an —OH group. Ketones do not have this hydrogen and do not react cleanly with oxidizing agents.
16.5 Oxidation of Aldehydes Because ketones cannot be oxidized, treatment with a mild oxidizing agent is used as a test to distinguish between aldehydes and ketones. Tollens’ reagent consists of a solution containing silver ion in aqueous ammonia. Treatment of an aldehyde with this reagent rapidly yields the carboxylic acid anion and metallic silver.
16.5 Oxidation of Aldehydes Benedict’s reagent contains blue copper(II) ion, which is reduced to give a precipitate of red copper(I) oxide in the reaction with an aldehyde. Benedict’s reagent does not unequivocally distinguish between ketones and aldehydes. At one time, Benedict’s reagent was extensively used as a test for sugars in the urine.
16.6 Reduction of Aldehydes and Ketones The reduction of a carbonyl group occurs with the addition of hydrogen across the double bond to produce an —OH group.
16.6 Reduction of Aldehydes and Ketones Aldehydes are reduced to primary alcohols, and ketones are reduced to secondary alcohols.
16.6 Reduction of Aldehydes and Ketones Aldehydes are reduced to primary alcohols, and ketones are reduced to secondary alcohols. Reductions occur by formation of a bond to the carbonyl carbon atom by a hydride ion H2– accompanied by bonding of a hydrogen ion H+ to the carbonyl oxygen atom.
16.6 Reduction of Aldehydes and Ketones A hydride ion has a lone pair of valence electrons. Both electrons are used to form a covalent bond to the carbonyl carbon. This leaves a negative charge on the carbonyl oxygen. Aqueous acid is then added, H+ bonds to the oxygen, and a neutral alcohol results.
16.6 Reduction of Aldehydes and Ketones
16.6 Reduction of Aldehydes and Ketones In biological systems, the reducing agent for a carbonyl group is often the coenzyme nicotinamide adenine dinucleotide (NAD+), which cycles between acting as a reducing agent (NADH) and an oxidizing agent (NAD+) by the loss and gain of a hydride ion.
16.6 Reduction of Aldehydes and Ketones How Toxic Is Toxic? The term dose refers to the amount of substance that enters the body at one time. One standard method for reporting toxicity is the LD50 or lethal dose, 50%, which is a measure of the toxicity of a single dose, known as acute toxicity. A substance is fed in varying doses to laboratory animals, frequently rats or mice, and the mortality rate of the animals recorded. The result of the test is reported as the the dose that kills 50% of the animals in a uniform testing laboratory population. By comparing LD50 values, relative toxicities of various laboratory chemicals can be evaluated. For therapeutic use, a compound must show a comfortably wide margin between the dose that produces the desired effect and the dose that produces an acute toxic effect. The LD50 test is controversial; it has many drawbacks and many advantages.
16.7 Addition of Alcohols: Hemiacetals and Acetals HEMIACETAL FORMATION: Aldehydes and ketones undergo addition reactions in which an alcohol combines with the carbonyl carbon and oxygen. The initial product of addition reactions with alcohols are known as hemiacetals. Hemiacetals have both an alcohol-like —OH group and an ether-like —OR group bonded to what was once the carbonyl carbon atom.
16.7 Addition of Alcohols: Hemiacetals and Acetals HEMIACETAL FORMATION: The H from the alcohol bonds to the carbonyl-group oxygen, and the OR from the alcohol bonds to the carbonyl-group carbon.
16.7 Addition of Alcohols: Hemiacetals and Acetals HEMIACETAL FORMATION: Hemiacetals rapidly revert back to aldehydes or ketones by loss of alcohol and establish an equilibrium with the aldehyde or ketone.
16.7 Addition of Alcohols: Hemiacetals and Acetals HEMIACETAL FORMATION: Hemiacetals are often too unstable to be isolated. A major exception occurs when the —OH and CHO functional groups that react are part of the same molecule. Because of their greater stability, most simple sugars exist mainly in the cyclic hemiacetal form.
16.7 Addition of Alcohols: Hemiacetals and Acetals ACETAL FORMATION: If a small amount of acid catalyst is added to the reaction of an alcohol with an aldehyde or ketone, the hemiacetal initially formed is converted into an acetal. An acetal is a compound that has two ether-like groups bonded to what was the carbonyl carbon atom.
16.7 Addition of Alcohols: Hemiacetals and Acetals
16.7 Addition of Alcohols: Hemiacetals and Acetals ACETAL HYDROLYSIS: Reversal requires an acid catalyst and a large quantity of water to drive the reaction back toward the aldehyde or ketone.
16.7 Addition of Alcohols: Hemiacetals and Acetals CARBONYL ADDITIONS A carbonyl is a polarized double bond such that the carbon carries a partial positive charge and the oxygen carries a partial negative charge. The electron poor end of the reagent will attach to the O of the carbonyl and the electron rich end to the C. While only a trace of acid is needed for formation of a hemiacetal, the presence of H+ is absolutely necessary for further conversion to the acetal.
Chapter Summary What is the carbonyl group? The carbonyl group is a carbon atom connected by a double bond to an oxygen atom, C O. Because of the electronegativity of oxygen, the C O group is polar, with a partial negative charge on oxygen and a partial positive charge on carbon. The oxygen and the two substituents on the carbonyl-group carbon atom form a planar triangle.
Chapter Summary, Continued How are ketones and aldehydes named? The simplest aldehydes and ketones are known by common names (formaldehyde, acetaldehyde, benzaldehyde, acetone). Aldehydes are named systematically by replacing the final -e in an alkane name with -al and when necessary numbering the chain starting with 1 at the CHO group. Ketones are named systematically by replacing the final -e in an alkane name with -one and numbering starting with 1 at the end nearer the C O group. The location of the carbonyl group is indicated by placing the number of its carbon before the name. Some common names of ketones identify each alkyl group separately.
Chapter Summary, Continued What are the general properties of aldehydes and ketones? Aldehyde and ketone molecules are moderately polar, do not hydrogen-bond with each other, but can hydrogen-bond with water molecules. The smaller ones are water-soluble, and the ketones are excellent solvents. In comparable series of compounds, aldehydes and ketones are higher boiling than alkanes but lower boiling than alcohols. Many aldehydes and ketones have distinctive, pleasant odors.
Chapter Summary, Continued What are some of the significant occurrences and applications of aldehydes and ketones? Aldehydes and ketones are present in many plants, where they contribute to their aromas. Such natural aldehydes and ketones are widely used in perfumes and flavorings. Formaldehyde (an irritating and toxic substance) is used in polymers, is present in smog-laden air, and is produced biochemically from ingested methanol. Acetone is a widely used solvent and is a by-product of food breakdown during diabetes and starvation. Many sugars (carbohydrates) are aldehydes or ketones.
Chapter Summary, Continued What are the results of the oxidation and reduction of aldehydes and ketones? Mild oxidizing agents convert aldehydes to carboxylic acids but have no effect on simple ketones. Tollens’ reagent is used to indicate the presence of an aldehyde while Benedict’s reagent will give a positive test result for both aldehydes and alpha hydroxy ketones. With reducing agents, hydride ion (H—) adds to the C of the C O group in an aldehyde or ketone and hydrogen ion (H+) adds to the O to produce primary or secondary alcohols, respectively.
Chapter Summary, Continued What are hemiacetals and acetals, how are they formed, and how do they react? Aldehydes and ketones establish equilibria with alcohols to form hemiacetals or acetals. The relatively unstable hemiacetals, which have an —OH and an —OR on what was the carbonyl carbon, result from addition of alcohol to the C O bond. The more stable acetals, which have two —OR groups on what was the carbonyl carbon, form by addition of a second alcohol molecule to a hemiacetal. The aldehyde or ketone can be regenerated from an acetal by treatment with an acid catalyst and a large quantity of water, which is an example of a hydrolysis reaction.