Chapter Fifteen Chapter 15 Lecture Fundamentals of General, Organic, and Biological Chemistry 8th Edition McMurry, Ballantine, Hoeger, Peterson Chapter Fifteen Aldehydes and Ketones Christina A. Johnson University of California, San Diego © 2017 Pearson Education, Inc.
Outline 15.1 The Carbonyl Group 15.2 Naming Simple Aldehydes and Ketones 15.3 Properties of Aldehydes and Ketones 15.4 Some Common Aldehydes and Ketones 15.5 Oxidation of Aldehydes 15.6 Reduction of Aldehydes and Ketones 15.7 Addition of Alcohols: Hemiacetals and Acetals
Concepts to Review Electronegativity and Molecular Polarity Sections 4.9 and 4.10 Oxidation and Reduction Section 5.6 Hydrogen Bonds Section 8.2 Functional Groups Section 12.2 Naming Alkanes Section 12.6 Types of Organic Reactions Section 13.5
15.1 The Carbonyl Group A carbonyl group is a functional group that has a carbon atom joined to an oxygen atom by a double bond. Learning Objective: Identify a carbonyl group and describe its polarity and shape.
15.1 The Carbonyl Group A carbonyl compound is any compound that contains a carbonyl group (C=O). Carbonyl compounds are classified according to what is bonded to the carbonyl carbon.
15.1 The Carbonyl Group
15.1 The Carbonyl Group Because 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°.
15.1 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
15.1 The Carbonyl Group 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
15.2 Naming Simple Aldehydes and Ketones Aldehydes and ketones are known by their common names as well as by using the IUPAC naming system. Learning Objective: Name and draw simple aldehydes and ketones given a structure or a name.
15.2 Naming Simple 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.
15.2 Naming Simple 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.
Worked Example 15.1 Give both the systematic (IUPAC) name and the common name for the following compound:
Worked Example 15.1 Cont. ANALYSIS: The compound is a ketone, as shown by the single carbonyl group bonded to two alkyl groups: an ethyl group on the left (CH3CH2–) and a propyl group on the right (– CH2CH2CH3). The common name uses the names of the two alkyl groups.
Worked Example 15.1 Cont. ANALYSIS Continued: The IUPAC system identifies and numbers carbon chains to indicate where the carbonyl group is located, counting in the direction that gives the carbonyl carbon the lowest number possible.
Worked Example 15.1 Cont. Solution: The IUPAC name is 3-hexanone. The common name is ethyl propyl ketone.
15.3 Properties of Aldehydes and Ketones The polarity of the carbonyl group makes aldehydes and ketones moderately polar. Learning Objective: Describe the polarity, hydrogen bonding, and water solubility of aldehydes and ketones.
15.3 Properties of Aldehydes and Ketones Aldehydes and ketones 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.
15.3 Properties of Aldehydes and Ketones In a series, the alkane is lowest boiling, the alcohol is highest boiling, and the aldehyde and ketone fall in between.
15.3 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.
15.3 Properties of Aldehydes and Ketones Hydrogen bonding with water of an aldehyde and ketone:
15.3 Properties of Aldehydes and Ketones Summary of Properties of Aldehydes and Ketones: Aldehyde and ketone molecules are polar due to the presence of the carbonyl group. Because aldehydes and ketones cannot hydrogen bond with one another, they have lower boiling points than alcohols but higher boiling points than alkanes because of dipole-dipole interactions. Common aldehydes and ketones are typically liquids.
15.3 Properties of Aldehydes and Ketones Summary of Properties of Aldehydes and Ketones (Continued): Simple aldehydes and ketones are water-soluble due to hydrogen bonding with water molecules, and ketones are good solvents for many polar and nonpolar solutes. Many aldehydes and ketones have distinctive odors. Simple ketones are less toxic than simple aldehydes.
15.4 Some Common Aldehydes and Ketones Many aromas and flavors derive largely from naturally occurring aldehydes and ketones. Learning Objective: Identify common aldehydes and ketones and their uses.
15.4 Some Common Aldehydes and Ketones Some naturally occurring aldehydes and ketones have distinctive odors:
15.4 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.
15.4 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.
15.4 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 reacting with amino groups in proteins, allowing for its use in disinfecting and sterilizing equipment. On standing, formaldehyde polymerizes into a solid known as paraformaldehyde.
15.4 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.
15.4 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.
15.4 Some Common Aldehydes and Ketones Benzaldehyde (PhCHO) Simplest Aromatic Aldehyde Benzaldehyde is a colorless liquid, pleasant almond or cherrylike odor; first extracted from bitter almonds. It is used as a flavoring and fragrance in food, cosmetics, pharmaceuticals, and soap and is “generally regarded as safe” by the Food and Drug Administration (FDA). It is used industrially as a forerunner to other organic compounds, ranging from pharmaceuticals to plastic additives.
15.5 Oxidation of Aldehydes Alcohols can be oxidized to aldehydes or ketones. Aldehydes can be further oxidized to carboxylic acids. Learning Objective: Identify the products formed from the oxidation of aldehydes (and see that ketones do not oxidize in the same way).
15.5 Oxidation of Aldehydes 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.
15.5 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.
15.5 Oxidation of Aldehydes
15.5 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.
15.5 Oxidation of Aldehydes At one time, Benedict’s reagent was extensively used as a test for sugars in the urine.
15.6 Reduction of Aldehydes and Ketones Aldehydes and ketones can be reduced to alcohols. Learning Objective: Identify the products of the reduction of aldehydes and ketones.
15.6 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.
15.6 Reduction of Aldehydes and Ketones Aldehydes are reduced to primary alcohols, and ketones are reduced to secondary alcohols.
15.6 Reduction of Aldehydes and Ketones Reductions occur by formation of a bond to the carbonyl carbon atom by a hydride ion :H– accompanied by bonding of a hydrogen ion H+ to the carbonyl oxygen atom.
15.6 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.
15.6 Reduction of Aldehydes and Ketones
15.6 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.
Worked Example 15.2 What product would you obtain by reduction of benzaldehyde?
Worked Example 15.2 Cont. ANALYSIS: First, draw the structure of the starting material, showing the double bond in the carbonyl group. Then rewrite the structure showing only a single bond between C and O, along with partial bonds to both C and O.
Worked Example 15.2 Cont. ANALYSIS Continued: Finally, attach hydrogen atoms to the two partial bonds and rewrite the product.
Worked Example 15.2 Cont. Solution: The product obtained is benzyl alcohol.
15.7 Addition of Alcohols: Hemiacetals and Acetals Aldehydes and ketones undergo addition reactions in which an alcohol combines with the carbonyl carbon and oxygen. Learning Objectives: Identify the differences between hemiacetals, hemiketals, acetals, and ketals. Predict the products of hemiacetal, hemiketal, acetal, and ketal formation and their hydrolysis.
15.7 Addition of Alcohols: Hemiacetals and Acetals Hemiacetal Formation: The initial product of addition reactions with alcohols are known as hemiacetals. Hemiacetals have both an alcohol-like –OH group and an etherlike –OR group bonded to what was once the carbonyl carbon atom.
15.7 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.
15.7 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.
15.7 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.
15.7 Addition of Alcohols: Hemiacetals and Acetals Hemiacetal Formation: Because of their greater stability, most simple sugars exist mainly in the cyclic hemiacetal form.
15.7 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 etherlike groups bonded to what was the carbonyl carbon atom.
15.7 Addition of Alcohols: Hemiacetals and Acetals
Worked Example 15.3 Write the structure of the intermediate hemiacetal and the acetal final product formed in the following reaction:
Worked Example 15.3 Cont. ANALYSIS: First, rewrite the structure showing only a single bond between C and O, along with partial bonds to both C and O.
Worked Example 15.3 Cont. ANALYSIS Continued: Next, add 1 molecule of the alcohol (CH3OH in this case) by attaching –H to the oxygen partial bond and –OCH3 to the carbon partial bond. This yields the hemiacetal intermediate. Finally, replace the –OH group of the hemiacetal with an –OCH3 from a second molecule of alcohol.
Worked Example 15.3 Cont. Solution: The reaction produces acetal and water.
Worked Example 15.4 Which of the following compounds are hemiacetals and which are hemiketals?
Worked Example 15.4 Cont. ANALYSIS: To identify a hemiacetal or a hemiketal, look for a carbon atom with single bonds to two oxygen atoms, with one being an –OH group and one an –OR group. Note that the O of the –OR group can be part of a ring. If the two remaining groups are carbons, it is a hemiketal; if one is a carbon and the other is a hydrogen, it is a hemiacetal.
Worked Example 15.4 Cont. Solution: Compound (a) contains two O atoms, but they are bonded to different C atoms; it is not a hemiacetal, rather a diol.
Worked Example 15.4 Cont. Solution Continued: Compound (b) has one ring C atom bonded to two oxygen atoms, one in the substituent –OH group and one bonded to the rest of the ring, which is the R group; the other two groups bonded to that carbon are a H and another C; it is a cyclic hemiacetal.
Worked Example 15.4 Cont. Solution Continued: Compound (c) also contains a C atom bonded to one –OH group and one –OR group, but here the other two bonded groups are carbons, so (c) is a hemiketal.
Worked Example 15.5 Which of the following compounds are acetals and ketals?
Worked Example 15.5 Cont. ANALYSIS: As in identifying hemiacetals and hemiketals, look for a carbon atom that has single bonds to two oxygen atoms, but in this case both of them will be –OR groups. Note that the O of the –OR group can be part of the ring. If the two remaining bonded groups are carbons, it is a ketal; if one is a carbon and the other a hydrogen, it is an acetal.
Worked Example 15.5 Cont. Solution: In (a), the central carbon atom is bonded to one –CH3, one –H, and two –OCH2CH3 groups, so the compound is an acetal.
Worked Example 15.5 Cont. Solution Continued: Compound (b) does have a carbon atom bonded to two oxygen atoms, but one of the bonds is a double bond rather than a single bond, so this is not an acetal.
Worked Example 15.5 Cont. Solution Continued: Compound (c) has an oxygen atom in a ring, making it also part of an –OR group, where R is the ring. Because one of the carbons connected to the O in the ring is also connected to an –OCH2CH3 group, compound (c) is an acetal.
Worked Example 15.5 Cont. Solution Continued: Compound (d) is sugar known as mannose; it too has an oxygen atom in a ring, making it part of an –OR group, where R is the ring. Because one of the carbons connected to the O in the ring is also connected to an H and an OH, compound (d) is a hemiacetal.
15.7 Addition of Alcohols: Hemiacetals and Acetals Acetal Hydrolysis: Hydrolysis: A reaction in which a bond or bonds are broken and the H– and –OH of water add to the atoms of the broken bond or bonds. Acetal hydrolysis (reversal of acetal formation) requires an acid catalyst and a large quantity of water to drive the reaction back toward the aldehyde or ketone.
15.7 Addition of Alcohols: Hemiacetals and Acetals Acetal Hydrolysis:
Worked Example 15.6 Write the structure of the aldehyde or ketone that forms by hydrolysis of the following acetal:
Worked Example 15.6 Cont. ANALYSIS: The products are the aldehyde or ketone plus two molecules of the alcohol from which the acetal could have been formed. First, identify the two C–O acetal bonds, redrawing the structure if necessary.
Worked Example 15.6 Cont. ANALYSIS Continued: Next, break the H–OH bond and one of the acetal C–OR bonds (in this case, it does not matter which one); move the water OH to the acetal carbon to form the hemiacetal and the water H to the OR to form one molecule of HOR.
Worked Example 15.6 Cont. ANALYSIS Continued:
Worked Example 15.6 Cont. ANALYSIS Continued: Remove the H and OR groups from the hemiacetal, and change the C–O single bond to a C=O double bond to give carbon the four bonds it must have. Combine the H and OR you removed to form the second alcohol molecule.
Worked Example 15.6 Cont. ANALYSIS Continued:
Worked Example 15.6 Cont. Solution: In this example, the product is an aldehyde. The procedure is identical if you start with a ketal rather than an acetal.
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