Lecture 4: Aldehydes, Ketones, and Chiral Molecules 14.1 Aldehydes and Ketones Carbonyl Group in Aldehydes and Ketones A carbonyl group (C=) In an aldehyde is attached to at least one atom. In a ketone is attached to two carbon groups. 1 2 Naming Aldehydes An aldehyde as an IUPAC name in which the -e in the alkane name is changed to -al. as a common name for the first four aldehydes that use the prefixes form- (1C), acet- (2C), propion- (3C), and butyr- (4C), followed by aldehyde. C C 3 C C 3 C 2 C methanal ethanal propanal (formaldehyde) (acetaldehyde) (propionaldehyde) Naming Aldehydes 3 4 1
Aldehydes in Flavorings Naming Ketones Several naturally occurring aldehydes are used as flavorings for foods and fragrances. C Benzaldehyde (almonds) C=C C Cinnamaldehyde (cinnamon) 5 Ketones are named In the IUPAC system by replacing the -e in the alkane name with -one. The carbonyl carbon is indicated by a number. With a common name by indicating the alkyl groups attached to the carbonyl group in alphabetical order followed by ketone. C 3 C C 3 C 3 C C 2 C 3 propanone 2-butanone 6 (dimethyl ketone) (ethyl methyl ketone) Ketones in Common Use Nail polish remover, Solvent Propanone, Dimethylketone, Acetone Butter flavoring 7 Classify each as 1) aldehyde or 2) ketone. A. C 3 C 2 C C 3 B. C 3 C C 3 C. C 3 C C 2 C D. C 3 8 2
Classify each as an aldehyde (1), ketone (2), alcohol (3), or ether (4). A. C 3 C 2 C C 3 B. C 3 C 3 C 3 C. C 3 C C 2 C D. C 3 C C 3 C 3 Name each of the following: 1. C 3 C 2 C 2 C 2 C 2. Cl C 2 C 2 C 3. C 3 C 2 C C 3 9 10 Draw the structural formulas for each: A. 4-methylpentanal B. 2,3-dichloropropanal C. 3-methyl-2-butanone Lecture 4: Aldehydes, Ketones, and Chiral Molecules 14.2 Physical Properties 11 12 3
Polar Carbonyl Group Boiling Points The polar carbonyl group Provides dipole-dipole interactions. δ + δ - δ + δ - C= C= Does not have on the oxygen atom. Cannot form hydrogen bonds. Aldehydes and ketones have Polar carbonyl groups (C=). δ + δ - C= Attractions between polar groups. δ + δ - δ + δ - C= C= igher boiling points than alkanes and ethers of similar mass. Lower boiling points than alcohols of similar mass. 13 14 Comparison of Boiling Points Select the compound in each pair that would have the igher boiling point. A. C 3 C 2 C 3 or C 3 C 2 58 B. or C. C 3 C 2 or C 3 C 3 15 16 4
Solubility in Water The electronegative atom of the carbonyl group in aldehydes and ketones forms hydrogen bonds with water. Indicate if each is soluble or insoluble in water. A. C 3 C 2 C 3 B. C 3 C 2 C. C 3 C 2 C 2 C D. C 3 C C 3 17 18 Lecture 4: Aldehydes, Ketones and Chiral Molecules xidation 14.3 xidation and Reduction Aldehydes are easily oxidized to carboxylic acids. [] C 3 C C 3 C Acetaldehyde Acetic acid 19 20 5
Tollens Test Benedict s Test Tollens reagent, which contains Ag +, oxidizes aldehydes, but not ketones. Ag + is reduced to metallic Ag, which appears as a mirror in the test tube. In the Benedict s Test, Cu 2+ reacts with aldehydes that have an adjacent group. Aldehydes are oxidized to carboxylic acids. Cu 2+ is reduced to give Cu 2 (s). 21 22 Lecture 4: Aldehydes, Ketones and Chiral Molecules Write the structure and name of the oxidized product when each is mixed with Tollens reagent. 1. butanal 2. acetaldehyde 3. ethyl methyl ketone 14.4 Addition Reactions δ+ δ- δ+ δ- C= + X Y C X Y 23 24 6
Addition Reactions Acetal Formation When a polar molecule adds to the carbonyl group of an aldehyde or ketone The negative part of the added molecule bonds to the positive carbonyl carbon. The positive part of the added molecule bonds to the negative carbonyl oxygen. Alcohols add to the carbonyl group of aldehydes and ketones. The addition of two alcohols forms acetals. δ + δ - δ + δ - C= + X Y C X Y 25 26 emiacetal Formation Cyclic emiacetals The addition of one alcohol to an aldehyde or ketone forms an intermediate called a hemiacetal. Usually, hemiacetals are unstable and difficult to isolate. A stable cyclic hemiacetal forms when the C= group and the are both part of a five- or six-atom carbon compound. 27 28 7
Identify each as a 1) hemiacetal or 2) acetal. C 3 A. C 3 C 2 C Draw the structure of the acetal formed by adding C 3 to butanal. C 2 C 3 B. C 3 C 2 C C 2 C 3 C 2 C 3 29 30 Lecture 4: Aldehydes, Ketones, and Chiral Molecules 15.5 Chiral Molecules Chiral Compounds Chiral compounds ave the same number of atoms arranged differently in space. ave one or more chiral carbon atoms bonded to four different groups. Are mirror images like your hands. Try to superimpose your thumbs, palms, back of hands, and little fingers. 31 32 8
Mirror Images The mirror images of Chiral compounds cannot be superimposed. Two compounds of bromochloroiodomethane can align the and I atoms, which places the Cl and Br atoms on opposite sides. Achiral Structures are Superimposable When the mirror image of an achiral structure is rotated, the structure can be aligned with the initial structure. Thus this mirror image is superimposable. Copyright 2007 by Pearson Education, Inc 33 Copyright 2007 by Pearson Education, Inc 34 Fischer Projections Identify each as a chiral or achiral compound. Cl Cl Cl C C 3 C C 3 C C 3 C 2 C 3 Br A Fischer projection Is a 2-dimensional representation of a 3-dimensional molecule. Places the most oxidized group at the top. Uses vertical lines in place of dashes for bonds that go back. Uses horizontal lines in place of wedges for bonds that come forward. A B C 35 36 9
Drawing Fischer Projections D and L Notations By convention, the letter L is assigned to the structure with the on the left. The letter D is assigned to the structure with the on the right. 37 Copyright 2007 by Pearson Education, Inc 38 Indicate whether each pair is a mirror image that cannot be superimposed. A. C 2 C C 3 and C 3 C 2 C Br Br Cl Cl B. C C 3 and C 3 C 39 10