Topic 9. Aldehydes & Ketones
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1 Chemistry 2213a Fall 2012 Western University Topic 9. Aldehydes & Ketones A. Structure and Nomenclature The carbonyl group is present in aldehydes and ketones and is the most important group in bio-organic chemistry. carbonyl group aldehyde ketone Both the C and are sp 2, and their p orbitals overlap to form a bond. Angles are 120. Most importantly, the C= is a polar double bond. It can react with both electrophiles and nucleophiles. In IUPAC nomenclature, for a compound named as an aldehyde, the C= is always position #1 (takes precedence over other groups) and the suffix al is used. propanal Br Br C - + 3,3-dibromo-2-methylbutanal
2 Aldehydes & Ketones 2 When the aldehyde group is a substituent on a ring, the suffix carbaldehyde is added to the ring name, and the point of attachment is given #1. C cyclopropanecarbaldehyde C cis-2-methylcyclopentanecarbaldehyde In ketones, the C= is given the lowest possible number in the carbon chain and the suffix one is added. 3-methyl-2-butanone 4,4-dimethyl-2-cyclohexenone In compounds with two or more functional groups, it may be necessary to name the carbonyl group as an ordinary substituent using oxo. 3-oxobutanal
3 Aldehydes & Ketones 3 Carbonyl compounds don t have a + hydrogen, so intermolecular hydrogen bonding is not possible. owever, the polar carbonyl group results in higher BP than ethers, but lower than that of alcohols. B. Spectroscopy In I spectroscopy, the carbonyl group has a strong, distinctive absorption between cm 1. (As we ll see later, the C= stretch appears in all functional groups containing the C= bond, including acids and derivatives).
4 Aldehydes & Ketones 4 In 1 NM, the aldehydes is shifted far downfield to about ppm. Although the carbonyl (d) splits (and is also split by) the two atoms c, the splitting is very small. Therefore, d looks like a singlet. Similarly, the signal for c looks like it is split only by the atoms b. Note that ketones do not have an on the C=, so the peak near ppm does not appear.
5 C. Nucleophilic Addition eactions Aldehydes & Ketones 5 The reactivity of carbonyl compounds can be explained by the structure of the carbonyl group. The bond is polar not only because of a difference in electronegativity between the two atoms, but also because of resonance. ' Nucleophiles will always add to the carbon, and electrophiles (typically a proton) will add to the oxygen. Therefore, only one regioisomer is formed. ' C ' Nu The mechanism of the reaction depends on whether the solution is acidic or basic. egardless of the mechanism, these are termed nucleophilic addition reactions because the DS is the breaking of the bond by the nucleophile.
6 Aldehydes & Ketones 6 In basic solution, where anions exist, nucleophilic attack occurs first. The electrophile is weakly electrophilic, while the nucleophile is strongly nucleophilic. A neutrally charged species, such as water, is the proton donor. ' Nu slow C Nu ' fast C ' Nu In acidic solution, where cations exist, protonation occurs to generate a strong electrophile prior to the addition of a weak nucleophile. Notice that a protonated C= is just like a resonance-stabilized carbocation. 2 2 ' fast ' Nu slow fast ' ' Nu 2 2 Nu
7 Aldehydes & Ketones 7 Some important considerations: o As in alkene reactions, an sp 2 carbon changes to sp 3, and a stereocentre may be formed, in which case both stereoisomers are formed in equal amounts. o Carbonyl reactions are not as exothermic as alkene reactions, so the reaction is easily reversed (this is a very useful biochemical property). o Aldehydes (less hindered) react faster than ketones. 1. Addition of carbon nucleophiles: Grignard eaction In the late 19 th century, Victor Grignard discovered that alkyl halides (Cl, Br, or I) reacted with Mg to form a highly polar, almost-ionic C Mg bond. + - ether - X Mg MgX f unctionally equivalent to MgX + These organomagnesium compounds, called Grignard reagents, behave as negatively charged carbons.
8 Grignard reagents are therefore nucleophilic. Aldehydes & Ketones 8 '' ' - + MgX ether '' MgX The ionic product is easily converted to an alcohol using some sort of proton donor, such as acid in water. ' '' MgX ' / 2 '' ' MgX owever, because carbanions are basic, they are destroyed by the presence of compounds that can act as acids. Acid is therefore only added in the second part of the sequence. MgBr ether / 2
9 Aldehydes & Ketones 9 The Grignard reaction is one of the most useful ways to create new carboncarbon bonds. The starting materials (carbonyl compounds and alkyl halides) are readily available, and the product is formed in good yield. There is also exceptional versatility: Two groups come from the carbonyl compound, and one from the alkyl halide. It doesn t matter which groups are from the carbonyl compound or from the alkyl halide. For example, two ways to make: C 3 MgBr C 3 ether / 2 product ether / 2 C 3 MgBr product
10 Aldehydes & Ketones 10 Grignard reagents even react with carbon dioxide, to yield carboxylic acids, and with epoxides, to give alcohols. eaction with epoxides occurs because of the polar C bond in addition to the reactivity of the strained 3-atom ring. MgX C ether / 2 C MgX - ether / 2 C C C C +
11 2. Addition of hydrogen nucleophiles: reduction Aldehydes & Ketones 11 The addition of a hydride reduces an aldehyde or a ketone to an alcohol. ' slow C ' fast C ' In the laboratory, there are two common hydride sources: o Weaker: NaB 4 in alcohol solvent, followed by 2 o Stronger: LiAl 4 in ether solvent, followed by 2 Aldehydes and ketones can also be reduced by catalytic hydrogenation, similar to the hydrogenation of alkenes. owever, metal hydrides usually do NT reduce alkenes because C=C bonds are typically non-polar.
12 + 2 / Ni 2 2 Aldehydes & Ketones LiAl Addition of oxygen (alcohol) nucleophiles The two previous reactions involved the addition of a good nucleophile (carbanion or hydride) to a neutrally charged aldehyde or ketone (a weak electrophile). Alcohols, which are weak nucleophiles, do not react very fast with aldehydes or ketones unless either an acid or a base catalyst is present. 1 2 or 1 C 2
13 Aldehydes & Ketones 13 The product of the reaction contains both and bonded to the same C, and is called a hemiacetal. The reaction is very readily reversible, even in nearly neutral solutions. Either an acid or a base is needed because: o Protonating the carbonyl group gives it a positive charge, making it a better electrophile that can attract even weak nucleophiles (). o Deprotonating an alcohol makes it a good nucleophile that can attack weak electrophiles (unprotonated C=). o That is, acid makes the C= more reactive, while base makes the more reactive.
14 Mechanism of base-catalyzed hemiacetal formation: Aldehydes & Ketones 14
15 Mechanism of acid-catalyzed hemiacetal formation: Aldehydes & Ketones 15
16 Aldehydes & Ketones 16 emiacetal and acetal formation is particularly favourable when five- or sixmembered rings are formed The cyclic form of monosaccharides contains a hemiacetal.
17 Aldehydes & Ketones 17 Under acidic conditions, hemiacetals can react with a second equivalent of alcohol an S N 1 reaction to yield an acetal, a carbon bonded to two groups. 1 C 2 1 C 2 Acid protonates, making it an excellent leaving group (water). The second alcohol acts as the nucleophile. The acid-catalyzed formation of an acetal is reversible, but in basic solution it is stable. Sucrose is a disaccharide that has acetal groups on both of its two rings, linking them together.
18 Aldehydes & Ketones 18 Because acetals are formed by a series of reversible steps in acidic solution, the acetal can revert back to the starting aldehyde and alcohol. Acetals are base-stable. ' ' ' acetal hemiacetal In the reversal, note that the C bonded to the two oxygens becomes the carbonyl, and the two oxygens become the groups of the alcohol. 4. Addition of nitrogen (amine) nucleophiles Because amines are moderate nucleophiles, they can react with aldehydes and ketones in the absence of a catalyst. 1 C N 1 C N N aminol
19 Aldehydes & Ketones 19 In the presence of an acid, the reaction occurs in a manner identical to that of hemiacetal formation, except the nucleophile is an amine instead of an alcohol. The addition product, an aminol, is unstable and decomposes to an imine in an E2-like reaction. The decomposition reaction is acid-catalyzed. 1 C 2 aminol N N imine Note that in the overall reaction, the double-bonded oxygen is replaced by a double-bonded nitrogen. Imines can be converted back to their parent carbonyl and amine compounds in the presence of water and acid. NC 2 C 3 2 +
20 Aldehydes & Ketones 20 Like alkenes, imines can be reduced. 1 2 N 2 Pt, Pd, or Ni 1 2 N The reaction of a carbonyl compound with an amine, followed by reduction of the imine, converts a carbonyl to a saturated amine. This process is called reductive amination. D. ther eactions 1. xidation of aldehydes Aldehydes are easily oxidized to carboxylic acids using common reagents such as 2 Cr 4 or KMn 4. []
21 Aldehydes & Ketones 21 The Tollen s Test for aldehydes relies on their easy oxidizability (Expt 5). The oxidizing agent is Ag + found in Tollens reagent Ag(N 3 ) 2. When Ag + oxidizes the aldehyde, it is reduced to Ag 0, which precipitates in the form of metallic silver. This is often observed in the form of a silver mirror on the glass. Ketones are resistant to common oxidizing agents. This is the principal difference between aldehydes and ketones. 2. Keto-enol tautomerism As seen in Chapter 5, the addition of water to alkynes results in the formation of an enol, which tautomerizes to the more-stable keto tautomer. 3 CC C 2 S gs 4 enol tautomer C 3 keto tautomer Keto-enol tautomerism involves the movement of a proton bonded to the -carbon, which is the carbon directly adjacent to the carbonyl group.
22 Aldehydes & Ketones 22 Although the keto form is more favourable due to the stronger C= double bond, aldehydes and ketones can tautomerize to enols under acidic or basic conditions. Keto-enol tautomerism is important in the chemistry of nucleic acids, where the nucleobases of DNA can adopt enol forms. This is important from a biological standpoint, as it allows of proper base pairing. Under acidic conditions, the carbonyl group is protonated and then the - hydrogen is removed. 2 C 2 C 2 C 2 2 3
23 Aldehydes & Ketones 23 Under basic conditions conditions, the -hydrogen is removed, and then the C= is protonated. C 2 C 2 C 2 C 2 esonance stabilization makes the -hydrogens of carbonyl compounds sufficiently acidic that they can be removed. C C C 3 C 2 3 C C 3 pka = 25 pka = 19 pka = 16 Because keto-enol tautomerism is an equilibrium, enol tautomers are always present, even though they may exist in very low concentrations. For example, for acetaldehyde (ethanal) one molecule out of is in the enol form.
24 Aldehydes & Ketones 24 If the -carbon is a stereocentre, an enantiomerically pure compound slowly converts to a racemic mixture via enol formation. This racemization is accelerated by acid or base. C 3 C 3 C 3 A visual test for methyl ketones, called the iodoform test (Expt 5), relies on - hydrogen acidity. These react with I 2 in base to give a carboxylic acid salt and the yellow ppt CI 3. CC 3 I2 Na CI 3 C iodoform ppt
25 Topic Summary Textbook pages covered: , , Suggested Quick Quiz questions: all Aldehydes & Ketones 25 Suggested problems: 14, 16, 17, 20, 22, 24, 26, 28, 30, 34, 35, 36, 40, 41, 43, 44, 46 a/b/c/e/f/i/j/k/l/n/o/p/q/r, 48, 51, 52, 53, 54. Suggested questions from final exams: o 2008: : o 2010: : Past Test Questions What is a correct IUPAC name for benzyl isopropyl ketone? o 3,3-dimethyl-1-phenylpropan-2-one o 3-methyl-1-phenylbutan-2-one o 2-methyl-1-phenylpropan-1-one o 2-methyl-4-phenylpropan-3-one o 1-phenylisopentanone
26 Aldehydes & Ketones 26 Which Grignard reaction gives the product shown in good yield?? 1. Ether MgBr MgBr MgBr MgBr MgBr
27 Aldehydes & Ketones 27 Which one of the following compounds is an acetal? During the course of the following multi-step process, the change in hybridization of the indicated atom is most accurately described as o sp 2 sp sp 3 sp 2 o sp 2 sp 3 sp 2 o sp 3 sp sp 3 + / C 3 C 3 C 3 o sp 2 sp 3 sp 2 sp 3 o sp 2 sp sp 3
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