hapter 20: Aldehydes and Ketones [hapter 20 Sections: 20.1-20.7, 20.9-10.10, 20.13] 1. Nomenclature of Aldehydes and Ketones ketone ' aldehyde 2. eview of the Synthesis of Aldehydes and Ketones Br Br f both aldehydes and ketones, the parent chain is the longest continuous carbon chain that contains the carbon of the carbonyl group numbering (locant) priity is given to the carbonyl carbon aldehydes always have the carbonyl group at carbon 1, so a number is not needed ketones have the "one" suffix, aldehydes have the "al" suffix P: 20.1(a-d), 20.2, 20.44(a,d), 20.45(b-d,g,h), 20.49 A. Friedel-rafts Acylation l All 3 B. xidation of Alcohols (most common method) P KMn 4 K 2 r 2 7, 2 S 4, 2. ydration of Alkynes 2 S 4, 2 gs 4 1. B 3 TF 2. 2 2, K P: 20.5(a-d,f)
3. eview of eactions of Aldehydes and Ketones A. xidation of Aldehydes P KMn 4 K 2 r 2 7, 2 S 4, 2 2 in air (slowly) KMn 4 K 2 r 2 7, 2 S 4, 2 aldehydes may be oxidized using KMn 4 Jones reagent to the carboxylic acid since aldehydes are usually synthesized via oxidation of 1 alcohols, the synthesis of carboxylic acids is usually accomplished via direct oxidation of 1 alcohols with the strong oxidizing agents ketones ANNT be oxidized further B. eduction of Aldehydes and Ketones Zn(g), l N 2 4, K, heat Zn(g), l N 2 4, K, heat lemmensen Wolff-Kishner reduction of aldehydes and ketones leads to reduction of the carbonyl group to a 2 group 2, Pt 2, Pd hydrogenation of ketones fms 2 alcohols (the opposite reaction of the oxidation process) hydrogenation of aldehydes fms 1 alcohols P: 20.25(predict product) 4. eactivity of Aldehydes and Ketones ' ' both aldehydes and ketones have partial positive charge at the carbon of the carbonyl group due to the polar = bond the partial charge on the aldehyde is greater since it is flanked by a single electron-donating alkyl group while both aldehydes and ketones are reactive towards nucleophilic addition, aldehydes are me reactive due to i) the greater partial positive charge and ii) less steric interference P: 20.50
5. Mode of Attack of Strong Nucleophiles on the arbonyl Group of Aldehydes and Ketones Nu Nu Nu ()' ()' ()' nucleophiles attack the partially positively charged carbon of the carbonyl group by pushing electrons into the polarized pi bond the pi electrons are fced onto the electronegative oxygen atom these reactions are often followed up by addition of acidic water (l, 2 3 + ) to protonate the negatively charged oxygen and fm a neutral product 6. Addition of Strong Nucleophiles to Aldehydes and Ketones A. Addition of rganometallic eagents NaN 2 1. 2. 3 + 1. 2. 3 + 1. 3 2 Li 2. 3 + 1. 3 MgBr 2. 3 + 1.? 2. 3 + these types of nucleophilic additions to aldehydes and ketones are critical reactions since they result in the fmation of NEW - bonds! these reactions, therefe, provide a way in which to create larger molecules from readily available smaller molecules P: 20.54
B. Addition of N 1. NaN 2. 3 + N catalytic NaN there are two methods f addition of N to a carbonyl group. The net result is the same the product of addition is referred to as a "cyanohydrin" to reflect the presence of both the cyanide AND hydroxyl groups as with the ganometallics, this reaction fms a new - bond. Addition of ydride (hydride reduction) hydride Na sodium hydride 3 ' a "hydride" is a hydrogen with an extra electron, giving it a negative charge sodium hydride is the simplest fm of hydride, but in this fm it acts only as a strong base and never as a nucleophile NaB 4 and LiAl 4 are both sources of nucleophilic hydride Na B Li Al NaB 4 sodium bohydride Examples LiAl 4 (LA) lithium aluminum hydride 1. NaB 4 2. 3 + 1. LiAl 4 2. l, 2 reduction of aldehydes with either NaB 4 LiAl 4 results in the fmation of 1 alcohols reduction of ketones with either NaB 4 LiAl 4 results in the fmation of 2 alcohols P: 20.31, 20.33
Summary of Nucleophilic Addition eactions of Aldehydes and Ketones '() nu product nucleophile catalyst type of product (hydride, NaB 4 LiAl 4 ) none alcohol (1 from aldehyde, 2 from ketone) '() (Grignard, alkyllithium) none alcohol (2 from aldehyde, 3 from ketone) '() N (cyanide) none cyanohydrin N '() D. Addition of Alcohols '() '() + '() alkoxides are strong nucleophiles but alcohols are po nucleophiles alcohols will not react with aldehydes ketones in the absence of an acid catalyst an acid catalyst protonates a lone pair of the oxygen atom of the carbonyl group protonation increases the partial positive charge on the carbon and enhances the reactivity of the carbonyl towards nucleophilic addition under acidic conditions, aldehydes and ketones will react with the weaker alcohol nucleophile the most common acid catalysts are 3 P 4, 2 S 4, l paratoluenesulfonic acid (Ts) '() '() unprotonated '() protonated 3 cat. 2 S 4 Et Ts cat. 2 S 4 reaction of aldehydes ketones with alcohols under acid catalysis results in the addition of TW equivalents of the alcohol to the carbonyl group. Τthe product is called a "ketal" if a diol such as ethylene glycol is used, both ends of the diol add across the carbonyl group to fm a cyclic ketal cyclic ketals are the most often utilized type of ketal
Ts ketals are "ether-like" compounds that behave like ethers, i.e., they are fairly unreactive the ketal fmation process can be reversed by addition of water under acidic conditions ketals act as carbonyl "protecting groups" in that they can be placed on a carbonyl group to keep it from reacting and then removed when desired 3 1 equivalent NaB 4 3 2 equivalents NaB 4 3 Glucose, Sbitol and other Sweeteners sbitol Sucrose (table sugar) l l Sucralose (Splenda ) glucose l P: 20.10, 20.12, 20.58, 20.63(c,e), 20.67(a,b) N S Saccharin (Sweet 'N Low ) α-d-glucose (hemiacetal) 2 Stevia (Truvia )
7. Addition/Elimination eactions of Aldehydes and Ketones A. The Wittig eaction '() verall eaction: (Ph) 3 P ' ' '() the Wittig reaction takes place between an aldehyde a ketone and a Wittig reagent the final product is replacement of the = double bond by a = double bond Making the Wittig eagent (Ph) 3 P + 3 Br (Ph) 3 P 3 Br strong base (Ph) 3 P 2 (Ph) 3 P 2 the initial reaction is an SN2 reaction between triphenylphosphine and an alkyl halide since it is an SN2 reaction 3 alkyl halides ANNT be used as substrates common bases to used deprotonate are typically NaN 2, Na n-buli eaction Steps (Ph) 3 P 3 3 3 3 3 (Ph) 3 P 3 3 3 3 3 3 (Ph) 3 P (Ph) 3 P 3 Examples (Ph) 3 P= 2 3 Br 1. Ph 3 P 2. nbuli P: 20.37, 20.51, 20.52, 20.53
B. Imine Fmation verall eaction: '() 2 N + N '() an imine when a 1 amine reacts with a ketone aldehyde, an imine product results an acid catalyst [usually Ts l] is often needed the final product is replacement of the = double bond by a =N double bond eaction Steps 3 N 2 3 3 + 3 3 Examples N Ph 3 2 N 2 Ts Ts P: 20.15-20.18 enzyme trans-retinal cis-retinal enzyme ENZYME N 2 ENZYME N LIGT ENZYME N P: 20.56(a-c,g,i-l), 20.60, 20.66(b,f)
Summary of Nucleophilic Addition eactions of Aldehydes and Ketones '() nu product nucleophile catalyst type of product single addition reactions (hydride, NaB 4 LiAl 4 ) none alcohol (1 from aldehyde, 2 from ketone) '() (Grignard, alkyllithium) none alcohol (2 from aldehyde, 3 from ketone) '() N (cyanide) none cyanohydrin N '() double addition reactions (alcohols) + acetal '() addition-elimination reactions (Ph) 3 P= 2 (Wittig reagents) none alkene '() -N 2 (1 amine) + sometimes imine N '()