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Page 1 of 18 ALDEHYDES AND KETONES: ch.19\p.503 O Carbonyl group is a functional group C O O CH3C C H Acetaldehyde (An aliphatic) H Benzaldehyde (An aromatic aldehyde) The ald. Group is often written RCHO for convenience O CH3C CH2CH3 O O C C CH3 2-Butanone (An aliphatic ketone) Acetophenone Benzophenone (An aryl alkyl ketone) (An aromatic ketone) -Naming: Common name, or IUPAC names: Common: CHO CH3CH2CHO Propionaldehyde CH3CH2CH2CHO Benzaldehyde Butyraldehyde 1 Page 2 of 18 -The position of substituents in the aldehyde can be indicated in the common name by Greek letters starting with ∝ at the position adjacent to the carbonyl group: δ 𝛾 𝛽 ∝ O C C C C C 𝛾 𝛽 ∝ ∝ Br CH2CH2CH2CHO CH3CHCHO H -For example: cl ∝-Chloropropionaldehyde Ȣ-Bromobutyraldehyde IUPAC names: replacing -e with -al. CH3 CH3ــCHــCHO CH3CHO 2 Ethanol CH3 1 clCH2CHــCH2CHO 4 3 2 1 2-methylpropanal 4-chloro-3-methyl butane (Acetaldehyde) (Isobutyraldehyde) H.W: Ex.19.1, 19.2, 19.3 Ex. 19.4, 19.5 P.506 The common name of Ketones: (the name are written as separate words) for example: 2 Page 3 of 18 O O O CH3CH2C CH3 CH 2 C CH3 CH3CH2C CH2CH3 Ethyl methyl ketone Diethyl ketone O Benzyl methyl ketone O CH3C CH3 O C C CH3 Acetone Acetophenone Benzophenone -The IUPAC names of ketones: -e replaced with –one O CH3CH2CH2CCH3 5 4 3 CH3 O C CHCCH3 2 1 5 CH3 4 2-pentanone O 3 21 cyclohexane 4-methyl-3-penten-2-one H.W: Ex. 19.6, 19.7, 19.8, 19.9 P.508 Physical properties: Read Table 19-1 P.509. Most simple aldehydes and Ketones are polar molecule strong intermolecular attractions in these compounds account for their boiling points, which intermediate between HC & alcohol (similar M.Wt). 3 Page 4 of 18 Preparing Aldehydes and Ketones: 1° alcohols oxid. Aldehydes further oxid. Carboxylic acids 2° alcohols oxid. Ketones (method of preparing ketones) OH O CH3ـــCHـــCH2CH3 2-Butanol 𝐾2Cr 2O7 𝐻+,𝐻2𝑂 OH CH3ـــCــCH2CH3 O Butanone 𝐾𝑀𝑛𝑂 4 𝐻+,𝐻2𝑂 Cyclohexanol cyclohexanone Preparing aldehydes: oxidation of 1° alcohols as a method of preparing aldehydes, it’s necessary to stop the reaction at the intermediate aldehyde stage. This can be done by removing the aldehyde from the reaction as soon as it’s formed quite easily by distillation, because aldehydes have lower boiling points than that of the starting alcohol. By carrying out the oxidation at a temperature slightly above the boiling point of the aldehyde, it can be removed by distillation as its forms and before it can be further oxidized. 4 Page 5 of 18 CH3CH2CH2OH 𝐶𝑟𝑂3,𝐻+ 65℃,𝐻2𝑂 1-propanol (b.p., 97℃ ) CH3CH2CHO propanaldehyde (b.p., 49℃) The fact that aldehydes are easily oxidized to carboxylic acids is used as a method of testing for the presence of an aldehyde. H.W.: Ex. 19-10 P.510 Tests for Aldehyde: Most of the reactions of aldehydes and ketones are very similar, but their ability to be oxidized differs greatly. Ketones are oxidized only under the most vigorous conditions, whereas aldehydes are easily oxidized, (ald. Are slowly oxidized by air). The ease of oxidation is the basis of several tests for aldehyde. 1-Tollen’s test: (an alkaline solution of di (ammine) silver (I) ion, Ag (NH3)2+ , to aldehyde. 2Ag (NH3)2++ RCHO+3OHSilver ion RCO2-+2Ag +4NH3+2H2O silver metal The silver metal coats the glass surface and forms a silver mirror, this reaction called silver mirror test for aldehydes. 5 Page 6 of 18 2-Fehling’s solution: (an alkaline solution of cupric ion (Cu+2) complexed with tartrate ion). Aldehyde oxidized carboxylate ion Cupric ion reduced cupric oxide (Cu2O) ppt. as brick-red solid Because alcohols, alkenes, ketones, and carbocyclic acids and their derivatives are not oxidized by either the Tollen’s or the Fehling’s reagent, then tests are highly specific for aldehyde. ADDITION REACTION OF ALDEHYDES & KETONES: :تفاعالت االضافة X ــC ــO ــY C=O + XــY Cδ+=Oδ- (the C=O is made up of two atoms of different electronegativities. (Diff. in polarity)). As a result of polarity, the carbon of carbonyl group forms bonds with the more nucleophilic part of the reagent XـــY. 𝛿+ 𝛿- C X C ــــX O Y O ــــY 𝛿- 𝛿+ 6 Page 7 of 18 Addition of water: Most aldehydes react with water to form an equilibrium mixture of the aldehyde and an aldehyde hydrate: - R H R H C OH C ــــOH O H O ــــH Aldehyde aldehyde hydrate1, 1-diols They are usually too unstable to isolate and purify because the easily lose water to reform the aldehyde. In certain special cases, the 1, 1-diol is sufficiently stable that it can be isolated. The carbonyl carbon is bonded to one or more highly electronegative group. Ex.: OH OH CF3 C CF3 Ccl3 CH OH Hexafluoroacetone hydrate OH chloral hydrate Addition of alcohols: ald. +alcohols CH3 H CH3 C O O H Acetaldehyde methanol CH3 H+ 𝑎𝑐𝑖𝑑 hemiacetal H C ـــOCH3 O ـــH Hemiacetal 7 Page 8 of 18 In the presence of excess alcohol, a Hemiacetal can react to form an Acetal and water. OCH3 OCH3 CH3CH + CH3OH H+ CH3 ـــCH + H2O OH methanol OCH 3 Hemiacetal Acetal 2steps summarized:H+ CH3CHO + 2CH3OH CH3CH (OCH3)2 + H2O Have 2 alkoxyl gr. In addition to forming Hemiacetals between 2 different molecules, such a reaction can occur within a molecule. 6 5 4 3 2 1 O CH3CHCH2CH2CH2C OH H 3CH3 H+ 4CH2 6CH3 2CH2 5C H 1CHOH O Carbohydrate: are compounds that form cyclic Hemiacetal. CH3 CH3 OCH3 C O + CH3OH CH3 Acetone H+ C + CH3OH CH3 OH methanol Hemikketal CH3 OCH3 H+ C CH3 + H2O OCH3 Ketal 8 Page 9 of 18 If adiol is used as the alcohol component, the acetal or the ketal will have a cyclic structure, for ex.: C O+ CH2 H+ 𝐶 H+ C 𝐶𝐻2 CH2 +H2O H.W: Ex. (19.11, 19.12, 19.13) P.514 Addition of ammonia and its derivatives :( Schiff bases) C NH2 O H 𝐶 𝐻𝑁 ــــ2 -H2O C NH Ketone or aldehyde Ammonia RHC==NR` Aldimines R2C==NR` Ketimines An imine Schiff bases H.W:19.14 P516 Reduction of aldehydes and ketones: Reduction RCHO Aldehyde O RCH2OH oxidation RCR OH R CH R primary alcohol ketone oxidation 2°alcohol 9 Page 10 of 18 CH3 𝛿+ CH3 𝑁𝑎𝐵𝐻 4 C==O CH3 Al ــــH CHOH 𝑜𝑟 𝐿𝑖𝐴𝑙𝐻 4 CH3 Acetone 𝛿- polarization of ALــH bond 2-propanol O C + H2 Pt. CH2OH Pt.: metal catalyst H ADDITION REACTIONS OF ALDEHYDES AND KETONES IN LIVING SYSTEM: OO=C=O+H2O 𝐸𝑛𝑧𝑦𝑚𝑒 (Carbon dioxide) O=C +H+ OH (bicarbonate ion) The enzyme that catalyzes this reaction is widely distributed in mammals. (Active in tissues that are involved in respiration), such as red blood cells. The formation of Schiff bases is an important reaction in the formation of many compounds in living systems. One example is the transamination reaction. 10 Page 11 of 18 CH3 CO2H CHNH2 + O=C CO2H CH3 𝐸𝑛𝑧𝑦𝑚𝑒 CH2 CO2H C=O +NH2CH CO2H CH2 CH2CO2H Alanine (A.A) CH2CO2H 𝛼-Ketoglutaric acid pyruvic acid Glutamic acid (An 𝛼-Keto acid) In such reactions, the 𝛼-amino group of an A.A is transferred to the 𝛼-carbon of an 𝛼-Keto acid. The first step in this transformation is the reaction of the amino group of the amino acid (A.A) with pyridoxal phosphate, the reactive part of the enzyme, to form a Schiff base. O O-ـــP ـــO- O O-ــPــO- CHO OCH3 CH3 CO2H OــCH2 CH=NCHCO2H OH+NH2CH N CH3 CH3 Pyridoxal phosphate Alanine OH +H2O N CH3 Schiff base This Schiff base is the amino group carrier between the amino acid and the Keto acid. 11 Page 12 of 18 The enzyme- catalyzed reduction of aldehydes and ketones occurs frequently in biological reactions. In these reactions, the same enzyme frequently catalyzes the oxidation of alcohols to aldehydes and ketones. The oxidation of L-malate is an example of such an oxidation-reduction reaction: H H - O2C CــOH+ CH2CO2- L-malate O C N+ O CO2- H H C +C=O 𝑂𝑥𝑖𝑑 .𝑜𝑓 𝑚𝑎𝑙𝑎𝑡𝑒 𝑟𝑒𝑎𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑜𝑥𝑎𝑙𝑜𝑎𝑐𝑒𝑙𝑎𝑡𝑒 NH2 N NH2 CH2 R R CO2- NAD NADH Oxidation is very stereospecific (only L-malate is oxidized). CONDENSATION REACTIONS In condensation reactions, two compounds are joined together (or condensed) to form one larger compound. 1) aldol condensation. 2) claisen condensation. Aldehyde and alcohol (the name derived from the structure of product) 12 Page 13 of 18 1) An aldol condensation reaction: - is a reaction in which the carbonyl carbon of one molecule forms a bond with the 𝛼 carbon of another carbonyl-containing molecule. O H O C CH2 C OــH O CــCH2 C R R Carbonyl carbon Ex. O H CH3C OــH OH- H Acetaldehyde 𝛼 carbon O CH2C H Aldol condensation product O CH3CـــCH2C H H Aldol The reaction is reversible. The reverse reaction is called a retro-aldol condensation. Most aldehydes and many ketones that have a hydrogen undergo the aldol condensation. All the products of an aldol condensation reaction have a common structural feature, A 𝛽-hydroxy carbonyl skeleton: OH O C CC This structure is the result of the way the two pieces join together. The 𝛼 carbon of one aldehyde molecule becomes bonded to what was the 13 Page 14 of 18 original carbonyl carbon of the second aldehyde molecule. For example:- O CH3CH2 C H O H C HCHO H CH3CH2 C C H CHO CH3 H CH3 The aldol condensation is an equilibrium. Formation of product is favorable for ald. than ketone. O OH O 2 CH3 C CH3 CH3 C CH2 C CH3 CH3 The following reaction is an example of mixed aldol condensation reaction:O C OH O C CH 2CCH3 H +CH3 C CH3 OHBenzaldehyde Acetone O H mixed aldol Condensation product Mixed aldol condensation reactions are successful only when the 𝛼 carbon of a ketone adds to the carbonyl group of an aldehyde that does not have 𝛼 hydrogens. 14 Page 15 of 18 Claisen condensation:Esters also undergo condensation reactions when treated with base. For example: O O CH3C + CH3C O NaOCH2CH3 CH3C OCH2CH3 OCH2CH3 O + CH3CH2OH CH2C OCH2CH3 Notice that in this reaction the ـــOCH2CH3 group of one ester molecule is replaced by the 𝛼 carbon of another ester molecule (the overall result of a claisen condensation is the transfer of an acyl group from one ester molecule to another). H.W: ex. 19.17 P 526 Condensation reactions in living systems: Carbohydrates are prepared in living systems by an enzyme-catalyzed aldol condensation. O HC C H HO HO H CHCCH2O P C OH CH2 OP HO CH2O P D-Glyceraldehyde 3-phosphate 1, 3- Dihydroxyacetone phosphate H C OH CHOH C=O CH2O P D-fructose 1, 6 diphosphate 15 Page 16 of 18 Show stereo specificity of enzyme. O ـــPـــO- P OThis condensation reaction again shows the stereospecificity of enzyme-catalyzed reactions. The acetyl group of acetyl coenzyme A also undergoes claisen condensations. An example is the formation of acetoacetyl coenzyme A by an enzyme-catalyzed reaction of acetyl coenzyme A: O CH3C O O CH2C ENZYME CH3C O + HــSCOA S H SCOA CH2C COA Acetyl Coenzyme A SCOA Acetyl coenzyme A Acetoacetyl coenzyme A coenzyme A Acidity of 𝜶 hydrogens: - saturated hydrocarbons are generally unreactive compounds. C – H bond is not acidic in these compounds, Ka = 10-40 for methane (CH4). Thus, there is little tendency for methane to ionize to form a proton at the ion –CH3 in which carbon bears –ve charge. Such anions are called carbanions. 16 Page 17 of 18 Carbanions: - a compound that contains a carbon atom bearing a negative charge and bonded to three groups of atoms. However, aldehydes, ketones, & esters do react with strong bases to form carbanions. The hydrogen 𝛼 to the carbonyl group is more acidic than most C – H bonds. The reason for this is that the enolate anion formed by loss of a 𝛼 hydrogen of an aldehyde or ketone is stabilized by resonance. The presence of ـــC ـــgroup makes the C – H bond 𝛼 to it much more acidic than usual. The reason for this is that the carbanion can be stabilized by resonance: for example: O CH3 C CH3 + OH- 𝑂 −CH2 C CH3 𝑂− CH2 = C CH3 Resonance-stabilized enolate anion This enolate anion is a resonance hybrid of 2 structures. One structure has the negative charge on carbon, whereas the other has the negative charge on oxygen. [Molecules or ions that can be written as a resonance hybrid of 2 or more structures are stabilized]. This stabilization of the enolate anion is the reason why the H 𝛼 to a carbonyl gr. is lost to a strong base. 17 Page 18 of 18 H.W.: Ex.19.19 p 528 In condensation reactions, the nucleophile is the enolate anion formed by a strong base removing the proton 𝛼 to the carbonyl group. The carbonyl group serves two functions in condensation reactions. It serves as the site where a nucleophile adds, and it makes its 𝛼 hydrogens more acidic than usual for a C – H bond. Many of the reactions of aldehyde and ketones in the laboratory are similar to their reactions in living system. 18