1. Both aldehydes and ketones are characterized by the presence of the "carboxyl group". © True O False 2. There is H-bonding in both aldehydes and ketone molecules. O True © False 3. The common name for Benzaldehyde is Benzenecarbaldehyde. © True O False 4. Many low molecular weight aldehydes also have pleasant odors and are also used in perfumes. © True O False 5. Both aldehydes and ketones can also be prepared by the hydration of alkenes. O True © False 6. As the number of hydrocarbons increases, aldehydes and ketones become more soluble in water. © True O False 7. The Cannizaro's Reaction is a specific reaction of an Aldehyde only. © True O False 8. This condensed structural formula: C6HCH2CH(OC2Hs)2 is a ketal. O True © False 9. Diethyl ketone has the same molecular formula as butanal. O True © False 10. The name for this structural formula: O=CH-CH=O is ethanedial. O True © False 11. Both aldehydes and ketones have lower boiling points than alcohols and carboxylic acids. © True O False 12. The Reduction of Aldehydes yields a secondary alcohol. O True © False 13 Ethanal may be distinguished from propanone by the use of Tollen's reagent which is a strong oxidizing agent. © True O False Aldehydes and Ketones are characterized by the presence of the “carbonyl group” (–C=O), perhaps the most important functional group in organic chemistry. The carbon atom of a carbonyl group must form two (2) other bonds in addition to the carbon oxygen double bond in order to have four bonds. Aldehyde – have a carbon group (R-) as one of the two attached bonds to the carbonyl group and at least one hydrogen atom. The other group may be to another hydrogen atom, an alkyl or cycloalkyl group, or an aromatic ring system. The –CH=O group characteristic of aldehydes is often called a “formyl group” . Ketone – are the organic compounds that have two other carbon groups connected to the carbonyl carbon atom. The two other carbon atoms must be a part of an alkyl, cycloalkyl or aromatic group. The carbonyl group of an aldehyde is flanked by a hydrogen atom, while the carbonyl group of a ketone is flanked by two carbon atoms Synthesis of Aldehydes and Ketones By Ozonolysis of Alkenes. Alkenes react with ozone to form ozonides which on subsequent reductive cleavage with zinc dust and water or H2/Pd give aldehydes, Ketones or a mixture of both depending on the substitution pattern of the alkene. Zinc dust removes H2O2 formed, which otherwise can further oxidize the aldehyde formed to acids. Using a suitable alkene, the desired aldehyde or ketone can be formed By Hydration of Alkynes. Ethynes adds water in the presence of H2SO4 and HgSO4 to give acetaldehyde. Hydration of other alkynes under similar condition gives ketones. By Oxidation of Methylbenzenes. Oxidation of an aromatic compound leaving a methyl group at the benzene ring with CrO3 in the presence of acetic anhydride followed by hydrolysis gives the corresponding benzaldehyde. By Oxidation of Methylbenzenes. Further oxidation of the benzaldehyde to benzoic acid is prevented as the aldehyde is trapped by acetic anhydride as a non-oxidizable benzylidene diacetate derivative. This reaction is called the “Etard Reaction”. From Nitriles. Partial reduction of nitriles with acidified stannous chloride SnCl2/HCl at room temperature gives aldehydes. In the first step imine hydrochloride is obtained which on subsequent hydrolysis with boiled water gives aldehyde. This specific type of reduction of nitriles is called “Stephen's reduction” . Similarly Benzonitrile gives Benzaldehyde. Ketones are obtained when Grignard's reagent add on to nitriles. The imine salt intermediate that is formed on hydrolysis gives ketone PROPERTIES OF ALDEHYDES & KETONES A. Aldehydes and Ketones are polar compounds and, as a result, have higher boiling points than ethers, hydrocarbons, and non-polar compounds of comparable molecular weight. B. There is No H-bonding between aldehyde or ketone molecules, and this lack of Hydrogen bonding is why these compounds have lower boiling points than alcohols and carboxylic acids – compounds in which there is H-bonding between molecules. C. Because the oxygen atom of the carbonyl groups of aldehydes and ketones are hydrogen bond acceptors, the low molecular weight aldehydes and ketones are more soluble in water than are nonpolar compounds of comparable M.W. D. As the number of hydrocarbons increases, aldehydes and ketones become less soluble in water. E. Most aldehydes and ketones have strong odors. The odors of ketones are generally pleasant, and many are used in perfumes and as flavoring agents. The odors of aldehydes vary from foul odor to pleasant ones. Many higher aldehydes, however, have pleasant odors and are also used in perfumes Examples of Aldehydes found in Nature: 2-Methylbutanal – nutty flavor of cheddar cheese Benzaldehyde - oil of almonds, used as food flavoring Cinnamaldehyde - oil of cinnamon, used as food flavoring Citronellal - citronella oils; used as flavoring Citral - lemon grass oil, used as flavoring Vanillin - vanilla bean, used as food flavoring Muscone - from the musk of deer used in perfumes ß-Ionone – oil of violets, used in perfumes Butanedione (Biacetyl) – flavoring for margarine Carvone – oil of spearmint, used as flavoring Cortisone – hormone used in protein metabolism; inflammation reducer. Progesterone – female sex hormone Testosterone – male sex hormone OXIDATION OF ALDEHYDES Aldehyde are oxidized to carboxylic acids by a variety of oxidizing agents, including potassium dichromate. Aldehyde are also oxidized to carboxylic acids by the oxygen in the air. Aldehydes can be oxidized to carboxylic acid with both mild and strong oxidizing agents. However, ketones can be oxidized to various types of compounds only by using extremely strong oxidizing agents. Typical oxidizing agents for aldehydes include either potassium permanganate (KMnO4) or potassium dichromate (K2Cr2O7) in acid solution and Tollens reagent. Peroxy acids, such as peroxybenzoic acid: Ketones in contrast resist oxidation by most oxidizing agents, including potassium dichromate and molecular oxygen. The fact that aldehydes are so easy to oxidize and ketone does not. To distinguish between these types of compounds simple chemical tests are carried out. Three Common Tests for Aldehydes: Benedict’s Test: Control (Red) Positive Test (Blue) Fehling’s Solution: Control (Red) Positive Test (Blue) Tollens Test: Control (Clear) Positive Test (Silver mirror) Aldehydes are reduced to primary alcohols and Ketones are reduced to secondary alcohol. The reduction of a C = O double bond under these conditions is slower than the reduction of a C = C double bond. Thus if the same molecule contains both C = O and C = C double bonds, the C = C double bond is reduced first. Formation of Acetals and Ketals It is well known that distilled alcoholic beverages contain a group of aromatic compounds called acetals. Acetals are formed by a reaction in which an aldehyde molecule adds a molecule alcohol to form a labile hemiacetal. This combines with a second molecule of the alcohol and yields a stable acetal. The reaction is catalyzed by acids but it is reversible, and consequently acetals are hydrolyzed in dilute alcoholic solution when acids are present. Ketones react in a similar way to form ketals. Acetal is "A molecule containing two OR groups bonded to the same carbon" . The acetal group serves as a good "protecting group“ - since the acetal is inert to most substances, this "protects" the molecules in question from undergoing unwanted reactions. A ketal - is a functional group derived from a ketone by replacement of the carbonyl (C=O) group by two alkoxy groups. The IUPAC once declared the term "ketal" obsolete, but later accepted it as a subclass of acetals. Therefore, a ketal can also be defined as "an acetal derived from a ketone. FORMATION OF ACETALS Aldehydes and ketones undergo a reversible reaction with alcohols in the presence of an acid catalyst to yield acetals, or ketals: R2C(OR)2 , These are compounds that have two ether like -OR groups bonded to the same carbon. Acetal formation involves the acid catalyzed addition of an alcohol to an aldehyde or ketone in a process analogous to that of the acid catalyzed addition of water. Acetals are more easily prepared from aldehydes than from ketones. Cyclic acetals are easier to form than acyclic acetals. Conjugation deactivates the carbonyl group towards acetalization. Sterically hindered carbonyls react more slowly. In aromatic aldehydes and ketones electron donating substituents on the arene ring retard acetal formation whereas electron withdrawing substituents facilitate it. Gem-dialkyl substitution as in 2,2-dimethylpropan-1,3-diol, promotes cyclic acetal Cannizzaro Reaction Cannizzaro’ reaction is named after Stanislao Cannizzaro. In cannizzaro reaction, the following takes place in the presence of base. HCHO + HCHO ----> HCOOH + CH3OH The characteristic features of Cannizzaro reaction are: 1) It is a disproportionate reaction. 2) It involves a reaction with aldehydes which have no alpha hydrogen atoms. 3) The products are an alcohol and a carboxylic acid Formation of alcohol is oxidation and formation of sodium salt is reduction. Since this reaction is shown by aldehydes that do not have alpha hydrogen atom, formaldehyde and benzaldehyde generally show these reactions, as they are not able to form the “enolate ion” after proton abstraction by base. Addition of Hydrogen Cyanide (HCN) The addition of hydrogen cyanide to a carbonyl group of an aldehyde or most ketones produces a Cyanohydrin. Sterically hindered ketones, however, don't undergo this reaction. The mechanism for the addition of hydrogen cyanide is a straightforward nucleophilic addition across the carbonyl carbony oxygen bond. SOME IMPORTANT ALDEHYDES AND KETONES Formaldehyde (Methanal), H-CHO It is a gas at room temperature. It is soluble in water and so is normally found in laboratories as a 37% aqueous solution called “formalin”. Formalin – is a preservative for biological specimens. It does this by denaturing proteins, thus, hardening them. Formalin It is used to embalm cadavers. But its use as a preservative and as an embalming liquid has declined, because it has been determined that it is carcinogenic. It is toxic to both humans and microorganism; therefore, it is used to sterilize surgical instruments. Industrially, formaldehyde is the most important aldehyde. Its principal use is in the production of polymers Bakelite or Phenol-formaldehyde, urea-formaldehyde, and melamine-formaldehyde polymers are not used only as plastics but even more importantly as adhesives and coatings. Besides Bakelite, the trade names Formica and Melmac are also well-known fiber-boards made from combining formaldehyde resins with wood chips or sawdust. Also, some formaldehyde polymers are produced as porous foams and are used for building insulation. However, in recent years, many of these products have been removed from the market because they contain tiny amounts of free formaldehyde, which can get into the air and possibly cause cancer. Acetaldehyde (Ethanal), CH3CHO Has a boiling point of 21°C, which is close to room temperature, because of this, acetaldehyde is not easy to handle, either as a liquid or as a gas. For this reason, it is often converted to a cyclic trimer (a molecule made of three units) called “paraldehyde”. Paraldehyde – was once used as a hypnotic or sleep- producer medication, but because of its disagreeable taste and odor it is seldom used today. Acetaldehyde is the common intermediate in alcohol production by fermentation and a key component in alcoholism. It is the key component in the symptoms of “alcoholic hangover”. The lethality of aldehyde is somewhere between NaCl and arsenic (As). Its LD50 value is 1900 mg/kg. USDA regulations set the acetaldehyde exposure limit by inhalation to 200 ppm over an 8-hr exposure period. ØThe regulatory limit for Formaldehyde exposure is even lower: 1.5 ppm. Acetone (2-Propanone), CH3COCH3 A liquid with a moderately low boiling point (56 °C). It is one of the few organic compounds that is infinitely soluble in water and also dissolves a great deal of many organic compounds. It is one of the most important industrial solvents. It is used in paints, varnishes, resins, coatings, and nail polish, among other things. Acetone is produced in the body as a product of one pathway of lipid metabolism. Normally it does not accumulate because the small amount produced is oxidized to CO2 and H2O. However, in diabetic patients a larger quantity of acetone is produced than the body can oxidize completely, so much of it is excreted in the urine A positive test for acetone in the urine is one way in which diabetes is diagnosed. In severe cases of diabetes, acetone is exhaled, and its sweet odor can be detected Lachrymators – are chlorine and bromine containing organic compounds that cause the eyes to tear. α-Chloroacetophenone has long been used as a tear gas, by police and other government officials to control unruly mobs. In recent years, this compound has been used as the active ingredient in “Mace” – a handheld aerosol tear gas projector, used by individuals to protect themselves from an attacker. Mace has a range of about 15 feet. Pheromones – substances secreted by animals that influence the behavior of the same species. They are mostly ketones Muscone – believed to be the sex pheromone of the male Himalayan musk deer Civetone – the sex hormone of the Ethiopian Civet cat Both are used in making perfumes Testosterone – a male sex hormone that is produced in the testes and is involved in the development of accessory sex function in the human male species. Progesterone – a female sex hormone secreted by the ovarian tissue, and is one of the sex hormones involved in stimulating the growth of the uterine mucosa in preparation for the implantation of the fertilized ovum. CARBOHYDRATES Commonly known as sugars They are generally defined as polyhydroxy aldehydes or polyhydroxy ketones or substances that yield these compounds when hydrolyzed. They are the most abundant of the four major classes of biomolecules. Most abundant constituents of plants and animals.’ They contain either an Aldehyde group (-COH) or a Ketone group (-COR’). They serve many vital functions such as: storehouses of chemical energy (e.g. glucose, starch, and glycogen), supportive structural component in plants (cellulose), and as essential component in the mechanism of genetic control and growth of cells (D- ribose and 2 – deoxyribose) MEDICAL PERSPECTIVE OF KETONES AND ALDEHYDES Medium Chain Acyl-CoA Dehydrogenase) Deficiency Children with any of these enzyme deficiencies have a significant risk (20%) of death during the first, clinical episode of hypoglycemia (low blood glucose). Those patients affected show episodes of acute, life-threatening attacks that are symptomatically consistent with Reye’s Syndrome and sometimes misdiagnosed as S.I.D.S. The most common of these in-born errors is MCAD Deficiency. ~1/50 Caucasians carry the gene. MCAD Enzyme (MCAD) is one of the enzymes involved in mitochondrial fatty acid b-oxidation, which fuels hepatic ketogenesis, a major source of energy once hepatic glycogen stores become depleted during prolonged fasting and periods of higher energy demands. Typically, a previously healthy child with MCAD deficiency presents with hypoketotic hypoglycemia, vomiting, liver dysfunction, skeletal muscle weakness and lethargy triggered by a common illness. On average, this occurs between 3 and 24 months of age. Ackee Fruit (Bligia Sapida) from Jamaica Ingestion of the unripe seeds from the fruit of the Jamaican Ackee tree causes a disruption of the dehydrogenase enzymes needed to metabolize fatty acids. This “vomiting sickness” is a result of the enzyme inhibitor Hypoglycin A.
