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(Carbonyl) Group - I Aldehydes, Ketones Compounds Containing a C=O Group

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(Carbonyl) Group - I Aldehydes, Ketones Compounds Containing a C=O Group ORGANIC CHEMISTRY Serkan SAYINER, DVM PhD. Assist. Prof. Near East University, Faculty of Veterinary Medicine, Department of Biochemistry [email protected] Compounds Containing a C=O (Carbonyl) Group - I Aldehydes, Ketones Compounds Containing a C=O Group Many different kinds of compounds contain a carbon– oxygen double bond (C=O, carbonyl group). Carbonyl compounds include aldehydes, ketones, carboxylic acids, esters, and amides. The type of atom bonded to the carbonyl carbon— hydrogen, carbon, or a heteroatom—determines the specific class of carbonyl compound. Asetaldehit Aseton Asetik asit Metil asetat Asetamid Compounds Containing a C=O Group Take special note of the condensed structures used to draw aldehydes, carboxylic acids, and esters. An aldehyde has a hydrogen atom bonded directly to the carbonyl carbon. Compounds Containing a C=O Group A carboxylic acid contains an OH group bonded directly to the carbonyl carbon. An ester contains an OR group bonded directly to the carbonyl carbon. Compounds Containing a C=O Group A ketone, contains two R groups bonded directly to the carbonyl carbon. or Aldehydes and Ketones Naming Aldehydes, Naming Ketones, Physical Properties, Interesting Aldehydes and Ketones, Preparation of Aldehydes and Ketones, Reactions of Aldehydes and Ketones (Addition of 1° Amines, Addition of 2° Amines, Addition of H2O—Hydration, Addition of Alcohols—Acetal Formation, Acetals as Protecting Groups, Cyclic Hemiacetals, Oxidation of Aldehydes) Aldehydes and Ketones Aldehydes (RCHO) and Ketones (RCOR or R2CO) are two families of compounds that contain a carbonyl group. Two structural features dominate the properties and chemistry of the carbonyl group. • The carbonyl carbon atom is trigonal planar, and all bond angles are 120°. • Since oxygen is more electronegative than carbon, a carbonyl group is polar. • The carbonyl carbon is electron poor (훅+) and the oxygen is electron rich (훅–). Naming Aldehydes In IUPAC nomenclature, aldehydes are identified by the suffix - al. To name an aldehyde using the IUPAC system: 1. Find the longest chain containing the CHO group, and change the -e ending of the parent alkane to the suffix -al. 2. Number the chain or ring to put the CHO group at C1, but omit this number from the name. 3. Apply all of the other usual rules of nomenclature. 4. Simple aldehydes have common names that are virtually always used instead of their IUPAC names. Common names all contain the suffix - aldehyde. Naming Ketones In the IUPAC system, ketones are identified by the suffix -one. To name an acyclic ketone using IUPAC rules: 1. Find the longest chain containing the carbonyl group, and change the -e ending of the parent alkane to the suffix -one. 2. Number the carbon chain to give the carbonyl carbon the lower number. Apply all of the other usual rules of nomenclature. Naming Ketones With cyclic ketones, numbering always begins at the carbonyl carbon, but the “1” is usually omitted from the name. The ring is then numbered clockwise or counterclockwise to give the first substituent the lower number. Most common names for ketones are formed by naming both alkyl groups on the carbonyl carbon, arranging them alphabetically, and adding the word ketone. Three simple ketones have widely used common names. Physical Properties Because aldehydes and ketones have a polar carbonyl group, they are polar molecules with stronger intermolecular forces than the hydrocarbons. • Aldehydes and ketones exhibit dipole–dipole interactions because of their polar carbonyl group. Since they have no O—H bond, two molecules of RCHO or RCOR are incapable of intermolecular hydrogen bonding, giving them weaker intermolecular forces than alcohols. Physical Properties As a result: • Aldehydes and ketones have higher boiling points than hydrocarbons of comparable size. • Aldehydes and ketones have lower boiling points than alcohols of comparable size. Physical Properties Based on the general rule governing solubility (i.e., “like dissolves like”), aldehydes and ketones are soluble in organic solvents. Moreover, because aldehydes and ketones contain an oxygen atom with an available lone pair, they can intermolecular hydrogen bond to water. Physical Properties Low molecular weight aldehydes and ketones (those having less than six carbons) are soluble in both organic solvents and water. Higher molecular weight aldehydes and ketones (those having six carbons or more) are soluble in organic solvents, but insoluble in water. Acetone and progesterone are two ketones that occur naturally in the human and animal body. Interesting Aldehydes and Ketones Many aldehydes with characteristic odors occur in nature. • Cinnamaldehyde, the major component of cinnamon bark, is a common flavoring agent. • Vanillin is the primary component of the extract of the vanilla bean. Because natural sources cannot meet the high demand, most vanilla flavoring agents are made synthetically from starting materials derived from petroleum. • Geranial has the lemony odor characteristic of lemon grass. Geranial is used in perfumery and as starting material for synthesizing vitamin A. • Citronellal gives the distinctive lemon odor to citronella candles, commonly used to repel mosquitoes. Interesting Aldehydes and Ketones Because it is a starting material for the synthesis of many resins and plastics, billions of pounds of formaldehyde are produced annually in the United States by the oxidation of methanol (CH3OH). Formaldehyde is also sold as a 37% aqueous solution called formalin, which has been used as a disinfectant, antiseptic, and preservative for biological specimens. Formaldehyde, a product of the incomplete combustion of coal and other fossil fuels, is partly responsible for the irritation caused by smoggy air. Interesting Aldehydes and Ketones Acetone [(CH3)2C=O, the simplest ketone] is produced naturally in cells during the breakdown of fatty acids. In diabetes, a disease where normal metabolic processes are altered because of the inadequate secretion of insulin, individuals often have unusually high levels of acetone in the bloodstream. The characteristic odor of acetone can be detected on the breath of diabetic patients when their disease is poorly controlled. Interesting Aldehydes and Ketones Ketones play an important role in the tanning industry. Dihydroxyacetone is the active ingredient in commercial tanning agents that produce sunless tans. Dihydroxyacetone reacts with proteins in the skin, producing a complex colored pigment that gives the skin a brown hue. In addition, many commercial sunscreens are ketones. • Examples include avobenzone, oxybenzone, and dioxybenzone. Preparation of Aldehydes and Ketones Common Methods to Synthesize Aldehydes • Aldehydes are prepared from 1° alcohols, esters, acid chlorides, and alkynes. • By oxidation of 1° alcohols • By reduction of esters and acid chlorides • By hydroboration-oxidation of an alkyne Preparation of Aldehydes and Ketones Common Methods to Synthesize Ketones • Ketones are prepared from 2° alcohols, acid chlorides, and alkynes. • By oxidation of 2° alcohols with Cr6+ reagents. • By reaction of acid chlorides with organocuprates. • By Friedel–Crafts acylation. • By hydration of an alkyne. Aldehydes and ketones are also both obtained as products of the oxidative cleavage of alkenes. Reactions of Aldehydes and Ketones Reaction at the carbonyl carbon • Electrophilic carbonyl carbon makes aldehydes and ketones susceptible to nucleophilic addition reactions. • The elements of H and Nu are added to the carbonyl group. Reaction at the α-carbon • A second general reaction of aldehydes and ketones involves reaction at the α-carbon. A C—H bond on the α carbon to a carbonyl group is more acidic than many other C—H bonds, because reaction with base forms a resonance-stabilized enolate anion. Addition of 1° Amines Treatment of an aldehyde or ketone with a 1° amine affords an imine (also called a Schiff base). Nucleophilic attack of the 1° amine on the carbonyl group forms an unstable carbinolamine, which loses water to form an imine. The overall reaction results in replacement of C—O by C—NR. Addition of 1° Amines Many imines play vital roles in biological systems. A key molecule in the chemistry of vision is the highly conjugated imine rhodopsin, which is synthesized in the rod cells of the eye from 11-cis-retinal and a 1° amine in the protein opsin. Addition of 2° Amines A 2° amine reacts with an aldehyde or ketone to give an enamine. • Enamines have a nitrogen atom bonded to a double bond (alkene + amine = enamine). • Enamines are versatile intermediates. Because imines and enamines are formed by a set of reversible reactions, both can be converted back to carbonyl compounds by hydrolysis with mild acid. Addition of H2O—Hydration Treatment of a carbonyl compound with H2O in the presence of an acid or base catalyst adds the elements of H and OH across the carbon–oxygen π bond, forming a gem-diol (geminal-diols) or hydrate. • Hydration of a carbonyl group gives a good yield of gem-diol only with an unhindered aldehyde like formaldehyde, and with aldehydes containing nearby electron-withdrawing groups. • Geminal diols are a subclass of the diols, which in turn are a special class of alcohols. • The two hydroxyls in a geminal-diol are easily converted to a carbonyl or keto group C=O by loss of one water molecule, thus turning the diol into a ketone. Addition of Alcohols—Acetal Formation Aldehydes and ketones react with two equivalents of alcohol to form acetals. In an acetal, the carbonyl carbon from the aldehyde or ketone is now singly bonded to two OR" (alkoxy) groups. • The term acetal refers to any compound derived from an aldehyde or ketone, having two OR groups bonded to a single carbon Addition of Alcohols—Acetal Formation Conversion of an aldehyde or ketone to an acetal is a reversible reaction, so an acetal can be hydrolyzed to an aldehyde or ketone by treatment with aqueous acid. Because this reaction is also an equilibrium process, it is driven to the right by using a large excess of water for hydrolysis. Acetals as Protecting Groups Acetals are valuable protecting groups for aldehydes and ketones.
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