Introduction of bioorganic chemistry. Classification, structure, chemical properties of organic compounds.
Organic chemistry is the study of hydrocarbons and their derivatives. Because of the unique property of catenation and isomerism, carbon forms à large number of organic compounds. Even towards the end of nineteenth century, the number of organic compounds known was so large that it became difficult to remember them by their common or trivial names.
Classification of organic compounds by a structure of hydrocarbon chain:
Saturated — compounds, which contain only s-bonds and nî p - bonds. Compound in molecule, which contain only simple – C – C – bonds.
Unsaturated — compounds which contain as s-bonds and p - bonds. Compound in molecule which contain one or more double bonds (– C = C –) or triple (– C @ C – ) bonds, and simple – C – C – bonds
Alkanes — saturated hydrocarbons that contain carbon-carbon and carbon-hydrogen s bonds only.
Alkenes — à hydrocarbons with à carbon-carbon double bond.
Alkynes - à hydrocarbons with à carbon-carbon triple bond
Diene — à compound with two double bonds
Carbocyclic - hydrocarbons containing à ring, which consist only carbon atoms.
Cycloalkanes — saturated hydrocarbons containing à ring.
Cycloalkenes — ansaturated hydrocarbons containing à ring.
Arene — an aromatic compound; often designated by the symbol Ar — Í.
Heterocyclic - hydrocarbons containing à ring, which consist carbon atoms and other atoms, such as S, O, N, therefore they are called sylphur-containing, nitro-containing, oxygen-containing. Example: furan, pyrrole, thiophene, pyridine.
General rules for IUPAC nomenclature. The IUPAC system is the most rational and widely used system of nomenclature in organic chemistry. The most important feature of this system is that any molecular structure has only one IUPAC name and any given IUPAÑ name denotes only one molecular structure.
The IUPAC name of any organic compound essentially consists of three parts: word root; suffix; prefix.
1. Word root. It is the basic unit of the name. It denotes the number of carbon atoms present in the principal chain (the longest possible continuous chain of carbon atoms including the functional group and the multiple bonds) of the organic molecule. For chains from one to four carbon atoms, special word roots (based upon the common names of alkanes) are used but for chains of five or more carbon atoms, Greek number roots are used as given below:
Extra 'à' given in parenthesis used only if the primary suffix (explained later) to be added to the word root begins with à consonant. In general, the word root for any carbon chain is alk.
The following examples illustrate the use of word root, primary suffix and secondary suffix in naming organic compounds:
The terminal 'å' from the primary suffix has been dropped because the secondary suffix begins with
3. Prefix. There are two types of prefixes:
(i) Primary prefix. À primary prefix is used simply to distinguish cyclic from acyclic compounds.
For example, in case of carbocyclic compounds, (cyclic compounds containing only carbon atoms in the ring), à primary prefix, cyclo is used immediately before the word root. Thus:
If the prefix cyclo is not used, it simply indicates that the compound is acyclic or open chain.
Rules for IUPAC names of complex aliphatic organic compounds. We shall now discuss the IUPAC names for complex organic compounds under the following categories.
1. Rules for IUPAC nomenclature of complex saturated hydrocarbons (Alkanes)
Alkanes which ñîn1àin à number of branched chains are called complex alkanes. These are usually named by the IUPAC system according to the following rules:
1. Longest chain rule. Select the longest continuous chain of carbon atoms in the molecule. This is called the parent chain while all other carbon atoms, which are not included in the parent chain are regarded as prefixes i.å. side chains or substituents. The complex alkane is then named as à derivative of the alkane representing the parent chain. It may be noted that the longest chain must always be continuous. It may or may not be straight.
Longest chain contains lx carbon atoms and hence Longest chain contains seven carbon atoms and hence
I is named as à derivative of hexane. II is named as à derivative of heptane.
2. Rule Rîr larger number of side chains. If two different chains of equal lengths are possible, select the one with larger number of side chains or alkyl groups. For example,
Named as hexane with two alkyl substituents (correct) Named as hexane with one alkyl substituent (wrong)
3. Lowest number rule or lowest locant rule. Number the carbon atoms î1 the parent chain as 1, 2, 3, 4, … etc. starting from that end which gives the lowest possible number to the carbon atom carrying the substituent. For example in structure (1), the numbering can be done in two different ways i.å., IA and IB.
The numbering of the carbon chain as assigned in structure IA is correct since it assigns à lower number (3) to the substituent (1.å. methyl group) while the numbering as given in structure IB is wrong since it assigns à higher number (4) to the substituent.
1À (Correct) IB (Wrong)
All the remaining functional groups such as halo (fluoro, chloro, bromo, iodo), nitroso (– NO), nitro (– NO2), and alkoxy (– OR) are always treated as substituent groups.
It may be noted that while writing the names of the polyfunctional compounds, the principal functional group is indicated by adding the secondary suffix to the word root while the secondary functional groups are indicated by adding suitable prefixes to the word root. The prefixes for some secondary functional groups are listed below:
2. Selecting the principal chain. While selecting the principal chain present in à polyfunctional compound care should be taken that it must contain the principal functional group and the maximum number of secondary functional groups and multiple bonds, if any.
3. Numbering the principal chain. The principal chain present in à polyfunctional compound must
double bond, triple bond and the substituents, i.å.,
Principal functional group > Double bond > Triple bond > Substituents
4. Alphabetical order. The prefixes for the secondary functional groups and other substituents should be placed in alphabetcial order before the word root as explained earlier. If, however, two groups of the same preference occupy identical positions from either end of the parent chain, the lower number must be given to the group whose prefix comes first in the alphabetical order.
Illustration. Let us now apply the above rules to name the following polyfunctional compound:
Step l. Identify the principal and secondary functional groups. Here, according to rule 1, the principal functional group is - ÑÎÎÍ and its suffix is oic acid while the secondary functional group is ÑÎÎÑ2Í5 and its prefix is carbethoxy.
Step 2. Identify the parent chain. Here the parent chain containing the principal functional group and double bond consists of four carbon atoms. Therefore, the word root for the parent chain is but and the primary suffix is ene.
Step 3. Number the parent chain. The parent chain is numbered as shown so that the principal functional group, i.e. - ÑÎÎÍ gets the lowest number, i.å. 1.
Step 4. Identify the substituents including the double bond and their numerical prefixes. There is only one substituent, i.å., - ÑÎÎÑ2Í5 and its numerical prefix is 3 and only one double bond and its numerical prefix is 2.
Step 5. Name the organic compound. Add the prefix or the substituent group along with its positional number to the word root followed by primary and secondary suffixes along with their positional numbers.
Polyfunctional compounds containing inîre than two like functional groups. According to latest convention (1993 recommendations for IUPAC nomenclature), if an unbranched carbon chain is directly linked more than two like functional groups, the organic compound is named as a derivative of the parent alkane which does not include the carbon atoms of the functional groups.
Bond-line notation of organic molecules. It is à simple and convenient method of representing structures. In this notation, bonds are represented by lines and carbon atoms by line ends and intersections. It is assumed that required number of Í-atoms are present wherever they are necessary to satisfy the tetracovalency of carbon. For example, buta-1, Ç-diene, penta-1, 4-diene and hexa-1, 3, 5-triene may be represented as follows:
2. If two or more alkyl groups or other subctituent groups are in the ring their positions are present positions are indicated by arabic numerals, i.e. 1, 2, 3, 4 …etc. while numbering the carbon atoms of the ring, the substituent which comes first in the alphabetical order is given the lowest number provided it does not violate the lowest sum rule.
3. If the ring compounds more or equal number of carbon atoms than the alkyl group attached to it, it is named as a derivative of cycloalkane and the alkyl group is treated as a substituent group it is named as a derivative of alkane and the cycloalkyl group is considered as à substituent group.
If, however, the side than contains a multiple bond or a functional group, the alicyclic ring is treated as the substituent irrespective of the size of the ring.
4. If a multiple (double or triple)bond and some other substituenta are present in the ring, the numbering is done in such à way that the multiple bond gets the lowest number.
5. If a compound contains an alicyclic ring directly linked to the benzene ring, it is named as a derivative of benzene, i.å., the compound having lowest state of hydrogenation.
6. If some functional group along with ather substituent groupa are present in the ring, it is indicated by some appropriate prefix or suffix and its position is indicated by numbering y\the carbon atoms of the ring in such a way that the functional group gets the lowest number.
Rules for IUPAC nomenclature of bicyclic compounds. Many hydrocarbons and their derivatives contain two or more rings. The carbon atoms common to both the rings are called bridge head atoms and each bond or chain of carbon atoms connecting both the bridge head atoms is called à bridge. The bridge may contain Î, 1, 2.....etc. carbon atoms.
The bicyclic compounds are named by attaching the prefix 'bicyclo' to the name of the hydrocarbon having the same total number of carbon atoms as in the two rings. The number of carbon atoms in each of these three bridges connecting the two bridge head carbon atoms is indicated by arabic numerals, i.å., 0, 1, 2.....etc. These arabic numerals are arranged in decending order; separated from one another by full stops and then enclosed in square brackets. The complete IUPAC name of the hydrocarbon is then obtained by placing these brackets containing the arabic numerals between the prefix bicyclo and the name of the alkane (containing the same total number of carbon atoms as in the two rings).
If à substituent is present, the bicyclic ring is numbered. The numbering begins with one of the bridge head atoms, proceeds first along the longest bridge to the second bridge head atom, continues along the next longest bridge to the first bridge head and is finally completed along the shortest path.
Rules for IUPAC nomenclature of spiro compounds. Compounds in which one carbon atom is ñoròîë to two different rings are called spiro compounds and the common carbon atom is called the spiro atom. The IUPAC name for à spiro compound begins with the word 'spiro' followed bybrackets containing the number of carbon atoms, in ascending order, in each ring connected to the common carbon atom and then by the name of the parent hydrocarbon containing the total number of carbon à1orà |n the two rings. The positions of substituents are indicated by arabic numerals; the numbering beginning with the ring atom next to the spiro atom, and proceeding first around the smaller and then to the spiro atom and finally around the larger ring.
IUPAC nomenclature of unbranched assemblies consisting of two or more identical cyclic hydrocarbon units joined by à single bond. These systems are named by placing à suitable numerical prefix such à by for two, ter for three, quater for four, quinque for five etc. before the name îéÜå repititive hydrocarbon unit. Starting from either end, the carbon atoms of each repititive hydrocarbon unit are numbered with unprimed and primed abrabic numerals such as 1, 2, 3......, 1', 2',3'....., 1",2",3"...... etc. The points of attachment of the repititive hydrocarbon units are indicated by placing the appropriate locants before the name. As an exception, unbranched assemblies consisting of benzene rings are named by using appropriate prefix with the name phenyl instead of benzene.
Nomenclatyre of aromatic compounds. The families of aromatic compounds are similar to those of aliphatic compounds with the only difference that each family of aromatic compounds, in fact, consists of two types of compounds with quite different chemical reactivities.
(1) Nuclear substituted - those in which the functional group è directly attached to the benzene ring. Most of these compounds are better known by their common and historical names. In the IUPAC system, they are named as derivatives of benzene. However, many of their common names have also been adopted by the IUPAC system. The positions of the substituents in disubstituted benzenes are indicated either by prefixes or by arabic numerals such as î-(ortho) for 1, 2; m-(meta) for 1, 3 andð-(para) for 1, 4. For tri- and higher substituted benzene derivatives, the positions of the substituents are indicated only by Arabic numerals.
(2) Side-chain substituted - those in which the functional group is present in the side chain of the
benzene ring. Both in the common and IUAC systems, there are usually named as henyl derivatives of the corresponding aliphatic compounds (except arenes which are named as derivatives of benzene in the IUPC system). The positions of the substituents on the side chain including the benzene ring are indicated by Greek letters i.å., a, b, g,... etc. in the common system, and by arabic numerals, i.å. 1, 2, 3 ... etc. in the UPAC system. However, many of these compounds are better known by their common names.
Types of bond fission or cleavage (breaking of à covalent bond).
(I) Homolytic (symmetrical) fission. If à covalent bond breaks in such à way that each atom takes away one electron of the shared pair, it is called homolytic or symmetrical fission. Homolytic fission is usually indicated by à fish arrow which denotes à one-electron displacement.
(II) Heterolytic (unsymmetrical) fission. When à covalent bond joining two atoms A and Â breaks in such a way that both the electrons of the covalent bond (i.e., shared pair) are taken away one of the bonded atoms, the mode of bond cleavage is called heterolitic fission. Heterolitic fission is usually indicated by à curved arrow which denotes à two-electron displacement.
(a) Electrophilic are electron loving chemical species. Their attraction for electrons is due to the presence of an electron-deficient atom in them. Electrophilic may be either positively charged or electrically neutral chemical species.
(b) Nucleophiles are nucleus loving chemical species. Since the nucleus of any atom is positively charged, therefore, nucleophiles must be electron rich chemical species containing at least one lone pair of electrons. They may be either negatively charged or neutral chemical species, å.g.,
(I) Negative nucleophiles: Í- (hydride 1în), ÑR-, Br-, I-, R- (carbanion), ÎÍ-, OR-, SR-, NÍ2-, CN-, RCOO-.
(II) Neutral nucleophiles: Í2Î, NH3, RNÍ2, ROH, RSH, ROR etc.
Since both negatively charged and neutral nucleophiles contain at least one unshared pair of electrons, they have à strong tendency to donate this pair of electrons to electron deficient species and hence behave as lewis bases. Nucleophiles always attack the substrate molecule at the site of lowest electron density.
Types of organic reactions.
All the organic reactions can be broadly classified into the following four types: (a) substitution reactions, (b) addition reactions, (c) elimination reactions, (d) rearrangement reactions.
(à) Substitution Reactions. À substitution reaction is that which involves the direct replacement (displacement or substitution) of an atom or a group of atoms in an organic molecule by another atom or group of atoms without any change in the remaining part of the molecule. The product obtained as à result of substitution is called the substitution product and the new atom or group of atoms which enters the molecule is called à substituent. Depending upon the nature of the attacking species (nucleophile, electrophile or free radical) the substitution reactions is of the following three types:
(1) Nucleophilic substitution reactions. Substitution reactions which are brought about by nucleophiles are called nucleophilic substitution reactions. In all these reactions, à stronger nucleophile usually displaces a weaker nucleophilie. These reactions are typical of alkyl halides.
(2) Electrophilic substitution reactions. Substitution reactions which are brought about by electropholes are called electrophilic substitution reactions. These reactions are typical of arenas and other aromatic compounds.
(3) Free radical substitution reactions. Substitution reactions brought about by free radicals are called free radical substitution reactions.
All alcohols, à principle, can be divided into two broad categories i.å. aliphatic alcohols und aromatic alcohols.
1. Aliphatic alcohols. Alcohols in which the hydraryl group is linked an aliphatic carbon chain are called aliphatic alcohols.
Methyl alcohol Ethyl alcohol Isopropyl alcohol
Methanol Ethanol 2-Propanol
2. Aromatic alcohols. Alcohols in which the hydroxyl group is present in the side chain of an aromatic hydrocarbon are called aromatic For example.
(benzyl alcohol) (b-phenylethyl alcohol)
Alcohols are further classified as monohydrtc, dihydric, trthydnc and ðî/óÜóéëñ according as their molecules contain one, two, three, or many hydroxyl groups respectively. For åõàøð1å,
Ethyl alcohol 1,2-Ethanediol 1,2,3-propanetriol
(Monohydric) (Dihydrtc) (Trihydric)
Classification of monohydric alcohols. As already mentioned, alcohols containing one ÎÍ group per molecule are called monohydric alcohols. These are further classified as primary (1'), secondary (2'), and tertiary (3') according as the ÎÍ group is attached to primary, secondary and tertiary carbon atoms respectively. For example:
Ethanol Isopropyl alcohol 2-Methylpropanan 2-ol
Primary alcohol Secondary alcohol Tertiary alcohol
Nomenclature of alcohols. As with most other classes of organic compounds, alcohols can be named in several ways. Common names are useful only for the simpler members of à class. However, common names are widely used in colloquial conversation and in the scientific literature. In order to communicate freely, the student must know common names. Since the systematic IUPAC names are often used for indexing the scientific literature, the student must be thoroughly familiar with systematic names in order to retrieve data from the literature.
1.Òhe alkyl alcohol system. In this system of common nomenclature, the name of an alcohol is derived by combining the name of the alkyl group with the word alcohol. The names are mitten as two words.
n-butyl alcohol isobutyl alcohol t-butyl alcohol
In this common system, the position of an additional substituent is indicated by use of the Greek alphabet rather than by numbers.
b-chloroethyl alcohol g-bromobutyl alcohol
This use of the Greek alphabet is widespread in organic chemistry and it is important to learn the first few letters, at least through delta. Many of the letters, small and capital, have evolved standard meanings in the mathematical and physical sciences (for example, the number p). In organic chemistry, the lower case letters are used more frequently than the capital letters.
The last letter of the Greek alphabet is omega, w. Correspondingly, this letter is used to refer to difunctional compounds when the secondary substituent is on the end carbon of the chain.
Br(CH2)nOH w-bromo alcohols
Any simple radical that has à common name may be used in the alkyl alcohol system, with one important exception. The grouping Ñ6Í5 - has the special name phenyl, but the compound C6H6OH is phenol, not phenyl alcohol.
The number of carbons attached to the carbinol carbon distinguishes primary, secondary, and tertiary carbinols. As in the case of the alkyl halides, this classification is useful because the different types of alcohols show important differences in reactivity under given conditions. The carbinol system of nomenclature has been falling into disuse in recent years. However, it is found extensively in the older organic chemical literature.
IUPAC rules for naming alcohols that contain à single hydroxyl group follow.
Rule 1: Name the longest carbon' chain to which the hydroxyl group is attached. The chain name is obtained by dropping the final -å from the alkane name and adding the suffix -ol. – alkanols.
Rule 2: Number the chain starting at the end nearest the hydroxyl group, and use the appropriate number to indicate the position of the - ÎÍ group. (In numberin of the longest carbon chain, the hydroxyl group has priority over double an triple bonds, as well as over alkyl, cycloalkyl, and halogen substituents.)
Rule 3: Name and locate any other substituents present.
Rule 4: In alcohols where the - ÎÍ group is attached to à carbon atom in à ring, the hydroxyl group is assumed to be on carbon 1.
In the naming of alcohols with unsaturated carbon chains, two endings are needed: one for the double or triple bond and one for the hydroxyl group. The -ol suffix always comes last in the name; that is, unsaturated alcohols are named as alkenols or alkynols.
Polyhydroxy alcohols — alcohols that possess more than one hydroxyl group - can be names with only à slight modification of the preceding IUPAC rules. An alcohol in which two hydroxyl groups are present is named as à diol, one containing three hydroxyl groups is named as à triol, and so on. In these names for diols, triols, and so forth, the final –å of the parent alkane name is retained for pronunciation reasons.
1,2-Ethanediol 1,2-propanediol 1,2,3-propanetriol
Preparation of alcohols Alcohols can be obtained from many other classes of compounds. Preparations from alkyl halides and from hydrocarbons will be discussed in this section. The following important ways of pråðàring alcohols will be discussed later, as reactions of the appropriate functional groups.
1. Reduction of aldehydes and ketones:
2. Reduction of carboxylic acids:
3. Reduction of esters:
4. One general method for preparing alcohols - the hydration of alkenes. Alkenes react with water (an unsymmetrical addition agent) in the presence of sulfuric acid (the catalyst) to form an alcohol. Markovnikov's rule is used to determine the predominant alcohol product.
5. preparation from alkyl halides:
Reactions of alcohols. Alcohols are classified as primary (1'), secondary (2'), or tertiary (3'), depending on the number of carbon atoms bonded to the carbon atom that bears the hydroxyl group. À primary alcohol is an alcohol in which the hydroxyl-bearing carbon atom is attached to only one other carbon atom. À secondary alcohol is an alcohol in which the hydroxyl-bearing carbon atom is attached to two other carbon atoms. À tertiary alcohol is an alcohol in which the hydroxyl-bearing carbon atom is attached to three other carbon atoms. Chemical reactions of alcohols often depend on alcohol class (1', 2', or 3').
The reactions of alcohols generally involve breaking one or more of three types of bonds in the carbinol structure and may be characterized as
In cases where the amount of elimination is high, the reaction is an important route to alkenes. Potassium t-butoxide and potassium t-pentoxide are frequently used as reagents for dehydrohalogenation because of their high basicity and because they are moderately soluble in nonpolar organic solvents such as benzene (Ñ6Í6).
The reaction is catalyzed by strong acids. Reaction with acyl halides is also an important way of preparing esters.
Various inorganic halides may be regarded as mixed anhydrides of some inorganic acid and ÍÑ1 or HBr. Important examples are
1.SÎÑ12 +Í2Î ® 2 HCl + SO2
2.PBr3 + Í2Î ® 2 HCl + H2PO3
3.PCl5+ 4Í2Î ® 5ÍÑ1 + Í3ÐO4
These compounds react readily with alcohols to form products that are esters of inorganic acids. Since the inorganic acids are strong acids, their anions are good leaving groups for subsequent SN1 and SN2 reactions. An example is the reaction of 1-butanol with thionyl chloride. The intermediate chlorosulfite ester may be isolated if desired. However, reaction with the chloride ion produced in the reaction occurs simply upon warming the alcohol with SÎÑ12. The productsare l-chlorobutane, SO2, and ÍÑ1.
Intermolecular Alcohol Dehydration. At à lower temperature (140 0Ñ) than that required for alkene formation (1800Ñ), an intermolecular rather than an intramolecular alcohol dehydration process could occur to produce ether. In ether formation, two alcohol molecules interact, an Í atom being lost from one and an - ÎÍ group from the other. The resulting "leftover" portions of the two alcohol molecules join to form the ether. This reaction, which gives useful yields only for primary alcohol reactants (2' and 3' alcohols yield predominantly alkenes), can be written as
The preceding reaction is an example of condensation. À condensation reaction is à molecule, usually water. In this case, two alcohol molecules combine to give an ether and water.
Secondary and tertiary alcohols undergo predominant dehydration when subjected to these conditions. Occasionally, some of the symmetrical ether is formed as à by-product in the case of secondary alcohols
major product by-product
The method is generally useless for the preparation of unsymmetrical ethers because complex mixtures are formed:
ROH + R'ÎÍ ® ROR + ROR'+ R'ÎR'
The tertiary cation may also be produced by protonation of an alkene in the presence of à primary or secondary alcohol.
g-bromopropyl t-butyl ether
Intramolecular Alcohol Dehydration. À dehydration reaction is à reaction in which the components of water (Í and ÎÍ) are removed from a single reactant or from two reactants (H from one and OH from the other) In intramolecular dehydration, both water components are removed from the same molecule.
Reaction conditions for the intramolecular dehydration of an alcohol are à temperature of 180'Ñ and the presence of sulfuric acid (Í2SO4) as à catalyst. The dehydration product is an alkene.
Intramolecular alcohol dehydration is an example of an elimination reaction, as contrasted to à substitution reaction and an addition reaction. An elimination reaction is à reaction in which two groups or two atoms on neighboring carbon atoms are removed, or eliminated, from à molecule, leaving à multiple bond between the carbon atoms.
What occurs in an elimination reaction is the reverse of what occurs in an addition reaction.
Dehydration of an alcohol can result in the production of more than one alkene product. This happens when here is more than one neighboring carbon atom from which hydrogen loss can occur. Dehydration of 2-butanol produces two alkenes.
Oxidation of alcohols: Oxidation. Primary and secondary alcohols readily undergo oxidation in the presence of mild oxidizing agents to produce compounds that contain à carbon — oxygen double bond (aldehydes, ketones, and carboxylic acids). À number of different oxidizing agents can be used for the oxidation, including potassium permanganate (ÊÌnO4), potassium dichromate (Ê2Ñr2Î7), and chromic acid (H2CrO4).
The net effect of the action of à mild oxidizing agent on à primary or secondary alcohol is the removal of two hydrogen atoms from the alcohol. One hydrogen comes from the - ÎÍ group, the other from the carbon atom to which the -ÎÍ group is attached. This Í removal generates à carbon — oxygen double bond. The two "removed" hydrogen atoms combine with oxygen supplied by the oxidizing agent to give H2O.
Primary alcohol = aldehyde = carboxylic acid
Secondary alcohol = ketone
Tertiary alcohol = no reaction
The general reaction for the oxidation of à primary alcohol is
Alcohol Aldehyde Carboxylic acid
In this equation, the symbol [O] represents the mild oxidizing agent. The immediate product of the oxidation of à primary alcohol is an aldehyde. Because aldehydes themselves are readily oxidized by the same oxidizing agents that oxidize alcohols, aldehydes are further converted to carboxylic acids. À specific example of à primary alcohol oxidation reaction is
The three classes of alcohols behave differently toward mild oxidizing agents.
The general reaction for the oxidation of à secondary alcohol is
As with primary alcohols, oxidation involves the removal of two hydrogen atoms. Unlike aldehydes, ketones are resistant to further oxidation. À specific example of the oxidation of à secondary alcohol is
Tertiary alcohols do not undergo oxidation with mild oxidizing agents. This is because they do not have hydrogen on the -ÎÍ-bearing carbon atom.
In inorganic chemistry, oxidation numbers are used to characterize oxidation - reduction, processes. This same technique could be used in characterizing oxidation-reduction processes involving organic compounds, but it is not. Formal use of the oxidation number rules with organic compounds is usually cumbersome because of the many carbon and hydrogen atoms present; often, fractional oxidation numbers for carbon result.
À better approach is to use the following set of operational rules, instead of oxidation numbers, in characterizing oxidation - reduction processes in organic chemistry.
1. À carbon atom in an organic compound is oxidized if it loses hydrogen atoms or gains oxygen atoms in à chemical reaction.
2. À carbon atom in an organic compound is reduced if it gains hydrogen atoms or loses oxygen atoms in à chemical reaction.
Adehyde and Ketene
Adehyde - à carbonyl compound containing two hydrogen atoms or hydrogen and alkyl group.
Acetaldehyde Propionaldehyde Butyraidehyde Benzaldehyde
Ketene - à carbonyl compound containing à pair of cumulative double bonds of which one is the carbonyl group, or ketone is à carbonyl compound containing two alkyl groups.
1-phenylethanone diphenylmethanon 5–methylhexan-3-one
When two alkyl groups are attached to the carbonyl, the compound is à ketone.
When two hydrogen atoms, or one hydrogen and one alkyl group are attached to the carbonyl, the compound is an aldehyde.
Lewis structure Kekule structure Condensed structure
R, R’ = Í or alkyl
The structure of formaldehyde, the simplest member of the class, is depicted below, along with its experimental bond lengths and bond angles.
The carbon atom is approximately sp2 hybridized and forms o bonds to two hydrogen atoms and one oxygen. The molecule is planar and the Í-Ñ-O and Í-Ñ-Í bond angles are close to 1200, the idealized sp2 angles. The remaining carbon p orbital overlaps with the oxygen ð, orbital, giving rise to à p-bond between these atoms. The oxygen atom also has two nonbonding electron pairs (the lone pairs) that occupy the remaining orbitals. Note the planarity of the carbonyl group. Also note that one Ñ-Í bond of the methyl group is eclipsed with the Ñ-O bond and that the carbonyl Ñ-Í is staggered with respect to the other two Ñ–Í bonds.
The actual structure is à composite of the normal octet structure, ÑÍ2 =Î and the polarized structure +ÑÍ2 - O-, which corresponds to à carbonium oxide. The composite structure may be represented with dotted line symbîlism which shows the partial charges in carbon and oxygen and the partial single bond character of the C –O bond
One physical consequence of this bond polarity is that carbonyl compounds generally have rather high dipole moments. The experimental dipole moments of formaldehyde and acetone are 2.27 D and 2.85 D, respectively.
The chemical consequences of this bond polarity will be are become apparent during our discussions of the reactions of carbonyl groups. We shall find that the positive carbon can react with bases and that much of the chemistry îf the carbonyl function corresponds to that of à relatively stable carbonium ion.
The ione pair electrons in the carbonyl oxygen have weakly basic properties. In acidic solution, acetone acts as à Lewis base and is protonated to à small but significant extent.
In fact, acetone is à much weaker Lewis base than is water. The material is one half protonated only in 82% sulfuric acid. This corresponds to an approximate pKa for the conjugate acid of acetone of - 7.2 (the approximate ðKa of ÍÎ+ is - 1.7). Even though the carbonyl group has only weakly basic properties, we shall find that this basicity plays an important role in the chemistry of aldehydes, ketones, and related compounds.
formic acid formaldehyde
Appendage groups are designated by the appropriate prefixes. The chain is labelled by using the Greek letters a, b, g, and so on, beginning with the carbon next to the carbonyl group.
The common names of ketones are derived by prefixing the word ketone by they names of the two alkyl-radical-groups; the separate parts are separate words.
methylethyl ketone di-sec-butyl ketone
Dimethyl ketone has the additional trivial name acetone, which is universally used.
As with aldehydes, appendages may be designated by à prefix using the Greek letter notational system.
b) The IUPAC rules for naming aldehydes are as follows:
1. Select as the parent carbon chain the longest chain that includes the carbon atom of the carbonyl group.
2. Name the parent chain by changing the -å ending of the corresponding alkane name to -al.
3. Number the parent chain by assigning the number 1 to the carbonyl carbon atom of the aldehyde group.
4. Determine the identity and location of any substituents, and append this information to the front of the parent chain name.
Nomenclature for Ketones. Assigning IUPAC names to ketones is similar to naming aldehydes except that the ending -one is used instead of -al. The rules for IUPAC ketone nomenclature follow.
1. Select as the parent carbon chain the longest carbon chain that includes the carbon atom of the carbonyl group.
2. Name the parent chain by changing the -å ending of the corresponding alkane name to -one.
3. Number the carbon chain such that the carbonyl carbon atom receives the lowest possible number. The position of the carbonyl carbon atom is noted by placing à number immediately before the parent chain name.
4. Determine the identity and location of any substituents, and append this information to the front of the parent chain name.
5. Cyclic ketones are named by assigning the number 1 to the carbon atom of the carbonyl group. The ring is then numbered to give the lowest number(s) to the atom(s) bearing substituents.
Occasionally, it is necessary to name à molecule containing à carbonyl group as à derivative of à more important function. In such à case, the prefix oxo- is used, along with à number, to indicate the position and nature of the group. One such example is shown below.
It is generally desirable that the common and IUPAC nomenclature systems not be mixed. Ambiguity can result because counting by Greek letters in the common system starts from the carbon next to the carbonyl group, whereas the numbers in the IUPAC system always include the carbonyl group.
Synthesis of aldehydes and ketones. The carbonyl group in aldehydes and ketones is one of the most important functional groups. In this section, we shall review several reactions that are good methods for the synthesis of aldehydes and ketones.
Oxidation of alcohol. As discussed, aldehydes and ketones may be obtained by the oxidation of primary and secondary alcohols, respectively.
In the latter case, the product is not easily oxidized further, so there is no special problem in controlling the reaction to obtain the ketone in good yield. Although many oxidants have been used, the most commonly employed ones are chromium (VI) compounds. However, in aqueous solution, the product aldehyde forms à hydrate, which is oxidized even more rapidly than the primary alcohol.
Oxidation of alkenes. Aldehydes and ketones may also be prepared by oxidative cleavage of Ñ -Ñ multiple bonds. À particularly useful reagent for this purpose is ozone. Hydrolysis of the ozonide, usually under reductive conditions, results in the production of two carbonyl compounds.
Hydration of alkynes. As was discussed, alkynes undergo hydration to yield an unstable vinyl alcohol, which immediately rearranges to the corresponding ketone. The reaction is usually catalyzed by mercuric ion and sulfuric acid.
Aldehydes may be prepared by the partial reduction of acyl halides or nitriles.
The reactions of aldehydes and ketones can be divided into the following types:
Keto – enol equilibrium. Aldehydes and ketones exist in solution as an equilibrium mixture of two isomeric forms, the keto form and the enol (from -ene + -ol, unsaturated alcohol) form. For simple aliphatic ketones, there is very little of the enol form present at equilibrium, as shown by the following examples.
This type isomerism, where the isomers differ only by the placement of à proton and the corresponding location of à double bond, is commonly referred to as tautomerism. The isomers are known as tautomers.
Even though the percentage of enol form at equilibrium is quite small, the enol is important in many reactions. As we shall soon see, many reactions of aldehydes and ketones occur by way of the unstable enol form.
Nucleophilic addition reaction of Aldehydes and Ketones. Many different substances can add to the carbon - oxygen double bond of à carbonyl group. In fact, because of its polarity, à carbon - oxygen double bond is even more susceptible to addition reactions than à carbon - carbon double bond.
Polarity is used to predict locations for the entering groups from addition. In à carbon-oxygen double bond, due to electronegativity differences, the carbon atom possesses à partial positive charge (d+), and the oxygen atom possesses à partial negative charge (d-).
An unsymmetrical addition agent also has partial charges.
The atom (or group of atoms) ø the addition agent with the positive partial charge (d+), bonds to the carbonyl oxygen atom (d-), and the atom (or group of atoms) with the partial negative charge (d-) bonds to the carbonyl carbon atom (d+).
Hemiacetals and Hemiketals. When an alcohol molecule adds across the carbon - oxygen double bond of an aldehyde or ketone, the Í atom from the alcohol adds to the carbonyl oxygen atom, and the R – Î portion of the alcohol adds to the carbonyl carbon atom.
The product of the addition of one molecule of an alcohol to an aldehyde is called à hemiacetal. Similarly, the addition of one molecule of alcohol to à ketone produces à hemiketal.
aldehyde alcohol hemiacetal ketone alcohol hemiketal
Hemiacetals and hemiketals both contain an alcohol group (hydroxyl group) and an ether group (alkoxy group) on the same carbon atom. What differentiates them is the presence or lack of à hydrogen atom on this same carbon atom. In à hemiacetal à hydrogen atom is present. In à bcmiketal no hydrogen atom is present.
Formally defined, à hemiacetal is à compound that has à carbon atom to which à hydroxyl group (-ÎÍ), an alkoxy group (-OR), and à hydrogen atom (-Í) are attached. À hemiketal differs from à hemiacetal in that an R group has replaced the Í atom. À hemiketal is à compound that has à carbon atom to which à hydroxyl group (-ÎÍ) and an alkoxy group (-OR), but no hydrogen atom (-Í) are attached.
Reaction mixtures containing hemiacetals and hemiketals are always in equilibrium with the alcohol and carbonyl compounds from which they are made, and the equilibrium lies to the carbonyl compound side of the reaction.
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