Lactam
A lactam is a cyclic amide. The term is a portmanteau of the words lactone + amide. Greek prefixes in alphabetical order indicate ring size: α-Lactam β-Lactam γ-Lactam δ-Lactam ε-Lactam This ring-size nomenclature stems from the fact that a hydrolyzed α-Lactam leads to an α-amino acid and a β-Lactam to a β-amino acid, etc. General synthetic methods exist for the organic synthesis of lactams. Lactams form by the acid-catalyzed rearrangement of oximes in the Beckmann rearrangement. Lactams form from hydrazoic acid in the Schmidt reaction. Lactams form from cyclisation of amino acids. Lactams form from intramolecular attack of linear acyl derivatives from the nucleophilic abstraction reaction. In iodolactamization an iminium ion reacts with a halonium ion formed in situ by reaction of an alkene with iodine. Lactams form by copper-catalyzed 1,3-dipolar cycloaddition of alkynes and nitrones in the Kinugasa reaction Diels-Alder reaction between cyclopentadiene and chlorosulfonyl isocyanate can be utilized to obtain both β- as well as γ-lactam.
At lower temp, β-lactam is the preferred product. At optimum temperatures, a useful γ-lactam known as Vince Lactam is obtained. A lactim is a cyclic carboximidic acid compound characterized by an endocyclic carbon-nitrogen double bond, they are formed. Lactams can polymerize to polyamides. Lactone, a cyclic ester. Β-Lactam β-Lactam antibiotics, which includes penicillins 2-Pyrrolidone 2-Piperidinone Caprolactam Media related to Lactams at Wikimedia Commons
Organometallic chemistry
Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkaline, alkaline earth, transition metals, sometimes broadened to include metalloids like boron and tin, as well. Aside from bonds to organyl fragments or molecules, bonds to'inorganic' carbon, like carbon monoxide, cyanide, or carbide, are considered to be organometallic as well; some related compounds such as transition metal hydrides and metal phosphine complexes are included in discussions of organometallic compounds, though speaking, they are not organometallic. The related but distinct term "metalorganic compound" refers to metal-containing compounds lacking direct metal-carbon bonds but which contain organic ligands. Metal β-diketonates, alkoxides and metal phosphine complexes are representative members of this class; the field of organometallic chemistry combines aspects of traditional inorganic and organic chemistry.
Organometallic compounds are used both stoichiometrically in research and industrial chemical reactions, as well as in the role of catalysts to increase the rates of such reactions, where target molecules include polymers and many other types of practical products. Organometallic compounds are distinguished by the prefix "organo-" e.g. organopalladium compounds. Examples of such organometallic compounds include all Gilman reagents, which contain lithium and copper. Tetracarbonyl nickel, ferrocene are examples of organometallic compounds containing transition metals. Other examples include organomagnesium compounds like iodomagnesium MeMgI, dimethylmagnesium, all Grignard reagents. In addition to the traditional metals, lanthanides and semimetals, elements such as boron, silicon and selenium are considered to form organometallic compounds, e.g. organoborane compounds such as triethylborane. Representative Organometallic Compounds Many complexes feature coordination bonds between a metal and organic ligands.
The organic ligands bind the metal through a heteroatom such as oxygen or nitrogen, in which case such compounds are considered coordination compounds. However, if any of the ligands form a direct M-C bond complex is considered to be organometallic, e.g. 2+. Furthermore, many lipophilic compounds such as metal acetylacetonates and metal alkoxides are called "metalorganics." A occurring transition metal alkyl complex is methylcobalamin, with a cobalt-methyl bond. This subset of complexes is discussed within the subfield of bioorganometallic chemistry. Illustrative of the many functions of the B12-dependent enzymes, the MTR enzyme catalyzes the transfer of a methyl group from a nitrogen on N5-methyl-tetrahydrofolate to the sulfur of homocysteine to produce methionine; the status of compounds in which the canonical anion has a delocalized structure in which the negative charge is shared with an atom more electronegative than carbon, as in enolates, may vary with the nature of the anionic moiety, the metal ion, the medium.
For instance, lithium enolates contain only Li-O bonds and are not organometallic, while zinc enolates contain both Zn-O and Zn-C bonds, are organometallic in nature. The metal-carbon bond in organometallic compounds is highly covalent. For electropositive elements, such as lithium and sodium, the carbon ligand exhibits carbanionic character, but free carbon-based anions are rare, an example being cyanide; as in other areas of chemistry, electron counting is useful for organizing organometallic chemistry. The 18-electron rule is helpful in predicting the stabilities of metal carbonyls and related compounds. Most organometallic compounds do not however follow the 18e rule. Chemical bonding and reactivity in organometallic compounds is discussed from the perspective of the isolobal principle; as well as X-ray diffraction, NMR and infrared spectroscopy are common techniques used to determine structure. The dynamic properties of organometallic compounds is probed with variable-temperature NMR and chemical kinetics.
Organometallic compounds undergo several important reactions: oxidative addition and reductive elimination transmetalation carbometalation hydrometalation electron transfer β-hydride elimination organometallic substitution reaction carbon-hydrogen bond activation cyclometalation migratory insertion nucleophilic abstraction Early developments in organometallic chemistry include Louis Claude Cadet's synthesis of methyl arsenic compounds related to cacodyl, William Christopher Zeise's platinum-ethylene complex, Edward Frankland's discovery of diethyl- and dimethylzinc, Ludwig Mond's discovery of Ni4, Victor Grignard's organomagnesium compounds. The abundant and diverse products from coal and petroleum led to Ziegler–Natta, Fischer–Tropsch, hydroformylation catalysis which employ CO, H2, alkenes as feedstocks and ligands. Recognition of organometalli
Lactone
Lactones are cyclic carboxylic esters, containing a 1-oxacycloalkan-2-one structure, or analogues having unsaturation or heteroatoms replacing one or more carbon atoms of the ring. Lactones are formed by intramolecular esterification of the corresponding hydroxycarboxylic acids, which takes place spontaneously when the ring, formed is five- or six-membered. Lactones with three- or four-membered rings are reactive, making their isolation difficult. Special methods are required for the laboratory synthesis of small-ring lactones as well as those that contain rings larger than six-membered. Lactones are named according to the precursor acid molecule, with a -lactone suffix and a Greek letter prefix that specifies the number of carbon atoms in the heterocycle — that is, the distance between the relevant -OH and the -COOH groups along said backbone; the first carbon atom after the carbon in the -COOH group on the parent compound is labelled α, the second will be labeled β, so forth. Therefore, the prefixes indicate the size of the lactone ring: α-lactone = 3-membered ring, β-lactone = 4-membered, γ-lactone = 5-membered, etc.
The other suffix used to denote a lactone is -olide, used in substance class names like butenolide, cardenolide or bufadienolide. To obtain the preferred IUPAC names, lactones are named as heterocyclic pseudoketones by adding the suffix ‘one’, ‘dione’, ‘thione’, etc. and the appropriate multiplicative prefixes to the name of the heterocyclic parent hydride. The name lactone derives from the ring compound called lactide, formed from the dehydration of 2-hydroxypropanoic acid CH3-CH-COOH. Lactic acid, in turn, derives its name from its original isolation from soured milk. An internal dehydration within the same molecule of lactic acid would have produced alpha-propiolactone, a lactone with a 3-membered ring. Occurring lactones are saturated and unsaturated γ- and δ-lactones, to a lesser extent macrocyclic lactones; the γ- and δ-lactones are intramolecular esters of the corresponding hydroxy fatty acids. They contribute to the aroma of fruits, butter and other foods. Cyclopentadecanolide is responsible for the musklike odor of angelica root oil.
Of the occurring bicyclic lactones, phthalides are responsible for the odors of celery and lovage oils, coumarin for woodruff. Lactone rings occur as building blocks in nature, such as in ascorbic acid, nepetalactone, hormones, neurotransmitters, anticancer drugs, phytoestrogens. Many methods in ester synthesis can be applied to that of lactones. In one industrial synthesis of oxandrolone the key lactone-forming step is an organic reduction - esterification. In halolactonization, an alkene is attacked by a halogen via electrophilic addition with the cationic intermediate captured intramolecularly by an adjacent carboxylic acid. Specific methods include Yamaguchi esterification, Shiina macrolactonization, Baeyer–Villiger oxidation and nucleophilic abstraction; the γ-lactones γ-octalactone, γ-nonalactone, γ-decalactone, γ-undecalactone can be prepared in good yield in a one-step process by radical addition of primary fatty alcohols to acrylic acid, using di-tert-butyl peroxide as a catalyst. The most stable structure for lactones are the 5-membered γ-lactones and 6-membered δ-lactones because, as in all organic cycles, 5 and 6 membered rings minimize the strain of bond angles.
Γ-lactones are so stable that, in the presence of dilute acids at room temperature, 4-hydroxy acids undergo spontaneous esterification and cyclisation to the lactone. Β-lactones can only be made by special methods. Α-lactones can be detected as transient species in mass spectrometry experiments. The reactions of lactones are similar to those of esters, as exemplified by gamma-lactone in the following sections: Heating a lactone with a base will hydrolyse the lactone to its parent compound, the straight chained bifunctional compound. Like straight-chained esters, the hydrolysis-condensation reaction of lactones is a reversible reaction, with an equilibrium. However, the equilibrium constant of the hydrolysis reaction of the lactone is lower than that of the straight-chained ester i.e. the products are less favored in the case of the lactones. This is because although the enthalpies of the hydrolysis of esters and lactones are about the same, the entropy of the hydrolysis of lactones is less than the entropy of straight-chained esters.
Straight-chained esters give two products upon hydrolysis, making the entropy change more favorable than in the case of lactones which gives only a single product. Lactones can be reduced to diols using lithium aluminium hydride in dry ether; the reduction reaction will first break the ester bond of the lactone, reduce the aldehyde group to the alcohol group. For instance, gamma-lactones will be reduced to butane-1,4-diol, (CH2-2-CH2. Lactones react with ethanolic ammonia, which will first break the ester bond and react with the acidic -COOH group, because of the basic properties of ammonia, to form a difunctional group, i.e. alcohol and amide. Gamma-lactones will react to yield CH2-2-CO-NH2. Lactones form polyesters according to the formula: Sesquiterpene lactones, found in many plants, can react with other molecules via a Michael reaction. Lactones contribute sig
Chemical polarity
In chemistry, polarity is a separation of electric charge leading to a molecule or its chemical groups having an electric dipole moment, with a negatively charged end and a positively charged end. Polar molecules must contain polar bonds due to a difference in electronegativity between the bonded atoms. A polar molecule with two or more polar bonds must have a geometry, asymmetric in at least one direction, so that the bond dipoles do not cancel each other. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Polarity underlies a number of physical properties including surface tension and melting and boiling points. Not all atoms attract electrons with the same force; the amount of "pull" an atom exerts on its electrons is called its electronegativity. Atoms with high electronegativities – such as fluorine and nitrogen – exert a greater pull on electrons than atoms with lower electronegativities such as alkali metals and alkaline earth metals. In a bond, this leads to unequal sharing of electrons between the atoms, as electrons will be drawn closer to the atom with the higher electronegativity.
Because electrons have a negative charge, the unequal sharing of electrons within a bond leads to the formation of an electric dipole: a separation of positive and negative electric charge. Because the amount of charge separated in such dipoles is smaller than a fundamental charge, they are called partial charges, denoted as δ+ and δ−; these symbols were introduced by Sir Christopher Ingold and Dr. Edith Hilda Ingold in 1926; the bond dipole moment is calculated by multiplying the amount of charge separated and the distance between the charges. These dipoles within molecules can interact with dipoles in other molecules, creating dipole-dipole intermolecular forces. Bonds can fall between one of two extremes – being nonpolar or polar. A nonpolar bond occurs when the electronegativities are identical and therefore possess a difference of zero. A polar bond is more called an ionic bond, occurs when the difference between electronegativities is large enough that one atom takes an electron from the other.
The terms "polar" and "nonpolar" are applied to covalent bonds, that is, bonds where the polarity is not complete. To determine the polarity of a covalent bond using numerical means, the difference between the electronegativity of the atoms is used. Bond polarity is divided into three groups that are loosely based on the difference in electronegativity between the two bonded atoms. According to the Pauling scale: Nonpolar bonds occur when the difference in electronegativity between the two atoms is less than 0.5 Polar bonds occur when the difference in electronegativity between the two atoms is between 0.5 and 2.0 Ionic bonds occur when the difference in electronegativity between the two atoms is greater than 2.0Pauling based this classification scheme on the partial ionic character of a bond, an approximate function of the difference in electronegativity between the two bonded atoms. He estimated that a difference of 1.7 corresponds to 50% ionic character, so that a greater difference corresponds to a bond, predominantly ionic.
As a quantum-mechanical description, Pauling proposed that the wave function for a polar molecule AB is a linear combination of wave functions for covalent and ionic molecules: ψ = aψ + bψ. The amount of covalent and ionic character depends on the values of the squared coefficients a2 and b2. While the molecules can be described as "polar covalent", "nonpolar covalent", or "ionic", this is a relative term, with one molecule being more polar or more nonpolar than another. However, the following properties are typical of such molecules. A molecule is composed of one or more chemical bonds between molecular orbitals of different atoms. A molecule may be polar either as a result of polar bonds due to differences in electronegativity as described above, or as a result of an asymmetric arrangement of nonpolar covalent bonds and non-bonding pairs of electrons known as a full molecular orbital. A polar molecule has a net dipole as a result of the opposing charges from polar bonds arranged asymmetrically.
Water is an example of a polar molecule since it has a slight positive charge on one side and a slight negative charge on the other. The dipoles do not cancel out resulting in a net dipole. Due to the polar nature of the water molecule itself, polar molecules are able to dissolve in water. Other examples include sugars, which have many polar oxygen–hydrogen groups and are overall polar. If the bond dipole moments of the molecule do not cancel, the molecule is polar. For example, the water molecule contains two polar O−H bonds in a bent geometry; the bond dipole moments do not cancel, so that the molecule forms a molecular dipole with its negative pole at the oxygen and its positive pole midway between the two hydrogen atoms. In the figure each bond joins the central O atom with a negative charge to an H atom with a positive charge; the hydrogen fluoride, HF, molecule is polar by virtue of polar covalent bonds – in the covalent bond electrons are displaced toward the more electronegative fluorine atom.
Ammonia, NH3, molecule. The molecule has two lone electrons in an orbital, that points towards the fourth apex of the approximate tetrahedron; this orbital is not participating in covalent bonding.
Lewis acids and bases
A Lewis acid is a chemical species that contains an empty orbital, capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base is any species that has a filled orbital containing an electron pair, not involved in bonding but may form a dative bond with a Lewis acid to form a Lewis adduct. For example, NH3 is a Lewis base. Trimethylborane is a Lewis acid. In a Lewis adduct, the Lewis acid and base share an electron pair furnished by the Lewis base, forming a dative bond. In the context of a specific chemical reaction between NH3 and Me3B, the lone pair from NH3 will form a dative bond with the empty orbital of Me3B to form an adduct NH3•BMe3; the terminology refers to the contributions of Gilbert N. Lewis; the terms nucleophile and electrophile are more or less interchangeable with Lewis base and Lewis acid, respectively. However, these terms their abstract noun forms nucleophilicity and electrophilicity, emphasize the kinetic aspect of reactivity, while the Lewis basicity and Lewis acidity emphasize the thermodynamic aspect of Lewis adduct formation.
In many cases, the interaction between the Lewis base and Lewis acid in a complex is indicated by an arrow indicating the Lewis base donating electrons toward the Lewis acid using the notation of a dative bond—for example, Me3B←NH3. Some sources indicate the Lewis base with a pair of dots, which allows consistent representation of the transition from the base itself to the complex with the acid: Me3B +:NH3 → Me3B:NH3A center dot may be used to represent a Lewis adduct, such as Me3B•NH3. Another example is boron trifluoride diethyl etherate, BF3•Et2O. Although there have been attempts to use computational and experimental energetic criteria to distinguish dative bonding from non-dative covalent bonds, for the most part, the distinction makes note of the source of the electron pair, dative bonds, once formed, behave as other covalent bonds do, though they have considerable polar character. Moreover, in some cases, the use of the dative bond arrow is just a notational convenience for avoiding the drawing of formal charges.
In general, the donor–acceptor bond is viewed as somewhere along a continuum between idealized covalent bonding and ionic bonding. Classically, the term "Lewis acid" is restricted to trigonal planar species with an empty p orbital, such as BR3 where R can be an organic substituent or a halide. For the purposes of discussion complex compounds such as Et3Al2Cl3 and AlCl3 are treated as trigonal planar Lewis acids. Metal ions such as Na+, Mg2+, Ce3+, which are invariably complexed with additional ligands, are sources of coordinatively unsaturated derivatives that form Lewis adducts upon reaction with a Lewis base. Other reactions might be referred to as "acid-catalyzed" reactions; some compounds, such as H2O, are both Lewis acids and Lewis bases, because they can either accept a pair of electrons or donate a pair of electrons, depending upon the reaction. Lewis acids are diverse. Simplest are those, but more common are those. Examples of Lewis acids based on the general definition of electron pair acceptor include: the proton and acidic compounds onium ions, such as NH4+ and H3O+ high oxidation state transition metal cations, e.g. Fe3+.
Again, the description of a Lewis acid is used loosely. For example, in solution, bare protons do not exist; some of the most studied examples of such Lewis acids are the boron trihalides and organoboranes, but other compounds exhibit this behavior: BF3 + F− → BF4−In this adduct, all four fluoride centres are equivalent. BF3 + OMe2 → BF3OMe2Both BF4− and BF3OMe2 are Lewis base adducts of boron trifluoride. In many cases, the adducts violate the octet rule, such as the triiodide anion: I2 + I− → I3−The variability of the colors of iodine solutions reflects the variable abilities of the solvent to form adducts with the Lewis acid I2. In some cases, the Lewis acid is capable of binding two Lewis base, a famous example being the formation of hexafluorosilicate: SiF4 + 2 F− → SiF62− Most compounds considered to be Lewis acids require an activation step prior to formation of the adduct with the Lewis base. Well known cases are the aluminium trihalides, which are viewed as Lewis acids. Aluminium trihalides, unlike the boron trihalides, do not exist in the form AlX3, but as aggregates and polymers that must be degraded by the Lewis base.
A simpler case is the formation of adducts of borane. Monomeric BH3 does not exist appreciably, so the adducts of borane are generated by degradation of diborane: B2H6 + 2 H− → 2 BH4−In this case, an intermediate B2H7− can be isolated. Many metal complexes serve as Lewis acids, but only after dissociating a more weakly bound Lewis base water. 2+ + 6 NH3 → 2+ + 6 H2O The proton is one of the strongest but is one of the most complicated Lewis acids. It is convention to ignore the fact that a proton is solvated (bound to solvent
Enone
An enone called an α,β-unsaturated carbonyl, is a type of organic compound consisting of an alkene conjugated to a ketone. The simplest enone is methyl vinyl ketone or CH2=CHCOCH3. An enal is an example being acrolein. Enones are produced using an Aldol condensation or Knoevenagel condensation; some commercially significant enones are produced by condensations of acetone, e.g. mesityl oxide and isophorone. In the Meyer–Schuster rearrangement the starting compound is a propargyl alcohol. Cyclic enones can be prepared via the Pauson–Khand reaction. Enones undergo many kinds of reactions, they are electrophilic at both the carbonyl carbon as well as the β-carbon. Depending on conditions, either site is attacked by nucleophiles. Additions to the alkene are called conjugate additions. Enones are good dienophiles in Diels-Alder reactions, they are activated by Lewis acids. Enones are reduced, they can undergo reduction of the carbonyl or the alkene selectively, or reduction of both functional groups. Enones in the Rauhut -- Currier reaction.
Sterically unhindered enones such as methyl vinyl ketone are prone to polymerization. Enones are good ligands for examples being Fe3 and trisdipalladium. Enone is not to be confused with ketene. An enamine is a cousin of an enone, with the carbonyl replaced by an amine group. Regiospecific formation is the controlled enolate formation by the specific deprotonation at one of the α-carbons of the ketone starting molecule; this provides one of the best understood synthetic strategies to introduce chemical complexity in natural product and total syntheses. A prominent example of its use is in the total synthesis of progesterone illustrated in Figure "Regiospecific enolate formation in the total synthesis of progesterone"; when ketones are treated with base, enolates can be formed by deprotonation at either α-carbon. The selectivity is determined by both the steric and electronic effects on the α-carbons as well as the precise base used. Enolate formation will be thermodynamically favoured at the most acidic proton which depends on the electronic stabilization of the resulting anion.
However, the selectivity can be reversed by sterically hindering the thermodynamic product and therefore kinetically favouring deprotonation at the other α-carbon centre. Traditional methods for regioselective enolate formation use either electronic activating groups or steric blocking groups. Enone can serve as a precursor for regiospecific formation of enolate, here the enone is a “masked functionality” for the enolate; this process is first described by Gilbert Stork, best known for his contributions to the study of selective enolate formation methods in organic synthesis. The Stork method uses enone as a “masked functionality” of enolate. Reacting enone with lithium metal generates the enolate at the α-carbon of the enone; the enolate product can either be alkylated. By using “masked functionality”, it is possible to produce enolates that are not accessible by traditional methods; the “masked functionality” approach to regiospecific enolate formation has been used in total synthesis of natural products.
For example, in the total synthesis of the steroid hormone progesterone, Stork and co-workers used the “masked functionality” to stereospecifically construct one of the quaternary carbons in the molecule. Acryloyl group
Resonance (chemistry)
In chemistry, resonance is a way of describing bonding in certain molecules or ions by the combination of several contributing structures into a resonance hybrid in valence bond theory. It has particular value for describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis structure. Under the framework of valence bond theory, resonance is an extension of the idea that the bonding in a chemical species can be described by a Lewis structure. For many chemical species, a single Lewis structure, consisting of atoms obeying the octet rule bearing formal charges, connected by bonds of positive integer order, is sufficient for describing the chemical bonding and rationalizing experimentally determined molecular properties like bond lengths and dipole moment. However, in some cases, more than one Lewis structure could be drawn, experimental properties are inconsistent with any one structure. In order to address this type of situation, several contributing structures are considered together as an average, the molecule is said to be represented by a resonance hybrid in which several Lewis structures are used collectively to describe its true structure.
For instance, in NO2–, nitrite anion, the two N–O bond lengths are equal though no single Lewis structure has two N–O bonds with the same formal bond order. However, its measured structure is consistent with a description as a resonance hybrid of the two major contributing structures shown above: it has two equal N–O bonds of 125 pm, intermediate in length between a typical N–O single bond and N–O double bond. According to the contributing structures, each N–O bond is an average of a formal single and formal double bond, leading to a true bond order of 1.5. By virtue of this averaging, the Lewis description of the bonding in NO2– is reconciled with the experimental fact that the anion has equivalent N–O bonds; the resonance hybrid represents the actual molecule as the "average" of the contributing structures, with bond lengths and partial charges taking on intermediate values compared to those expected for the individual Lewis structures of the contributors, were they to exist as "real" chemical entities.
The contributing structures differ only in the formal apportionment of electrons to the atoms, not in the actual physically and chemically significant electron or spin density. While contributing structures may differ in formal bond orders and in formal charge assignments, all contributing structures must have the same number of valence electrons and the same spin multiplicity; because electron delocalization lowers the potential energy of a system, any species represented by a resonance hybrid is more stable than any of the contributing structures. The difference in potential energy between the actual species and the energy of the contributing structure with the lowest potential energy is called the resonance energy or delocalization energy; the magnitude of the resonance energy depends on assumptions made about the hypothetical "non-stabilized" species and the computational methods used and does not represent a measurable physical quantity, although comparisons of resonance energies computed under similar assumptions and conditions may be chemically meaningful.
Molecules with an extended π system such as linear polyenes and polyaromatic compounds are well described by resonance hybrids as well as by delocalised orbitals in molecular orbital theory. Resonance is to be distinguished from isomerism. Isomers are molecules with the same chemical formula but are distinct chemical species with different arrangements of atomic nuclei in space. Resonance contributors of a molecule, on the other hand, can only differ in the way electrons are formally assigned to atoms in the Lewis structure depictions of the molecule; when a molecular structure is said to be represented by a resonance hybrid, it does not mean that electrons of the molecule are "resonating" or shifting back and forth between several sets of positions, each one represented by a Lewis structure. Rather, it means that the set of contributing structures represents an intermediate structure, with a single, well-defined geometry and distribution of electrons, it is incorrect to regard resonance hybrids as interconverting isomers though the term "resonance" might evoke such an image.
Symbolically, the double headed arrow A ⟷ B is used to indicate that A and B are contributing forms of a single chemical species. A non-chemical analogy is illustrative: one can describe the characteristics of a real animal, the narwhal, in terms of the characteristics of two mythical creatures: the unicorn, a creature with a single horn on its head, the leviathan, a large, whale-like creature; the narwhal is not a creature that goes back and forth between being a unicorn and being a leviathan, nor do the unicorn and leviathan have any physical existence outside the collective human imagination. Describing the narwhal in terms of these imaginary creatures provides a reasonably good description of its physical characteristics. Due to confusion
