In organic chemistry , a carbanion is an anion in which carbon is negatively charged.
90-449: The Brønsted–Lowry theory (also called proton theory of acids and bases ) is an acid–base reaction theory which was first developed by Johannes Nicolaus Brønsted and Thomas Martin Lowry independently in 1923. The basic concept of this theory is that when an acid and a base react with each other, the acid forms its conjugate base , and the base forms its conjugate acid by exchange of
180-512: A Lewis acid , a compound that can accept an electron pair. Lewis's proposal explains the Brønsted–Lowry classification using electronic structure. In this representation both the base, B, and the conjugate base, A, are shown carrying a lone pair of electrons and the proton, which is a Lewis acid, is transferred between them. Lewis later wrote "To restrict the group of acids to those substances that contain hydrogen interferes as seriously with
270-513: A in DMSO of CHPh 3 = 30.6) in THF at low temperatures followed by 12-crown-4 results in a red solution and the salt complex [Li(12-crown-4)] [CPh 3 ] precipitates at −20 °C. The central C–C bond lengths are 145 pm with the phenyl ring propellered at an average angle of 31.2°. This propeller shape is less pronounced with a tetramethylammonium counterion. A crystal structure for
360-406: A phosphate–phosphorane rearrangement to phosphorane 3 which on reaction with acetic acid gives alcohol 4 . Once again in the range of −78 °C to 0 °C the chirality is preserved in this reaction sequence. ( Enantioselectivity was determined by NMR spectroscopy after derivatization with Mosher's acid .) A carbanionic structure first made an appearance in the reaction mechanism for
450-629: A proton (the hydrogen cation, or H). This theory generalises the Arrhenius theory . In the Arrhenius theory , acids are defined as substances that dissociate in aqueous solutions to give H (hydrogen ions or protons ), while bases are defined as substances that dissociate in aqueous solutions to give OH (hydroxide ions). In 1923, physical chemists Johannes Nicolaus Brønsted in Denmark and Thomas Martin Lowry in England both independently proposed
540-445: A proton donor which can lose a proton to become its conjugate base, A. The base, B, is a proton acceptor which can become its conjugate acid, HB. Most acid–base reactions are fast, so the substances in the reaction are usually in dynamic equilibrium with each other. Consider the following acid–base reaction: Acetic acid , CH 3 COOH , is an acid because it donates a proton to water ( H 2 O ) and becomes its conjugate base,
630-410: A salt and water. acid + base ⟶ salt + water {\displaystyle {\text{acid}}\ +\ {\text{base}}\ \longrightarrow \ {\text{salt}}\ +\ {\text{water}}} In this traditional representation an acid–base neutralization reaction is formulated as a double-replacement reaction . For example,
720-477: A values allow the prediction of whether a proton transfer process will be thermodynamically favorable: In order for the deprotonation of an acidic species HA with base B to be thermodynamically favorable ( K > 1), the relationship p K a (BH) > p K a (AH) must hold. These values below are p K a values determined in dimethylsulfoxide (DMSO), which has a broader useful range (~0 to ~35) than values determined in water (~0 to ~14) and better reflect
810-408: A well below −10. On the other end of the scale, hydrocarbons bearing only alkyl groups are thought to have p K a values in the range of 55 to 65. The range of acid dissociation constants for carbon acids thus spans over 70 orders of magnitude. The acidity of the α-hydrogen in carbonyl compounds enables these compounds to participate in synthetically important C–C bond-forming reactions including
900-460: A base in liquid sulfuric acid: HNO 3 base + 2 H 2 SO 4 ⟶ NO 2 + + H 3 O + + 2 HSO 4 − {\displaystyle {\underset {\text{base}}{{\ce {HNO3}}}}+{\ce {2 H2SO4 -> NO2+ + H3O+ + 2 HSO4-}}} The unique strength of this definition shows in describing
990-415: A carbanion is the conjugate base of a carbon acid : where B stands for the base . The carbanions formed from deprotonation of alkanes (at an sp carbon), alkenes (at an sp carbon), arenes (at an sp carbon), and alkynes (at an sp carbon) are known as alkyl , alkenyl ( vinyl ), aryl , and alkynyl ( acetylide ) anions, respectively. Carbanions have a concentration of electron density at
SECTION 10
#17327839917751080-402: A commercial baking powder might use sodium acid pyrophosphate as one of the two acidic components instead of sodium aluminium sulfate. Another typical acid in such formulations is cream of tartar ( KC 4 H 5 O 6 ), a derivative of tartaric acid . The Brønsted–Lowry definition, formulated in 1923, independently by Johannes Nicolaus Brønsted in Denmark and Martin Lowry in England,
1170-435: A compound that can receive this electron pair. For example, boron trifluoride , BF 3 is a typical Lewis acid. It can accept a pair of electrons as it has a vacancy in its octet . The fluoride ion has a full octet and can donate a pair of electrons. Thus BF 3 + F − ⟶ BF 4 − {\displaystyle {\ce {BF3 + F- -> BF4-}}}
1260-483: A hybrid orbital, the carbanionic lone pair may instead occupy a p orbital (or an orbital of high p character). A p orbital has a more suitable shape and orientation to overlap with the neighboring π system, resulting in more effective charge delocalization. As a consequence, alkyl carbanions with neighboring conjugating groups (e.g., allylic anions, enolates, nitronates, etc.) are generally planar rather than pyramidized. Likewise, delocalized alkenyl carbanions sometimes favor
1350-551: A linear instead of bent geometry. More often, a bent geometry is still preferred for substituted alkenyl anions, though the linear geometry is only slightly less stable, resulting in facile equilibration between the ( E ) and ( Z ) isomers of the (bent) anion through a linear transition state . For instance, calculations indicate that the parent vinyl anion or ethylenide, H 2 C=CH , has an inversion barrier of 27 kcal/mol (110 kJ/mol), while allenyl anion or allenide, H 2 C=C=CH ↔ H 2 C −C≡CH ), whose negative charge
1440-470: A pair of electrons to the metal ion. The reaction [ Ag ( H 2 O ) 4 ] + + 2 NH 3 ⟶ [ Ag ( NH 3 ) 2 ] + + 4 H 2 O {\displaystyle {\ce {[Ag(H2O)4]+ + 2 NH3 -> [Ag(NH3)2]+ + 4 H2O}}} can be seen as an acid–base reaction in which
1530-452: A polar covalent bond, though with electron density heavily polarized toward the carbon atom. The more electropositive the attached metal atom, the closer the behavior of the reagent is to that of a true carbanion. In fact, true carbanions (i.e., a species not attached to a stabilizing covalently bound metal) without electron-withdrawing and/or conjugating substituents are not available in the condensed phase, and these species must be studied in
1620-401: A stronger base (ammonia) replaces a weaker one (water). The Lewis and Brønsted–Lowry definitions are consistent with each other since the reaction H + + OH − ↽ − − ⇀ H 2 O {\displaystyle {\ce {H+ + OH- <=> H2O}}}
1710-767: A water molecule is split into a hydrogen ion, which is donated to a pyridine molecule, and a hydroxide ion. In the Brønsted–Lowry model, the solvent does not necessarily have to be water, as is required by the Arrhenius Acid–Base model . For example, consider what happens when acetic acid , CH 3 COOH , dissolves in liquid ammonia . CH 3 COOH + NH 3 ↽ − − ⇀ NH 4 + + CH 3 COO − {\displaystyle {\ce {CH3COOH + NH3 <=> NH4+ + CH3COO-}}} An H ion
1800-459: A weak acid in the Brønsted–Lowry sense. According to the Lux–Flood theory , oxides like MgO and SiO 2 in the solid state may be called acids or bases. For example, the mineral olivine may be known as a compound of a basic oxide, MgO, and silicon dioxide, SiO 2 , as an acidic oxide. This is important in geochemistry . Acid%E2%80%93base reaction In chemistry , an acid–base reaction
1890-456: Is proton NMR . A spectrum of cyclopentadiene in DMSO shows four vinylic protons at 6.5 ppm and two methylene bridge protons at 3 ppm whereas the cyclopentadienyl anion has a single resonance at 5.50 ppm. The use of Li and Li NMR has provided structural and reactivity data for a variety of organolithium species. Any compound containing hydrogen can, in principle, undergo deprotonation to form its conjugate base. A compound
SECTION 20
#17327839917751980-455: Is a carbon acid if deprotonation results in loss of a proton from a carbon atom. Compared to compounds typically considered to be acids (e.g., mineral acids like nitric acid , or carboxylic acids like acetic acid ), carbon acids are typically many orders of magnitude weaker, although exceptions exist (see below). For example, benzene is not an acid in the classical Arrhenius sense, since its aqueous solutions are neutral. Nevertheless, it
2070-540: Is a chemical reaction that occurs between an acid and a base . It can be used to determine pH via titration . Several theoretical frameworks provide alternative conceptions of the reaction mechanisms and their application in solving related problems; these are called the acid–base theories, for example, Brønsted–Lowry acid–base theory . Their importance becomes apparent in analyzing acid–base reactions for gaseous or liquid species, or when acid or base character may be somewhat less apparent. The first of these concepts
2160-507: Is a convenient approximation, although these species are generally clusters or complexes containing highly polar, but still covalent bonds metal–carbon bonds (M –C ) rather than true carbanions. Absent π delocalization , the negative charge of a carbanion is localized in an sp hybridized orbital on carbon as a lone pair . As a consequence, localized alkyl, alkenyl/aryl, and alkynyl carbanions assume trigonal pyramidal, bent, and linear geometries, respectively. By Bent's rule , placement of
2250-534: Is a hydrogen-containing compound whose hydrogen can be replaced by a metal. This redefinition was based on his extensive work on the chemical composition of organic acids , finishing the doctrinal shift from oxygen-based acids to hydrogen-based acids started by Davy. Liebig's definition, while completely empirical, remained in use for almost 50 years until the adoption of the Arrhenius definition. The first modern definition of acids and bases in molecular terms
2340-1232: Is a strong acid. In liquid sulfur dioxide ( SO 2 ), thionyl compounds (supplying SO ) behave as acids, and sulfites (supplying SO 2− 3 ) behave as bases. The non-aqueous acid–base reactions in liquid ammonia are similar to the reactions in water: 2 NaNH 2 base + Zn ( NH 2 ) 2 amphiphilic amide ⟶ Na 2 [ Zn ( NH 2 ) 4 ] 2 NH 4 I acid + Zn ( NH 2 ) 2 ⟶ [ Zn ( NH 3 ) 4 ] I 2 {\displaystyle {\begin{aligned}{\underset {\text{base}}{{\ce {2 NaNH2}}}}+{\underset {{\text{amphiphilic}} \atop {\text{amide}}}{{\ce {Zn(NH2)2}}}}&\longrightarrow {\ce {Na2[Zn(NH2)4]}}\\[4pt]{\underset {\text{acid}}{{\ce {2 NH4I}}}}\ +\ {\ce {Zn(NH2)2}}&\longrightarrow {\ce {[Zn(NH3)4]I2}}\end{aligned}}} Nitric acid can be
2430-421: Is a typical Lewis acid, Lewis base reaction. All compounds of group 13 elements with a formula AX 3 can behave as Lewis acids. Similarly, compounds of group 15 elements with a formula DY 3 , such as amines , NR 3 , and phosphines , PR 3 , can behave as Lewis bases. Adducts between them have the formula X 3 A←DY 3 with a dative covalent bond , shown symbolically as ←, between
2520-435: Is an acid in both Lewis and Brønsted–Lowry classifications and shows that the theories agree with each other. Boric acid is recognised as a Lewis acid because of the reaction In this case the acid does not split up but the base, H 2 O, does. A solution of B(OH) 3 is acidic because hydrogen ions are given off in this reaction. There is strong evidence that dilute aqueous solutions of ammonia contain minute amounts of
2610-422: Is an acid–base reaction in both theories. One of the limitations of the Arrhenius definition is its reliance on water solutions. Edward Curtis Franklin studied the acid–base reactions in liquid ammonia in 1905 and pointed out the similarities to the water-based Arrhenius theory. Albert F.O. Germann , working with liquid phosgene , COCl 2 , formulated the solvent-based theory in 1925, thereby generalizing
2700-441: Is based upon the idea of protonation of bases through the deprotonation of acids – that is, the ability of acids to "donate" hydrogen ions ( H ) – otherwise known as protons – to bases, which "accept" them. An acid–base reaction is, thus, the removal of a hydrogen ion from the acid and its addition to the base. The removal of a hydrogen ion from an acid produces its conjugate base , which
2790-782: Is demonstrated in the image below: Here, one molecule of water acts as an acid, donating an H and forming the conjugate base, OH , and a second molecule of water acts as a base, accepting the H ion and forming the conjugate acid, H 3 O . As an example of water acting as an acid, consider an aqueous solution of pyridine , C 5 H 5 N . C 5 H 5 N + H 2 O ↽ − − ⇀ [ C 5 H 5 NH ] + + OH − {\displaystyle {\ce {C5H5N + H2O <=> [C5H5NH]+ + OH-}}} In this example,
Brønsted–Lowry acid–base theory - Misplaced Pages Continue
2880-586: Is destabilizing. However, relatively modest stabilizing effects can render them bound. For example, cyclopropyl and cubyl anions are bound due to increased s character of the lone pair orbital, while neopentyl and phenethyl anions are also bound, as a result of negative hyperconjugation of the lone pair with the β-substituent (n C → σ* C–C ). The same holds true for anions with benzylic and allylic stabilization. Gas-phase carbanions that are sp and sp hybridized are much more strongly stabilized and are often prepared directly by gas-phase deprotonation. In
2970-437: Is not to be confused with baking soda , which is sodium bicarbonate ( NaHCO 3 ). Baking powder is a mixture of baking soda (sodium bicarbonate) and acidic salts. The bubbles are created because, when the baking powder is combined with water, the sodium bicarbonate and acid salts react to produce gaseous carbon dioxide . Whether commercially or domestically prepared, the principles behind baking powder formulations remain
3060-525: Is removed from acetic acid, forming its conjugate base, the acetate ion, CH 3 COO . The addition of an H ion to an ammonia molecule of the solvent creates its conjugate acid, the ammonium ion, NH + 4 . The Brønsted–Lowry model calls hydrogen-containing substances (like HCl ) acids. Thus, some substances, which many chemists considered to be acids, such as SO 3 or BCl 3 , are excluded from this classification due to lack of hydrogen. Gilbert N. Lewis wrote in 1938, "To restrict
3150-399: Is stabilized by delocalization, has an inversion barrier of only 4 kcal/mol (17 kJ/mol), reflecting stabilization of the linear transition state by better π delocalization. Carbanions are typically nucleophilic and basic. The basicity and nucleophilicity of carbanions are determined by the substituents on carbon. These include Geometry also affects the orbital hybridization of
3240-1134: Is still used in modern geochemistry and electrochemistry of molten salts . This definition describes an acid as an oxide ion ( O ) acceptor and a base as an oxide ion donor. For example: (base) (acid) MgO + CO 2 ⟶ MgCO 3 CaO + SiO 2 ⟶ CaSiO 3 NO 3 − + S 2 O 7 2 − ⟶ NO 2 + + 2 SO 4 2 − {\displaystyle {\begin{array}{ccccl}_{\text{(base)}}&&_{\text{(acid)}}\\[4pt]{\ce {MgO}}&+&{\ce {CO2}}&\longrightarrow &{\ce {MgCO3}}\\[4pt]{\ce {CaO}}&+&{\ce {SiO2}}&\longrightarrow &{\ce {CaSiO3}}\\[4pt]{\ce {NO3-}}&+&{\ce {S2O7^2-}}\!\!&\longrightarrow &{\ce {NO2+ + 2 SO4^2-}}\end{array}}} Carbon acid Formally,
3330-399: Is the acid with a hydrogen ion removed. The reception of a proton by a base produces its conjugate acid , which is the base with a hydrogen ion added. Unlike the previous definitions, the Brønsted–Lowry definition does not refer to the formation of salt and solvent, but instead to the formation of conjugate acids and conjugate bases , produced by the transfer of a proton from the acid to
3420-427: Is the acid. One feature of the Brønsted–Lowry theory in contrast to Arrhenius theory is that it does not require an acid to dissociate. The essence of Brønsted–Lowry theory is that an acid is only such in relation to a base, and vice versa . Water is amphoteric as it can act as an acid or as a base. In the image shown at the right one molecule of H 2 O acts as a base and gains H to become H 3 O while
3510-471: Is very weak Brønsted acid with an estimated p K a of 49 which may undergo deprotonation in the presence of a superbase like the Lochmann–Schlosser base ( n -butyllithium and potassium t -butoxide ). As conjugate acid–base pairs, the factors that determine the relative stability of carbanions also determine the ordering of the p K a values of the corresponding carbon acids. Furthermore, p K
3600-826: The HCl produces the chloride ion, Cl , the conjugate base of the acid. The addition of H to the H 2 O (acting as a base) forms the hydronium ion, H 3 O , the conjugate acid of the base. Water is amphoteric – that is, it can act as both an acid and a base. The Brønsted–Lowry model explains this, showing the dissociation of water into low concentrations of hydronium and hydroxide ions: H 2 O + H 2 O ↽ − − ⇀ H 3 O + + OH − {\displaystyle {\ce {H2O + H2O <=> H3O+ + OH-}}} This equation
3690-427: The acetate ion ( CH 3 COO ). H 2 O is a base because it accepts a proton from CH 3 COOH and becomes its conjugate acid, the hydronium ion, ( H 3 O ). The reverse of an acid–base reaction is also an acid–base reaction, between the conjugate acid of the base in the first reaction and the conjugate base of the acid. In the above example, ethanoate is the base of the reverse reaction and hydronium ion
Brønsted–Lowry acid–base theory - Misplaced Pages Continue
3780-494: The acid dissociation constants of carbon-containing molecules. Because DMSO accepts protons more strongly than H 2 O the acid becomes stronger in this solvent than in water. Indeed, many molecules behave as acids in non-aqueous solutions but not in aqueous solutions. An extreme case occurs with carbon acids , where a proton is extracted from a C−H bond. Some non-aqueous solvents can behave as acids. An acidic solvent will make dissolved substances more basic. For example,
3870-545: The aldol reaction and Michael addition . With the molecular geometry for a carbanion described as a trigonal pyramid the question is whether or not carbanions can display chirality , because if the activation barrier for inversion of this geometry is too low any attempt at introducing chirality will end in racemization , similar to the nitrogen inversion . However, solid evidence exists that carbanions can indeed be chiral for example in research carried out with certain organolithium compounds. The first ever evidence for
3960-467: The ammonium ion and that, when dissolved in water, ammonia functions as a Lewis base. The reactions between oxides in the solid or liquid states are excluded in the Brønsted–Lowry theory. For example, the reaction is not covered in the Brønsted–Lowry definition of acids and bases. On the other hand, magnesium oxide acts as a base when it reacts with an aqueous solution of an acid. Dissolved silicon dioxide , SiO 2 , has been predicted to be
4050-428: The benzoin condensation as correctly proposed by Clarke and Arthur Lapworth in 1907. In 1904 Wilhelm Schlenk prepared [Ph 3 C] [NMe 4 ] in a quest for tetramethylammonium (from tetramethylammonium chloride and Ph 3 CNa ) and in 1914 he demonstrated how triarylmethyl radicals could be reduced to carbanions by alkali metals The phrase carbanion was introduced by Wallis and Adams in 1933 as
4140-521: The hydroxide anion. On the other hand, for cyclopentadiene , the numerical values are comparable: the p K a in water is 15, while the p K a in DMSO is 18. As indicated by the examples above, acidity increases (p K a decreases) when the negative charge is delocalized. This effect occurs when the substituents on the carbanion are unsaturated and/or electronegative. Although carbon acids are generally thought of as acids that are much weaker than "classical" Brønsted acids like acetic acid or phenol,
4230-2170: The Arrhenius definition to cover aprotic solvents. Germann pointed out that in many solutions, there are ions in equilibrium with the neutral solvent molecules: For example, water and ammonia undergo such dissociation into hydronium and hydroxide , and ammonium and amide , respectively: 2 H 2 O ↽ − − ⇀ H 3 O + + OH − 2 NH 3 ↽ − − ⇀ NH 4 + + NH 2 − {\displaystyle {\begin{aligned}{\ce {2 H2O}}&{\ce {\, <=> H3O+ + OH-}}\\[4pt]{\ce {2 NH3}}&{\ce {\, <=> NH4+ + NH2-}}\end{aligned}}} Some aprotic systems also undergo such dissociation, such as dinitrogen tetroxide into nitrosonium and nitrate , antimony trichloride into dichloroantimonium and tetrachloroantimonate, and phosgene into chlorocarboxonium and chloride : N 2 O 4 ↽ − − ⇀ NO + + NO 3 − 2 SbCl 3 ↽ − − ⇀ SbCl 2 + + SbCl 4 − COCl 2 ↽ − − ⇀ COCl + + Cl − {\displaystyle {\begin{aligned}{\ce {N2O4}}&{\ce {\, <=> NO+ + NO3-}}\\[4pt]{\ce {2 SbCl3}}&{\ce {\, <=> SbCl2+ + SbCl4-}}\\[4pt]{\ce {COCl2}}&{\ce {\, <=> COCl+ + Cl-}}\end{aligned}}} A solute that causes an increase in
4320-524: The Arrhenius model. The calculation of pH under the Arrhenius model depended on alkalis (bases) dissolving in water ( aqueous solution ). The Brønsted–Lowry model expanded what could be pH tested using insoluble and soluble solutions (gas, liquid, solid). The general formula for acid–base reactions according to the Brønsted–Lowry definition is: HA + B ⟶ BH + + A − {\displaystyle {\ce {HA + B -> BH+ + A-}}} where HA represents
4410-1007: The acid, B represents the base, BH represents the conjugate acid of B, and A represents the conjugate base of HA. For example, a Brønsted–Lowry model for the dissociation of hydrochloric acid (HCl) in aqueous solution would be the following: HCl acid + H 2 O base ↽ − − ⇀ H 3 O + conjugate acid + Cl − conjugate base {\displaystyle {\underset {\text{acid}}{{\ce {HCl_{\,}}}}}\ +\ {\underset {\text{base}}{{\ce {H2O}}}}\quad {\ce {<=>}}\quad {\underset {{\text{conjugate }} \atop {\text{acid }}}{{\ce {H3O+}}}}\ +{\underset {{\text{conjugate}} \atop {\text{base}}}{{\ce {Cl_{\,}-}}}}} The removal of H from
4500-498: The analogous diphenylmethanide anion ([Li(12-crown-4)] [CHPh 2 ] ), prepared form diphenylmethane (p K a in DMSO of CH 2 Ph 2 = 32.3), was also obtained. However, the attempted isolation of a complex of the benzyl anion PhCH − 2 from toluene (p K a in DMSO of CH 3 Ph ≈ 43) was unsuccessful, due to rapid reaction of the formed anion with the THF solvent. The free benzyl anion has also been generated in
4590-505: The atoms A (acceptor) and D (donor). Compounds of group 16 with a formula DX 2 may also act as Lewis bases; in this way, a compound like an ether , R 2 O , or a thioether , R 2 S , can act as a Lewis base. The Lewis definition is not limited to these examples. For instance, carbon monoxide acts as a Lewis base when it forms an adduct with boron trifluoride, of formula F 3 B←CO . Adducts involving metal ions are referred to as co-ordination compounds; each ligand donates
SECTION 50
#17327839917754680-447: The base. In this approach, acids and bases are fundamentally different in behavior from salts, which are seen as electrolytes, subject to the theories of Debye , Onsager , and others. An acid and a base react not to produce a salt and a solvent, but to form a new acid and a new base. The concept of neutralization is thus absent. Brønsted–Lowry acid–base behavior is formally independent of any solvent, making it more all-encompassing than
4770-440: The basicity of the carbanions in typical organic solvents. Values below less than 0 or greater than 35 are indirectly estimated; hence, the numerical accuracy of these values is limited. Aqueous p K a values are also commonly encountered in the literature, particularly in the context of biochemistry and enzymology. Moreover, aqueous values are often given in introductory organic chemistry textbooks for pedagogical reasons, although
4860-413: The carbanion: On heating the reaction to 0 °C the optical activity is lost. More evidence followed in the 1960s. A reaction of the cis isomer of 2-methylcyclopropyl bromide with s -butyllithium again followed by carboxylation with dry ice yielded cis -2-methylcyclopropylcarboxylic acid. The formation of the trans isomer would have indicated that the intermediate carbanion was unstable. In
4950-579: The carbanionic lone pair electrons in an orbital with significant s character is favorable, accounting for the pyramidalized and bent geometries of alkyl and alkenyl carbanions, respectively. Valence shell electron pair repulsion (VSEPR) theory makes similar predictions. This contrasts with carbocations, which have a preference for unoccupied nonbonding orbitals of pure atomic p character, leading to planar and linear geometries, respectively, for alkyl and alkenyl carbocations. However, delocalized carbanions may deviate from these geometries. Instead of residing in
5040-575: The charge-bearing carbanion. The greater the s-character of the charge-bearing atom, the more stable the anion. Carbanions, especially ones derived from weak carbon acids that do not benefit sufficiently from the two stabilizing factors listed above, are generally oxygen- and water-sensitive to varying degrees. While some merely degrade and decompose over several weeks or months upon exposure to air, others may react vigorously and exothermically with air almost immediately to spontaneously ignite ( pyrophoricity ). Among commonly encountered carbanionic reagents in
5130-435: The compound CH 3 COOH is known as acetic acid since it behaves as an acid in water. However, it behaves as a base in liquid hydrogen fluoride , a much more acidic solvent. In the same year that Brønsted and Lowry published their theory, G. N. Lewis created an alternative theory of acid–base reactions. The Lewis theory is based on electronic structure . A Lewis base is a compound that can give an electron pair to
5220-456: The concentration of the solvent ions. Under this definition, pure H 2 SO 4 and HCl dissolved in toluene are not acidic, and molten NaOH and solutions of calcium amide in liquid ammonia are not alkaline. This led to the development of the Brønsted–Lowry theory and subsequent Lewis theory to account for these non-aqueous exceptions. The reaction of an acid with a base is called a neutralization reaction. The products of this reaction are
5310-413: The concentration of the solvonium ions and a decrease in the concentration of solvate ions is defined as an acid . A solute that causes an increase in the concentration of the solvate ions and a decrease in the concentration of the solvonium ions is defined as a base . Thus, in liquid ammonia, KNH 2 (supplying NH − 2 ) is a strong base, and NH 4 NO 3 (supplying NH + 4 )
5400-428: The condensed phase only carbanions that are sufficiently stabilized by delocalization have been isolated as truly ionic species. In 1984, Olmstead and Power presented the lithium crown ether salt of the triphenylmethanide carbanion from triphenylmethane , n -butyllithium and 12-crown-4 (which forms a stable complex with lithium cations) at low temperatures: Adding n -butyllithium to triphenylmethane (p K
5490-537: The cumulative (additive) effect of several electron accepting substituents can lead to acids that are as strong or stronger than the inorganic mineral acids. For example, trinitromethane HC(NO 2 ) 3 , tricyanomethane HC(CN) 3 , pentacyanocyclopentadiene C 5 (CN) 5 H , and fulminic acid HCNO, are all strong acids with aqueous p K a values that indicate complete or nearly complete proton transfer to water. Triflidic acid , with three strongly electron-withdrawing triflyl groups, has an estimated p K
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#17327839917755580-428: The existence of chiral organolithium compounds was obtained in 1950. Reaction of chiral 2-iodooctane with s -butyllithium in petroleum ether at −70 °C followed by reaction with dry ice yielded mostly racemic 2-methylbutyric acid but also an amount of optically active 2-methyloctanoic acid, which could only have formed from likewise optically active 2-methylheptyllithium with the carbon atom linked to lithium
5670-474: The gas phase. For some time, it was not known whether simple alkyl anions could exist as free species; many theoretical studies predicted that even the methanide anion CH − 3 should be an unbound species (i.e., the electron affinity of •CH 3 was predicted to be negative). Such a species would decompose immediately by spontaneous ejection of an electron and would therefore be too fleeting to observe directly by mass spectrometry. However, in 1978,
5760-424: The group of acids to those substances that contain hydrogen interferes as seriously with the systematic understanding of chemistry as would the restriction of the term oxidizing agent to substances containing oxygen ." Furthermore, KOH and KNH 2 are not considered Brønsted bases, but rather salts containing the bases OH 2 and NH 2 . The hydrogen requirement of Arrhenius and Brønsted–Lowry
5850-477: The hydronium ion in water and the amide ion, NH − 2 in ammonia, to the hydroxide ion in water. Ammonium salts behave as acids, and metal amides behave as bases. Some non-aqueous solvents can behave as bases, i.e. accept protons, in relation to Brønsted–Lowry acids. where S stands for a solvent molecule. The most important of such solvents are dimethylsulfoxide , DMSO, and acetonitrile , CH 3 CN , as these solvents have been widely used to measure
5940-403: The issue of solvent dependence is often glossed over. In general, p K a values in water and organic solvent diverge significantly when the anion is capable of hydrogen bonding. For instance, in the case of water, the values differ dramatically: the p K a in water of water is 14.0, while the p K a in DMSO of water is 31.4, reflecting the differing ability of water and DMSO to stabilize
6030-473: The laboratory, ionic salts of hydrogen cyanide ( cyanides ) are unusual in being indefinitely stable under dry air and hydrolyzing only very slowly in the presence of moisture. Organometallic reagents like butyllithium (hexameric cluster, [BuLi] 6 ) or methylmagnesium bromide (ether complex, MeMg(Br)(OEt 2 ) 2 ) are often referred to as "carbanions," at least in a retrosynthetic sense. However, they are really clusters or complexes containing
6120-519: The lack of oxygen in hydrogen sulfide ( H 2 S ), hydrogen telluride ( H 2 Te ), and the hydrohalic acids . However, Davy failed to develop a new theory, concluding that "acidity does not depend upon any particular elementary substance, but upon peculiar arrangement of various substances". One notable modification of oxygen theory was provided by Jöns Jacob Berzelius , who stated that acids are oxides of nonmetals while bases are oxides of metals. In 1838, Justus von Liebig proposed that an acid
6210-425: The methanide anion was unambiguously synthesized by subjecting ketene to an electric discharge, and the electron affinity (EA) of •CH 3 was determined by photoelectron spectroscopy to be +1.8 kcal/mol, making it a bound species, but just barely so. The structure of CH − 3 was found to be pyramidal (C 3v ) with an H−C−H angle of 108° and inversion barrier of 1.3 kcal/mol, while •CH 3
6300-481: The negatively charged carbon, which, in most cases, reacts efficiently with a variety of electrophiles of varying strengths, including carbonyl groups , imines / iminium salts , halogenating reagents (e.g., N -bromosuccinimide and diiodine ), and proton donors . A carbanion is one of several reactive intermediates in organic chemistry . In organic synthesis, organolithium reagents and Grignard reagents are commonly treated and referred to as "carbanions." This
6390-453: The other acts as an acid and loses H to become OH . Another example is illustrated by substances like aluminium hydroxide , Al(OH) 3 . The hydrogen ion, or hydronium ion, is a Brønsted–Lowry acid when dissolved in H 2 O and the hydroxide ion is a base because of the autoionization of water reaction An analogous reaction occurs in liquid ammonia Thus, the ammonium ion, NH + 4 , in liquid ammonia corresponds to
6480-436: The other hand, solvent system theory has been criticized as being too general to be useful. Also, it has been thought that there is something intrinsically acidic about hydrogen compounds, a property not shared by non-hydrogenic solvonium salts. This acid–base theory was a revival of the oxygen theory of acids and bases proposed by German chemist Hermann Lux in 1939, further improved by Håkon Flood c. 1947 and
6570-491: The reaction of hydrochloric acid (HCl) with sodium hydroxide (NaOH) solutions produces a solution of sodium chloride (NaCl) and some additional water molecules. HCl ( aq ) + NaOH ( aq ) ⟶ NaCl ( aq ) + H 2 O {\displaystyle {\ce {HCl_{(aq)}{}+ NaOH_{(aq)}-> NaCl_{(aq)}{}+ H2O}}} The modifier ( aq ) in this equation
6660-737: The reaction produces carbon dioxide by the following stoichiometry : 14 NaHCO 3 + 5 Ca ( H 2 PO 4 ) 2 ⟶ 14 CO 2 + Ca 5 ( PO 4 ) 3 OH + 7 Na 2 HPO 4 + 13 H 2 O {\displaystyle {\ce {14 NaHCO3 + 5 Ca(H2PO4)2 -> 14 CO2 + Ca5(PO4)3OH + 7 Na2HPO4 + 13 H2O}}} A typical formulation (by weight) could call for 30% sodium bicarbonate, 5–12% monocalcium phosphate , and 21–26% sodium aluminium sulfate . Alternately,
6750-560: The reactions in aprotic solvents; for example, in liquid N 2 O 4 : AgNO 3 base + NOCl acid ⟶ N 2 O 4 solvent + AgCl salt {\displaystyle {\underset {\text{base}}{{\ce {AgNO3}}}}+{\underset {\text{acid}}{{\ce {NOCl_{\ }}}}}\longrightarrow {\underset {\text{solvent}}{{\ce {N2O4}}}}+{\underset {\text{salt}}{{\ce {AgCl_{\ }}}}}} Because
6840-429: The same manner the reaction of (+)-( S )- l -bromo- l -methyl-2,2-diphenylcyclopropane with n -butyllithium followed by quenching with methanol resulted in product with retention of configuration : Of recent date are chiral methyllithium compounds: The phosphate 1 contains a chiral group with a hydrogen and a deuterium substituent. The stannyl group is replaced by lithium to intermediate 2 which undergoes
6930-490: The same. The acid–base reaction can be generically represented as shown: NaHCO 3 + H + ⟶ Na + + CO 2 + H 2 O {\displaystyle {\ce {NaHCO3 + H+ -> Na+ + CO2 + H2O}}} The real reactions are more complicated because the acids are complicated. For example, starting with sodium bicarbonate and monocalcium phosphate ( Ca(H 2 PO 4 ) 2 ),
7020-476: The solution phase by pulse radiolysis of dibenzylmercury. Early in 1904 and 1917, Schlenk prepared two red-colored salts, formulated as [NMe 4 ] [CPh 3 ] and [NMe 4 ] [PhCH 2 ] , respectively, by metathesis of the corresponding organosodium reagent with tetramethylammonium chloride. Since tetramethylammonium cations cannot form a chemical bond to the carbanionic center, these species are believed to contain free carbanions. While
7110-491: The solvent system definition depends on the solute as well as on the solvent itself, a particular solute can be either an acid or a base depending on the choice of the solvent: HClO 4 is a strong acid in water, a weak acid in acetic acid, and a weak base in fluorosulfonic acid; this characteristic of the theory has been seen as both a strength and a weakness, because some substances (such as SO 3 and NH 3 ) have been seen to be acidic or basic on their own right. On
7200-426: The structure of the former was verified by X-ray crystallography almost a century later, the instability of the latter has so far precluded structural verification. The reaction of the putative " [NMe 4 ] [PhCH 2 ] " with water was reported to liberate toluene and tetramethylammonium hydroxide and provides indirect evidence for the claimed formulation. One tool for the detection of carbanions in solution
7290-428: The symbol H is interpreted as a shorthand for H 3 O , because it is now known that a bare proton does not exist as a free species in aqueous solution. This is the species which is measured by pH indicators to measure the acidity or basicity of a solution. The Arrhenius definitions of acidity and alkalinity are restricted to aqueous solutions and are not valid for most non-aqueous solutions, and refer to
7380-491: The systematic understanding of chemistry as would the restriction of the term oxidizing agent to substances containing oxygen ." In Lewis theory an acid, A, and a base, B, form an adduct , AB, where the electron pair forms a dative covalent bond between A and B. This is shown when the adduct H 3 N−BF 3 forms from ammonia and boron trifluoride , a reaction that cannot occur in water because boron trifluoride hydrolizes in water. The reaction above illustrates that BF 3
7470-401: The theory named after them. In the Brønsted–Lowry theory acids and bases are defined by the way they react with each other, generalising them. This is best illustrated by an equilibrium equation. With an acid, HA, the equation can be written symbolically as: The equilibrium sign, ⇌, is used because the reaction can occur in both forward and backward directions (is reversible). The acid, HA, is
7560-479: Was determined to be planar (D 3h point group). Simple primary, secondary and tertiary sp carbanions (e.g., ethanide CH 3 CH − 2 , isopropanide (CH 3 ) 2 CH , and t -butanide (CH 3 ) 3 C were subsequently determined to be unbound species (the EAs of CH 3 CH 2 • , (CH 3 ) 2 CH• , (CH 3 ) 3 C• are −6, −7.4, −3.6 kcal/mol, respectively) indicating that α substitution
7650-522: Was devised by Svante Arrhenius . A hydrogen theory of acids, it followed from his 1884 work with Friedrich Wilhelm Ostwald in establishing the presence of ions in aqueous solution and led to Arrhenius receiving the Nobel Prize in Chemistry in 1903. As defined by Arrhenius: This causes the protonation of water, or the creation of the hydronium ( H 3 O ) ion. Thus, in modern times,
7740-576: Was first proposed in 1754 by Guillaume-François Rouelle , who introduced the word " base " into chemistry to mean a substance which reacts with an acid to give it solid form (as a salt). Bases are mostly bitter in nature. The first scientific concept of acids and bases was provided by Lavoisier in around 1776. Since Lavoisier's knowledge of strong acids was mainly restricted to oxoacids , such as HNO 3 ( nitric acid ) and H 2 SO 4 ( sulfuric acid ), which tend to contain central atoms in high oxidation states surrounded by oxygen, and since he
7830-454: Was implied by Arrhenius, rather than included explicitly. It indicates that the substances are dissolved in water. Though all three substances, HCl, NaOH and NaCl are capable of existing as pure compounds, in aqueous solutions they are fully dissociated into the aquated ions H , Cl , Na and OH . Baking powder is used to cause the dough for breads and cakes to "rise" by creating millions of tiny carbon dioxide bubbles. Baking powder
7920-465: Was not aware of the true composition of the hydrohalic acids ( HF , HCl , HBr , and HI ), he defined acids in terms of their containing oxygen , which in fact he named from Greek words meaning "acid-former" (from Greek ὀξύς (oxys) 'acid, sharp' and γεινομαι (geinomai) 'engender'). The Lavoisier definition held for over 30 years, until the 1810 article and subsequent lectures by Sir Humphry Davy in which he proved
8010-487: Was provided by the French chemist Antoine Lavoisier , around 1776. It is important to think of the acid–base reaction models as theories that complement each other. For example, the current Lewis model has the broadest definition of what an acid and base are, with the Brønsted–Lowry theory being a subset of what acids and bases are, and the Arrhenius theory being the most restrictive. The concept of an acid–base reaction
8100-479: Was removed by the Lewis definition of acid–base reactions, devised by Gilbert N. Lewis in 1923, in the same year as Brønsted–Lowry, but it was not elaborated by him until 1938. Instead of defining acid–base reactions in terms of protons or other bonded substances, the Lewis definition defines a base (referred to as a Lewis base ) to be a compound that can donate an electron pair , and an acid (a Lewis acid ) to be
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