Lewis acids and bases - wikipedia Lewis acids and bases Acids and bases pH Reaction Titration Extraction Dissociation constant ( acid ) Acid strength Acidity function Buffer solutions Proton affinity Self - ionization of water Amphoterism Acid types Brønsted Lewis Mineral Organic Strong Superacids Weak Monobasic acid Dibasic acid Tribasic acid Solid Base types Brønsted Lewis Organic Strong Superbases Non-nucleophilic Weak Diagram of some Lewis bases and acids A Lewis acid is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any species that has a filled orbital containing an electron pair which is not involved in bonding but may form a dative bond with a Lewis acid to form a Lewis adduct. For example, NH is a Lewis base, because it can donate its lone pair of electrons. Trimethylborane ( Me B) is a Lewis acid as it is capable of accepting a lone pair. 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 NH and Me B, the lone pair from NH will form a dative bond with the empty orbital of Me B to form an adduct NH BMe. The terminology refers to the contributions of Gilbert N. Lewis. Contents 1 Depicting adducts 2 Examples 2.1 Simple Lewis acids 2.2 Complex Lewis acids 2.3 H as Lewis acid 2.4 Applications of Lewis acids 3 Lewis bases 3.1 Applications of Lewis bases 4 Hard and soft classification 5 ECW model 6 There is no single order of Lewis base strengths 7 History 7.1 Reformulation of Lewis theory 7.2 Comparison with Brønsted -- Lowry theory 8 See also 9 References 10 Further reading Depicting adducts 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, Me B ← NH. Some sources indicate the Lewis base with a pair of dots ( the explicit electrons being donated ), which allows consistent representation of the transition from the base itself to the complex with the acid : Me B + : NH → Me B : NH A center dot may also be used to represent a Lewis adduct, such as Me B NH. Another example is boron trifluoride diethyl etherate, BF Et O. The center dot is also used to represent hydrate coordination in various crystals, as in MgSO 7H O for hydrated magnesium sulfate. In general, however, the donor -- acceptor bond is viewed as simply somewhere along a continuum between idealized covalent bonding and ionic bonding. Examples Major structural changes accompany binding of the Lewis base to the coordinatively unsaturated, planar Lewis acid BF. Classically, the term `` Lewis acid '' is restricted to trigonal planar species with an empty p orbital, such as BR where R can be an organic substituent or a halide. For the purposes of discussion, even complex compounds such as Et Al Cl and AlCl are treated as trigonal planar Lewis acids. Metal ions such as Na, Mg, and Ce, which are invariably complexed with additional ligands, are often sources of coordinatively unsaturated derivatives that form Lewis adducts upon reaction with a Lewis base. Other reactions might simply be referred to as `` acid - catalyzed '' reactions. Some compounds, such as H O, 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 that react directly with the Lewis base. But more common are those that undergo a reaction prior to forming the adduct. Examples of Lewis acids based on the general definition of electron pair acceptor include : the proton ( H ) and acidic compounds onium ions, such as NH and H O high oxidation state transition metal cations, e.g., Fe ; other metal cations, such as Li and Mg, often as their aquo or ether complexes, trigonal planar species, such as BF and carbocations H C pentahalides of phosphorus, arsenic, and antimony electron poor π - systems, such as enones and tetracyanoethylenes. Again, the description of a Lewis acid is often used loosely. For example, in solution, bare protons do not exist. Simple Lewis acids Some of the most studied examples of such Lewis acids are the boron trihalides and organoboranes, but other compounds exhibit this behavior : BF + F → BF In this adduct, all four fluoride centres ( or more accurately, ligands ) are equivalent. BF + OMe → BF OMe Both BF and BF OMe are Lewis base adducts of boron trifluoride. In many cases, the adducts violate the octet rule, such as the triiodide anion : I + I → I The variability of the colors of iodine solutions reflects the variable abilities of the solvent to form adducts with the Lewis acid I. In some cases, the Lewis acids are capable of binding two Lewis bases, a famous example being the formation of hexafluorosilicate : SiF + 2 F → SiF Complex Lewis acids 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 widely viewed as Lewis acids. Aluminium trihalides, unlike the boron trihalides, do not exist in the form AlX, but as aggregates and polymers that must be degraded by the Lewis base. A simpler case is the formation of adducts of borane. Monomeric BH does not exist appreciably, so the adducts of borane are generated by degradation of diborane : B H + 2 H → 2 BH In this case, an intermediate B H can be isolated. Many metal complexes serve as Lewis acids, but usually only after dissociating a more weakly bound Lewis base, often water. ( Mg ( H O ) ) + 6 NH → ( Mg ( NH ) ) + 6 H O H as Lewis acid The proton ( H ) is one of the strongest but is also one of the most complicated Lewis acids. It is convention to ignore the fact that a proton is heavily solvated ( bound to solvent ). With this simplification in mind, acid - base reactions can be viewed as the formation of adducts : H + NH → NH H + OH → H O Applications of Lewis acids A typical example of a Lewis acid in action is in the Friedel -- Crafts alkylation reaction. The key step is the acceptance by AlCl of a chloride ion lone - pair, forming AlCl and creating the strongly acidic, that is, electrophilic, carbonium ion. RCl + AlCl → R + AlCl Lewis bases A Lewis base is an atomic or molecular species where the highest occupied molecular orbital ( HOMO ) is highly localized. Typical Lewis bases are conventional amines such as ammonia and alkyl amines. Other common Lewis bases include pyridine and its derivatives. Some of the main classes of Lewis bases are amines of the formula NH R where R = alkyl or aryl. Related to these are pyridine and its derivatives. phosphines of the formula PR A, where R = alkyl, A = aryl. compounds of O, S, Se and Te in oxidation state 2, including water, ethers, ketones The most common Lewis bases are anions. The strength of Lewis basicity correlates with the pK of the parent acid : acids with high pK ' s give good Lewis bases. As usual, a weaker acid has a stronger conjugate base. Examples of Lewis bases based on the general definition of electron pair donor include : simple anions, such as H and F. other lone - pair - containing species, such as H O, NH, HO, and CH complex anions, such as sulfate electron rich π - system Lewis bases, such as ethyne, ethene, and benzene The strength of Lewis bases have been evaluated for various Lewis acids, such as I, SbCl, and BF. Heats of binding of various bases to BF Lewis base Donor atom Enthalpy of complexation ( kJ / mol ) Et N 135 quinuclidine 150 pyridine 128 Acetonitrile 60 Et O O 78.8 THF O 90.4 acetone O 76.0 EtOAc O 75.5 DMA O 112 DMSO O 105 Tetrahydrothiophene 51.6 Trimethylphosphine 97.3 Applications of Lewis bases Main article : homogeneous catalysis Nearly all electron pair donors that form compounds by binding transition elements can be viewed as a collections of the Lewis bases -- or ligands. Thus a large application of Lewis bases is to modify the activity and selectivity of metal catalysts. Chiral Lewis bases thus confer chirality on a catalyst, enabling asymmetric catalysis, which is useful for the production of pharmaceuticals. Many Lewis bases are `` multidentate, '' that is they can form several bonds to the Lewis acid. These multidentate Lewis bases are called chelating agents. Hard and soft classification Main article : HSAB theory Lewis acids and bases are commonly classified according to their hardness or softness. In this context hard implies small and nonpolarizable and soft indicates larger atoms that are more polarizable. typical hard acids : H, alkali / alkaline earth metal cations, boranes, Zn typical soft acids : Ag, Mo ( 0 ), Ni ( 0 ), Pt typical hard bases : ammonia and amines, water, carboxylates, fluoride and chloride typical soft bases : organophosphines, thioethers, carbon monoxide, iodide For example, an amine will displace phosphine from the adduct with the acid BF. In the same way, bases could be classified. For example, bases donating a lone pair from an oxygen atom are harder than bases donating through a nitrogen atom. Although the classification was never quantified it proved to be very useful in predicting the strength of adduct formation, using the key concepts that hard acid -- hard base and soft acid -- soft base interactions are stronger than hard acid -- soft base or soft acid -- hard base interactions. Later investigation of the thermodynamics of the interaction suggested that hard -- hard interactions are enthalpy favored, whereas soft -- soft are entropy favored. ECW model Main article : ECW model The ECW model is quantitative model that describes and predicts the strength of Lewis acid base interactions, − ΔH. The model assigned E and C parameters to many Lewis acids and bases. Each acid is characterized by an E and a C. Each base is likewise characterized by its own E and C. The E and C parameters refer, respectively, to the electrostatic and covalent contributions to the strength of the bonds that the acid and base will form. The equation is − ΔH = E E + C C + W The W term represents a constant energy contribution for acid -- base reaction such as the cleavage of a dimeric acid or base. The equation predicts reversal of acids and base strengths. The graphical presentations of the equation show that there is no single order of Lewis base strengths or Lewis acid strengths. There is no single order of Lewis base strengths Cramer -- Bopp plots show graphically using the E and C parameters of the ECW model that there is no one single order of Lewis base strengths ( or acid strengths ). Single property or variable scales are limited to a small range of acids or bases. History MO diagram depicting the formation of a dative covalent bond between two atoms. The concept originated with Gilbert N. Lewis who studied chemical bonding. In 1923, Lewis wrote An acid substance is one which can employ an electron lone pair from another molecule in completing the stable group of one of its own atoms. The Brønsted -- Lowry acid -- base theory was published in the same year. The two theories are distinct but complementary. A Lewis base is also a Brønsted -- Lowry base, but a Lewis acid does n't need to be a Brønsted -- Lowry acid. The classification into hard and soft acids and bases ( HSAB theory ) followed in 1963. The strength of Lewis acid - base interactions, as measured by the standard enthalpy of formation of an adduct can be predicted by the Drago -- Wayland two - parameter equation. Reformulation of Lewis theory Lewis had suggested in 1916 that two atoms are held together in a chemical bond by sharing a pair of electrons. When each atom contributed one electron to the bond it was called a covalent bond. When both electrons come from one of the atoms it was called a dative covalent bond or coordinate bond. The distinction is not very clear - cut. For example, in the formation of an ammonium ion from ammonia and hydrogen the ammonia molecule donates a pair of electrons to the proton ; the identity of the electrons is lost in the ammonium ion that is formed. Nevertheless, Lewis suggested that an electron - pair donor be classified as a base and an electron - pair acceptor be classified as acid. A more modern definition of a Lewis acid is an atomic or molecular species with a localized empty atomic or molecular orbital of low energy. This lowest energy molecular orbital ( LUMO ) can accommodate a pair of electrons. Comparison with Brønsted -- Lowry theory A Lewis base is often a Brønsted -- Lowry base as it can donate a pair of electrons to H ; the proton is a Lewis acid as it can accept a pair of electrons. The conjugate base of a Brønsted -- Lowry acid is also a Lewis base as loss of H from the acid leaves those electrons which were used for the A -- H bond as a lone pair on the conjugate base. However, a Lewis base can be very difficult to protonate, yet still react with a Lewis acid. For example, carbon monoxide is a very weak Brønsted -- Lowry base but it forms a strong adduct with BF. In another comparison of Lewis and Brønsted -- Lowry acidity by Brown and Kanner, 2, 6 - di - t - butylpyridine reacts to form the hydrochloride salt with HCl but does not react with BF. This example demonstrates that steric factors, in addition to electron configuration factors, play a role in determining the strength of the interaction between the bulky di - t - butylpyridine and tiny proton. A Brønsted -- Lowry acid is a proton donor, not an electron - pair acceptor. See also Acid Base ( chemistry ) Acid -- base reaction Brønsted -- Lowry acid -- base theory Chiral Lewis acid Frustrated Lewis pairs Gutmann -- Beckett method ECW model References Jump up ^ IUPAC Gold Book − Lewis acid Jump up ^ Lewis, Gilbert ( 1923 ). Valence and the Structure of Atoms and Molecules. ^ Jump up to : March, J. `` Advanced Organic Chemistry '' 4th Ed. J. Wiley and Sons, 1992 : New York. ISBN 0 - 471 - 60180 - 2. Jump up ^ Greenwood, N.N. ; & Earnshaw, A. ( 1997 ). Chemistry of the Elements ( 2nd Edn. ), Oxford : Butterworth - Heinemann. ISBN 0 - 7506 - 3365 - 4. ^ Jump up to : Traditionally, but not precisely, H ions are referred as `` protons ''. See IUPAC, Compendium of Chemical Terminology, 2nd ed. ( the `` Gold Book '' ) ( 1997 ). Online corrected version : ( 2006 -- ) `` hydron ''. Jump up ^ Christian Laurence and Jean - François Gal `` Lewis Basicity and Affinity Scales : Data and Measurement '' Wiley, 2009. ISBN 978 - 0 - 470 - 74957 - 9. Jump up ^ Vogel G.C. ; Drago, R.S. ( 1996 ). `` The ECW Modeldoi = 10.1021 / ed073p701 ''. Journal of Chemical Education. 73 : 701 -- 707. Bibcode : 1996JChEd... 73... 701V. doi : 10.1021 / ed073p701. CS1 maint : Uses authors parameter ( link ) Jump up ^. Cramer, R.E., and Bopp, T.T. ( 1977 ) The Great E & C Plot. A graphical display of the enthalpies of adduct formation for Lewis acids and bases... Journal of Chemical Education 54 612 -- 613. Jump up ^ Lewis, G.N., Valence and the Structure of Atoms and Molecules ( 1923 ) p. 142. Jump up ^ Miessler, L.M., Tar, D.A., ( 1991 ) p. 166 -- Table of discoveries attributes the date of publication / release for the Lewis theory as 1923. Jump up ^ Brown HC and Kanner B. `` Preparation and Reactions of 2, 6 - Di - t - butylpyridine and Related hindered Bases. A case of Steric Hindrance toward the Proton. '' J. Am. Chem. Soc. 88, 986 ( 1966 ). Further reading Jensen, W.B. ( 1980 ). The Lewis acid - base concepts : an overview. New York : Wiley. ISBN 0 - 471 - 03902 - 0. Yamamoto, Hisashi ( 1999 ). Lewis acid reagents : a practical approach. New York : Oxford University Press. ISBN 0 - 19 - 850099 - 8. Retrieved from `` https://en.wikipedia.org/w/index.php?title=Lewis_acids_and_bases&oldid=843496473 '' Categories : Acid -- base chemistry Acids Bases ( chemistry ) Hidden categories : CS1 maint : Uses authors parameter All articles with unsourced statements Articles with unsourced statements from October 2014 Talk Contents About Wikipedia Български Català Čeština Dansk Deutsch Eesti Ελληνικά Español فارسی Français Hrvatski Italiano עברית Latviešu മലയാളം Nederlands Polski Português Română Русский Simple English Slovenščina Српски / srpski Srpskohrvatski / српскохрватски தமிழ் Türkçe Українська Tiếng Việt 中文 21 more Edit links This page was last edited on 29 May 2018, at 14 : 39. Text is available under the Creative Commons Attribution - ShareAlike License ; additional terms may apply. By using this site, you agree to the Terms of Use and Privacy Policy. Wikipedia ® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization. About Wikipedia

Question: define the terms lewis acid and lewis base
Answer: a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct