Research: Science and Education Using Hydrocarbon Acidities To Demonstrate Principles W of Organic Structure and Bonding Andrew P. Dicks Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada; [email protected] A recent article in this Journal (1) discusses the proper- Hybridization Effects ties of acidic substances not containing a formal carboxylic acid functionality. The authors elaborate on a number of car- Acidities of alkanes, alkenes, and alkynes are discussed bon-based acids, such as Meldrum’s acid and dimedone. in most introductory organic chemistry textbooks (4). The 3 Other compounds that could be considered in this context pKa of ethyne (acetylene) is well established (25.5 in DMSO ; are hydrocarbons (substances containing only hydrogen and ref 5). Acidities of ethane (pKa = 49–62; ref 6) and ethene carbon). Conant and Wheland undertook the first “study of (ethylene, pKa = 42–45; ref 7) are less well known owing to extremely weak acids” in 1932 and by colorimetric methods experimental difficulty in accurately measuring pKa values of were able to arrange 10 hydrocarbons in a scale of increasing such weakly acidic compounds.4 acid strength (2). McEwen (3) extended this work by quan- Hydrocarbon acidity increases as the percentage s char- tifying the acidity of these compounds (and many others) acter in hybrid orbitals of carbon increases; that is, a corre- with respect to the acid dissociation constant of methanol. lation exists between hybridization and acidity as shown in The most acidic hydrocarbon noted by these researchers was Table 1. Walsh (8) has suggested that carbon electronegativ- 1 9-phenylfluorene (Figure 1) with a pKa = 21(3). ity towards hydrogen is greatest for an acetylenic carbon atom. 2 Ϫ The pKa values of many structurally diverse hydrocar- A C H bond in ethyne therefore has a comparatively high bons have been measured during the last 70 years. A striking bond dipole of sign Cδ−ϪHδ+. The ability of an sp hybrid- feature of the collected data is the enormous acidity range ized carbon to attract electrons is related to conjugate base covered. Hydrocarbon acidities encompass an astonishing 50 stabilization. The relative base stability order formed by pro- orders of magnitude (minimum) and thus span the majority ton loss from each hydrocarbon is shown in Figure 2. of known acid behavior. Examination of such wide-ranging The acetylide anion formed by deprotonation of ethyne acidities facilitates a classroom discussion concerning funda- is more stable than corresponding anions derived from ethene mental principles of organic structure and bonding. This re- and ethane. An sp hybrid orbital (present in the acetylide view article highlights the acidity of different types of anion) possesses 50% s character and 50% p character. As hydrocarbons and rationalizes observed trends. Emphasis is an s orbital is lower in energy and closer to the carbon nucleus placed upon undergraduate concepts of hybridization, reso- than a p orbital, the negative charge developed on hydrocar- nance, induction, and aromaticity. bon deprotonation is stabilized most for the acetylide anion. This effect leads to increased dissociation of ethyne compared to ethene and ethane and thus increased acidity. A sufficiently H Ph strong base such as an amide ion can deprotonate ethyne (Scheme I). The equilibrium constant for this equilibrium is 3.16 × 1010 meaning that ethyne is essentially completely deproto- nated (> 99.9%) under these conditions. This behavior is ex- Figure 1. 9-Phenylfluorene. The most acidic proton is highlighted tended to any terminal alkyne (one possessing an acetylenic by a bold H. hydrogen, RϪCϵCϪH). Anions derived from terminal alkynes are useful in constructing carbon chains, as they act as excellent nucleophiles in SN2 reactions (9) (Scheme II). H H An sp3 hybrid orbital (25% s character) cannot stabilize a ؊ ؊ ؊ H CC > CC > H3CC negative charge developed in the resulting anion as effectively H H H and alkanes are observed to be much less acidic than alkynes. A linear relationship between acidities of 10 non- decreasing stability conjugated hydrocarbons and hybrid orbital composition has Figure 2. Relative order of conjugate base stability of two-carbon been reported (10). In this study hybridization parameters hydrocarbons. were calculated by the scaled maximum overlap (SMO) method.5 This approach defines hybrid orbitals in the form spn, where n often represents a noninteger hybridization in- dex exponent (second-order hybridization). A general wave Table 1. pKa and Hybridization of Two-Carbon function for an spn hybrid (n being an integer or not) is given Hydrocarbons by the expression Mnolecule Cparbon Hybridizatio Ka Epthane s 3 49–62 1 1 ψ =+22s n 2 p ()n + 1 − 2 Epthene s 2 42–45 () Epthyne s525. Even for a simple molecule such as ethyne the use of integer 1322 Journal of Chemical Education • Vol. 80 No. 11 November 2003 • JChemEd.chem.wisc.edu Research: Science and Education ؉ ؊ ؊ ؉ H CCH NH2 H CC NH3 hybrids to describe covalent bonds has no quantitative basis. acetylide There is no reason to suppose the “best” hybrid orbital to pK = 25.5 pK = 36 a anion a form a CϪH bond in ethyne has an identical s:p ratio as present in the triple bond. According to the SMO method Scheme I. Formation of an acetylide anion. an acetylenic carbon forms a CϪH bond with a hybrid or- bital described as sp1.29, corresponding to 43.6% s character. When hybrid orbitals are idealized (sp, sp2, or sp3; first-or- ؊ der hybridization) the correlation with hydrocarbon acidity ' R CC ؉ R' CH XCHRR C C 2 2 is much poorer (Figure 3). R' = H, 1° alkyl Highly strained cycloalkanes possess CϪH bonds con- or alkenyl new C–C bond taining an unusually high degree of s character. Carbon or- X = Cl, Br, I bital hybridization used for CϪH bonding in cyclopropane 2.36 3 Scheme II. Carbon–carbon bond formation. is calculated as sp rather than sp (11). The hybridization index for an spn hybrid orbital is related to the NMR 13CϪH coupling constant (J13C-H) by (12): ؊؉ 1 1 Li 500 n-BuLi J 13 ()Hz = H C-H n + 1 H H 2 2 Cycloalkanes forming CϪH bonds with hybrid orbitals of 2,7 tricyclo[4.1.0.0. ]- greater than 25% s character (J13C-H > 125 Hz) exhibit en- heptane D2O hanced acidity. The strained tricyclic hydrocarbon 2,7 1 tricyclo[4.1.0.0. ]heptane (Scheme III) should theoretically be more acidic than a saturated alkane such as n-heptane. D Ϫ J13C-H for the C1 H bond has been measured as 200 Hz, H while J13 for the C2ϪH bond is 146 Hz (13). This corre- 2 C-H sponds to a hybrid orbital of composition sp1.50 involved in the C1ϪH bond (40% s character) and a hybrid orbital of 2,7 Scheme III. Acidity of tricyclo[4.1.0.0. ]heptane composition sp2.42 involved in the C2ϪH bond (29% s char- acter). Increased acidity of tricyclo[4.1.0.0.2,7]heptane is veri- fied by salt formation on treatment with n-butyllithium, 50 followed by deuteration on addition of D2O (Scheme III). Streitwieser has noted a correlation between J13C-H and kinetic acidities of cycloalkanes (14). Rates of hydrogen–tri- tium (T) isotopic exchange were measured in N-tritiated cy- 40 clohexylamine catalyzed by cesium cyclohexylamide (Table a 2). The dominant factor determining cycloalkane acidity is K p the quantity of s character in the CϪH bond hybrid orbital. ethyne 30 Cycloheptane and cyclooctane are slightly less acidic than second-order cyclohexane (smaller s character). More highly strained rings first-order appear more acidic than cyclohexane (larger J13C-H, greater s character). 20 20 30 40 50 Resonance and Inductive Effects Percent s Character The preceding section highlights factors increasing an- Figure 3. Plot of hybrid orbital % s character versus measured acidity ionic stabilization that promote increased hydrocarbon acid- for first-order hybridization and second-order hybridization (10a). ity. Another stabilizing effect is resonance delocalization of charge. This effect is illustrated by comparing the acidities of cyclohexane (pKa = 52; ref 15) and cyclohexene (pKa = Table 2. Relationship between the Rates of H–T Isotopic 46; ref 16 ). Deprotonation at an allylic position in cyclo- Exchange and 13C–H Coupling Constantsa hexene forms a resonance stabilized anion (Figure 4). For- b He y d r o c a r b o n E x c h a n g e R a t J13C-H /Hz mation of this conjugate base is energetically more favorable 4 than generation of the corresponding anion from cyclohex- C0yclopropane 7.0 x 1 161 ane, hence cyclohexene exhibits greater acidity. C8yclobutane 2413 Similarly, formation of the resonance stabilized benzyl C7yclopentane 58. 12 anion rationalizes the greater acidity of toluene (pKa = 42; C1yclohexane 123, 12 4ref 17) compared to methane (pKa ≈ 55; ref 18). Toluene is C6ycloheptane 03.7 12 at least 13 orders of magnitude more acidic than methane. C4yclooctane 02.6 12 Four canonical forms can be drawn for the benzyl anion (Fig- aData from ref 14. bThe exchange rate is relative to cyclohexane. ure 5), where the negative charge is delocalized to ortho and JChemEd.chem.wisc.edu • Vol. 80 No. 11 November 2003 • Journal of Chemical Education 1323 Research: Science and Education para ring positions. A greater degree of charge dispersion in ؊ this anion than the cyclohexenyl anion leads to toluene ap- 4 pearing more acidic than cyclohexene by a factor of 10 . ؊ Implicit in construction of such canonical forms is co- planarity of the ring with an sp2 hybridized carbon, as maxi- mal p orbital overlap will ensue. Charge delocalization lowers Figure 4.