pKa 酸度表pKa table — acidity of common acids in water and DMSO
pKa = −lg Ka，衡量一个质子给出的难易：pKa 越小，酸越强，其共轭碱越弱。pKa 每差 1，酸性相差 10 倍。判断酸碱反应方向的经验规则：平衡偏向 pKa 较大的那个酸一侧，即较强的酸把质子交给较强的碱。水中可测的范围大约是 −2 到 16，超出部分（强酸与碳氢酸）靠外推或在 DMSO 等溶剂中测定。
pKa = −log Ka measures how readily a proton is given up: the lower the pKa, the stronger the acid and the weaker its conjugate base. Each unit is a factor of ten. A working rule for acid–base reactions: the equilibrium lies on the side of the acid with the higher pKa, so the stronger acid gives its proton to the stronger base. Water allows direct measurement only from about −2 to 16; values outside that range (strong acids, C–H acids) are extrapolated or measured in solvents such as DMSO.
酸性强弱的规律What makes an acid strong
- 电负性：同周期越靠右，X–H 越酸（CH₄ ≈ 50 < NH₃ ≈ 38 < H₂O 14 < HF 3.2）Electronegativity: across a period X–H acidity rises (CH₄ ≈ 50 < NH₃ ≈ 38 < H₂O 14 < HF 3.2)
- 原子半径：同族越往下 X–H 键越弱、越酸（HF 3.2 < HCl −7；H₂O 14 < H₂S 7）Atom size: down a group the X–H bond weakens and acidity rises (HF 3.2 < HCl −7; H₂O 14 < H₂S 7)
- 共振：负电荷能离域的共轭碱更稳定（乙醇 16 → 苯酚 10 → 乙酸 4.8）Resonance: delocalising the negative charge stabilises the base (ethanol 16 → phenol 10 → acetic acid 4.8)
- 诱导效应：吸电子基使酸性增强，越近越强（乙酸 4.76 → 氯乙酸 2.87 → 三氯乙酸 0.66）Induction: electron-withdrawing groups raise acidity, more so when closer (acetic 4.76 → chloroacetic 2.87 → trichloroacetic 0.66)
- 杂化：s 成分越多，碳负离子越稳定（乙烷 ≈ 50 > 乙烯 ≈ 44 > 乙炔 25）Hybridisation: more s character stabilises the carbanion (ethane ≈ 50 > ethylene ≈ 44 > acetylene 25)
- 芳香性：去质子后形成芳香体系的酸特别强（环戊二烯 ≈ 16）Aromaticity: acids whose conjugate base is aromatic are unusually strong (cyclopentadiene ≈ 16)
酸度阶梯Acidity ladder
点击任意一行跳到下方对应分类。中间的色带是水中能直接测量的范围。Click a row to jump to its group below. The shaded band is the range measurable directly in water.
查表Look-up table
结构图中高亮的氢就是给出的质子；“≈”表示外推或估计值。The highlighted H is the proton that is lost; “≈” marks extrapolated or estimated values.
无机酸 Inorganic acids
| 结构 Structure | 化合物 Compound | pKa (H₂O) | pKa (DMSO) | 说明 Notes |
|---|---|---|---|---|
| 氢碘酸Hydroiodic acidHI | ≈ −10 | — | ||
| 高氯酸Perchloric acidHClO₄ | ≈ −10 | — | ||
| 氢溴酸Hydrobromic acidHBr | ≈ −9 | — | ||
| 盐酸Hydrochloric acidHCl | ≈ −7 | 1.8 | 文献估计值在 −5.9 到 −8 之间literature estimates range from −5.9 to −8 | |
| 硫酸（第一级）Sulfuric acid (1st)H₂SO₄ | ≈ −3 | — | ||
| 水合氢离子Hydronium ionH₃O⁺ | −1.74 | — | 按热力学惯例也常取 00 under the thermodynamic convention | |
| 硝酸Nitric acidHNO₃ | −1.4 | — | ||
| 亚硫酸（第一级）Sulfurous acid (1st)H₂SO₃ | 1.85 | — | 第二级 7.22nd: 7.2 | |
| 硫酸氢根Hydrogen sulfateHSO₄⁻ | 1.99 | — | ||
| 磷酸（第一级）Phosphoric acid (1st)H₃PO₄ | 2.15 | — | 第二级 7.20，第三级 12.352nd 7.20, 3rd 12.35 | |
| 氢氟酸Hydrofluoric acidHF | 3.17 | 15 | H–F 键很强，所以是弱酸weak because the H–F bond is very strong | |
| 亚硝酸Nitrous acidHNO₂ | 3.25 | — | ||
| 碳酸（表观）Carbonic acid (apparent)H₂CO₃ | 6.35 | — | 按溶解的 CO₂ 总量计；第二级 10.33based on total dissolved CO₂; 2nd 10.33 | |
| 氢硫酸Hydrogen sulfideH₂S | 7 | — | 第二级很弱，文献值在 12 到 19 之间不一2nd is very weak; reported 12–19 | |
| 磷酸二氢根Dihydrogen phosphateH₂PO₄⁻ | 7.2 | — | 磷酸盐缓冲液的基础basis of phosphate buffers | |
| 次氯酸Hypochlorous acidHClO | 7.53 | — | ||
| 氢氰酸Hydrocyanic acidHCN | 9.21 | 12.9 | ||
| 硼酸Boric acidB(OH)₃ | 9.24 | — | 路易斯酸：接受 OH⁻ 而不是给出 H⁺a Lewis acid: it accepts OH⁻ rather than releasing H⁺ | |
| 铵离子Ammonium ionNH₄⁺ | 9.25 | 10.5 | ||
| 碳酸氢根Hydrogen carbonateHCO₃⁻ | 10.33 | — | ||
| 过氧化氢Hydrogen peroxideH₂O₂ | 11.6 | — | ||
| 磷酸氢根Hydrogen phosphateHPO₄²⁻ | 12.35 | — | ||
| 水WaterH₂O | 14 | 31.4 | 有机化学书中常见 15.7（把水的浓度 55.5 M 计入）organic texts often give 15.7 (including [H₂O] = 55.5 M) | |
| 氨AmmoniaNH₃ | ≈ 38 | 41 | NaNH₂ 能使端炔去质子why NaNH₂ deprotonates terminal alkynes |
羧酸 Carboxylic acids
| 结构 Structure | 化合物 Compound | pKa (H₂O) | pKa (DMSO) | 说明 Notes |
|---|---|---|---|---|
| 三氟乙酸Trifluoroacetic acidCF₃COOH | 0.23 | 3.45 | ||
| 三氯乙酸Trichloroacetic acidCCl₃COOH | 0.66 | — | ||
| 草酸（第一级）Oxalic acid (1st)(COOH)₂ | 1.25 | — | 第二级 4.272nd: 4.27 | |
| 二氯乙酸Dichloroacetic acidCHCl₂COOH | 1.35 | — | ||
| 氯乙酸Chloroacetic acidCH₂ClCOOH | 2.87 | — | ||
| 水杨酸Salicylic acido-HOC₆H₄COOH | 2.97 | — | 邻位 OH 与羧酸根形成分子内氢键，稳定共轭碱the ortho OH hydrogen-bonds to the carboxylate | |
| 柠檬酸（第一级）Citric acid (1st)citric | 3.13 | — | 第二级 4.76，第三级 6.402nd 4.76, 3rd 6.40 | |
| 对硝基苯甲酸4-Nitrobenzoic acidp-O₂NC₆H₄COOH | 3.44 | — | ||
| 甲酸Formic acidHCOOH | 3.75 | — | ||
| 乳酸Lactic acidCH₃CH(OH)COOH | 3.86 | — | ||
| 苯甲酸Benzoic acidC₆H₅COOH | 4.2 | 11.1 | 哈米特方程的参比酸the reference acid of the Hammett equation | |
| 乙酸Acetic acidCH₃COOH | 4.76 | 12.3 | ||
| 丙酸Propanoic acidC₂H₅COOH | 4.87 | — |
醇、酚、硫醇 Alcohols, phenols & thiols
| 结构 Structure | 化合物 Compound | pKa (H₂O) | pKa (DMSO) | 说明 Notes |
|---|---|---|---|---|
| 苦味酸Picric acid2,4,6-(O₂N)₃C₆H₂OH | 0.42 | — | 三个硝基共振吸电子，酸性接近强酸three nitro groups make it nearly a strong acid | |
| 苯硫酚ThiophenolPhSH | 6.6 | 10.3 | ||
| 对硝基苯酚4-Nitrophenolp-O₂NC₆H₄OH | 7.15 | 10.8 | ||
| 苯酚PhenolPhOH | 9.99 | 18 | 能溶于 NaOH 而不溶于 NaHCO₃dissolves in NaOH but not NaHCO₃ | |
| 对甲苯酚4-Methylphenolp-cresol | 10.26 | — | ||
| 乙硫醇EthanethiolEtSH | 10.6 | — | ||
| 三氟乙醇2,2,2-TrifluoroethanolCF₃CH₂OH | 12.4 | 23.5 | ||
| 甲醇MethanolMeOH | 15.5 | 29 | ||
| 乙醇EthanolEtOH | 15.9 | 29.8 | ||
| 叔丁醇tert-Butanolt-BuOH | ≈ 17 | 32.2 | 溶剂化受位阻影响，水中酸性弱于乙醇hindered solvation makes it weaker than ethanol in water |
C–H 酸 C–H acids
| 结构 Structure | 化合物 Compound | pKa (H₂O) | pKa (DMSO) | 说明 Notes |
|---|---|---|---|---|
| 乙酰丙酮AcetylacetoneCH₃COCH₂COCH₃ | 8.99 | 13.3 | 1,3-二羰基，负电荷分散到两个氧上1,3-dicarbonyl: charge spread over two oxygens | |
| 硝基甲烷NitromethaneCH₃NO₂ | 10.2 | 17.2 | ||
| 乙酰乙酸乙酯Ethyl acetoacetateCH₃COCH₂COOEt | 10.7 | 14.2 | ||
| 丙二腈MalononitrileCH₂(CN)₂ | 11.2 | 11.1 | ||
| 丙二酸二乙酯Diethyl malonateCH₂(COOEt)₂ | 13.3 | 16.4 | 丙二酸酯合成中用 NaOEt 即可去质子NaOEt suffices in the malonic ester synthesis | |
| 环戊二烯CyclopentadieneC₅H₆ | 16 | 18 | 共轭碱是 6π 芳香的环戊二烯负离子the conjugate base is the aromatic 6π cyclopentadienide | |
| 乙醛AcetaldehydeCH₃CHO | 16.7 | — | ||
| 丙酮AcetoneCH₃COCH₃ | 19.3 | 26.5 | 烯醇化需要 LDA 等强碱才能完全进行complete enolate formation needs a strong base such as LDA | |
| 乙酸乙酯（α-H）Ethyl acetate (α-H)CH₃COOEt | ≈ 25 | — | ||
| 乙腈AcetonitrileCH₃CN | ≈ 25 | 31.3 | ||
| 乙炔AcetyleneHC≡CH | ≈ 25 | — | sp 杂化，50% s 成分sp carbon, 50% s character | |
| 三苯甲烷TriphenylmethanePh₃CH | ≈ 31.5 | 30.6 | ||
| 二甲亚砜Dimethyl sulfoxideDMSO | — | 35.1 | DMSO 标度的上限the upper limit of the DMSO scale | |
| 甲苯（苄位 H）Toluene (benzylic H)PhCH₃ | ≈ 41 | 43 | ||
| 苯BenzeneC₆H₆ | ≈ 43 | — | ||
| 乙烯EthyleneCH₂=CH₂ | ≈ 44 | — | ||
| 甲烷MethaneCH₄ | ≈ 50 | — | 烷烃的估计值在 48–53 之间alkane estimates range 48–53 |
N–H 酸 N–H acids
| 结构 Structure | 化合物 Compound | pKa (H₂O) | pKa (DMSO) | 说明 Notes |
|---|---|---|---|---|
| 邻苯二甲酰亚胺Phthalimidephthalimide | 8.3 | 13.4 | 加布里埃尔合成中用 KOH 去质子deprotonated by KOH in the Gabriel synthesis | |
| 丁二酰亚胺Succinimidesuccinimide | 9.6 | 14.7 | ||
| 乙酰胺AcetamideCH₃CONH₂ | 15.1 | 25.5 | ||
| 吡咯PyrroleC₄H₄NH | 17.5 | 23 | 氮孤对参与芳香 6π，所以吡咯碱性很弱而 N–H 偏酸the N lone pair is part of the aromatic sextet, so pyrrole is barely basic and its N–H is fairly acidic | |
| 二异丙胺Diisopropylaminei-Pr₂NH | ≈ 36 | — | 其锂盐即 LDAits lithium salt is LDA |
碱的共轭酸（pKaH） Conjugate acids of bases (pKaH)
| 结构 Structure | 化合物 Compound | pKa (H₂O) | pKa (DMSO) | 说明 Notes |
|---|---|---|---|---|
| 质子化丙酮Protonated acetone(CH₃)₂C=OH⁺ | ≈ −7.2 | — | 羰基氧是很弱的碱carbonyl oxygen is a very weak base | |
| 质子化乙酸Protonated acetic acidCH₃C(OH)₂⁺ | ≈ −6 | — | ||
| 质子化乙醇Protonated ethanolEtOH₂⁺ | −2.4 | — | ||
| 对硝基苯胺鎓4-Nitroaniliniump-O₂NC₆H₄NH₃⁺ | 1 | — | ||
| 苯胺鎓AniliniumPhNH₃⁺ | 4.6 | — | 苯胺氮孤对与苯环共轭，碱性比烷基胺弱约一百万倍conjugation with the ring makes aniline ~10⁶ times less basic than alkylamines | |
| 吡啶鎓PyridiniumC₅H₅NH⁺ | 5.25 | 3.4 | sp² 孤对不参与芳香体系the sp² lone pair is not part of the aromatic sextet | |
| 咪唑鎓ImidazoliumC₃H₄N₂H⁺ | 6.95 | — | 接近生理 pH，组氨酸侧链的作用基础near physiological pH — the basis of histidine's role | |
| 吗啉鎓Morpholiniummorpholine·H⁺ | 8.4 | — | ||
| DMAP 共轭酸DMAP conjugate acidDMAP·H⁺ | 9.6 | — | 二甲氨基共振给电子，碱性远强于吡啶the NMe₂ group donates by resonance, far more basic than pyridine | |
| 甲铵MethylammoniumCH₃NH₃⁺ | 10.64 | — | ||
| 三乙铵TriethylammoniumEt₃NH⁺ | 10.75 | 9 | ||
| 二乙铵DiethylammoniumEt₂NH₂⁺ | 10.98 | — | ||
| 胍鎓GuanidiniumC(NH₂)₃⁺ | 13.6 | — | 正电荷在三个氮上共振离域，是很强的中性有机碱the charge is delocalised over three nitrogens — a very strong neutral base |
氨基酸 Amino acids
| 结构 Structure | 化合物 Compound | pKa (H₂O) | pKa (DMSO) | 说明 Notes |
|---|---|---|---|---|
| 甘氨酸 α-COOHGlycine α-COOHGly | 2.34 | — | ||
| 天冬氨酸侧链Aspartate side chainAsp | 3.65 | — | ||
| 谷氨酸侧链Glutamate side chainGlu | 4.25 | — | ||
| 组氨酸侧链Histidine side chainHis | 6 | — | 唯一 pKa 接近 7 的侧链，常作酶的酸碱催化基团the only side chain near pH 7; a common acid–base catalyst in enzymes | |
| 半胱氨酸侧链Cysteine side chainCys | 8.18 | — | ||
| 甘氨酸 α-NH₃⁺Glycine α-NH₃⁺Gly | 9.6 | — | ||
| 酪氨酸侧链Tyrosine side chainTyr | 10.07 | — | ||
| 赖氨酸侧链Lysine side chainLys | 10.53 | — | ||
| 精氨酸侧链Arginine side chainArg | 12.48 | — | 胍基，生理条件下始终带正电a guanidinium group, always protonated at physiological pH |
DMSO 中阴离子不能被氢键溶剂化，所以多数酸在 DMSO 中的 pKa 比在水中大几个单位（例如乙酸 4.76 → 12.3）；带正电的酸（铵离子）变化小得多。比较不同化合物时，务必使用同一种溶剂的数据。
In DMSO anions are not stabilised by hydrogen bonding, so most acids have pKa values several units higher than in water (acetic acid 4.76 → 12.3); cationic acids such as ammonium ions change much less. Compare compounds only with values from the same solvent.
数据为 25 °C 下的常用文献值（DMSO 为 Bordwell 标度）；不同来源可能相差零点几个单位，强酸与碳氢酸的数值仅为量级估计。Common literature values at 25 °C (DMSO on the Bordwell scale); sources differ by a few tenths, and values for very strong acids and hydrocarbons are order-of-magnitude estimates.