定义Definition
乙酸是结构简式为 CH₃COOH 的饱和一元羧酸，是食醋酸味的来源（食醋含乙酸约 3–5%）。纯乙酸在 16.6 °C 以下凝结成冰状晶体，称为冰醋酸。它是弱酸，也是重要的溶剂和化工原料。
Acetic acid, CH₃COOH, is a saturated monocarboxylic acid and the source of vinegar's sourness (vinegar is about 3–5% acetic acid). Pure acid freezes below 16.6 °C into ice-like crystals, hence glacial acetic acid. It is a weak acid and an important solvent and feedstock.
结构与性质Structure and properties
羧基中羰基与羟基共轭，电离后生成的乙酸根负电荷在两个氧原子上离域，因此乙酸的酸性远强于醇（pKa 4.76 对比乙醇 15.9）。乙酸在气相和非极性溶剂中通过两个氢键形成环状二聚体，所以测得的相对分子质量接近 120。
In the carboxyl group the carbonyl is conjugated with the hydroxyl, and in the acetate ion the negative charge is delocalised over both oxygens, so acetic acid is far more acidic than an alcohol (pKa 4.76 versus 15.9 for ethanol). In the gas phase and in nonpolar solvents it forms cyclic dimers held by two hydrogen bonds, so its apparent molar mass is close to 120.
历史History
醋是人类最早使用的酸，古代各文明都以酒发酵制醋。8 世纪贾比尔已描述过醋的蒸馏浓缩。1845 年科尔贝由无机原料（经二硫化碳、四氯化碳等）合成乙酸，是打破“生命力论”的重要实验之一。
Vinegar was the first acid people used, made by fermenting wine in many ancient cultures. In the eighth century Jabir described concentrating vinegar by distillation. In 1845 Kolbe synthesised acetic acid from inorganic starting materials via carbon disulfide and carbon tetrachloride, one of the key experiments against vitalism.
工业生产与用途Industrial production and uses
目前主要用甲醇羰基化法生产：CH₃OH + CO → CH₃COOH，1970 年代孟山都法用铑-碘催化剂，1996 年英国石油公司推出铱催化的卡蒂瓦法。乙酸用于生产乙酸乙烯酯（聚乙烯醇）、对苯二甲酸（PTA 溶剂）、乙酸酐和乙酸纤维素等。
Most acetic acid is now made by methanol carbonylation, CH₃OH + CO → CH₃COOH: the Monsanto process of the 1970s with a rhodium–iodide catalyst, and BP's iridium-based Cativa process of 1996. It goes into vinyl acetate (for polyvinyl alcohol), terephthalic acid (as solvent), acetic anhydride and cellulose acetate.
反应Reactions
乙酸具有酸的通性，并能发生酯化、还原和 α-卤代。
It shows the general properties of acids and undergoes esterification, reduction and α-halogenation.
与碳酸氢盐反应，说明酸性强于碳酸
Reaction with bicarbonate shows it is stronger than carbonic acid
酯化：酸脱羟基、醇脱氢
Esterification: the acid loses OH and the alcohol loses H
用醋除水垢
Removing limescale with vinegar
化学性质Chemical properties
弱酸，水溶液能使石蕊变红，与活泼金属、碱、碱性氧化物和碳酸盐反应。
A weak acid: it turns litmus red and reacts with active metals, bases, basic oxides and carbonates.
被 LiAlH₄ 还原为乙醇；与 PCl₃ 或 SOCl₂ 反应生成乙酰氯。
LiAlH₄ reduces it to ethanol; PCl₃ or SOCl₂ converts it to acetyl chloride.
浓乙酸有腐蚀性，能灼伤皮肤。
Concentrated acetic acid is corrosive and burns skin.
年表Timeline
- 约 8 世纪
贾比尔描述蒸馏浓缩醋
Jabir describes concentrating vinegar by distillation
- 1845
科尔贝由无机物合成乙酸
Kolbe synthesises acetic acid from inorganic materials
- 1970
孟山都甲醇羰基化工艺投产
Monsanto's methanol carbonylation process starts up
- 1996
卡蒂瓦法（铱催化）工业化
The iridium-catalysed Cativa process is commercialised
相关公式Formulas
例题Worked example
0.10 mol/L 乙酸（Ka = 1.8×10⁻⁵）：[H⁺] = 1.34×10⁻³ mol/L，pH = 2.87，电离度 1.3%。
0.10 M acetic acid: pH = 2.87, 1.3 % ionised.