Acid anhydride - Sanmusuibutsu

[ I ] Also known as anhydrous acid. A compound with a chemical formula in which _H2O has been removed from the chemical formula of an acid. It reacts with water to produce an acid. This term is confusingly similar to acid without water, so it is best not to use it.
(1) In the case of inorganic acids, there are anhydrous nitric acid N ₂ O ₅ and anhydrous phosphoric acid P ₂ O ₅ , etc., and they are named as oxides, i.e. dinitrogen pentoxide and diphosphorus pentoxide, respectively.
(2) Of the organic acids, only four types are permitted for use: acetic anhydride, succinic anhydride, phthalic anhydride, and maleic anhydride. Others are named with the word "anhydride" at the end of the name, such as benzoic anhydride. [ II ] A compound formed when one molecule of water is removed from two carboxyl groups and bonded together. In the case of a monobasic acid, two molecules condense to form a compound represented by (RCO) _2O , and in the case of a dibasic acid where the two carboxyl groups are adjacent, an intramolecular cyclic acid anhydride with the structure shown in the diagram is obtained. An example of the former is acetic anhydride, and typical examples of the latter are succinic anhydride and phthalic anhydride. The production method is as follows:
(1) Condensation of a carboxylic acid with an acid chloride in pyridine.
(2) Reacting a carboxylate with an acid chloride.
(3) Direct dehydration condensation of carboxylic acids,
(4) Reacting ketene with carboxylic acid,
Linear saturated monocarboxylic acid anhydrides up to _C12 are liquids and have a pungent odor. Those with more carbon atoms are generally colorless, odorless solids. They react with water, primary and secondary alcohols, ammonia, primary and secondary amines, hydrogen halides, etc. to produce acids, esters, amides, and acid halides, respectively. They also react with aromatic hydrocarbons in the presence of aluminum chloride to produce aromatic ketones (Friedel-Crafts reaction).

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

【Ⅰ】無水酸ともいう．酸の化学式から，H₂Oが失われた形の化学式をもつ化合物をいう．水と反応して酸を生じる．水分を含まない酸の意味とまぎらわしく，使わないほうがよい．
(1)無機酸の場合，無水硝酸N₂O₅，無水リン酸P₂O₅などであるが，それぞれ五酸化二窒素，五酸化二リンのように酸化物として命名する．
(2)有機酸では，無水酢酸，無水コハク酸，無水フタル酸，無水マレイン酸の4種類のみは使用が許されるが，それ以外の場合は，安息香酸無水物のように語尾に無水物を付けて命名する．【Ⅱ】カルボキシル基2個から水1分子がとれて結合してできた化合物．一塩基酸の場合は2分子が縮合し，(RCO)₂Oで表される化合物となり，二塩基酸で2個のカルボキシル基が隣接する場合は，図の構造をもつ分子内の環状酸無水物が得られる．前者の例としては無水酢酸が，後者の代表的例としては無水コハク酸，無水フタル酸がある．製法としては，
(1)カルボン酸と酸塩化物とをピリジン中で縮合させる，
(2)カルボン酸塩と酸塩化物とを反応させる，
(3)カルボン酸の直接脱水縮合，
(4)カルボン酸にケテンを作用させる，
などがある．C₁₂ までの直鎖状飽和モノカルボン酸無水物は液体で，刺激臭をもつ．炭素数がそれ以上のものは一般に無色，無臭の固体である．水，第一級および第二級アルコール，アンモニア，第一級および第二級アミン，ハロゲン化水素などと反応し，それぞれ酸，エステル，アミド，酸ハロゲン化物となる．また，塩化アルミニウムの存在下で芳香族炭化水素と反応し，芳香族ケトンを生成する(フリーデル-クラフツ反応)．

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