Ortho

Japanese: オルト

A prefix meaning "regular". [ I ] A prefix indicating that a substituent is present in adjacent positions on the benzene nucleus, i.e., positions 1 and 2. Usually indicated by o- . Other disubstituted benzenes include meta, m- (positions 1 and 3) and para, p- (positions 1 and 4). [ II ] A prefix given to the compound with the highest degree of hydration among oxo acids with the same central atom and oxidation number. It is used like ortho-acid, but is also called normal-acid. Examples are orthoboric acid H ₃ BO ₃ , orthophosphoric acid H ₃ PO ₄ , and orthosilicic acid H ₄ SiO ₄ . Acids with a low degree of hydration include pyroic acid and meta-acid. Organic compounds include orthoformic acid CH(OH) ₃ (which does not exist as a free acid, but exists as an ester), and orthoaldehyde RCH(OH) ₂ (which does not usually exist in a free form, but exists as an acetal). However, chloral hydrate, CCl ₃ CH(OH) ₂ , is an exceptionally stable orthoaldehyde. [ III ] A prefix that distinguishes the nuclear spin state of an atom. In ortho-hydrogen, the nuclear spins of the two hydrogen atoms are in the same direction, while in para-hydrogen they are in opposite directions.

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

Japanese:

“正規の”を意味する接頭辞．【Ⅰ】ベンゼン核の隣接する位置，すなわち，1，2位に置換基をもつことを示す接頭辞．普通，o-で示す．このほかにベンゼンの二置換体には，メタ，m-(1,3位)とパラ，p-(1,4位)がある．【Ⅱ】中心原子とその酸化数が同一のオキソ酸のなかで，水和度のもっとも高い化合物につける接頭辞．オルト――酸のように使うが，正――酸ともいう．たとえば，オルトホウ酸H₃BO₃，オルトリン酸H₃PO₄，オルトケイ酸H₄SiO₄．水和度の低いものにはピロ酸，メタ酸などがある．有機化合物では，オルトギ酸CH(OH)₃(遊離酸として存在せず，エステルとして存在する)，オルトアルデヒドRCH(OH)₂(普通，遊離型では存在せず，アセタールとして存在する)がある．しかし，抱水クロラールCCl₃CH(OH)₂は例外的に安定なオルトアルデヒドである．【Ⅲ】原子の核スピン状態を区別する接頭辞．オルト水素では，2個の水素原子の核スピンは同方向で，パラ水素では逆方向である．