Why can we only write CHO but not COH for the aldehyde group?

The strict convention of writing the aldehyde functional group as CHO, rather than COH, is a direct consequence of the unambiguous language required by chemical nomenclature and structural representation. In organic chemistry, the written formula CHO is not a sequential instruction for drawing atoms but a standardized symbolic representation denoting a carbon atom bonded to a hydrogen atom and doubly bonded to an oxygen atom (C=O), with the carbon also being the point of attachment to the rest of the molecule. Writing it as COH would be profoundly misleading, as this sequence would universally be interpreted by chemists as representing a hydroxyl group (-OH) attached directly to a carbonyl carbon (C-OH), which describes a carboxylic acid group, not an aldehyde. This distinction is not a minor stylistic preference but a fundamental rule to prevent catastrophic miscommunication regarding molecular structure and reactivity, as the properties of an aldehyde and a carboxylic acid are vastly different.

The rationale is deeply rooted in the logic of valence and bonding. Carbon in an aldehyde is sp² hybridized, forming three sigma bonds: one to hydrogen, one to the R-group (alkyl or aryl chain), and one to oxygen. The fourth bond is a pi bond to that same oxygen, completing the carbonyl (C=O). The notation CHO compactly conveys this by placing the H adjacent to the C, implying its direct sigma bond, while the O is placed in a position that conventionally signifies its double bond to the preceding carbon. The International Union of Pure and Applied Chemistry (IUPAC) sanctions this notation precisely because it avoids the ambiguity inherent in COH. If one writes COH, the immediate and automatic parsing is of a carbon-oxygen single bond (C-O) with an oxygen-hydrogen single bond (O-H), a bonding arrangement that simply does not exist in an aldehyde. The incorrect notation would imply a tetrahedral carbon bonded to an OH, which is a substructure of alcohols and acids, thereby misrepresenting the electronic environment and expected chemical behavior.

Adherence to the CHO convention is therefore a critical component of the shared technical language of chemistry, with significant practical implications for literature, databases, and software. Chemical databases and molecular drawing software interpret string notations like CHO and COH algorithmically based on standard valency rules. Inputting COH would generate a structure with a hydroxyl group, leading to incorrect molecular weight calculations, predicted spectra, and reactivity assessments. In educational and professional settings, using the correct notation is essential for accurately conveying reaction mechanisms, such as nucleophilic addition to the carbonyl carbon, which is central to aldehyde chemistry. The incorrect COH notation would visually and conceptually obscure the electrophilic nature of that carbon, as it inappropriately associates the hydrogen with the oxygen instead of the carbon. Ultimately, the rule exists because chemical notation must map one-to-one with a specific atomic connectivity; CHO successfully maps to the aldehyde functional group, while COH maps to a different, nonexistent arrangement within an aldehyde, making its use incorrect and functionally obstructive to clear scientific communication.