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Should chemical formula of ethanol be written as $\ce{CH3CH2OH}$ or $\ce{C2H5OH}$? And whats the logic behind it? I have seen the former being used more often, so why is ethane represented by $\ce{C2H6}$ than $\ce{CH3CH3}$?

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$\ce{CH3CH2OH}$, and $\ce{C2H5OH}$ or $\ce{EtOH}$ (Ethyl OH) tells you specifically what are the bonds in the molecule. $\ce{CH3H6O}$, which would be the equivalent of writing ethane like $\ce{C2H6}$, on the other hand, tells you nothing, since it can be both ethanol or dimethyl ether. Bigger formulas can represent thousends of different compounds if written like that.

The reason why $\ce{C2H6}$ is used instead of $\ce{CH3CH3}$ is because it can only represent ethane - it's a very simple compound with no isomers, so you can write it both ways.

You can write ethanol both ways you suggested, although I've never seen $\ce{C2H5}$ instead of $\ce{Et}$ anywhere but in examples. What you shouldn't do is write it like $\ce{C2H6O}$.

If you were asking what is the logic of $\ce{CH3CH2OH}$:

What it's telling you is that the compound it is representing [ethanol] consists of a carbon atom bonded to 3 hydrogen atoms and to another carbon atom, which is bonded to 2 hydrogen atoms and 1 oxygen atom, which is finally bonded to 1 hydrogen atom, as follows:

CH3 CH2 OH

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  • $\begingroup$ Good. But didn't you mean ‘C₂H₆O' instead of ‘CH₃H₆O'(??) in the 1st paragraph, too? $\endgroup$ – mykhal Oct 26 '18 at 21:28
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I think that organic chemists tend do prefer $\ce{CH3CH2OH}$ (condensed) formula over $\ce{C2H5OH}$ (not mentioning favorite abbr. EtOH) because we/they like to see each carbon atom separated in the formula.*
$\ce{C2H5OH}$ looks like some ugly hybrid between molecular formula ($\ce{C2H6O}$) and nice condensed formula.

I speculate that most organic chemists are, for some reason, more comfortable with $\ce{C6H5\bond{-}}$ for phenyl (not speaking of $\ce{Ph\bond{-}}$) than $\ce{C2H5\bond{-}}$ for ethyl, even though it can mean multiple different structural isomers, e.g. $\ce{HC#C-CH2-C#C-CH2\bond{-}}$ (hexa-2,5-diyn-1-yl).

$\ce{C2H5\bond{-}}$ is unambiguous and represents only ethyl group. I think we should learn to be more comfortable with it.


*) In formulae like $\ce{(CH3)3C\bond{-}}$ (tert-butyl) or $\ce{CH3[CH2]7\bond{-}}$ (octyl), many carbons are not explicitly written (repeating units), but they are still written separately. No subscript index number $\ce{C_x}$ used.

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