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When I read about the different kinds of functional groups, they usually have formulas like $\ce{-NH2} $, $\ce{-COOH}$, $\ce {-OH}$, etc.

What does the bar represent? Any bond? Or can it only be a covalent one?

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    $\begingroup$ Yes, the bar represents any bond, which in this case can only be a covalent one. $\endgroup$ – Ivan Neretin Oct 20 '17 at 15:16
  • $\begingroup$ @IvanNeretin Why? What about compunds like $\ce {NaOH}$? $\endgroup$ – Qwedfsf Oct 20 '17 at 15:18
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    $\begingroup$ @Qwedfsf because OH is not a "functional" group of "Na". $\endgroup$ – ParaH2 Oct 20 '17 at 15:19
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    $\begingroup$ @Qwedfsf You will not see anyone write $\ce{Na-OH}$. $\endgroup$ – Zhe Oct 20 '17 at 15:20
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    $\begingroup$ I can't think of any -OH group that wouldn't be a hydroxyl group. A chemist would never consider a -COOH group to be a combination of a -C=O (a ketone) and a -OH group. // I'd also stipulate that "functional groups" are abstractions for organic chemistry. Hence the bar in organic chemistry is used for covalent bonds. $\endgroup$ – MaxW Oct 20 '17 at 15:52
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Functional groups are a unifying idea in organic chemistry. There are tens of millions of different organic chemicals but many of their key properties are determined by the functional groups they contain. For example, amines (compounds with the $\ce{-NH2}$ group) are basic and often smelly; carboxylic acids (containing the $\ce{-COOH}$ group) are acidic.

The bar in the symbol represents the bond to the rest of the molecule. Sometimes chemists write this slightly differently by including a specific symbol for the rest of the molecule like this: R-OH where R is the rest of the alcohol. So a chemist might talk about a series of similar alcohols by defining R as $\ce{CH3}$ (methanol) $\ce{CH3CH2}$ (ethanol) or $\ce{(CH3)2CH}$ (isopropanol).

This simplifying terminology allows chemists to talk about large groups of related compounds without having to be too verbose and writing out all the details of every structure.

This is an organic chemistry concept so the functional group attaches covalently to a carbon. Some groups can also occur in inorganic compounds but the terminology would be different. For example, Sodium Hydroxide contains an isolated OH but this isn't an alcohol and wouldn't be written or described using terminology related to functional groups.

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  • $\begingroup$ So what about N-hydroxyphthalimide? Does that contain the hydroxy functional group or not? $\endgroup$ – Jan Oct 22 '17 at 3:30
  • $\begingroup$ @jan It isn't an alcohol and the functional group would probably be described as something like an "N-hydroxy" or a hydroxyimide. The alcohol functional group is defined by an OH connected to a carbon. $\endgroup$ – matt_black Oct 22 '17 at 12:18

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