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My textbook categorizes them as functional isomers (phenol and aromatic alcohol) whereas

The above mentioned isomers are given as position isomers on 'chemguide.co.uk'

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    $\begingroup$ Welcome to Chem SE. I can understand your confusions, such classifications are vague and often overlap. You can call it as both positional and functional. By the way, have a tour of this site. $\endgroup$ – Pritt says Reinstate Monica Jun 9 '17 at 11:53
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The chloride example is interesting because it highlights a fine line separating both.

The distinction between functional and position isomers isn't as simple as the typical examples make it to be - sometimes, what a substituent is attached to is critical to its functional group, so the same sequence of atoms with the same connectivity can behave, chemically speaking, very differently depending on what they are bonded to. And, conversely, position isomers in general don't have exactly the same chemistry either - you know, for instance, that n-butanol, 2-butanol and t-butanol will behave similarly but not identically.

So where do we draw the line? Technically, it's true that both are position isomers. In both cases the same substituent is attached to different positions (unlike, say, acetone and propenol, where the connectivity is clearly different). But it's also true that their chemistry is distinct - aryl compounds, in general, have different reactivity than their equivalent alkyl compounds, which is why we talk about the reactivity of "phenols" and "alcohols" as separate ideas.

So, from your examples, I'd say benzyl alcohol and o-cresol are clearly functional isomers, as their chemistry is quite different. For instance, compare pKa values:

o-cresol: $\mathrm pK_\mathrm a=10.3$

benzyl alcohol: $\mathrm pK_\mathrm a=15.4$

2-ethylphenol (similar to o-cresol): $\mathrm pK_\mathrm a=10.2$

2-phenylethanol (similar to benzyl alcohol): $\mathrm pK_\mathrm a=15.9$

Chloride, on the other hand, is less sensitive to the substrate; but I'd still classify chlorotoluenes and benzyl chloride as functional isomers, since I think they are different enough to merit that distinction, but it's in the twilight zone of vague definitions.

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