# What does IUPAC mean by these rules?

I don't understand IUPAC's rules/recommendations to name the compound.

P-14.4 NUMBERING

When several structural features appear in cyclic and acyclic compounds, low locants are assigned to them in the following decreasing order of seniority:

(…)

(e) saturation/unsaturation:

(i) low locants are given to hydro/dehydro prefixes (…) and ‘ene’ and ‘yne’ endings;

(ii) low locants are given first to multiple bonds as a set and then to double bonds (…);

(f) detachable alphabetized prefixes, all considered together in a series of increasing numerical order;

(…)

Does hydro or dehydro refer to the $\ce{OH}$ group?

The rule first says that low locants are given to ene and yne endings. The next part of the rule says locants are first given to multiple bonds and then to double bonds. Aren't double bonds multiple bonds? Do multiple bonds refer to triple bonds?

• Hydro/dehydro means having added or removed hydrogen, usually a pair. For example: tetrahydrofuran. I think they mean that triple bonds have higher "seniority" than double bonds, but not both of those are lower than double and triple bonds indicated by "ene" or "yne" in the name. – Zhe Oct 18 '16 at 3:19

The prefixes ‘hydro’ and ‘dehydro’ are not related to hydroxy groupds ($\ce{-OH}$). They are used to indicate addition and subtraction, respectively, of hydrogen atoms (see Subsection P-31.2.1).