"You can't call that a 2-butyl". You have to call it sec-butyl or 1-methylpropyl.
Edit: 1-methylpropyl, not 1-methylpentyl - thanks AWK
Quote: "One recent development in substitutive
nomenclature is to place locants immediately before the part of the name
(suffix, infix, prefix or descriptor) that they qualify. The CNOC (Commission on
Nomenclature of Organic Chemistry) Guide also recommends that the
traditional names of alkyl groups be supplemented by alkanyl names that are
derived by adding the suffix -yl to the name of the parent hydrocarbon, rather
than replacing the ending -ane by the suffix -yl, as in current practice. For
example, we can write three names for (CH3)2CH- : isopropyl, 1-methylethyl
and propan-2-yl. The last systematic name is short, easy to understand and to
use and fully able to cope with further substitution. The substituted group
(ClCH2)2CH- can be named 2-chloro-1-(chloromethyl)ethyl or, in the new
form, 1,3-dichloropropan-2-yl. This last name is shorter and quite appealing."