Option [(a)] to get alphabet numbering when applied to enumerate doesn't work

According to this popular guide http://texblog.wordpress.com/2008/10/16/lists-enumerate-itemize-description-and-how-to-change-them/, I should be able to apply an option [(a)] to get small case alphabet numbering. As it is below, I cannot:

\documentclass[12pt]{article}
\usepackage[version=3]{mhchem}

\date{}
\begin{document}

\begin{enumerate}[(a)]
\item {\bf \ce{CH3Br + KOH -> CH3OH + KBr}}
\end{enumerate}
\end{document}


Error message:

! LaTeX Error: Something's
wrong--perhaps a missing \item.

See the LaTeX manual or LaTeX
Companion for explanation. Type  H
<return>  for immediate help.  ...

l.22 \item {
\bf \ce{CH3Br + KOH -> CH3OH + KBr}} ?

-

You need to \usepackage{enumerate}.
You should consider using \usepackage[shortlabels]{enumitem} as I belive that that is a newer package. See What are the differences between using paralist vs. enumitem for a comparison of the paralist and enumitem
The enumitem package offers substantially more flexibility that the enumerate package, but for simple things like changing the labels, the enumerate package does a fine job as well. – Alan Munn Jul 1 '11 at 17:02