[Tex/LaTex] A set of equations, vertically aligned, individually tagged, with subequations, and separate label

amsmathmathtools

Can the following be achieved using just amsmath and mathtools? (If not, what other tools do I need?)

I have a collection of equations

\begin{align}
A &= B \\
C &= D \\
E &= F
\end{align}

I want the following display:

  1. The three equations are all aligned at the = sign (like above)
  2. The equation numbers for the first two are "subequations", that is the first equation is numbered (1a), the second (1b), but the third (2). (The numbering should be dynamical, I don't want to hard-code an equation number that I'd have to change every time.)

  3. Since I am already putting the equation number on the left, I want to group the first two equations and mark them with a brace. Something similar to what can be done with 

    \begin{equation*}
\left.\begin{aligned}
A &= B\\
C &= D
\end{aligned}\right\}\text{Some description}
\end{equation*}

I know how to do each of 1, 2, and 3 separately. I am at a loss on how to put any two of them together. I would love to know how to achieve all three at the same time.

Edit: To do 1 and 2 together, I can use one of these answers.

Edit 2: A partial solution toward doing 1 and 3 together is to use the empheq package as described here. However best as I can tell the brace is an "all or nothing" thing. I can't just brace the first two out of three expressions.

Best Answer

The following is taken from Mixed (sub)equation numbering within an array (which satisfies 1 & 2), with the addition of grouping via braces (satisfying 3):

enter image description here

\documentclass{article}
\usepackage[leqno]{amsmath}% http://ctan.org/pkg/amsmath
\newcounter{subeqn} \renewcommand{\thesubeqn}{\theequation\alph{subeqn}}%
\makeatletter
\@addtoreset{subeqn}{equation}
\makeatother
\newcommand{\subeqn}{%
  \refstepcounter{subeqn}% Step subequation number
  \tag{\thesubeqn}% Label equation
}
\begin{document}
\begin{align}
  A + B &\to C + D \\
  E + F &\to G + H \label{eq:EFGH} 
          \smash{\raisebox{\dimexpr.5\normalbaselineskip+.5\jot}{$%
            \left.\begin{array}{@{}c@{}}\\[\jot]\\[\jot]\end{array}\right\}\text{Description 1}
          $}}\\
  I + J &\to K + L \refstepcounter{equation}\subeqn \\
        &\to M + N \subeqn \label{eq:MN}
          \smash{\left.\begin{array}{@{}c@{}}\\[\jot]\\[\jot]\\[\jot]\end{array}\right\}\text{Description 2}}\\
        &\to O + P \subeqn
\end{align}

The EFGH equation is \eqref{eq:EFGH} and the MN is equation~\eqref{eq:MN}.
\end{document}

For grouping an odd number of equations, insert a \smashed array in the middle equation. For an even number of equations, insert a \smashed array in the equation just below the middle, and move it up .5\normalbaselineskip+.5\jot, putting it right in the middle (vertically).

The array has a @{}c@{} column specification, thereby have width zero. However, the \nulldelimiterspace on the left (from \left.) pushes it slightly away from the equations. The vertical line skip of \\[\jot] within each array is to maintain the same vertical height as that of align.

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