mjhjmtu

I would like to introduce a line break in text line inside an equation.

What I have done is:

beginalign*
 fractextReactividad relativa \ Hidr'ogeno secundariotextReactividad relativa\ Hidr'ogeno secundario&=d
endalign*

And what I would like to reproduce is the left side of the equal:

A variant, with the Centerstack command,, which automatically starts text mode, and can take an optional argument to set the alignment of the lines:

documentclassarticle
usepackage[T1]fontenc
usepackage[utf8]inputenc
usepackagegeometry
usepackageamsmath
usepackage[usestackEOL]stackengine

begindocument

 beginequation %% Longstack is probably better, look at the manual for stackengine...
 frac,Centerstack[l]Relative reactivity of \ a secondary hydrogen,%
 Centerstack[l]Relative reactivity of \ a primary hydrogen
 = frac,Centerstack[l]yield of product from \ secondary hydrogen abstractionBigm / Centerstack[l]number of \ secondary hydrogens,%
 Centerstack[l]yield of product from \ primary hydrogen abstractionBigm /Centerstack[l]number of \ primary hydrogens%
 endequation

enddocument 

Define a suitable command for those “textual variables”:

documentclassarticle
usepackageamsmath

newcommandtvar[1]%
 textup%
 begintabular@l@
 #1
 endtabular%
 %


begindocument

beginequation*
fractvarReactividad relativa \ Hidr'ogeno secundario
 tvarReactividad relativa\ Hidr'ogeno primario
=
frac
 left(tvarAbc def \ ghi klmright)bigg/
 left(tvarAbc def \ ghi klmright)
 
 
 left(tvarAbc def \ ghi klmright)bigg/
 left(tvarAbc def \ ghi klmright)
 
endequation*

enddocument

You can do it, which does not mean you should do it. I agree with @Johannes_B, using proper variables will increase the readability a lot.

You can use stackengine for these things:

and still, this is IMHO better:

Full MWE:

documentclassarticle
usepackage[T1]fontenc
usepackage[utf8]inputenc
usepackageamsmath
usepackagestackengine
begindocument
 beginequation %% Longstack is probably better, look at the manual for stackengine... 
 fractextstackunderRelative reactivity ofa secondary hydrogen%
 textstackunderRelative reactivity ofa primary hydrogen cdots
 endequation

 bigskip

 But this is better: 

 Call $R_2$ the relative reactivity of a secondary hydrogen, $R_1$ the relative reactivity of a primary hydrogen, dots
 beginequation
 fracR_2R_1cdots
 endequation
enddocument