Concept Summary

Structural Isomers

Stereoisomers

Structural  isomers share   only    a   molecular   formula.

They    have    different   physical    and chemical    properties.

Conformational  isomers differ  by  rotation    around  a   single  (σ) bond.

Staggered   conformations   have    groups  60° apart,  as  seen    in  a   Newman  projection. In  anti

staggered   molecules,  the two largest groups  are 180°    apart,  and strain  is  minimized.  In

gauche staggered    molecules,  the two largest groups  are 60° apart.

Eclipsed    conformations   have    groups  directly    in  front   of  each    other   as  seen    in  a   Newman

projection. In  totally eclipsed    conformations,  the two largest groups  are directly    in  front   of

each    other   and strain  is  maximized.

The strain  in  cyclic  molecules   comes   from    angle   strain  (created    by  stretching  or  compressing

angles  from    their   normal  size),  torsional   strain  (from   eclipsing   conformations), and

nonbonded   strain  (from   interactions    between substituents    attached    to  nonadjacent carbons).

Cyclic  molecules   will    usually adopt   nonplanar   shapes  to  minimize    this    strain.

Substituents    attached    to  cyclohexane can be  classified  as  axial   (sticking   up  or  down    from    the

plane   of  the molecule)   or  equatorial  (in the plane   of  the molecule).  Axial   substituents    create

more    nonbonded   strain.

In  cyclohexane molecules   with    multiple    substituents,   the largest substituent will    usually take

the equatorial  position    to  minimize    strain.

Configurational isomers can only    be  interchanged    by  breaking    and reforming   bonds.

Enantiomers are nonsuperimposable   mirror  images  and thus    have    opposite    stereochemistry

at  every   chiral  carbon. They    have    the same    chemical    and physical    properties  except  for

rotation    of  plane-polarized light   and reactions   in  a   chiral  environment.