In organic chemistry csi/trans isomerism (also known
as geometric isomerism, or E/Z isomerism) is a
form of stereoisomerism describing the orientation
of functional groups within a molecule. In general,
such isomers contain double bonds, which cannot
rotate, but they can also arise from ring structures,
wherein the rotation of bonds is greatly
restricted. Cis and trans isomers occur both in organic
molecules and in inorganic coordination complexes.
The terms cis and trans are from Latin, in
which cis means "on the same side" and trans means
"on the other side" or "across". The term "geometric
isomerism" is considered an obsolete synonym of
"cis/trans isomerism" by IUPAC. It is sometimes used
as a synonym for general stereoisomerism (e.g., optical
isomerism being called geometric isomerism); the
correct term for non-optical stereoisomerism
is diastereomerism.The cis is german word zusamen
which mean together represent by z and trans is
Entgegen which mean opposite.
- They can have different dipole moments.
In a trans compound, the dipole can cancel but not
in cis compound.
- The trans compound is more symmetric and can
pack well, but not the cis compound. So the trans
compound will generally have a high melting point.
Cis-2-butene have high boiling point [4(3.73)] because
they have high dipole moment.
Trans-2-butene have high melting point [1(0.9)] due to
less dipole moment.
Cis M.P (- 139)
Cis gives addition reaction.
Addition of halogens.
Addition of hydrogen.
Addition of sulphuric acid.e.t.c.
Trans gives elimination reaction.
Only E2 reaction.
Why trans is more stable than cis?
Because in trans the large substituents (methyl groups,
etc.) are farther apart and therefore have less steric
In cis alkyl group present on the same side of double bond
repulsive force occur.but in trans alkyl group present on
opposite side while repulsive force dose not occur.
Those isomers which have smaller value of heat of
combustion per mole ,they have more stable so by this
reason trans is more stable than cis because low value
of com/per mole.
convertion cis-2-butene into trans-2-butene
cis and trans are structural isomers. The double bond
does not allow free rotation so the configuration is
fixed and cannot be converted unless bonds are broken
and reformed in the new configuration. If we rotate
one carbon of the double bond to 90˚ the p orbital
become prependicular to each other and then there is
no net overlap between them.As a result,the pi bond
breaks and only the sigma bond remains.Neither of
doubly bonded carbon atoms can be rotated about the
double bond with out breaking the pi bond.the
strength of pi bond is 263kj/mol which is the barrier to
rotation about the double bond.