In the name of ALLAH, The Most Beneficent and

Merciful

NAME:SIDRA SALEEMR0ll no 97BS-CHEMISTRY(5th SEMESTER)DATE:3-4-2013

Geometrical Isomerism

2-butene Isomerism

IntroductionIn 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.

Structures

Cis-but-2-eneTrans-but-2-

ene

NomenclatureThe 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.

Structures

CIS TRANS

PropertiesDIPOLE MOMENTPHYSICAL PROPERTYCHEMICAL PROPERTY

Dipole moment- 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.

Structures

cis Trans

Physical PropertyCis-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)Trans B.P(-

105.5)

Chemical PropertyCis 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 strain.

Explation:-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.

StabilityThose 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.

Cis-2-butene(2712)

Trans-2-butene(2707)

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.

Structures