SPATIAL STRUCTURE OF MOLECULES II
Stereoisomerism
When the isomers have the same structural formulas, have the same constitution, but differ in relative arrangement of their atoms or groups in space within the molecule, they are known as stereoisomers, and the phenomena as stereoisomerism. It can be further classified into three types:
- Conformational isomerism
- Geometrical isomerism
- Optical isomerism
CONFORMATIONS OF ALKANES AND CYCLOALKANES
1.1. Conformation of ethane
Ethane is the simplest hydrocarbon that possesses distinct conformations. Two conformations of ethane, called the eclipsed (I) and the staggered conformations (II), merit special attention.
(I)eclipsedconformation (II) staggered conformation
In bothexamples,thetwocarbonatomsarebonded to each other,andeachcarbon atom is bonded totreehydrogenatoms.Thetworepresentations differ in howthehydrogenatoms of one carbon atomarepositionedrelativetothehydrogenatoms of the othercarbonatom.Which of the two formsrepresentsethane?Theansweristhatbothdo this to the samedegree. Whenasp³hybridorbitalofonecarbonatomoverlaps a sp³ hybridorbitalofanother carbon atom, itdoessoalongtheaxisbetweenthetwonuclei. Each carbonatomanditsthree bonded hydrogenatomscanrotateaboutthecarbon-carbonσ-bond. The carbonatomsarestill bonded but the spatialpositionofhydrogenatomsisaltered.
Moleculessuchasethaneareconstantlyrotating. A moleculecanexistin different orientationsorconformationsbyrotationaboutthesingle C-C bond.Differentorientations or conformationsofthesamemoleculearesometimesreferred to as conformersorrotamers.
Eclipsed(1)andstaggered(II)formsofethane can be representedbydrawing Newman projection formulasofethane:
These projections show that a rotation of 60oaround the carbon-carbon axis converts the staggered form to the eclipsed structure. As the rotation is continued foranother 60, a new o staggered conformation is produced, which is identical with the first staggeredconformation.
A plot of potential energy versus degree of rotation for one complete 360orotation around the carbon-carbon bond in ethane is shown in figure:
Cyclopropane is generally more reactive than other alkanes. Reactions that open the cyclopropane ring are realized.
Cyclobutane:
Cyclobutane has less angle strain that cyclopropane andcan reduce the torsional strain inherent in a planar geometry by adopting the nonplanar “puckered”conformation:
Cyclopentane:
Angle strain in the planar conformation of cyclopentane is relativelysmallbecause the 108o
2. GEOMETRICAL ISOMERISM Geometrical isomerism occurs in compounds that have the same sequential arrangement of atoms but different spatial arrangements.
2.1. Cis - trans isomerism. In an alkane there is free rotation about the carbon - carbon bonds, and the molecule can exist in many conformations. The double bond in alkenes does not permit free rotation. In order to maintain the π-bond of an alkene, the two p-orbitals must remain side by side. If the two carbon atoms were rotated around the bond axis, the σ-bond would remain, but π-bond would be broken. This process can occur only if adequate energy is supplied to break the π-bond. Under ordinary conditions, the two carbon atoms do not rotate in relation to each other.