L6 - Stereoisomerism 1 - E/Z Isomerism

objectives

Understandings

• Stereoisomers are subdivided into two classes—conformational isomers, which interconvert by rotation about a σ bond and configurational isomers that interconvert only by breaking and reforming a bond.
• Configurational isomers are further subdivided into cis-trans and E/Z isomers and optical isomers.
• Cis-trans isomers can occur in alkenes or cycloalkanes (or heteroanalogues) and differ in the positions of atoms (or groups) relative to a reference plane. According to IUPAC, E/Z isomers refer to alkenes of the form R1R2C=CR3R4 (R1 ≠ R2, R3 ≠ R4) where neither R1 nor R2 need be different from R3 or R4.

Applications and Skills

• Construction of 3-D models (real or virtual) of a wide range of stereoisomers.
• Explanation of stereoisomerism in non-cyclic alkenes and C3 and C4 cycloalkanes.

Guidance

• The term geometric isomers as recommended by IUPAC is now obsolete and cis-trans isomers and E/Z isomers should be encouraged in the teaching programme.
• In the E/Z system, the group of highest Cahn–Ingold–Prelog priority attached to one of the terminal doubly bonded atoms of the alkene (ie R1 or R2) is compared with the group of highest precedence attached to the other (ie R3 or R4). The stereoisomer is Z if the groups lie on the same side of a reference plane passing through the double bond and perpendicular to the plane containing the bonds linking the group

Molecular Shape of Alkenes and Alkynes

Alkenes

• spatial arrangement around the C=C is planar
• the bond angles are 120°
• This is because the Double bond counts as ONE ELECTRON DOMAIN
• This means there are 3 different electron domains
• Resulting in a TRIGONAL PLANAR Shape

Alkynes

• Spatial arrangement around the C≡C is Linear
• The bond angles are 180°
• This is because the Triple bond counts as ONE ELECTRON DOMAIN
• This means there are 2 different electron domains
• Resulting in a LINEAR Shape

Structure of Alkenes

• The two 2p orbitals also overlap to form a second bond. This is known as a PI (π) bond.
• For maximum overlap and hence the strongest bond, the 2p orbitals are in line.
• This gives rise to the planar arrangement around C=C bonds.

Overlapping Orbitals in Ethene

Stereoisomerism

• Stereoisomers are compounds with the same structural formula but with different arrangements of atoms in space.
• There are two types in Alkenes:
  • E/Z Isomerism
  • Cis/Trans Isomerism

E/Z Isomerism

• an example of stereoisomerism
• Found in some, but not all, alkenes
• occurs due to the RESTRICTED ROTATION OF C=C bonds
• get two forms...

RESTRICTED ROTATION OF C=C BONDS

ALL THESE STRUCTURES ARE THE SAME BECAUSE C-C BONDS HAVE ‘FREE’ ROTATION

C=C bonds have restricted rotation so the groups on either end of the bond are ‘frozen’ in one position; it isn’t easy to flip between the two.

This produces two possibilities. The two structures cannot interchange easily so the atoms in the two molecules occupy different positions in space.

E/Z or CIS-TRANS

CIS/TRANS:

• Should only be used when there are two H’s and two non-hydrogen groups attached to each carbon.
• cis non-hydrogen groups / atoms on the SAME SIDE of C=C bond
• trans non-hydrogen groups / atoms on OPPOSITE SIDES of C=C bond

E/Z:

• Z (zusammen) higher priority groups / atoms on the SAME SIDE of C=C bond
• E (entgegen) higher priority groups / atoms on OPPOSITE SIDES of C=C bond

CAHN, INGOLD & PRELOG CONVENTION

Determines the priority of atoms/groups around a double bond

An Example using CIP Convention

This is a Z Isomer as the High Priority group on the left of the double bond is the Methyl (15) group and the High Priority on the right of the double bond is the Ethyl Group (29)

• As both High Priority Groups are above the bond (and on the same 'side' this is called a Z isomer
• It's name will be Z-3-methylpent-2-ene

Below you can see the comparison of the isomers:

Cis/Trans Isomerism in Cyclopropanes and Cyclobutanes

• Cyclic molecules such as cyclopropane or cyclobutane also have restricted rotation about the carbon-carbon bonds, as rotation would break the ring structure.
• This gives the possibility of geometric isomerism, with a difference between sites above the plane of the heterocycle and those below.
• An example of this is 1,2-dichlorocyclobutane.
• When the two chlorine atoms are on the same side of the ring structure, i.e. both above or both below, the molecule is designated using the prefix 'cis-'.
• Once again, when the substituents are on opposite sides the molecule is designated using the prefix 'trans-'.