S3.2.7 Isomerism

Cis-trans isomerism

Arises from restricted rotation in double bonds or rings

Requires two different groups on each carbon of the C=C bond or ring structure.

When there are no cis-trans isomers

Only one carbon in the C=C bond has two different groups

Cis-trans isomerism needs both carbons to be bonded to two different groups.

Cis and trans isomers of but-2-ene

Cis has CH₃ groups on same side of C=C; trans has CH₃ groups on opposite sides

Caused by restricted rotation around the pi bond.

3D difference in cis/trans but-2-ene

Cis shows CH₃ groups on same side in 3D; trans shows CH₃ groups on opposite sides

Affects polarity and boiling point.

Cis-trans isomerism in cyclic compounds

Rotation restricted by ring

Different substituents can be on same side (cis) or opposite sides (trans) of the ring.

Cis/trans pent-2-ene

Cis has same substituents on same side of double bond

Trans has them on opposite sides

Occurs in unsymmetrical alkenes.

Cis/trans 1,2-dibromocyclobutane

Cis has both Br on same face of the ring

Trans has Br on opposite faces

Classic cyclic isomer pair.

Optical isomerism

Occurs in chiral molecules with a carbon bonded to 4 different groups

Carbon is called a chiral center or asymmetric carbon.

Chiral center (example in butan-2-ol)

Carbon is bonded to CH3, OH, H, and C2H5 making it chiral

Creates two non-superimposable mirror images.

Enantiomers

Non-superimposable mirror images of a chiral molecule

They differ in the direction they rotate plane-polarized light.

Physical properties of enantiomers

Same melting and boiling points

Differ only in how they rotate plane-polarized light (optically active).

Chemical properties of enantiomers

Identical unless reacting with other chiral molecules like those in the human body (e.g., enzymes, receptors).

Examples of chiral molecules

2-chlorobutane, pentan-2-ol, and alanine all have a carbon bonded to four different groups, making them chiral.

Plane-Polarized Light

Light waves vibrating in one plane only

Produced by passing ordinary light through a polarizer.

Enantiomers and Light

Rotate plane-polarized light equally but in opposite directions

One clockwise (dextrorotatory), the other anti-clockwise (levorotatory).

Polarimeter

Instrument that measures the angle and direction by which a chiral compound rotates plane-polarized light.

Racemic Mixture

Contains equal amounts of both enantiomers

Optically inactive as rotations cancel each other.