Symmetry The World of Organic Molecules

What is Symmetry?

Symmetry is likeness of shape or arrangement about a center.

Does this Design Have Symmetry?

Symmetry About a Plane Anything that contains an internal mirror plane is identical with its mirror image. Most things that lack an internal mirror plane are not identical to their mirror image.

Symmetry About an Inversion Center An inversion center takes (x,y,z) to (-x,-y,-z) Anything that contains an inversion center is identical with its mirror image. This is a rare example of an object being different from its mirror image without having a mirror plane.

Symmetry About an Inversion Center An inversion center takes (x,y,z) to (-x,-y,-z)

Chirality Asymmetric objects have an interesting property called Chirality which means “handedness”. Chiral objects can have two different forms, a right- handed and left-handed form which are mirror images of each other but are not exactly alike.

Examples of Chiral Objects Your Hands and Feet Gloves and Shoes Golf Clubs Computer keyboard

Are the Following Objects Chiral or Achiral? Baseball Bat Refrigerator Car Bicycle Your Face Fork Nail Screw

Chiral Molecules? A carbon atom with four different groups bonded to it creates a stereocenter in the molecule. Most molecules which contain one or more stereocenters lack an internal plane of symmetry and are chiral.

Enantiomers The right-handed and left-handed forms of a chiral molecule make up a pair of stereoisomers called enantiomers. Enantiomers are not identical, but they have the same physical properties such as melting point boiling point, etc. So why do we care about this subtle form of stereoisomerism?

Chirality in the Biological World When chiral organic compounds interact with living systems, the two enantiomers often interact very differently. This is because living systems are chiral at many different levels in many ways: - our hands and feet are chiral (of course) - our taste buds are chiral - our olfactory sensors are chiral - our enzymes are chiral

Carvone

Caraway Spearmint

Limonene

OrangesLemons

Aspartame

160 Times Sweeter than Sugar Bitter

Ibuprofen

Analgesic No Activity

Phthalidomide

(R) -Thalidomide Sedative and Hypnotic (S)-Thalidomide Teratogen Birth Defects

Summary Molecules which lack plane of symmetry (and inversion center) are Chiral Chiral molecules have right-handed and left-handed forms Right-handed and left-handed molecules interact with living systems in very different ways. –Different smell –Different taste –Different drug effects