Chirality and Chiral Centers

• A carbon atom attached to four different atoms or groups of atoms is a chiral carbon or chiral center.
• "Chira" comes from a Greek word for hand, describing molecules with a handedness.
• A carbon atom is asymmetric, meaning there's no plane of symmetry in the molecule.
• Compounds with one chiral center (chiral molecules) exist as two optical isomers, called enantiomers.
• Enantiomers are non-superimposable, similar to how the left hand cannot be superimposed on the right hand.
• Enantiomers are mirror images of each other.

Chirality

• A molecule has a chiral center when a carbon atom bonds to four different atoms or groups of atoms, leading to enantiomers.
• Two optical isomers are obtained from one chiral center.
• Molecules with two chiral centers will have two pairs of optical isomers.
• Isomers differ in how they interact with plane-polarized light and react with other chiral molecules.
• A molecule with three chiral centers results in eight stereoisomers.
• The relationship is defined as: Number of isomers = 2^n, where 'n' is the number of chiral centers.
• When drawing optical isomers, always draw mirror images, including wedge and dashed bonds.

Properties of Optical Isomers

• Chemical properties of optical isomers are generally identical, with one exception
• Optical isomers interact with biological sensors differently.
• For example, one enantiomer of carvone smells of spearmint, while the other smells of caraway.
• Optical isomers have identical physical properties except for their ability to rotate the plane of polarized light.
• Unpolarized light becomes polarized when passed through a polarizer; the waves vibrate in one plane only.
• Enantiomers differ in how they rotate plane-polarized light; one rotates it clockwise, the other counterclockwise.
• (+) and (-) or d and l, D and L, or R and S are commonly used to differentiate the isomers.
• The rotation of plane-polarized light identifies an optical isomer of a single substance by passing it through a sample.
• Depending on the isomer the sample contains, the plane of polarized light will be rotated either clockwise or anticlockwise by a fixed number of degrees.
• Each enantiomer rotates the plane of polarised light in a different direction.

Racemic Mixture

• A racemic mixture (or racemate) contains equal amounts of each enantiomer.
• One enantiomer rotates light clockwise, while the other rotates it anticlockwise.
• A racemic mixture is optically inactive, with enantiomers canceling each other's effects.
• This means that the plane of polarized light won't change.
• In the pharmaceutical industry, producing synthetic drugs as racemic mixtures is easier than producing a single enantiomer.
• Approximately 56% of all drugs in use are chiral, and 88% of those are sold as racemic mixtures.
• Separating enantiomers results in an enantiopure compound, containing only one enantiomer.
• Because it is expensive and time-consuming, separating is only worthwhile, even though only half the drug is pharmacologically active.
• The pain reliever Ibuprofen is sold a racemic mixture.

SN1 and SN2 Reactions

• Optical activity suggests the mechanism of a chemical reaction is especially important in nucleophilic substitution.
• Nucleophilic substitution occurs via SN1 or SN2 mechanisms.

SN1 Mechanism details

• The SN1 mechanism is a two-step reaction.
• In the first step, the C-X bond breaks heterolytically, and the halogen leaves as an X- ion.
• The first step leaves a trigonal planar, tertiary carbocation.
• In the second step, the planar, tertiary carbocation is attacked by the nucleophile.
• The nucleophile attacks from either side of the trigonal planar carbocation, forming a racemic mixture.
• A reaction with an SN1 mechanism will produce a racemic mixture.

SN2 Mechanism

• The SN2 mechanism is a one-step reaction.
• The nucleophile donates a pair of electrons to the δ+ carbon atom of the halogenoalkane to form a new bond.
• Simultaneously, the C-X bond breaks, with the halogen (X) taking both electrons in the bond.
• The halogen leaves as an X- ion.
• An example of the nucleophilic substitution of bromoethane by hydroxide ions to form ethanol.
• The bromine atom of the bromoethane causes steric hindrance.
• This means the hydroxide ion nucleophile can only attack from the opposite side of the C-Br bond.
• Attack from the same side as the bromine atom is sometimes called frontal attack.
• Attack from the opposite side is sometimes called backside or rear-side attack.
• As the C-OH bond forms, the C-Br bond breaks, causing the bromine atom to leave as a bromide ion.
• As a result, the molecule has undergone an inversion of configuration.
• A common comparison is the umbrella turning inside out in the wind.
• Therefore, a reaction with an SN2 mechanism that starts with an enantiopure reactant will form enantiopure products.