Functional Groups and Isomers

Structural Isomers

  • Compounds with the same molecular formula but different atomic arrangements.

    • Differ in connectivity: atoms are connected in a different order.

    • Affects physical properties: boiling point, melting point, density, etc., can vary significantly.

    • Example: Butane and 2-methylpropane (C<em>4H</em>10C<em>4H</em>{10})

  • Also known as Constitutional Isomers.

Functional Groups

  • Atoms (other than Carbon) or bonds (alkenes, alkynes) in organic molecules.

    • Impart specific chemical properties to the molecule.

    • Participate in chemical reactions.

  • Responsible for chemical reactivity and physical properties.

Stereoisomers: Enantiomers

  • Chiral Center: Carbon bonded to 4 different substituents.

    • Asymmetric center or stereocenter.

    • Causes molecule to be non-superimposable on its mirror image.

    • Essential for enantiomer formation.

  • Enantiomers: Non-superimposable mirror images.

    • Distinguished by their interaction with polarized light.

    • Rotate plane-polarized light in opposite directions.

  • Physical properties are the same except in chiral environments.

    • Melting/boiling points, reaction with achiral reagents, and magnitude of optical rotation (opposite directions) are the same.

Thalidomide Example:

  • Racemic mixture with one isomer causing issues.

    • One enantiomer treated morning sickness, while the other caused birth defects.

    • Illustrates the importance of stereochemistry in drug action.

  • Chirality is important in drug design.

Chirality in Biological Systems

  • Most drugs are chiral.

    • Affects binding to target proteins/enzymes.

    • Can result in different pharmacological effects.

  • Amino acids (L-form) are chiral.

    • Building blocks of proteins.

    • Only L-amino acids are used in protein synthesis.

  • DNA is a chiral double helix.

    • The sugar component (deoxyribose) is chiral.

    • The helical structure introduces chirality.

Geometric Isomers (cis-trans)

  • Restricted rotation around a double bond (C=CC=C).

    • Double bond prevents free rotation.

    • Requires two different groups on each carbon of the double bond.

  • Cis: groups on the same side.

  • Trans: groups on opposite sides.

Geometric Isomers and Fats:

  • Trans fats: pack tightly, higher melting point, 'bad' fats.

    • Increase LDL cholesterol and decrease HDL cholesterol.

    • Associated with increased risk of cardiovascular diseases.

  • Cis fats: don't pack well, lower melting point, 'good' fats.

    • Usually liquid at room temperature.

    • Considered healthier than saturated and trans fats.

Biological Consequences of Geometric Isomers

  • Light can weaken the π\pi bond in alkenes, enabling cis-trans isomerism.

    • Involved in vision (retinal).

    • Isomerization is crucial for light detection.

Isomers: Summary

  • STEREOISOMERS

    • Carbon with 4 different substituents = CHIRAL CENTRE

    • Asymmetric carbon atom attached to four different atoms or groups of atoms

    • Critical for determining the molecule's stereochemistry

    • Enantiomers: 2 molecules that are nonsuperimposable mirror images of each other

  • GEOMETRIC ISOMERS: identifying cis and trans isomers

  • STRUCTURAL ISOMERS

    • Chain

    • Variation in the arrangement of the carbon chain, leading to different branching patterns

    • Positional

    • Occurs when a functional group changes position on the same carbon chain

    • Functional Isomers

    • Molecules with the same molecular formula but different functional groups, resulting in distinct chemical properties

Types of Isomerism

  • Isomers: Molecules with the same molecular formula but different structural or spatial arrangements.

Structural Isomerism:

  • Chain Isomers: Different arrangement of the carbon skeleton (e.g., butane vs. 2-methylpropane).

    • Variation in the carbon chain's branching structure.

    • Affects physical properties such as boiling point and melting point.

  • Position Isomers: Differing position of the same functional group (e.g., but-1-ene vs. but-2-ene).

    • The carbon skeleton remains the same, but the position of the functional group changes.

    • Alters the chemical reactivity of the molecule.

  • Functional Isomers: Differing positions of atoms give a different functional group (e.g., cyclobutane).

    • Different functional groups lead to vastly different chemical properties.

Stereoisomerism:

  • Geometric Isomers: Different substituents around a bond with restricted rotation (cis/trans).

    • Arise from the restricted rotation around a double bond or a ring structure.

    • E = oppposite side, Z = same side.

  • Optical Isomers: Non-superimposable mirror images (enantiomers).

    • Enantiomers have identical physical properties except for their interaction with plane-polarized light.

    • Display optical activity, rotating the plane of polarized light either to the right (dextrorotatory) or to the left (levorotatory).