Isomerism: Types and Examples

Isomerism

• Isomers are compounds that have the same numbers and kinds of atoms

Types of Isomers

• Structural isomers have different bond patterns
• Stereoisomers have the same bond pattern
• Conformers (rotamers) are interconvertible by single bond rotation
• Configurational isomers are not interconvertible by bond rotation

Constitutional (Structural) Isomers

• They exhibits different CONNECTIVITY patterns
• Skeletal isomers have different carbon skeletons, butane and 2-methylpropane for example
• Functional isomers have different functional groups, dimethyl ether has a boiling point of -23°C and Ethanol has a boiling point = 78.4°C for instance
• Positional isomers show a different position of functional groups, 1-propanamine and 2-propanamine for example
• The number of constitutional isomers increases dramatically with the number of carbons in alkanes
  • C3H8 has 1
  • C4H10 has 2
  • C5H12 has 3
  • C6H14 has 5
  • C7H16 has 9
  • C8H18 has 18
  • C9H20 has 35
  • C10H22 has 75
  • C15H32 has 4,347
  • C20H42 has 366,319
  • C30H62 has 4,111,846,763
  • C40H82 has 62,481,801,147,341

Activity 1

• Using 5 toothpicks, construct a line structure using HEXANE
• Move the sticks to make a different structure, without cyclic or multiple bonds
• Do not add or reduce sticks
• Name it and list down all of the isomers on a separate bond paper
• Repeat until you complete five different isomers of hexane (including the original hexane)
• The five (5) Isomers of Hexane:
  • Hexane
  • 2-methylpentane
  • 3-methylpentane
  • 2,2-dimethylbutane
  • 2,3-dimethylbutane

Stereochemistry

• A branch of chemistry concerned with the three-dimensional structures of molecules

Stereoisomers

• They have the same molecular formula and constitution but a different spatial (3D) arrangement of atoms
• Exhibit the SAME CONNECTIVITY between each other

Activity 2

• Isomers
• Construct a ball and stick 3D model of ETHANE using molding clay (atoms) and toothpicks (bonds)
• Take a snapshot after 90° of rotation, looking directly in front of a C-C bond end on
• On a bond paper, sketch a drawing of ETHANE using skeletal structures and label the H-atoms

Representations of Ethane

• Wedge and dash
• Sawhorse
• Newman projection
  • Includes both Front and back carbons

Wedge and Dash Diagrams

• Solid lines are in the plane
• Dashed lines indicate that it goes backwards out of the plane of the page, and behind
• Wedged lines come forwards out of the plane of the page, meaning its infront

Drawing Newman projection exercise

• Viewed from the angle indicated, Newman projection drawn is generated
• CH3 will be facing the FRONT with CI and BR respectively
• Methyl group on the BACK

Conformations of Ethane

• Staggered conformations make up 99% of the structure
  • These are are the lowest in energy and the most stable conformation
  • All six C-H bonds are as far away from one another as possible
• Eclipsed conformations make up 1%
  • They are the highest in energy and lease stable
  • Six C-H bonds are as close as possible
• Dihedral angle measures 60° and is defined as the TORSIONAL ANGLE

Conformations of Butane

• Anti conformations have methyl groups that is farthest apart
• Gauche conformations have methyl groups that experience a gauche interaction
• Steric strain is the repulsion as a result of trying to occupy the same space

Geometric (Cis-Trans) Isomers

• Show a lack of rotation in C=C bonds
• Are present for disubstituted alkenes
• such as, but-2-ene

Geometric Isomers Exercise

• Need to be able to name the structure and assign the double bond configuration as either cis- or trans-
• trans-1,2-dibromoethene
• cis-1,2-dibromoethene
• An example of a trans-isomer with the structure provided
• Aconitic acid is a cis-isomer and is involved in metabolism
• A molecule is neither isomeric if two identical groups are connected to the same position
• 1,1-dichloroprop-1-ene as an example
• An example of a neither isomer is shown with the structure provided

Cis v Trans

• Cis-but-2-ene is less stable than trans-but-2-ene due to steric strain
• Interconversion of cis and trans isomers doesn't occur spontaneously
• Conversion can occur by treating the alkene with a strong acid catalyst

Chirality

• Chemistry concerned with the 3D structures
• Focus on chirality (ky-ral-i-tee), or handedness
• Cheir greek meaning "hand"
• Any object can be viewed in a mirror, revealing its mirror image

Chirality - SUPERIMPOSABLE

• The mirror image is IDENTICAL to the actual object

Chirality - NONSUPERIMPOSABLE

• The object and its mirror image are DIFFERENT.

Chirality Finding Handedness in Molecules

• The presence of a carbon atom bonded to four different groups
  • stereocenter, or chirality center
• 5-Bromodecane (chiral) is an example for this
  • substituents on carbon 5

Chirality and Symmetry

• Plane of symmetry
  • a plane that cuts through the middle of a molecule or other object
  • one half of the object is a mirror image of the other half
• A molecule is not chiral (achiral) if it contains a plane of symmetry.

Optical Activity

• Ability to rotate a beam of plane-polarized light
• Polarimeter
  • Measures the amount of rotation
• Some optically active molecules rotate plane-polarized light
  • to the right (clockwise) → Dextrorotatory (+)
  • to the left (counterclockwise) → Levorotatory (-)

Racemetes

• A.k.a. racemic (rah-see-mic) mixtures
• 50:50 mixture of enantiomers

Chirality Centres and Stereoisomers

• Maximum number of stereoisomers = 2n
• Where n = number of chirality centers

Enantiomers

• Stereoisomers that are nonsuperimposable mirror image

Diastereomers

• Stereoisomers that are NOT mirror images of each other

Meso Compounds

• Compounds that are achiral, yet contain stereocenters

Stereogenic Centers and the R-S Convention

• Chirality
  • Objects that are not superimposable on their mirror images are called chiral objects
• CHIRALITY CENTER
  • The most common source of molecular chirality is the presence of a carbon atom bearing four different groups
  • Chiral center, stereocenter, stereogenic center, asymmetric center, CHIRAL CARBON
• 2-butanol is a classic example chiral
• Shown in this molecule
• The carbon atoms cannot be chirality centers are shown

Enantiomers

• When a compound are be chiral, it should have one nonsuperimposable mirror image, called its enantiomer, from the Greek word meaning "opposite"

Chirality in Nature

• Limonene, the (+) enantiomer of limonene has the odor of oranges, but the (-) enantiomer has a piney scent.
• The pure S enantiomer fluoxetine, works remarkably well in preventing migraine and is now undergoing clinical evaluation.
• For biological action, a chiral molecule must fit into a chiral receptor at a target site