Isomerism: Structural, Chain, and Functional

Isomerism

• Occurs when different compounds have their same molecular formula
• However, despite having equal molecular formulas, their physical and chemical properties diverge
• The field of isomerism can be divided broadly into two subcategories: structural isomerism and stereo isomerism

Structural or Constitutional Isomerism

• A compound is classified as a structural isomer when it shares a molecular formula with another compound, but its structural arrangement differs
• It's important to remember that different compounds will have different naming

Chain Isomerism

• A compound displays chain isomerism when it possesses the same molecular formula of another compound, but differing chain structure

Position Isomerism

• Position isomerism occurs when compounds share a molecular formula but differ in the placement of a functional group

Functional Isomerism

• Functional isomerism arises when two compounds exhibit the same molecular formula but belong to different functional group classifications

Metamerism

• Metamerism appears when compounds have the same molecular formula with an atom chain, but exhibit different numbers of carbon atoms

Ring-Chain Isomerism

• Ring-chain isomerism emerges when a compound with the same molecular formula can exist either in a chain form or a ring structure

Tautomerism

• Dynamic Isomers - tautomers exist in a dynamic state of equilibrium
• Geometric Isomers Not Applicable – tautomers have differing geometries, but aren't geometrical isomers due to low energy differences
• Intramolecular Acid-Base Reaction - tautomerization involves an intramolecular acid-base reaction impacting the stability

Diad System

• A hydrogen atom moves between two atoms in this system

Triad System

• A hydrogen atom moves between three atoms here

Tautomerism in Basic Median

• Reaction - In a basic medium, tautomerism involves a series of proton transfers, leading to the formation of tautomers

Tautomerism in Acidic Medium

• Enol stability - In acidic catalyzed mechanisms, the stability of enol forms influences the tautomeric process

Enol Content

• Thermodynamic and Equilibrium Controlled - Enol content relies on favorable thermodynamically condition and equilibrium
• Aromaticity Enhancement - Stability correlates with the enol's ability to establish an aromatic system

Bond Strength Influence

• Stability of enol content is determined by the strength of bonds within the molecule and the strength of the bonds

Hydrogen Bonding Basis

• Intramolecular Stabilization - Enol content increases when intramolecular hydrogen bonding stabilizes the enol form, lowering energy

Extended Conjugation Effects

• Delocalization and Stability - Enol content increases with extended conjugation. Greater delocalization of electrons stabilizes the enol

Enol content definition

• Bond Strength, Aromaticity, Hydrogen bonding and extendend conjugation - In summary, enol content depends on bond strength, aromaticity, hydrogen bonding, and extended conjugation

Bond Dissociation & Keto/Enol Forms

• Higher dissociation - The form with higher bond dissociation energy is more prevalent

Bond Strength

• Double Bond Formation - Stability depends on the formation and breakage of double bonds

Hydrogen Bonding

• Acetyl Acetone details - Acetyl acetone shows keto-enol variations with varied stability depending on solvent-mediated hydrogen bonding

Enol Content

• Percentage in Acetylacetone-Percentage of enol is greater than keto due to strong hydrogen bonds in acetyl acetone

PPS Details

• Polar Protic Solvent Effects – increased keto form due to weaker hydrogen bonding: weak hydrogen bonds

Gaseous Phase Influence

• Enol Preference- In gaseous phase, enol form preference is observed because no effects are taking place: 96%

Which Form is More Stable Summary

• Enol form is favored primarily through extended conjugation and hydrogen bonding stabilization

Stereoisomerism

• Space-Arrangement Dependent - Stereoisomerism differs in spatial arrangement of atoms or groups

Configurational Isomerism

• Subtypes - Configurational isomerism includes geometrical and optical isomerism

Geometrical Isomerism

• Restricted rotation: Occurs due to limited rotation around a bond, creating spatial arrangements
• Structural similarities with 3-D variance: Geometrical isomers share formulas and linkages, differing only in 3D arrangement

Geometric Isomer Identification Rules: Overview

• Restricted Rotation, Different Groups, Suitable Distance, Distinct Existence- Molecules has to display limited rotation, have unlike groups on each carbon, be structurally different at room conditions

Terminal Atoms

• Terminal atoms need to connect to to two variant chains for GI to take place

Arial distance

• Arial distance between atoms has to be close or similar for GI to place otherwise it wont be able to rotate like how it wants

Cycloalkenes & Geometric Isomerism

• Cyclopentene and larger rings show geometric isomerism, while smaller rings do not

Restricted Bond Rotation

• Key criteria: restricted rotation that stems from double bonds

Following Groups

• Groups where rotation happens : C double bond C (Alkene), C double bond N (Oxime), or N double bond N (Azo group)

Cyclic Molecule

• Key Feature - Geometric isomerism requires restricted rotation, which can be achieved in cyclic compounds

Bulky Group

• A requirement: spatial constraints can induce geometric isomerism in molecules with bulky groups

Essential Condition to show GI (geometric isomer Jugnoo/isomerism)

• Essential for geometric isomerism is restricted rotation

Bond rotations

• Types of bonds - sigma bonds and pi bonds

Naming Geometric Isomers

• Cis-Trans system , E-Z system and syn-Anti system

Cis system

• Cis-Group on same side

Stereo system with the trans

• Trans-Grouping on opposite side.

Trans form

• Trans forms tend to have higher stability than cis forms due to reduced dipole moment

Higher Priority Side

• HIGHER PRIORITY SIDE => Z = Zame Zide

Higher Priority Opposite Side

• HIGHER PRIORITY SIDE =E = Ends

E-z Priority

• Priority guided by atomic number
• Higher priority to group with atom having higher atomic number

Atomic Mass Influence

• Mass over number - If atomic numbers are equal, assess atomic masses

No fixed relation

• Cis and trans is independent to E and Z

Immediate atom rules

• If immediate atoms are identical, see what the next atoms are and go from there

Imaginary molecules

• Absolute prioritization goes towards real atoms if they are also connected to imaginary atoms

High Priority

• System - In E-Z system, Z has higher priority than E

GI (Geometric Jugnoo/Isomerism)

• In cyclolalkanes and cyclo-olefines , is more prevalent, where there are differences in the location of carbon groups

Gi in Cycloalkenes

• Is most prevalent when there a is bond using 8, 9 or 10 Carbons , with cis forms being more stable

Number of chain and position isomers in total

• Number of chain isomers follows a numbering, in 4 Carbon compounds there are 2, increasing each carbon from 5 to 8, there are 3,5, 6,9 increasing respectively. As compounds with 10 Carbons and many after that there are 35 or 75 respectively

Cummelene Details

• Smallest member – Not showing GI. (allene, > back to back double bond)

Double Cyclo rings

• Double and cyclic rings can produce GI depending on if atoms are present on the right direction

Spiro-compounds

• Show or don't show GI depending on the amount of bonds the compound features

Bicyclo Compounds – GI

• trans cis variations can be prevalent depending on what way molecules are resonating and arranged in.

Resonating structures-GI

• trans cis relationship can be altered within a molecule so depending on resonance and where a bonds shift and re-situate, molecules can change position

Enol forms-GI

• Enol forms - Geometric Isomers is influenced in acidic medium, by way the molecule rotates within the compound there may be geometric isomers present that depend on the structure. Aldehyde structures show this.

CARBONYL COMPUNDS-GI

• Geometric Isomers can also happen for symmetric but not asymmetric states. 1

Alternating double bonds-GI

• Alternating double bonds can induce changes and rotations along compounds creating a geometric interaction. Ends have to be dissimilar for GI to occur. If they are similar the same rules apply and then the pi bonds have to follow

Calculate total number of

• For every double bond that may appear, there must be the number of total chain pi counts to determine the different isomers that may appear,

Stereocenters and isomers and

• To know how to find stereoisomers, the R and S patterns have to be analyzed for where methyl chain groups align, and there position

STEREOISOMERS

• In stereoisomers and how reactions act on them depends heavily on steric hindrances or bond shifts that would block or provide assistance for reactions to take place

Syn and anti cyclo

• syn and Anti cyclo form is another method of evaluating geometric isomers

Oxime Geometric Details

• For OH, look at the opposite side to see if any atoms (H) are present that are on the same side of the molecule to determine if it will have geometric properties

Dipole moment

• Dependant on what type of force a chain that may attract carbon bonds is to then direct the orientation of dipole moments when an EWG is on the ends

BOILING POINT

• Is impacted by molecular weight, surface Area or Vanderwaals forces of bonding within the molecule, as well as, hydrogen bonds and Dipole moment.

MELTING POINTS

• Is all about efficient packing, and symmetric layout with hydrogen bonds will increase MP as well as Vanderwaals increases it.

Distortions will

• Increase and or decrease melting, Higher Symmetry increases Mp. A trans form of these usually has better melting points that are more stable. The exception being rings have some of the highest stabilities

STABILIZATIONS

• Double bonds over than 7 are more stable as well as dipoles

Optical Isomerism

• Plane Polarized Dependency - Optical isomerism is based on 3D configuration with plane polarised light
• When in doubt you should perform an experiment when evaluating optical reactions

Essential Condition

• An important aspect and quality is to assess the symmetry of a optical reaction

Symmetry of what the outcome will be

• Symmetry relies on four properties. These four traits are plane, axis point and axis to compare

Plane

• There can have many of varying forms to then impact the structure of a isomer

Symmetry Of Molecular Structure

• Symmetry in molecules can be categorized and evaluated at planes on axis with a mirror structure

Axis

• Symmetry with in a system that would make axis that do not match due to arrangement

Chiral Center

• Is a sp3 hybrid atom connection that connects four differing groups or atoms to then form a chiral center

Chiral Centers versus what it could do as a carbon

• When assessing if chiral changes then the formula to eval if chiral will change has to first look to find the chiral carbon to then make those shifts

RS Nomenclature

• Assessment must be done to see what types of chiral shifts are taking place, so first make a chart with the compound connections and where its positioning relative to other components, then make sure the fourth priority connections have a value to then see what shifts and changes must take place

Representation system

• fisher and wedge based assessment methods to then organize shifts and movements

D and L relationships

• Assesses the placement to make movements

TOPo system

• the top most section of formula to see the bonds in place

Interconversion

• with the position the molecule moves and how that affects how is all affected by where and what groups are present within the molecule to twist

Newman projection

• to review all the rules in place of the previous system we can compare where groups sit on newman diagrams based first what position we are aiming it at.

Anti relationship

• with the bond groups and their relationship you must first assess using newman or wedge,

Stereo and more

• is when at the same time atoms are not only chiral, bit but have some stereoisomerism and this also takes into account where in rings, pi and more compounds sit and move in, This section can become all encompassing

Isomer properties

• is the relation of what the newman and wedge systems are going to turn into for new groups

Enationmers

• When assessing that structures are enationmers, they will be mirror images of each other that will display unique physical structures