Isomers and Physical Properties

Isomers and Physical Properties

Concept and Types of Isomers

• Isomers: Compounds with the same condensed formula but different developed or semi-developed formula (arrangement in space).
• Molecules are made of the same type and number of elements (atoms) but organized differently.

Importance of Isomerism

• Molecules with the same number and type of atoms can have unique and different properties.
• Example:
  • D-Glucose is a sugar providing energy.
  • L-Glucose causes diarrhea.

How to Identify Isomers

• Determine the condensed formula of each molecule and compare them by counting all atoms.

Types of Isomers

• Structural or chain isomers
• Positional isomers
• Functional isomers
• Geometrical isomers

Structural or Chain Isomers

• Isomers where the main chain and branches differ between molecules.
• This is common and easy to identify.

Example: Butane ($C<em>4H</em>{10}$

• n-butane (straight chain)
• 2-methylpropane (branched chain)
• These are different compounds with different physical properties.

Generating Chain Isomers

• Change the position of carbons from the main chain until all options are exhausted.
• Avoid drawing the same molecule multiple times.

Example: Isomers for Pentane ($C<em>5H</em>{12}$)

• Pentane ($CH<em>3CH</em>2CH<em>2CH</em>2CH_3$)
• 2-methylbutane (Isopentane) ($CH<em>3CH</em>2CH(CH<em>3)CH</em>3$)
• 2,2-dimethylpropane (Neopentane) ($CH<em>3C(CH</em>3)<em>2CH</em>3$)

Number of Isomers of Some Hydrocarbons

Rules to Draw Isomers

1. Use all atoms from the condensed formula.
2. Consider the valence of every atom to exhaust all bonds.
   • Every atom must complete its octet (or duet for hydrogen).

• Connect carbons with single bonds as stated in the condensed formula.
• If the quantity of hydrogens and other atoms does not match, change single bonds for double or triple bonds to reduce the number of atoms.
  • Start by adding double bonds one by one until the numbers match.

Positional Isomers

• Positional isomers have the functional group in a different position; the carbon chain is not modified.
• Example: For the condensed formula $C<em>4H</em>9Cl$, the following are positional isomers:
  • These isomers have the same length in carbon atoms, differing only in the position of the chlorine atom.

Key Point

• Positional isomers must share the same functional group.

Additional Information

• All chemical families present chain or structural isomerism, but not all have positional isomerism.
• Only functional groups can have positional isomers (alkanes cannot create positional isomers).
• Example: Ketones
  • Both compounds have the same quantity and arrangement of carbon atoms, only differing in the position of $C=O$; both are ketones and positional isomers.

Functional Isomers

• They have the same condensed formula but a different functional group in the molecule due to the arrangement of atoms different from carbon, which are responsible for the functional groups.
• Example: $C<em>2H</em>6O$ has two isomers:
  • Ethanol (an alcohol)
  • Dimethyl ether (an ether)

Another Type of Isomers

• Enantiomers
• Example in daily life
  • Teratogenic: a teratogenic agent is a substance, physical agent or organism that can cause a congenital defect in the fetus during the gestation period.

Geometric Isomers

1. They are a type of “stereoisomers,” differing in the spatial orientation of substituents attached to carbons with DOUBLE BONDS (or in rings).
2. Two types of geometrical isomers:
   • cis isomer: substituents are on the same side of the double bond (e.g., cis-2-butene).
   • trans isomer: substituents are on opposite sides of the double bond (e.g., trans-2-butene).

Additional Information

• The groups at the end of the bonds don't necessarily have to be the same.

Nomenclature and Representation

• cis-but-2-ene
• trans-but-2-ene

Key Requirement

• For cis-trans isomerism, there must be only two different carbons connected to the double bond.
• Example of a molecule that does not have cis-trans isomerism: 1-propene.

Fats: Cis and Trans Fats

• Elaidic acid (9-trans) - Bad
• Vaccenic acid (11-trans) - Brutal
• CLA (9-cis, 11-trans) - EPICO
• Oleic acid (9-cis) - Good

Omega Acids

• Omega 3
• Omega 6
• Omega 9

Physical Properties of Isomers (and how to predict them)

• Boiling point: one of the most evident properties that change between isomers.

Boiling Point

Observe the Following Linear Alkanes

Observe the Isomers for Pentane and How Their Physical Properties Change Between Them

• Isomers are like toy soldiers:
  • Linear molecule = Soldier standing straight
  • Branched molecule = Soldier in a cool pose

Analogy

• Which soldiers will fit more easily in a box?

  • Linear molecule = Soldier standing straight
  • Branched molecule = Soldier in a cool pose
  • More soldiers = more molecules = more energy needed to boil them.

• The more branched an isomer is, the easier it is to boil:

  • Neopentane (very branched)
  • Isopentane (branched)
  • Pentane (linear)

Summary

• Structural or chain isomers: Move carbons from the main chain.
• Positional isomers: "Freeze" the chain of carbons; just move the functional group to a different place (adjust the hydrogens).
• Functional isomers: Carbons are moved; look for a different functional group (e.g., alcohol-ether, aldehyde-ketone, carboxylic acid-ester).
• Geometric isomers (cis-trans): Only for alkenes; search for carbons with a double bond that have different substituents (e.g., cis-2-butene, trans-2-butene).

REFERENCES

• Bailey, P. S., & Bailey, C. A. (1995). Química orgánica: Concepto y aplicaciones. México: Pearson.
• Wade, L. G. (2006). Química orgánica (5ª. ed.). México: Pearson.