Isomerisation and intermolecular forces

What is isomerism?

The existence of molecules with the same molecular formula but different structural formulas or spatial arrangements of atoms.

• same number and type of atoms

Structural- Chain isomerism

• different arrangement of a molecules carbon skeleton

• Creates branched carbon chains off the main chain

• Makes them more compact = lower boiling point and higher melting point

• Same molecular formula

Structural- Position isomerism

• Differing position of the same functional group in a molecule

• The name of the molecule changes to reflect its new position

• Same molecular formula

Structural- Functional group isomerism

• Isomers have the same molecular formula but different functional groups

• Differing positions of atoms (rearranged)

Sterioisomerism

Where molecules have the same molecular formula but different structural formulas in 3D orientations of their atoms.

Stero- Geometric

• the substituents around a bond with restricted rotation are different

• Two types: cis and trans

• Usually around a restricted bond e.g.C double bond

What are cis/trans stereoisomers?

• Cis stereoisomers = substituents on the same side of the double bond

• Trans stereoisomers = substituents on opposite sides of the double bond

Stereo- Optical

• non-superimposable mirror images of the same molecule

• Cannot be perfectly aligned onto another

• Identical physical properties except interaction with plane-polarised light

Enantiomers

• Stereoisomers

• Differ in arrangement at positions called chiral centres (central carbon)

• Must have at least one chiral centre

Chiral centres

• Central carbon atom bonded to 4 groups

• Cannot be bonded to two+ of the same group E.g. 2H

• Cannot be a double bond

• Cannot have any symmetry

Dipoles in polar bonds

• a polar covalent bond has a permanent dipole (slight -ve charge and slight +ve)

• Dipole = molecule whose ends have opposite charges e.g. HCl

• H2O is a polar molecule, with a dipole +ve at the H2 end and -ve at the O end

• Polar bonds may not have a Dipole e.g. CCl4 (-ve charges spread out around C so NO NET DIPOLE)

• Medium strength, stronger when molecules are closer together

London dispersion/Vander Walls

• attraction between 2 instantaneous dipoles

• Asymmetrical electron distribution possible at any point

• All atoms and molecules

• Weakest intermolecular force

• Increase in strength as molar mass increases

Hydrogen bonding

• attraction between molecules with NH, OH or FH bonds

• Extremely polar bonds = very strong dipole-dipole forces

• H atom is attached to an electronegative atom, but also attracted to another molecules electronegative atom

• E.g. water

• Strongest intermolecular force

How do polar solvents interact?

WITH A POLAR SOLUTE

• e.g. water H2O

• Hydrogen bonding occurs (attraction to -ve atoms) so the solution is mixed

WITH A NONPOLAR SOLUTE

• e.g. hexane and water

• Solute and solvent stay separate