S2 Fulton L5-7

Kinetic isotope effects

Kinetic isotope effects can tell us whether a particular bond is broken either before or during the RDS

Consider C-H and C-D bonds. These bonds have the same chemical nature and they react in the same way. Because of different masses the bonds have different vibration frequencies and dissociation energies. The C-D is stronger bond and C-D will be slower to break. If a C-H bind is broken in the rate determining step, the rate of reaction should be slower when the C-D is broken

Requires two experiments one using H and one using D. We then work out the rate constants from these two reactions and work out the value of kH/kD. A higher ratio between 2-8 suggests a primary KIE and supports the idea that C-H is broken in the RDS

Isotope exchange experiment can test for processes where there is fast reversible removal of a proton such as E1cB

Can help us distinguish between E1 and E2 mechanisms

In the E1cB mechanism, a carbanion intermediate is formed by removal of an acidic proton. If the reaction is run in D₂O and deuterium replaces hydrogen at the α-carbon before elimination occurs, this shows: The proton loss is reversible and The carbanion exists long enough to undergo isotopic exchange. This deuterium incorporation is strong evidence for the E1cB pathway,

Polymers are large molecules made up of simple repeating units

Polymers are synthesised from simple molecules called monomers

Polymerisation

Covalent bonds

To be classed as a polymer, n is typically greater than 50. Anything less that 50 is considered an oligomer.

polymer architecture - how the monomer units are arranged within the polymer chain.

Archetypical polymer Alternating copolymer Random copolymer Block copolymer

Made from only one type of monomer repeated throughout the chain.

Two monomers alternate in a regular ABAB pattern.

Two or more monomers are randomly distributed along the chain.

Long, continuous segments (blocks) of one monomer followed by blocks of another.

Hyperbranched polymers Comb polymers Dendrimers

Has a high degree of branching, but not perfectly symmetric.

A linear backbone with regularly spaced side chains (like teeth on a comb).

Highly branched, tree-like structure with perfect symmetry and layers (called generations).

because samples of synthetic polymers are heterogeneous mixtures (they are polydisperse).

The number average molecular weight - describe the average size of polymer chains in a sample, especially when those chains have different lengths.