2.7 Reaction Mechanisms Flashcards
What does the mechanism of an organic reaction show?
How electrons move around to break bonds and make new ones
(Can be single or multiple steps)
What is a kinetic product?
The one with the lowest Ea
What is the thermodynamic product?
The most stable
What mechanism is where bonds break involving the movement of a single electron?
Free radical mechanisms
Produce radical intermediates (1 unpaired electron) - very reactive
How is the movement of a single electron shown?
A curly arrow with a single head (fish hook arrow)
What are the characteristics of bonds that break into radicals?
Tend to be non-polar
Energy input required to break bond
This can be in form of heat or light (uv)
How is two radicals pairing up illustrated in a mechanism?
Two single headed arrows come together to create an electron pair
What are the steps of a free radical mechanism?
Initiation: free radical formation - more radicals on right of equation
Propergation: same number of radicals on either side of reaction
Termination: when 2 radicals pair to form an unreactive molecule
What enzymes use free radicals in their mechanisms to functionalise biological molecules?
Prostaglandin - start from arachidonic acid - tyrosine side chain radical
Fatty acids double bonds - introduced with iron cofactor
Metabolism of drugs/steroids - rely on iron cofactor (which creates radical)
How does water form radicals
OH bonds split to give hydroxy radical - causes aging
Give an example of a vitamin that uses free radicals
Vitamin E - antioxidants
What does a polar/Ionic mechanisms show?
Movement of pairs of electrons - indicated by double headed curly arrow
What direction do electrons tend to go in?
Direction of most electronegative atom
What are electrophiles?
Electron-loving
Short of electron - what to gain
Will have a + charge or a partial positive charge
Eg. Cations, carbonation
What are nucleophiles?
Electron rich
Might be - charged, partially negativity charged, or have lone pair of electrons
Eg. Anions, carbanions
O, N
What is an adition reaction?
Where a small molecule is incorporated into a larger starting material
What is a classic feature of addition reactions?
Pi bond broken in starting material
2 new sigma bonds formed as reagent added to starting material
(Happens in molecules with double bonds)
What are electrophilic and nucleophilic addition reactions?
Electrophilic - double bond is attached by electrophilic agent
Nucleophilic - double bond attached by a nuceophilic agent
Give an example of a nucleophic addition reaction
fumaric acid ——-> malic acid
O: in water acts as nuceophile
Catalysed by fumarase
What is an elimination reaction?
The reverse of an addition reaction
2 sigma bonds are broken to form a pi bond
Give an example of an elimination reaction
2-phophoglycerate ——->phosphoenolpyruvate
Water is being removed
Catalysed by Enolase enzyme
What 2 type of reactions can be seen as special cases of addition and elimination?
Oxidation and reduction
Reduction - usually addition of a hydrogen (H2) molecule
Oxidation - usually elimination/removal of H2 molecule
What are condensation reactions?
Joining 2 molecules together. Often involves removal of small molecule.
Biologically usually H20
Chemistry can be things like HCl
Give examples of condensation reactions
- Formation of peptides/proteins
- Formation of esters (carboxylic acid + alcohol)
- Formation of nucleosides
- Formation of phosphate esters
What is a substitution reaction?
Where a functional group is directly replaced by another one
What are the 2 types of substitution reactions?
Nucleophilic - more biological relevant - happens on sp3 hybridised C atoms - polar bond is attacked by nucleophile and replaces
Electrophilic - characteristic reaction of aromatic rings - allows to maintain stability
What is a rearrangement (isomerisation) reaction?
Formula of molecule doesn’t change, but arrangement of atoms in space does
Example of rearrangement reaction
Succinyl CoA ——-> (R)-Methylmalonyl CoA
Catalysed by methylmalonyl-CoA mutase
