7 Organic Reaction Mechanisms Flashcards
What needs to be specified in an organic biochemical reaction?
substrate/starting material
reagent
product
In the following reaction, state the:
Br₂
Cyclohexene → 1,2 - Dibromo cyclohexane
hV or Δ
CCl₄
a) reagent
b) solvent (for alkene to dissolve in to react)
c) reaction conditionsa) Br₂
b) CCl₄
c) hV = light
Δ = heat
What two things must you look at to identify the type of reaction that took place?
what bonds change
what groups move
What do the steps of an organic reaction mechanism show and what does it lead to?
bond breaking and bond making (movement of electrons)
leads to product formation
What does a single-step organic reaction mechanism mean?
it is a kinetically straightforward reaction
What does a multiple-step organic reaction mechanism mean?
it involves transitory intermediate(s)
the slowest step controls reaction kinetics and determines the rate equation (R.D.S)
What does the overall reaction of an organic reaction mechanism determine?
The thermodynamics of the reaction
State the general structure of a 2-step organic reaction mechanism.
substrate (or starting material) → intermediate (measurable unstable lifetime) → product
On the following (see document) energy profile diagram fill in labels 1-5 as:
a) ΔHr
b) First activation energy
c) Second activation energy
d) First transition state
e) Second transition state
f) Intermediate
1 - second activation energy 2 - first activation energy 3 - first transition state 4 - second transition state 5 - ΔHr 6 - intermediate
What is bond-breaking by homolysis.
electrons equally divided between the 2 atoms in the bond
Where is homolysis common?
in non-polar bonds
What does homolysis produce?
radicals (X*)
How do radical reactions occur?
in distinct stages
Draw out the mechanism for homolysis from a C-Z bond (include single-headed ‘fish-hook’ arrows).
see document
What conditions are required for CH₄ and Br₂ to react?
(UV) light
Write out the initiation reaction between CH₄ and Br₂
Br₂ → Br* + Br*
Write out and explain the propagation reaction between CH₄ and Br₂
radicals react with substrate
new radicals are produced - reaction continues
CH₄ + Br* → HBr + H₃C*
Br₂ + H₃C* → CH₃Br + Br*
Write out and explain the termination reaction between CH₄ and Br₂
2 radicals combine to terminate the reaction
Br* + Br* → Br₂
Br* + H₃C* → CH₃Br
H₃C* + H₃C* → CH₃-CH₃
What happens at the very end of the reaction between CH₄ and Br₂?
bond formation between radicals
each donate one electron to form a 2-electron bond
Draw out the free radical use in prostaglandin and fatty acid biosynthesis in the body, naming and drawing both of the compounds.
(first compound = Hint: caterpillar head and then another and few more lines and carboxyl)
(PGH2 = Hint: tilted wizard head sneezing with headphones and poop and carboxyl)
see document for diagrams
Give 2 examples of free radical formation in the body that can cause DNA damage.
HO radicals from H₂O₂ (metabolism) or H₂O (irradiation)
Antioxidants form unreactive radicals i.e. vitamin E (see document for diagrams)
What does a polar/ionic mechanism involve?
the attack of electrophilic or nucleophilic reagents on polar substrate molecules
How do valence electrons move and where is this most common?
valence electrons move in pairs
most common in the lab
Draw out the mechanism for heterolysis from a C-X bond and a C-A bond (include double-headed arrows).
see document
Define ‘electrophile.’
fully + or δ+
Define ‘nucleophile.’
Lewis bases – pair of electrons
Why are carbocations electrophiles?
because the carbon centre is electron-deficient
Are radicals electrophiles or nucleophiles and why?
radicals are electron-deficient and may act as electrophiles
Why are carboanions nucleophiles?
because the carbon centre is electron-rich
Explain how bond-breaking by heterolysis occurs.
electrons are unequally divided between the atoms in the bond
most electronegative atom/best leaving group gets 2 electrons
produces ions
In which type of bonds does heterolysis common?
in polar bonds
State how bond formation between organic ions occurs.
two non-metal ions with unlike charges can come together
with the negatively-charged ion donating both electrons to form the resulting two-electron bond
Draw a diagram of bond formation between organic ions.
see document
For each of the following bonds 1-4 state how they break and why.
1 - homolysis (O-O is a non-polar bond)
2 - heterolysis (C-I is a non-polar bond)
3 - heterolysis (C-H is a non-polar bond)
4 - heterolysis (O-H is a polar bond)
Define ‘addition.’
a reaction where a small molecule is added to the starting material
State the general reaction for addition of general molecule X-Y to a 2,3- dimethyl but-2-ene.
see document for diagram
State the bonds broken and formed in the addition in alkenes.
1 x (double) π-bond broken 2 x new (single) σ-bonds formed
State and draw an equation for an example of an addition reaction and state the roles of the molecules involved.
fumaric acid → malic acid
(H₂O)
(fumarase)
H₂O is added
fumarase is catalyst
(see document for symbol equation)Define ‘elimination.’
involves the removal of atoms or groups from the 2 adjacent atoms in the substrate molecule to form multiple bonds
State the general reaction for elimination of general molecule X-Y from 2,3- X,Y but-2-ane.
see document for diagram
State and draw an equation for an example of an elimination reaction and state the roles of the molecules involved.
2-phosphoglycerate → phosphophoenol pyruvate
(H₂O)
(enolase)
H₂O is added
enolase is catalyst
(see document for symbol equation)What is the relationship between ‘elimination’ and ‘addition’ reactions?
they are exact opposites
a (double) π-bond is formed in elimination reactions
a (double) π-bond is broken in addition reactions
Define ‘oxidation.’
removal of H₂ (or introduction of oxygen)
Define ‘reduction.’
addition of H₂
State and draw an equation for an example of a reduction reaction and state the roles of the molecules involved.
pyruvate → lactate
(lactate
dehydrogenase)
lactate dehydrogenase is a catalyst
Define ‘condensation.’
reaction that involves combination of two molecules with elimination of a second smaller molecule
Mechanistically, is condensation addition or elimination?
often a combination
State a category of general reactions that are an example of condensation reactions.
peptide formation - 2 amino acid residues join together and a molecule of water is eliminated
Write out the general reaction for peptide formation.
see document for this
Define ‘substitution.’
an atom or group directly attached to a carbon in the substrate molecule is replaced by another atom or group
State and draw an equation for a general substitution reaction of Y in the place of X.
see document for this
What is meant by a heteroatom?
An atom other than C or H in the place of them
State and draw an equation for an example of a substitution reaction and state the roles of the molecules involved.
ROH is also eliminated
see document for this
Define ‘rearrangement’ reaction.
where molecule undergoes reorganisation of bonds to yield an isomeric product
What migrate in a ‘rearrangement’ reaction?
groups or bonds
State and draw an equation for an example of a rearrangement reaction and state the roles of the molecules involved.
1-butene → 2-butene
(H⁺)
Succinyl CoA ⇌ (R-)Methylmulonyl CoA
(Methylmalonyl-CoA mutase)
see document for symbol equations
