Ch. 6 – Organic Reactions Flashcards
T/F: When two ions with unlike charge bond, an ionic bond is always formed.
False. Many organic reactions involve the combination of two ions that form a covalent bond. For example, if a carbocation is attracted to an ion with a negatively charged oxygen atom, the oxygen will contribute two bonding electrons to the carbon, and a covalent bond will be formed.
Why are reagents often drawn on top of arrow?
to focus on organic starting materials
Where is the solvent or temp of a reaction added in an equation?
above or below arrow
What is the symbol for light?
hv
What is the symbol for heat?
∆
What do numbers above or below the arrow signal?
sequential steps of a reaction
T/F: Organic byproducts are often omitted.
True
What occurs during a substitution reaction?
atom/group is replaced by another atom/group
one sigma bond breaks, another one forms
When do substitution reactions most likely occur?
most commonly occur when a heteroatom more EN than carbon replaces a hydrogen
What occurs during an elimination reaction?
elements of starting materials are lost
2 sigma bonds are broken, one pi bond is formed
(two groups removed)
When does elimination reaction most commonly occur?
when one group is hydrogen and another group on adjacent carbon is a heteroatom more EN than carbon
What occurs during an addition reaction?
elements are added to starting materials
a pi bond is broken and two sigma bonds form
What is a reaction mechanism?
detailed description of how bonds are broken and formed as starting materials are converted to products
What is a concerted reaction?
a one-step reaction
What is a stepwise reaction?
A multiple-step reaction where starting materials are converted to unstable intermediates (reactive) and then form product
What is bond cleavage?
breaking of a bond, electrons can either be divided equally or unequally between two atoms of the bond
What is homolytic cleavage?
equal dividing of electrons (each atom gets one electron)
What does homolysis generate?
uncharged reactive intermediates with unpaired electrons
What is heterolytic cleavage?
unequal dividing of electrons (each atom gets both or none electrons)
Where do electrons go in heterolysis?
usually to more EN atom
What does heterolysis generate?
charged intermediates that are reactive
What are fishhook arrows?
used to show movement of one electron
What are radicals?
reactive intermediates with a single unpaired electron
generated by homolysis, highly unstable, no charge
What is a carbocation?
electrophile carbon with plus charge (electron deficient)
result of heterolysis where electrons go to more EN heteroatom
+ intermediate, unstable, reactive
What is a carbanion?
nucleophile carbon with negative charge (electron rich)
has lone pair of e- (- intermediate, reactive, unstable)
generated by heterolysis where two electrons are given to C and none are given to heteroatom
Are radicals, carbocations and carbanions electrophilic or nucleophilic?
electrophilic: radials and carbocations
nucleophilic: carbanions
Does forming bonds absorb or release energy?
bond formation ALWAYS RELEASES energy
What is bond dissociation energy? What is symbol?
energy needed to homolytically cleave a covalent bond
∆Hº
BDE is always positive!
What is enthalpy? What is symbol?
∆Hº
energy absorbed or released in a reaction
What is a positive/negative ∆Hº indicate?
positive = endothermic negative = exothermic
How does BDE change with bond strength?
as bond strength increases, BDE increases
How does BDE change with ptable?
going down ptable, atom size increases and BDE decreases
How does BDE change with % s character?
as % s character increases, so does BDE
How is BDE determined for a reaction?
∆Hº = (sum of bonds broken) + (sum of bonds formed (-))
What are the limitations of BDE?
only give overall energy change
don’t give info about reaction mechanism
only for gas phase
What is thermodynamics?
relation of energy and equilibrium
What is kinetics?
reaction rates
How is Keq calculated?
[products]/[reactants]
What does Keq>1 mean?
products are favored
What does Keq<1 mean?
reactants favored
What determines the positive of equilibrium?
relative energies of reactants and products
What is Gibbs Free Energy?
free energy in a molecule (Gº)
What does ∆Gº represent?
overall E difference between R and P
How is ∆Gº calculated?
Gº products – Gº reactants
or
-2.303RT log Keq
What does it mean if ∆Gº is positive or negative?
positive: Keq < 1, endothermic, reactants favored
negative: Keq > 1, exothermic, products favored
When does equilibrium favor products?
when Keq > 1 and when products are more stable (have less E) than reactants
What is entropy?
measure of a system’s disorder
What causes higher entropy? (five factors)
1) more freedom of motion
2) more disorder
3) gas > liquid > solid
4) acyclic > cyclic
5) more than one product
What is ∆Sº?
change in disorder/entropy between R and P
What does positive/negative ∆Sº mean?
positive: products more disordered
negative: reactants more disordered
Is positive or negative ∆Sº reaction favored?
positive, nature favors disorder
How is ∆Gº calculated including entropy?
∆Gº = ∆Hº – T∆Sº
What contributes to total energy change for reaction?
change in bonding E, change in disorder
How does temperature affect ∆Sº?
at high T, entropy is small compared to ∆Hº and can be neglected
What does negative/positive ∆Hº signal?
positive: starting materials favored (endothermic)
negative: products favored (exothermic)
What is an energy diagram?
represents E change when R becomes P
What is a transition state?
unstable energy max where bonds are partially broken and formed; can never be isolated, caused by e-e repulsion in cloud when atoms get too close (high energy)
What is activation energy?
E difference between starting materials and transition state (minimum amount of energy needed to break R bonds)
How are transition states drawn?
in brackets, partial bonds and charges, add double dagger to outside bracket
How many humps are in a multistep reaction?
humps = number of steps
In energy diagram, what is rate-determining step?
step with highest Ea
What is the reaction rate?
how fast a reaction proceeds
What does a larger Ea mean for reaction rate?
larger Ea = slower reaction
How does concentration affect reaction rate?
higher concentration = faster rate
How does temp affect reaction rate?
higher temp = faster rate
How many degrees increase (in ºC) are needed to double reaction rate?
10 ºC
What is a rate law?
shows relationship between reactant concentration and reaction progress
What is rate law equation?
rate = k[reactants]
How does size of k affect rate?
large k = fast
small k = slow
For multistep reaction, which reactants should be put in rate law?
those included in rate-determining step
What is a bimolecular reaction?
reaction with 2 reactants
What is the order of a reaction (kinetics)?
sum of exponents of concentration terms
How are exponents determined for rate law?
experimentally
What is a unimolecular reaction?
reaction involving only one reactant
What is a catalyst?
a substance that speeds up a reaction rate and does not change during a reaction, not a product
Where are catalysts written in rxn equation?
above or below arrow
Does a catalyst affect Keq?
nope
What are enzymes?
biochemical catalysts composed of amino acids held together in very specific 3D shape, have active site
