Exam 3 Flashcards
(ΔG) Gibbs Free Energy
(Basic Definition and Types of Reaction)
Ultimate Determinant for whether a reaction will happen
(+) Endergonic reaction (unfavorable)
(-) Exergonic reaction (favorable)
(TΔS) Entropy
(Basic Definition and Reaction Types)
“Disorder”/ Change in number of molecules
(+ΔS)= favorable
(-ΔS) = unfavorable
(ΔH) Enthalpy
(Basic Defintion and Reaction Types)
bonds breaking/ forming
(+) = Endothermic Rxn (unfavorable)
(-) = Exothermic Rxn (favorable)
Enthalpy (ΔH or q)
Definition
The heat energy exchange between the reaction and it surroundings
Breaking a bond (bond cleavage) requires the system to absorb energy
Bond Cleavage
(Two Types)
- Homolytic: occurs with equal sharing of electrons. Forms two uncharged radicals
- Heterolytic: unequal splitting/ sharing of electrons. Forms two ions
Bond Dissociation Energy (BDE)
or ΔH for bond breaking refers to homolytic cleavage
How do you calculate the heat of reaction?
can calculate by subtracting BDEs for bonds formed from the sum of BDEs of bonds broken.
Entropy ΔS
- exothermic and endothermic rxns can occur spontaneously
What two factors most significantly affect ΔS and make it positive?
- When there are more moles of product than reactant
- When a cyclic compound becomes acyclic
ΔG
- Gibbs Free Energy
- A negative value of G means the reaction is spontaneous
- A positive value is nonspontaneous
- temperature sensitive
Negative ΔG
Spontaneous, Exergonic
Positive ΔG
non spontaneous, endergonic
Spontaneous Reaction
- ΔG < 0
- [products]>[reactants]
Keq
Keq=[products/reactants]
- Keq>1, the rxn favors the products
- Keq <1, the reaction favors the reactants
What are the five factors that influence reaction kinetics?
- concentrations of the reactants
- activation energy
- temperature
- Geometry and sterics
- presence of a catalyst
What does the reaction constant k depend on?
the activation energy, temperature, and sterics
larger k = faster rate
Catalyst
- speed up a reaction but are not consumed in the process
- act to decrease the activation energy (altering kinetics) but having no impact on the delta G
Transition States
- fleeting existence, cannot be isolated
- represents every maxima, point of bonds breaking/forming
Hammond Postulate
- For an exothermic reaction, transition state resembles reactants
- For an endothermic reaction, transition state resembles products
Nucleophile
- electron rich species, can donate a pair of electrons
- Lewis bases (carry a lone pair of electrons)
- More polarizable nuleophile=stronger nucleophile
Electrophile
- electron deficient species
- can accept a pair of electrons
- Lewis acids
- Carbocations and partially positive atoms are electrophilic
What is the purpose of a reaction mechanism?
Stepwise description of chemical reactions
What are the four major types of arrow pushing patterns?
- nucleophilic attack
- Loss of a leaving group
- proton transfer
- carbocation rearrangement
Nucleophilic Attack
the electron pusing arrow starts at the nucleophile and points towards the electrophile
Loss of a Leaving Group
Electrons from a bond relocate to the departing atom/group
Proton Transfer
Any time hydrogen is moving around, likely a proton transfer
Carbocation rearrangements
- positive charge on a carbon is stabilized by having more alkyl groups around (hyperconjugation)
Methyl Shift or Hydride Shift
methyl groups or hydrogens can rearrange to produce a more stable carbocation
Hyperconjugation
having more electron density near by (more stuff) stabilizes a (+) charge
Two major types of organic reactions
substitution and elimination
- frequently involving alkyl halides
Alkyl Halides
- aka haloalkanes or organohalides
- great for substitution because:
- the carbon adjacent to the halogen is electrophilic
- Some halogens are great leaving groups (conjugate base of a strong acid)
Sn2 Reactions
a bimolecular substitution reactoin that has a concerted mechanism
- Rate=k[alkyl halide][nucleophile]
alpha carbon
- ## the electrophile attached to the halogen
What happens if the alpha carbon is a chiral center?
the steriochemistry is inverted
What is the effect of more substitution on the alpha and beta carbons?
It decreases the reactivity of the alkyl halide.
3 prime is unreactive, 1 prime is most reactive
What is Nucleophilicity?
the rate at which a nucleophile will attack an electrophile
- strong nucleophile needed for SN2 Reaction
E2 Reaction
- What happens
- Outcome
- Reaction Rate
- favored in the presence of a strong base (OH-, OEt -, t-BuO-)
- Alkyl halides lose both the halogen and a hydrogen from the beta carbon in a process called B elimination
- Outcome: double bond between alpha and beta carbon
- Rate=k[strong base][alkyl halide]
- prefers more substituted alkyl halides
Stability of Alkenes
- trans alkenes are more stable than cis
- more substituted alkanes are more stable to due hyperconjugation (stabilizing the electrons in the double bond)
What is regiochemistry?
an area of a reaction where the changes occur
What is regioselective?
reaction favors one direction/location for the reaction to occur on the molecule
Zaitsev Product
- double bond forms towards the MORE substituted side (more stable product)
- Favored by using small, strong bases
- Ex: (OH-, OEt-)
Hoffman Product
- Double bond forms towards the Less substituted side (less stable product)
- Favored by using big, bulky bases (t-BuO-)
Stereoselectivity
- When there is more than one beta hydrogen on the same carbon, can form cis or trans product
- selective for trans product
Stereospecific
- If there is only one beta hydrogen, only one product can form
- Double bond forms when the halogen and beta hydrogen are coplanar or periplanar (aligned in space)
What type of mechanism is SN2 and E2 reactions
- concerted mechanism, one step
What type of mechanism is SN1 and E1
multistep mechanism
What is solvolysis?
- a reaction with a solvent
What are some traits of SN1 and E1 reactions?
- primarily seen with tertiary alkyl halides
- frequently a reaction with a solvent
- proceeds through a carbocation intermediate
- unimolecular kinetics
- Rate=[alkyl halide]
What is unimolecular kinetics?
a rate that depends on the concentration of only one reactant
What type of reaction does heat favor?
elimation reaction
