Exam 3

Question 1

(ΔG) Gibbs Free Energy

(Basic Definition and Types of Reaction)

Answer 1

Ultimate Determinant for whether a reaction will happen

(+) Endergonic reaction (unfavorable)

(-) Exergonic reaction (favorable)

Question 2

(TΔS) Entropy

(Basic Definition and Reaction Types)

Answer 2

“Disorder”/ Change in number of molecules

(+ΔS)= favorable

(-ΔS) = unfavorable

Question 3

(ΔH) Enthalpy

(Basic Defintion and Reaction Types)

Answer 3

bonds breaking/ forming

(+) = Endothermic Rxn (unfavorable)

(-) = Exothermic Rxn (favorable)

Question 4

Enthalpy (ΔH or q)

Definition

Answer 4

The heat energy exchange between the reaction and it surroundings

Breaking a bond (bond cleavage) requires the system to absorb energy

Question 5

Bond Cleavage

(Two Types)

Answer 5

• Homolytic: occurs with equal sharing of electrons. Forms two uncharged radicals
• Heterolytic: unequal splitting/ sharing of electrons. Forms two ions

Question 6

Bond Dissociation Energy (BDE)

Answer 6

or ΔH for bond breaking refers to homolytic cleavage

Question 7

How do you calculate the heat of reaction?

Answer 7

can calculate by subtracting BDEs for bonds formed from the sum of BDEs of bonds broken.

Question 8

Entropy ΔS

Answer 8

• exothermic and endothermic rxns can occur spontaneously

Question 9

What two factors most significantly affect ΔS and make it positive?

Answer 9

1. When there are more moles of product than reactant
2. When a cyclic compound becomes acyclic

Question 10

ΔG

Answer 10

• Gibbs Free Energy
• A negative value of G means the reaction is spontaneous
• A positive value is nonspontaneous
• temperature sensitive

Question 11

Negative ΔG

Answer 11

Spontaneous, Exergonic

Question 12

Positive ΔG

Answer 12

non spontaneous, endergonic

Question 13

Spontaneous Reaction

Answer 13

• ΔG < 0
• [products]>[reactants]

Question 14

Keq

Answer 14

Keq=[products/reactants]
- Keq>1, the rxn favors the products
- Keq <1, the reaction favors the reactants

Question 15

What are the five factors that influence reaction kinetics?

Answer 15

1. concentrations of the reactants
2. activation energy
3. temperature
4. Geometry and sterics
5. presence of a catalyst

Question 16

What does the reaction constant k depend on?

Answer 16

the activation energy, temperature, and sterics
larger k = faster rate

Question 17

Catalyst

Answer 17

• 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

Question 18

Transition States

Answer 18

• fleeting existence, cannot be isolated
• represents every maxima, point of bonds breaking/forming

Question 19

Hammond Postulate

Answer 19

• For an exothermic reaction, transition state resembles reactants
• For an endothermic reaction, transition state resembles products

Question 20

Nucleophile

Answer 20

• electron rich species, can donate a pair of electrons
• Lewis bases (carry a lone pair of electrons)
• More polarizable nuleophile=stronger nucleophile

Question 21

Electrophile

Answer 21

• electron deficient species
• can accept a pair of electrons
• Lewis acids
• Carbocations and partially positive atoms are electrophilic

Question 22

What is the purpose of a reaction mechanism?

Answer 22

Stepwise description of chemical reactions

Question 23

What are the four major types of arrow pushing patterns?

Answer 23

• nucleophilic attack
• Loss of a leaving group
• proton transfer
• carbocation rearrangement

Question 24

Nucleophilic Attack

Answer 24

the electron pusing arrow starts at the nucleophile and points towards the electrophile

Question 25

Loss of a Leaving Group

Answer 25

Electrons from a bond relocate to the departing atom/group

Question 26

Proton Transfer

Answer 26

Any time hydrogen is moving around, likely a proton transfer

Question 27

Carbocation rearrangements

Answer 27

- positive charge on a carbon is stabilized by having more alkyl groups around (hyperconjugation)

Question 28

Methyl Shift or Hydride Shift

Answer 28

methyl groups or hydrogens can rearrange to produce a more stable carbocation

Question 29

Hyperconjugation

Answer 29

having more electron density near by (more stuff) stabilizes a (+) charge

Question 30

Two major types of organic reactions

Answer 30

substitution and elimination - frequently involving alkyl halides

Question 31

Alkyl Halides

Answer 31

- 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)

Question 32

Sn2 Reactions

Answer 32

a bimolecular substitution reactoin that has a concerted mechanism - Rate=k[alkyl halide][nucleophile]

Question 33

alpha carbon

Answer 33

- the electrophile attached to the halogen -

Question 34

What happens if the alpha carbon is a chiral center?

Answer 34

the steriochemistry is inverted

Question 35

What is the effect of more substitution on the alpha and beta carbons?

Answer 35

It decreases the reactivity of the alkyl halide. | 3 prime is unreactive, 1 prime is most reactive

Question 36

What is Nucleophilicity?

Answer 36

the rate at which a nucleophile will attack an electrophile - strong nucleophile needed for SN2 Reaction

Question 37

E2 Reaction | - What happens - Outcome - Reaction Rate

Answer 37

- 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

Question 38

Stability of Alkenes

Answer 38

- trans alkenes are more stable than cis - more substituted alkanes are more stable to due hyperconjugation (stabilizing the electrons in the double bond)

Question 39

What is regiochemistry?

Answer 39

an area of a reaction where the changes occur

Question 40

What is regioselective?

Answer 40

reaction favors one direction/location for the reaction to occur on the molecule

Question 41

Zaitsev Product

Answer 41

- double bond forms towards the MORE substituted side (more stable product) - Favored by using small, strong bases - Ex: (OH-, OEt-)

Question 42

Hoffman Product

Answer 42

- Double bond forms towards the Less substituted side (less stable product) - Favored by using big, bulky bases (t-BuO-)

Question 43

Stereoselectivity

Answer 43

- When there is more than one beta hydrogen on the same carbon, can form cis or trans product - selective for trans product

Question 44

Stereospecific

Answer 44

- 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)

Question 45

What type of mechanism is SN2 and E2 reactions

Answer 45

- concerted mechanism, one step

Question 46

What type of mechanism is SN1 and E1

Answer 46

multistep mechanism

Question 47

What is solvolysis?

Answer 47

- a reaction with a solvent

Question 48

What are some traits of SN1 and E1 reactions?

Answer 48

- primarily seen with tertiary alkyl halides - frequently a reaction with a solvent - proceeds through a carbocation intermediate - unimolecular kinetics - Rate=[alkyl halide]

Question 49

What is unimolecular kinetics?

Answer 49

a rate that depends on the concentration of only one reactant

Question 50

What type of reaction does heat favor?

Answer 50

elimation reaction