Ch. 5: Alkenes, Thermodynamics, and Kinetics

Question 1

What is an alkene?

Answer 1

hydrocarbons that contain and C–C double bond

Question 2

What is the general molecular formula of an alkene?

Answer 2

CnH2n

The general molecular formula for a hydrocarbon is CnH2n+2 - 2 Hs for every pi bond or ring in the molecule

Question 3

What is “degrees of unsaturation?”

Answer 3

the total number of pi bonds and rings in a compound

Question 4

What is a vinylic carbon?

Answer 4

the sp2 carbons of an alkene

Question 5

What is a allylic carbon?

Answer 5

an sp3 carbon that is adjacent to a vinylic carbon

Question 6

What is a vinylic hydrogen?

Answer 6

a H attached to a vinylic carbon

Question 7

What is a allylic hydrogen?

Answer 7

a H attached to a allylic carbon

Question 8

What is the functional group of an alkene and how is it denoted?

Answer 8

The functional group is the double bond, is denoted by the suffix “-ene”

Question 9

How many isomers can a compound with two double bonds have and what are its configurations?

Answer 9

four isomers, EE, ZZ, EZ, ZE

Question 10

What is a function group?

Answer 10

determines the kinds of reactions a compound undergoes

Question 11

What is an electrophile?

Answer 11

an electron-deficient species, so it is electron loving, also are Lewis acids

Question 12

What is a nucleophile?

Answer 12

an electron-rich species, has electrons to share, also are Lewis bases

Question 13

Is an alkene an electrophile or a nucleophile?

Answer 13

nucleophile

Question 14

What are characteristic reactions of alkenes?

Answer 14

Electrophilic addition reactions, the addition of an electrophile (H+) to the alkene

Question 15

How do you calculate Keq of a reaction?

Answer 15

[products]/[reactants]

Question 16

What does it mean if Keq is greater than 1?

Answer 16

It means that the products are more stable (have a lower free energy) at equilibrium than the reactants so at equilibrium, there is a higher concentration of products than reactants.

Question 17

What does it mean if Keq is less than 1?

Answer 17

It means that the reactants are more stable (have a lower free energy) at equilibrium than the products, so at equilibrium there is a higher concentration of reactants than products.

Question 18

What are the standard conditions?

Answer 18

1atm, 25C, and [ ] = 1M

Question 19

How is Gibbs free energy calculated?

Answer 19

Question 20

What does it mean to have a positive delta G value?

Answer 20

that the products have a higher free energy (less stable) than the reactants and the reaction will consume more energy than it releases (an endergonic reaction)

Question 21

What does it mean to have a negative delta G value?

Answer 21

that the reactants have a higher free energy (less stable) than the products and the reaction will release more energy than it consumes (an exergonic reaction)

Question 22

What equations relates Keq and delta G?

Answer 22

Question 23

How is Gibbs standard free-energy change related to enthalpy and entropy?

Answer 23

Question 24

What is enthalpy?

Answer 24

The heat given off or the heat consumed during the course of a reaction. Heat is given off when bonds are formed, and heat is consumed when bonds are broken. Thus, delta H is a measure of the bond-making and bond-breaking processes that occur as reactants are converted to products.

Question 25

What is entropy?

Answer 25

A measure of the freedom of motion in a system. Restricting the freedom of motion of a molecule decreases its entropy. Por ejemplo, in a reaction in which two molecules come together to form a single molecule, the entropy of the product will be less than the entropy of the reactants because two separate molecules can move in ways that are not possible when they are bound together in a single molecule. In such a reaction, delta S will be negative. In a reaction in which a single molecule is cleaved into two separate molecules, the products will have greater freedom of motion than the reactant and delta S will be positive.

Question 26

What is La Chatalier’s principle?

Answer 26

if an equilibrium is disturbed, then the system will adjust to offset the disturbance. A way to ensure the value of the equilibrium constant remains constant.

Question 27

What does exergonic mean?

Answer 27

A reaction is exergonic when it releases energy to the environment. The reactants have more free energy (delta G) than the products, so energy is released as the reaction proceeds.

Question 28

What does endergonic mean?

Answer 28

Energy is stored in the products of a reaction. The reactants have less energy than the products, so energy must be added to make the reaction happen.

Question 29

What’s the difference between endergonic and exergonic vs. endothermic and exothermic?

Answer 29

Endergonic and exergonic refer to free energy changes (delta G). Endothermic and exothermic refer to the changes in internal energy of molecules, measured as heat given off or taken up, delta H.

Question 30

What is a reduction reaction?

Answer 30

The addition of hydrogen to a compound is a reduction reaction. This type of reaction increases the number of C–H bonds and/or decreases the number oof C–O, C–N, or C–X bonds (X=halogen)

Question 31

What is a catalytic hydrogenation?

Answer 31

The addition of a hydrogen is called hydrogenation. Because hydrogenation reactions require a catalyst, they are called catalytic hydrogenation.

Question 32

Explain the mechanism for catalytic hydrogenation.

Answer 32

Hydrogen is absorbed on the surface of the metal and that the alkene interacts with the metal by overlapping its p orbitals with the vacant orbitals of the metal. All the bond-breaking and bond-forming events occur on the surface of the metal. As the alkane product forms, it diffuses away from the metal surface.

Question 33

What is the heat of hydrogenation?

Answer 33

The heat released in a hydrogenation reaction. it is customary to give it a positive value even though hydrogenation reactions are exothermic. The heat of hydrogenation is the value of delta H without the negative sign.

Question 34

How does the stability of an alkene change with the number of alkyl substituent groups bonded to the sp2 carbons?

Answer 34

As the number of alkyl substituent groups attached to the sp2 carbons increases, the stability of the molecule increases. ( can also say that the stability of an alkene increases as the number of hydrogens bonded to its sp2 carbons decreases)

Question 35

Evaluate the stability of cis and trans isomers in relation to large substituent groups.

Answer 35

When large groups are on the same side of the double bond, as in a cis isomer, their electron clouds can interfere with each other, causing steric strain in the molecule. Steric strain makes a compound less stable. When large groups are on opposite sides of the double bond, as in a trans isomer, their electron clouds cannot interact, so there is no destabilizing steric strain.

Question 36

What is the free energy of activation?

Answer 36

The energy barrier of a reaction. It is the difference between the free energy of the transition state and the free energy of the reactants. As this value decreases, the rate of reaction increases. Thus, anything that makes the reactants less stable or makes the transition state more stable will make the reaction go faster.

delta G dd = (free energy of the transition state) - (free energy of the reactants)

Question 37

How does the free energy of activation relate to the enthalpy and entropy?

Answer 37

Question 38

What’s the difference between thermodynamic stability and kinetic stability?

Answer 38

Question 39

What are the three factors that rate of reaction depends on?

Answer 39

1) The number of collisions that take place between the reaction molecules in a given period of time. The rate of the reaction increases as the number of collisions increases.

2) The fraction of collisions that occur with sufficient energy to get the reacting molecules over the energy barrier. If the free energy of activation is small, then more collisions will lead to reaction than if the free energy of activation is large.

3) The fraction of collisions that occur with the proper orientation. 2-Butene and HBr will react only if the molecules collide with the hydrogen of HBr approaching the pi bond of 2 butene. If a collision occurs with the hydrogen approaching a methyl group of 2-butene, no reaction will take place, regardless of the energy of collision.

Question 40

What is the rate law of a reaction?

Answer 40

When you know the relationship between the rate of a reaction and the concentration of the reaction, you can write the rate law for the reaction. The rate is proportional to the concentration of the reactants.

Question 41

What is the Arrhenius equation?

Answer 41

This equation relates the rate constant of a reaction to the experimental energy of activation and to the temperature at which the reaction is carried out.

Question 42

Explain the rate constant for a reaction (k).

Answer 42

The rate constant tells us how easy it is to reach the transition state (how easy it is to get over the energy barrier). Low energy barriers are associated with large rate constants. High energy barriers are associated with small rate constants. The rate of a reaction is a measure of the amount of product that is formed per unit of time. The rate of reaction depends on concentration, whereas the rate constant is independent of concentration.

Question 43

How is rate constant related to Keq?

Answer 43

Question 44

What is an intermediate?

Answer 44

A chemical species that is a product of one step of a reaction and a reactant for the next step. Intermediates have fully formed bonds.

Question 45

What is a transition state?

Answer 45

represent the highest-energy structures that are involved in the reaction. They exist only fleetingly, and they can never be isolated.

Question 46

What is the rate-determining step?

Answer 46

If a reaction has two or more steps, the step that has its transition state at the highest point on the reaction coordinate is the rate-determining step. The rate-determining step controls the overall rate of reaction, because the overall rate cannot exceed the rate of the rate-determining step.

Question 47

What is a catalyst?

Answer 47

A catalyst increases the rate of reaction by giving the reactants a new pathway to follow– one with a smaller delta G dd. In other words, a catalyst decreases the energy barrier that must be overcome in the process of converting the reactants to products. A catalyst does not change the relative concentrations of products and reactants when the system changes equilibrium. Therefore, it does not change the amount of product formed; it changes only the rate at which it is formed.

Question 48

Answer 48

Question 49

What is an enzyme?

Answer 49

Most biological catalysts are proteins called enzymes. Enzymes are chains of amino acids linked together by covalent bonds. Each biological reaction is catalyzed by a different enzyme.

Question 50

What is a substrate?

Answer 50

the reactant of an enzyme-catalyzed reaction

Question 51

What is the active site?

Answer 51

The enzyme binds to the substrate in a pocket of the enzyme called the active site. All bond-making and bond-breaking steps of the reaction occur while the substrate is bound to the active site.