Lecture 37: Reaction Models

In the next few sections, we’ll:

• Describe models of reactions that explain the value of the rate constant

• See how temperature affects reactions

• Look at catalysts

Effect of Temperature on Reactions

• We know from practical experience that reactions go faster as the temperature increases, right?– Ice melts– Cells grow faster– Things dissolve

• These are qualitative observations, but we need something quantitative

The Arrhenius Equation

• In the 1800’s, a Swedish chemist found:

€

ln k = Intercept + slope 1T ⎛ ⎝ ⎜

⎞ ⎠ ⎟

Intercept = ln A (A is the pre - exponential function)

Slope = - E A

R (EA is the Activation Energy and R is the gas constant)

The Arrhenius Equation

€

ln k = ln A - E A

RT

A and EA are dependent unique for each specific reaction

We can take the antilog of both sides and get:

€

k = Ae−

EA

RT

Solving an Arrhenius Problem

• You’ll be given data consisting of rate constants at various temperatures

• Calculate ln k and 1/T• Plot the values you obtain

– Y-intercept = ln A– Slope = -EA/R

How can we use the Arrhenius Equation?

• We can predict the rate constant at one temperature from its value at another

€

ln k '

k= E A

R1T

− 1T'

⎛ ⎝ ⎜

⎞ ⎠ ⎟

•We can determine the effect of temperature on a reaction and use that to model the reaction

•Low slope: Low EA; Reaction not dependent on temperature

•High slope: High EA; Rxn is dependent on temperature

Collision Theory (13.12)

• Your textbook goes into WAAAYYY too much detail for our purposes

• What we want from this section is not the mathematical bits, but the concepts behind them– We want to think about the relationships b/w: EA,

kinetic energy, A and the extent of reaction• Collision theory deals with gas phase

reactions

Collision Theory. Huh?• For gas phase reactions, EA is

the minimum kinetic energy required for a reaction to occur

• At higher temperatures, more of the molecules are moving at higher speeds (notice the spread of Ek)– The preexponential

parameter, A, is a measure of how often they collide

• Only a certain number of molecules will have a kinetic energy equal to EA

– The higher the EA value, the less likely the reaction will happen without a “push”

Reaction CoordinateFor endothermic reactions, the EA is higher in the forward reaction than that of the reverse

•The rate constant for the forward reaction will depend heavily on temperature

•As we increase the temperature, K increases (we can think of temp as pushing the reaction)

The opposite is true for exothermic reactions

Steric Influences on Reactions

Not all collisions in the gas phase will be successful

We have to consider the shape of the reacting molecules

Transition State Theory

• The Collision Model applies to the gas phase, but not very much biology happens in the gas phase

• Remember: In solutions, molecules move ______ and influence each other much ______ than in the gas phase

• Transition State Theory takes Arrhenius’ equation into the solution phase

Transition State Theory

In this theory, the molecules:

1. Wander through solution and meet, but they may not have enough energy to distort their electron clouds

2. Solvent molecules may collide with the potential reactants and “kick” them together

3. The molecules form an Activated Complex or Transition State that will either move forward to form products or will fall apart to reactants

Transition State Theory

• The activated complex is also called the transition state

• The transition state is a blend of the reactant and product structures with some of the bonds shortening and the bonds in the products partially forming

• The Activation Energy, EA, is the free energy of the transition state.

Transition State Theory

In Transition State theory, a reaction will only take place if two molecules acquire enough energy (perhaps from solvent) to form an activated complex and cross an energy barrier

Catalysis•A catalyst is a substance that increases the rate of a reaction without being consumed

•They may lock reactants into a particular conformation

•They may immobilize fast moving reactants

•They may provide a surface for reactants to meet and react upon

•Catalysts offer alternative reaction mechanisms

•New mechanism has a lower EA in both directions, so Keq doesn’t change

Catalysis

Catalysis: An example

Reduction of ethene

Enzymes

Enzymes are protein molecules with distinct regions for binding reactant(s)

Enzymes and Drugs

•Many of the medicines we have developed function by inhibiting, or shutting down, enzymes

•Antibiotics: Penicillin (beta-lactamase)

Chloramphenicol (bacterial ribosomes)

Kanamycin (bacterial ribosomes)

•Ibuprofen: COX-1 and COX-2

•Statins: HMGCoA Reductase

Chapter 18: Hydrocarbons

• Hydrocarbons are compounds consisting of only carbon and hydrogen

• Aromatic Hydrocarbons: Contain a benzene ring

• Aliphatic Hydrocarbons: No benzene ring; usually a chain

• Some molecules have both (remember surfactants?)

Aliphatic HydrocarbonsBecause we’re talking about chains and carbon will

have 4 bonds, we need to define some termsSaturated Hydrocarbons: No multiple carbon-carbon

bonds– The carbons are saturated with hydrogens

Unsaturated Hydrocarbons: One or more multiple carbon-carbon bonds exist

Dealing with Organic Formulas and Structures

1. Condensed Structural Formula: Shows how the atoms are grouped together

2. Line Structure: Represents the chain as a zig-zag line

Aliphatic Hydrocarbons

• Alkanes: Saturated hydrocarbons (all single bonds)

• Alkenes: Unsaturated hydrocarbons with carbon-carbon double bond(s)

• Alkynes: Unsaturated hydrocarbons with carbon-carbon triple bond(s)

Naming Hydrocarbons

These are the base names

Naming Hydrocarbons: Rules

1. Alkanes end with _____. Alkenes end with _____. Alkynes end with _____

2. Branched chain hydrocarbons are based on the longest continuous carbon chain in the molecule

3. When you have substituents, the carbons in the longest chain are numbered consecutively starting at the end that gives the lower number to the substituent

4. The prefixes di-, tri-, tetra-, penta-, hexa-, … indicate how many of each substituent are in the molecule

5. For alkenes and alkynes, number the molecule such that the lowest numbered carbon has the multiple bond

Isomers

Structural Isomers: Same atoms, different binding arrangements.

A-B-C or C-A-B