Chem 321 - Chapter 3: Chemical Reactivity and Mechanisms

Using the bond dissociation energies given, calculate ΔH° for the following reaction.

+20 kJ/mol (In this reaction, a C-Cl and H-O bond has broken, and a C-O and H-Cl bond has formed. From Equation 3.1 in the text, the ΔH° for a reaction is the sum of the energy of broken bonds (352 + 497) plus the sum of the energy of formed bonds (-398 + (-431); Note the negative signs are in place because we are forming bonds as opposed to breaking bonds). Therefore, ΔH° = 352 + 497 - 398 - 431 = +20 kJ/mol.)

Using the rate law below, determine the order of the reaction with respect to NaOH. Rate = k​[CH2Br]​[NaOH]

First-order

reaction mechanism

the step-by-step sequence of reactions by which the overall chemical change occurs

^Hrnx =

{^Hbonds broken + {^Hbonds formed

How many transition states are present in the reaction in the energy diagram?

2 (A mountain represents a transition state in the energy diagram. Therefore, there are two transition states here.)

Use the following diagram to answer the questions below. Which letter corresponds to the activation energy of the reaction? Which letter corresponds to the change in energy for the overall reaction? Which letter corresponds to the activation energy for the reverse reaction? Which letter corresponds to the transition state?

A C B Y

What is a transition state?

A transition state is a mountain on the reaction energy diagram. It is a high-energy compound that cannot be isolated.

Which of the following statements is true? Ionic intermediates are formed in radical reactions Radicals are intermediates in polar reactions Carbocations are electrophiles Radicals are nucleophiles

Carbocations are electrophiles (A carbocation is an electron-poor carbon -- perfect for an electrophile.)

For a reaction carried out at 298 K and 1 atm and for which ΔH° = +30 kJ/mol and ΔS° = +153 J/mol, determine if the reaction favors reactants or products.

Favors products (Including the values into the equation to calculate ΔG° will give a negative value. This means that the reaction favors the products.)

Which of the following is true about the reaction energy diagram?

It represents an endothermic reaction with ΔH = +50 kJ (Since the products are higher in energy than the reactants, this reaction is endothermic. The net gain in energy is +50 kJ.)

What kind of reaction does the conversion of A to B represent?

Substitution reaction (In this reaction, the σ bond between the carbon and chlorine atoms has broken, and the σ bond between a carbon and an oxygen has formed. This is a substitution reaction. Put more simply, in the overall course of a reaction, two groups have switched places.)

Rate law

an expression relating the rate of a reaction to the concentration of the reactants

What kind of reaction does the conversion of A to B represent?

Addition reaction (In this reaction, a π-bond has broken, and two new σ-bonds have formed. This is an addition reaction. More specifically, this is an acid-catalyzed hydration reaction, which is discussed in more detail in future chapters.)

Addition reaction

Addition reactions occur when two reactants add together to form a single new product with all parts of the added reagents appearing in the product. In effect, two new atoms or groups of atoms are added to a starting material, a π bond is broken, and two new σ bonds are formed.

What is an intermediate

An intermediate refers to a local minima. The one indicated is the first intermediate.

Using Table 3.1, rank each of the indicated bonds in order of increasing BDE.

Bond 2 Bond 3 Bond 1 (As you go down a column in the periodic table, the bond dissociation energy decreases.)

Using Table 3.1, rank each of the indicated bonds in order of increasing BDE.

Bond 3 Bond 2 Bond 1 (Bond dissociation energy is related to bond strength. A triple bond is stronger than a single bond, so it will have a higher bond dissociation energy.)

Bond formation

Bond formation releases energy (exothermic) and occurs in two ways. A bond can form in an asymmetrical fashion where both bonding electrons are donated by one reactant (nucleophile) to form the new bond. This asymmetrical bond formation is called heterogenic bond formation. A full-headed curved arrow is used to show the movement of two electrons in heterogenic bond formation Alternatively, a bond may be formed in a symmetrical fashion where each reactant (radical) donates an electron to form the new covalent bond. This symmetrical bond formation is called homogenic bond formation. A half-headed curved arrow is used to show each electron's movement in homogenic bond formation

Match the following reactions with their reaction type.

Elimination Substitution Addition Substitution (In the first scheme there is the elimination of water. In the second scheme, a hydrogen is substituted with a bromine. In the third scheme, two hydrogen atoms have been added to the carbon-oxygen double bond. In the fourth scheme, a chlorine atom has been replaced with a hydrogen.)

What kind of reaction does the conversion of A to B represent?

Elimination reaction (In this reaction, a π-bond has formed, and two σ-bonds have been broken. Two groups have been eliminated from the molecule and replaced with an alkene functional group: This is an elimination reaction. More specifically, this is an unimolecular elimination reaction that will be discussed in future chapters.)

Elimination reaction

Elimination reactions occur when a single reactant splits into two products. That is, two groups are eliminated from the starting material to form a new product. As a result of the elimination, two σ bonds are broken in the starting material, and a π bond is formed between the adjacent atoms.

Given the data Keq = 2.7, determine if the reaction favors reactants or products.

Favors products Keq is the ratio of the products over the reactants at equilibrium. If it is greater than 1, it means that the products must have a higher concentration. Hence this reaction favors the products.

Given the data ∆G° = +1.49 kJ/mol, determine if the reaction favors reactants or products.

Favors reactants (A positive ∆G° indicates a Kvalue less than one. This indicates a reaction that favors the reactants.)

Given the data K = 0.6, determine if the reaction favors reactants or products.

Favors reactants (Since Keq is the ratio of the products divided by the reactants, if it is less than 1, it means the reactants have a higher concentration. This reaction favors the reactants.)

For an endothermic reaction with a negative value of ΔS°sys, determine if the reaction favors reactants or products.

Favors reactants (The sign for ΔH° in an endothermic reaction is positive. To calculate ΔG°, we take ΔH° and subtract ΔS°T. Since ΔS° is negative, this is like adding a positive number to an already positive ΔH°. Hence, ΔG*° must be positive, and the reaction favors the reactants.)

Factors affecting reaction rate

For a multistep reaction, the slow step is called the rate-limiting step or the rate-determining step. Other factors that affect reaction rates are concentration, temperature, correct orientation of the molecules during the collision and steric hindrance. Thermodynamic quantities such as ΔG°, ΔH°, and Keq do not affect reaction rate.

Hammond-Leffler Postulate

Generally the transition state looks most like the species it is closest to in energy

Which potential energy diagram represents an exothermic reaction?

Graph D (An exothermic reaction is characterized by the products being lower in energy than the starting material. This is the case for Reaction D only. Reactions A and C are neither exothermic nor endothermic, as no energy is absorbed or released. Reaction B is an endothermic reaction, since the products are higher in energy than the starting material.)

Which of the following statements about bond breaking is true? Select an answer and submit. Homolysis and heterolysis require energy In homolysis, the electrons in the bond are divided unequally In heterolysis, the electrons in the bond are divided equally Homolysis generates charged intermediates

Homolysis and heterolysis require energy (In heterolytic bond cleavage, both electrons go to one atom, creating two charged intermediates. In homolytic bond cleavage, one electron goes to each atom, creating two radical species. Both of these processes require energy.)

∆G < 0

If ∆G° < 0, then the products will be favored at equilibrium (Keq> 1). The energy of the products is lower than the energy of the reactants. Such processes are spontaneous and are called exergonic.

Choose the reaction that shows the correct electron pushing arrows to generate the indicated product.

Reaction A (The highest electron density is the bromide ion, and the lowest electron density is the carbocation. The arrow should start at the bromine and end at the carbon.)

Choose the reaction that shows the correct electron pushing arrows to generate the indicated product.

Reaction B (Neutral starting material looses neutral N2 which takes away two electrons, leaving a positive charged cation.)

Match the following reactions with their reaction type.

Rearrangement Addition Elimination Substitution (Rearrangements are characterized by the movement of bonds and atoms in a structure. substitutions are characterized by a group being replaced by another group. Eliminations are characterized by the formation of a double bond and the loss of at least one atom. Lastly, additions are characterized by the opposite of eliminations: adding atoms to a double bond.)

Rearrangement reaction

Rearrangement reactions occur when a single reactant undergoes reorganization of its atoms and bonds to yield a single product that is an isomer of the original reactant. The overall number of bonds does not change in rearrangement reactions. An example is the conversion of cis-2-butene into trans-2-butene by treating with an acid catalyst

Substitution reaction

Substitution reactions occur when an atom or a group of atoms is replaced by another atom or group of atoms. In these reactions, the overall number of bonds does not change.

hemolytic cleavage

Symmetrical bond cleavage, or homolytic cleavage, results from homolysis, where each fragment takes away one of the bonding electrons (Figure 3.6). Homolytic cleavage produces fragments with unpaired electrons called radicals. A half-headed curved arrow is used to show the movement of each individual electron in homolysis.

Activation energy

The activation energy (Ea or Eact) is the energy difference between reactants and the transition state (TS). Reactants must overcome the energy barrier to form products. A slow reaction has a larger activation energy while a fast reaction has a smaller activation energy (Figure 3.16).

Which of the following statements are true based on the energy diagram below and your knowledge of the Hammond postulate?

The structure of the first transition state more closely resembles the structure of the intermediate

Which of the following is an accurate statement of the Hammond postulate? The transition state of an exothermic reaction will resemble the starting materials (reactants) more than it resembles the product The transition state of an exothermic reaction will resemble the products more than the starting materials (reactants) The transition state of an endothermic reaction will resemble the starting materials (reactants) more than the product The difference in energy between the starting materials (reactants) and transition state controls the rate of a reaction

The transition state of an exothermic reaction will resemble the starting materials (reactants) more than it resembles the product

Keq

[products]/[reactants]

enthalpy (∆H)

[thermochemistry] indicates heat changes at constant pressure

∆G

change in free energy

heterolysis

cleavage of a bond in which both electrons are given to the same atom

Bonds may break evenly, right in the middle, leaving each side with one electron (_____ cleavage); or bonds may break unevenly, leaving one side with both bond electrons (with a negative charge) and the other side with an empty orbital (with a positive charge) (_____ cleavage).

homolytic heterolytic

Match reactions 1 and 2 with the type of bond cleavage (homolytic or heterolytic cleavage). 1: 2:

homolytic heterolytic (In reaction 1, the electrons in the C-Br bond were split evenly to produce two radical species. This is an example of homolytic bond cleavage. In reaction 2, the electrons in the C-Br bond are split unevenly to produce two ions: this is an example of heterolytic bond cleavage.)

Select the pair of words that make the following statement true: The catalyzed pathway in a reaction mechanism has a_____ activation energy which causes a _____reaction rate.

lower, faster

A spontaneous reaction will occur if the process gives off heat (ΔH° is _____, exothermic) and becomes more disordered (ΔS° is positive).

negative (Conditions that favor products: Keq > 1, ΔH° < 0, ΔG° < 0, ΔS° > 0.)

Organic reactions fall into four broad types:

substitutions, eliminations, additions, and rearrangements.