Organic Chemistry I: Chapter 4
For a given reaction, if ΔG° is greater than zero, then:
0 < Keq < 1
Consider the reaction of A being converted into B at 25°C. If the ΔG° of this reaction is +1.0 kcal/mol, the Keq is ________ and the % conversion is ________.
0.18, 15%
Consider the reaction of A being converted into B at 25°C. If the ΔG° of this reaction is +0.5 kcal/mol, the Keq is ________ and the % conversion is ________
0.43, 30%
Assume the reaction A + B → C + D proceeds to equilibrium. Calculate the equilibrium concentration of D at 25°C, given that the starting concentrations of A and B are 2M and that ΔG° for the reaction is 1.0 kcal/mol. R = 1.987 cal/mol∙K.
0.60M
How many distinct monochlorinated products can result when cyclopentane is subjected to free radical chlorination?
1
What is the name of the major monobrominated product which results when methylcyclohexane is subjected to Br2/hν conditions?
1-bromo-1-methylcyclohexane
Given a reaction in which reactant A is converted only to product B at 25°C, what Keq results if at equilibrium 80% of A has become B?
4
Given a DG° of 0.8 kJ/mol at 25°C for the equilibrium shown below, calculate the percentage of the axial conformer at 25°C. [R = 8.314 J/K∙ mol]
42%
For the reaction A + B → C + D, ΔG° = -5.00 kcal/mol. What is the corresponding equilibrium constant at 25°C? R = 1.987 cal/mol∙K.
4648
List the following radicals in order of increasing stability (i.e., from least stable to most stable) (CH3)3C∙, CH2CHCH2∙, CH3CH2∙, CH3∙, (CH3)2CH∙
CH3∙ < CH3CH2∙ < (CH3)2CH∙ < (CH3)3C∙ < CH2CHCH2∙
Describe the hybridization of the cationic center and predict the CCC bond angle in (CH3)3C+
Cationic center is sp2 hybridized, therefore the CCC bond angle is 120°.
Write an equation to describe the initiation step in the chlorination of methane.
Cl-Cl + photon (hν) → 2 Cl∙
Given that tertiary H atoms react with a chlorine radical about 5.5 times faster than primary ones, estimate the ratio of the two monochlorinated products that result when 2,3-dimethylbutane undergoes free radical chlorination
1-chloro-2,3-dimethylbutane, 52%; 2-chloro-2,3-dimethylbutane, 48%
Predict the signs of DH° and DS° in the reaction of cyclohexene with H2 to form cyclohexane.
DH° < 0 and DS° < 0
Predict the sign of DS° in the combustion of propane.
DS°>0
What is the CCC bond angle in (CH3)3C+?
120 degrees
How many distinct monochlorinated products can result when isobutane is subjected to free radical chlorination?
2
Predict the major monobromination product in the following reaction.
Look at picture
Given a K of 0.45 at 25°C, calculate the corresponding DG° in kJ/mol. [R = 8.314 J/K∙ mol]
2.0 kJ/mol
Consider the reaction of A being converted into B at 25°C. If the ΔG° of this reaction is -0.5 kcal/mol, the Keq is ________ and the % conversion is ________.
2.3, 70%
Given a DG° of -8.0 kJ/mol at 25°C, calculate the corresponding K. [R = 8.314 J/K∙ mol]
25
How many distinct dichlorination products can result when isobutane is subjected to free radical chlorination?
3
In the hydrocarbon shown below, how many tertiary hydrogens are present?
3
What is the name of the major monobrominated product which results when 3-methylpentane is subjected to Br2/hν conditions?
3-bromo-3-methylpentane
Provide the major organic product that results when 2,2,4-trimethylpentane is subjected to free radical bromination.
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Provide the major organic product(s) in the reaction below
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Provide the major organic product(s) in the reaction below.
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Provide the structure of the carbene that results when diazomethane (shown below) decomposes.
Look at picture
Provide the structure of the transition state in the first propagation step of the free radical chlorination of ethane.
Look at picture
Provided the following pKa values for the two acids below, draw an energy diagram for the acid-base reaction. Be sure to label each axis correctly.
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Remove an H+ from the following structure to create the most reactive (least stable) carbanion
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When acetaldehyde (CH3CHO) is deprotonated, the resulting anion is stabilized by resonance. Draw the major resonance contributing forms of this anion
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Which is a measure of the randomness of a system?
entropy
Which of the halogens below undergoes free radical halogenation with ethane most rapidly?
fluorine
The bond dissociation energy is the amount of energy required to break a bond ________.
homolytically
When compound I, C7H16, was treated with chlorine and light it yielded 3 monochlorination products that could be separated by chromatography. Two of the products were primary alkyl halides and the other was a secondary alkyl halide. Provide a possible structure for compound I.
Look at picture
Which of the following depictions most closely resembles the structure of the transition state for the following acid-base reaction?
Look at picture
Which of the presented mechanisms would be the most energetically favorable and thus the most likely mechanism to actually occur for the following free radical chain reaction? (bond dissociation energies -- H-H = 104 kcal/mol; Cl-Cl = 58 kcal/mol; H-Cl = 103 kcal/mol)
Look at picture
Write a detailed, stepwise mechanism for the following reaction.
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Write the structures of all of the monobromination products of 1,1,3,3-tetramethylcyclobutane
Look at picture
Chlorination of methane can result in a mixture of chlorinated products. What experimental conditions should be used to favor the production of chloromethane over the other chlorinated products?
Make sure the molar ratio of methane to chlorine is relatively large. Additionally, lower reaction temperature also helps prevent multiple chlorination.
The chlorination of methane is characterized by a high quantum yield. Explain what this means.
Many molecules of product are formed for every photon of light which is absorbed by the reaction mixture.
Given that the theoretical reaction below was found to be second order and bimolecular, provide a rate equation for the reaction. A-B + C-D → A-C + B-D
Rate = kr[AB][CD]
In the reaction of Cl2 with ethane and UV light, which of the following reactions would be a chain termination event(s)?
Reaction III
In the reaction of Cl2 with ethane and UV light, which of the following reactions would be a propagation event(s)?
Reactions II and IV
Which of the following statements is the best statement of the Hammond Postulate?
Related species that are similar in energy are also similar in structure
What is the relative reactivity of 2° vs 1° hydrogens in the free radical bromination of n-butane if the ratio of 1-bromobutane to 2-bromobutane formed is 7:93?
The 2° hydrogens are 20 times more reactive than the 1° ones.
Consider the following substitution reaction with a ΔG° value of -91.1 kJ/mole. HO- + CH3Cl ↔ CH3OH + Cl- Given this information which of the following statements must be true? (R = 8.315 J/mole K)
The Keq at 25°C for this reaction is very large, in other words this reaction proceeds to near completion as written, left to right under standard conditions
________ is the minimum kinetic energy reacting molecules must possess to overcome the repulsions between their electron clouds when they collide.
The activation energy or Ea
Which of the following reactive intermediate species maintains sp3 hybridization?
methyl carbanion
Given the chlorination of acetone shown below, choose the correct rate law. CH3COCH3 + Cl2 → CH3COCH2Cl + HCl
cannot be determined from stoichiometry; must be determined experimentally
Which of the following reactive intermediates can best be described as both nucleophilic and strongly basic?
carbanions
What reactive intermediate is formed when CHBr3 is treated with hydroxide or when diazomethane is irradiated?
carbene
When 1,1,3,3-tetramethylcyclobutane is brominated at 125°C, the relative reactivity of the 1°: 2°:3° hydrogens is approximately 1:82:1600. Estimate the amount of each monobromination product.
primary bromide, 3.5%; secondary bromide, 96.5%
Species with unpaired electrons are called ________.
radicals or free radicals
Consider the elementary step in the solvolysis of isopropyl chloride shown below and write the rate equation for this step (CH3)2CHCl → (CH3)2CH+ + Cl-
rate = k[(CH3)2CHCl]
The following reaction occurs readily: Experimentally one finds that if the concentration of I- is doubled, the rate doubles. Also if the concentration of CH3Br is halved, the rate is halved. What is the rate equation for this reaction?
rate = k[CH3Br][I-]
Energy is ________ when bonds are formed and is ________ when bonds are broken; therefore, bond dissociation energies are always ________.
released / consumed / endothermic
What is the CCC bond angle in (CH3)3C:-?
slightly less than 109.5 degrees
What is the hybridization of the positively charged carbon in (CH3)3C+?
sp2
What is the hybridization of the negatively charged carbon in (CH3)3C:-?
sp3
When the reaction between methane and chlorine is photochemically initiated, which of the following compounds cannot be formed through a termination reaction?
HCl
Which H atom in the molecule shown will be most readily abstracted by a bromine radical?
Hc
Rank the free radicals (I-III) shown below in order of decreasing stability (i.e., from most stable to least stable)
I > III > II
If ΔG° for a given reaction at 25°C is less than zero, which of the following statements also correctly describes this reaction at this temperature?
Keq is greater than zero.
________ is the study of reaction rates.
Kinetics
Consider the three-step mechanism for the reaction of A through intermediates B and C to produce D shown below A → B Ea = 15 kcal/mol, ΔH° = 13 kcal/mol B → C Ea = 10 kcal/mol, ΔH° = -6 kcal/mol C → D Ea = 2 kcal/mol, ΔH° = -20 kcal/mol What's the enthalpy difference between reactant A and intermediate C?
+7 kcal/mol
Of the two C-H bonds shown, which has the smaller bond dissociation energy? Explain your choice. (CH3)2CH-H vs. CH3CH2-H
(CH3)2CH-H has the smaller bond dissociation energy. This homolytic bond cleavage yields a more stable secondary radical.
Predict the major monobromination product in the following reaction.
(CH3)3CCHBrCH3
What C5H12 isomer will give only a single monochlorination product?
(CH3)4C or neopentane or 2,2-dimethylpropane
Predict the enthalpy (ΔH) value for the theoretical reaction below, and indicate whether it is endothermic or exothermic. The bond dissociation energy for each bond in Kcal/mol is shown below each reactant and product. A-B + C-D → A-C + B-D 63 88 47 96
+8 Kcal/mol, endothermic
Consider the transformation of A to B (i.e., A → B). If at equilibrium at 25°C the concentration of A is 20% of the initial concentration of A, determine the value of ΔG° (in kcal/mol) for this reaction. R = 1.987 cal/mol∙K.
-0.82 kcal/mol
Given the bond dissociation energies below (in kcal/mol), calculate the overall ΔH° for the following reaction: (CH3)3CH + Br2 → (CH3)3CBr + HBr (CH3)3CH 91 (CH3)3CBr 65 BrBr 46 HBr 88 CH3Br 70
-16 kcal/mol
Given a K of 2.2 at 25°C, calculate the corresponding DG° in kJ/mol. [R = 8.314 J/K∙ mol]
-2.0 kJ/mol
Given the bond dissociation energies below (in kcal/mol), estimate the ΔH° for the propagation step CH3)2CH∙ + C12 → (CH3)2CHCl + Cl∙ CH3CH2CH2H 98 (CH3)2CHH 95 ClCl 58 HCl 103 CH3CH2CH2Cl 81 (CH3)2CHCl 80
-22 kcal/mol
Consider the conversion of C to D via a one-step mechanism. The activation energy of this conversion is 3 kcal/mol. The energy difference between D and the transition state of the reaction is 7 kcal/mol. Estimate ΔH° for the reaction C → D
-4 kcal/mol
A certain free radical chlorination reaction of 2,3-dimethylbutane leads to the production of two major monochlorinated products 1-chloro-2,3-dimethylbutane and 2-chloro-2,3-dimethylbutane. When run through a Gas chromatograph, it was determined that the product distribution was 51.7% 1-chloro-2,3-dimethylbutane and 48.3% 2-chloro-2,3-dimethylbutane. If we assign the primary H's a reactivity of 1.0, what is the relative reactivity of the tertiary H's?
5.6 times more reactive
How many secondary hydrogens are present in the hydrocarbon below?
6
Which of the following is a carbene?
:CCl2
In the first propagation step of the free radical chlorination of methane, which of the following occurs?
A chlorine radical abstracts a hydrogen.
Which of the following species is not formed through a termination reaction in the chlorination of methane?
H2
Within the visible spectrum, it has been experimentally determined that blue light is the most effective in initiating the chlorination of methane. What is the mechanistic significance of this observation?
Blue light is effectively absorbed by chlorine but not by methane. Thus the initiating step is most likely the production of two chlorine atoms from molecular chlorine.
Provide the two propagation steps in the free-radical chlorination of ethane
CH3CH3 + Cl∙ → CH3CH2∙ + HCl CH3CH2∙ + ClCl → CH3CH2Cl + Cl∙
When light is shined on a mixture of chlorine and chloromethane, carbon tetrachloride is one of the components of the final reaction mixture. Propose a series of mechanistic steps which explain this observation.
CH3Cl + Cl∙ → ∙ CH2Cl + HCl ∙CH2Cl + Cl-Cl → CH2Cl2 + Cl∙ CH2Cl2 + Cl∙ → ∙ CHCl2 + HCl ∙CHCl2 + Cl-Cl → CHCl3 + Cl∙ CHCl3 + Cl∙ → ∙ CCl3 + HCl ∙CCl3 + Cl-Cl → CCl4 + Cl
When Br radical reacts with 1-butene (CH3CH2CH=CH2), the hydrogen atom which is preferentially abstracted is the one which produces a resonance stabilized radical. Draw the major resonance contributing forms of this radical.
CH3HCHCH2 ↔ CH3CHCH H2
The relative reactivity of the 1°: 2°: 3° hydrogens of (CH3)3CCH2CH3 in free radical chlorination is 1: 3.8: 5.0. Provide the structure of each monochlorination product, and estimate the relative amount of each in the mixture of monochlorinated products.
ClCH2C(CH3)2CH2CH3, 45.9%; (CH3)3CCHClCH3, 38.8%; (CH3)3CCH2CH2Cl, 15.3%
If stronger bonds are formed and weaker bonds are broken, then the reaction is ________.
Exothermic
If the equilibrium constant (Keq) of a reaction is 0.5 then which of the following that must be true?
Gibbs free energy (G) is positive.
In an exothermic reaction, are stronger bonds broken and weaker bonds formed or are weaker bonds broken and stronger bonds formed?
In an exothermic reaction, weaker bonds are broken and stronger bonds are formed
Explain how the termination step in a free-radical chain reaction stops the chain
In order for a free-radical chain to propagate, each mechanistic step must yield a free-radical species as one of its products. A mechanistic step that does not yield a free-radical stops the chain. Such a step is known as a termination step
Free radical bromination of pentane results in poor yields of 1-bromopentane, while cyclopentane can be readily brominated under similar conditions to yield bromocyclopentane. Offer an explanation.
In the bromination of pentane, the lowest energy reaction pathways go through secondary free radical intermediates to produce secondary alkyl bromides (2-bromopentane and 3-bromopentane). Thus 1-bromopentane is a very minor product. All of the hydrogen atom abstractions of cyclopentane lead to the same secondary radical which eventually leads to bromocyclopentane.
The Arrhenius equation mathematically models which of the following statements?
Increasing the temperature of a chemical reaction increases the number of particles in the reaction that have the minimum energy required to meet the Ea. & The rate of a chemical reaction is exponentially related to the Ea and relatively small differences in the Ea can dramatically affect the reaction rates of similar reactions at the same temperature.
Consider the one-step conversion of F to G. Given that the reaction is endothermic by 5 kcal/mol and that the energy difference between G and the transition state for the process is 15 kcal/mol, sketch a reaction-energy diagram for this reaction. Make sure to show how the given energy differences are consistent with your sketch.
Look at picture
Consider the reaction: CH3CH2∙ + Br2 → CH3CH2Br + Br∙ Given that this reaction has an activation energy of +6 kcal/mol and a ΔH° of -22 kcal/mol, sketch a reaction-energy diagram for this reaction. Label the axes and show Ea and ΔH° on your drawing
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Draw all monochlorination products expected from reaction of chlorine with 3,4-dimethylhexane. Circle the major product
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Draw an energy diagram for a two step reaction where the structure of the transition state of the rate determining step most closely resembles the starting material and the overall reaction is exothermic.
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Do you expect the initiation step in the free radical chlorination of 2,2-dimethylpropane to have a positive or negative DS? Explain briefly
One expects a positive DS for this reaction step. A single chlorine molecule is cleaved into two chlorine atoms (chlorine radicals). There is more disorder in the product
Which compound has the smaller bond dissociation energy for its carbon-chlorine bond, CH3Cl or (CH3)3CCl? Explain your reasoning.
The carbon-chlorine bond of (CH3)3CCl has the smaller bond dissociation energy. The carbon radical that results upon bond cleavage is a tertiary radical. Tertiary alkyl radicals are more stable than primary due to hyerconjugative or inductive stabilization
Consider the conversion of X to Z through the sole intermediate Y. Given the reaction-energy diagram shown below, which step is the rate-limiting step? Explain your reasoning
The conversion of Y to Z has a higher Ea than does the conversion of X to Y. The conversion of Y to Z is therefore rate-limiting
The hydrogen atom abstraction step in the free radical bromination of methane is endothermic. Use the Hammond Postulate to speculate on the extent of bond formation and bond cleavage in the transition state.
The endothermic nature of this step means that the energy of the transition state is closer to that of the step's products than to its reactants. Using the Hammond Postulate this means the transition state more closely resembles a carbon radical than a bromine radical. The carbon-hydrogen bond is almost completely broken and the hydrogen-bromine bond completely formed in the transition state.
Which of the following statements correctly describes the contribution of ΔS° to ΔG°?
The entropy term makes a greater contribution to ΔG° at high temperatures.
The hydrogen atom abstraction step in the free radical chlorine of methane is exothermic. Use the Hammond Postulate to speculate on the extent of bond formation and bond cleavage in the transition state
The exothermic nature of this step means that the energy of the transition state is closer to that of the step's reactants than to its products. Using the Hammond Postulate this means the transition state more closely resembles a chlorine radical than a carbon radical. The carbon-hydrogen bond has broken very little and the hydrogen-chlorine bond has formed very little in the transition state.
Explain the significance of the exponential factor e-Ea/RT in the Arrhenius equation
The exponential factor e-Ea/RT in the Arrhenius equation corresponds to the fraction of collisions in which the reacting molecules have the appropriate activation energy to react at a given temperature.
Use the Hammond Postulate to explain why free radical brominations are more selective than free radical chlorinations
The first propagation step in free radical bromination is endothermic while the analogous step in free radical chlorination is exothermic. From the Hammond Postulate, this means that the transition state for the bromination is product-like (ie, radical-like) while the transition state for the chlorination is reactant-like. The product-like transition state for bromination has the C-H bond nearly broken and a great deal of radical character on the carbon atom. The energy of this transition state reflects most of the energy difference of the radical products. This is not true in the chlorination case where the transition state possesses little radical character.
Explain the significance of the frequency factor A in the Arrhenius equation.
The frequency factor accounts for the number of collisions between reacting molecules and the fraction of these collisions with the proper orientation for the reaction to happen. There is a linear relationship between the number of these collisions and the rate constant of the reaction. For a given reaction A is a constant
Do you expect the initiation step in the free radical chlorination of 2,2-dimethylpropane to be endo- or exothermic? Explain briefly, and comment on the sign of DH.
The initiation step of this mechanism is the homolytic bond cleavage of the chlorine-chlorine bond. Energy is required to break a bond. The reaction step is endothermic with a positive DH.
What is meant by the mechanism of a chemical reaction?
The mechanism of a reaction is the complete, step-by-step description of exactly which bonds break, which bonds form, and the order in which these events occur in the transformation of reactants into products.
Does one expect ΔS° in a propagation step of the free-radical chlorination of methane to be greater than zero, less than zero, or approximately equal to zero? Briefly explain your choice. CH4 + Cl∙ → CH3∙ + HCl CH3∙ + ClCl → CH3Cl + Cl∙
The propagation steps of the free-radical chlorination of methane are shown below. In each of the steps, two reactant molecules generate two product molecules. This similarity in the number of molecular species means that the disorder in the reaction is neither greatly increased nor diminished. Therefore, one expects ΔS° in either propagation step to be approximately equal to zero
Consider the reaction (CH3)3CBr + CH3CH2OH → (CH3) 3COCH2CH3 + HBr. Experimentally one finds that if the concentration of (CH3)3CBr is tripled, the rate of the reaction triples. One also finds that if the concentration of CH3CH2OH is doubled, the rate of the reaction is unchanged. Which of the following correctly describes the kinetics of this reaction?
The reaction is first order overall.
Consider the three-step mechanism for the reaction of A through intermediates B and C to produce D shown below A → B Ea = 15 kcal/mol, ΔH° = 13 kcal/mol B → C Ea = 10 kcal/mol, ΔH° = -6 kcal/mol C → D Ea = 2 kcal/mol, ΔH° = -20 kcal/mol Which of the three steps is rate-limiting?
The reaction of A to B.
Given an activation energy of 15 kcal/mol, use the Arrhenius equation to estimate how much faster the reaction will occur if the temperature is increased from 100°C to 120°C. R = 1.987 cal/mol∙K
The reaction will occur about 2.8 times faster.
What accounts for the relatively high stability of the benzyl radical?
The unpaired electron is delocalized through a conjugated pi system.
What reactive species is produced in the initiation step of the free radical chlorination of 2,2-dimethylpropane?
a chlorine atom
The major monobrominated product which results when ethylcyclohexane is subjected to free radical bromination is:
a tertiary bromide.
When two carbenes collide, they immediately dimerize to give ________.
an alkene
How do alkyl substituents stabilize a carbocationic center to which they are attached?
both A and C
Consider the bond dissociation energies listed below in kcal/mol. CH3-Br 70 CH3CH2-Br 68 (CH3)2CH-Br 68 (CH3)3C-Br 65 These data show that the carbon-bromine bond is weakest when bromine is bound to a ________.
tertiary carbon
The difference in energy between reactants and the transition state is known as ________
the activation energy or Ea
The rate of a reaction typically increases as the temperature increases because:
the fraction of molecules with kinetic energy greater than Ea increases.
What term describes the highest-energy structure in a molecular collision which leads to reaction?
transition state
Which of the following correctly expresses the standard Gibbs free energy change of a reaction in terms of the reaction temperature (T) and equilibrium constant (K)?
ΔG° = -RTlnK
Which of the following correctly expresses the standard Gibbs free energy change of a reaction in terms of the changes in enthalpy and entropy?
ΔG° = ΔH° - TΔS°
If a reaction is exothermic, then:
ΔH° < 0
Which of the following is true for the termination step of a free radical chlorination reaction?
ΔH° < 0 and ΔS° < 0
Which of the following is true for the initiation step of a free radical chlorination reaction?
ΔH° > 0 and ΔS° > 0
The hydrogenation of acetylene to produce ethane is shown below. Is ΔS° for this reaction positive, negative, or impossible to predict? Explain your reasoning C2H2 (g) + 2H2 (g) → C2H6 (g)
ΔS° < 0. The three reactant molecules have significantly more freedom of motion, and randomness, than does the single molecule of product into which they are converted.
Which of the following is not a possible termination step in the free radical chlorination of methane?
∙CH3 + Cl2 → CH3Cl + Cl∙
Which of the following is a propagation step in the free radical chlorination of dichloromethane?
∙CHCl2 + Cl2 → CHCl3 + Cl∙
