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Worksheetschapter 4 exam
Total questions: 128
Worksheet time: 2hrs 33mins
(a) is the study of reaction rates.
What is meant by the mechanism of a chemical reaction?
(a)
Which of the following species is not formed through a termination reaction in the
chlorination of methane?
CH3Cl
HCl
H2
CH3CH3
Draw the important resonance form/s of the following free radical.
Draw the product of the monochlorination of methane with Cl2 in the presence of light or
heat.
(a)
In the presence of a small amount of bromine, the following light-promoted reaction has been
observed. Write a mechanism for this reaction, making sure to explain how both products are
formed.
Species with unpaired electrons are called (a) .
Which of the following is not a possible termination step in the free radical chlorination of
methane?
∙CH3 + Cl2 → CH3Cl + Cl∙
∙CH3 + Cl∙ → CH3Cl
∙CH3 + ∙CH3 → CH3CH3
∙CH3 + wall → CH3-wall
Cl∙ + wall → Cl-wall
In the first propagation step of the free radical chlorination of methane, which of the following
occurs?
Cl2 dissociates.
A chlorine radical abstracts a hydrogen.
A carbon radical reacts with Cl2.
A carbon radical reacts with a chlorine radical.
Two chlorine radicals combine
In the reaction of Cl2 with ethane and UV light, which of the following reactions would be a
chain termination event(s)?
I) Cl∙ + CH3-CH3 → CH3-CH2-Cl + H∙
II) Cl∙ + CH3-CH3 → CH3-H2C∙ + HCl
III) Cl∙ + CH3-H2C∙ → CH3-CH2-Cl
IV) Cl2 + CH3-H2C∙ → CH3-CH2-Cl + Cl∙
V) Cl2 + UV light → C l∙ + Cl∙
reaction V
reactions I and IV
reactions III and IV
reactions I and II
reaction III
In the reaction of Cl2 with ethane and UV light, which of the following reactions would be a
propagation event(s)?
I) Cl∙ + CH3-CH3 → CH3-CH2-Cl + H∙
II) Cl∙ + CH3-CH3 → CH3-H2C∙ + HCl
III) Cl∙ + CH3-H2C∙ → CH3-CH2-Cl
IV) Cl2 + CH3-H2C∙ → CH3-CH2-Cl + Cl∙
V) Cl2 + UV light → C l∙ + Cl∙
reactions I and V
reactions II, III and IV
reactions I and IV
reactions II and IV
reactions I, II and IV
Which of the following is a propagation step in the free radical chlorination of
dichloromethane?
∙ CHCl2 + Cl2 → CHCl3 + Cl∙
∙ CHCl2 + Cl∙ → CHCl3
CH2Cl2 + Cl∙ → CHCl3 + H∙
Cl2 + UV light → 2 Cl∙
∙ CHCl2 + ∙ CHCl2 → CHCl2CHCl2
Provide the two propagation steps in the free-radical chlorination of ethane.
(a)
Write an equation to describe the initiation step in the chlorination of methane.
(a)
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?
(a)
When the reaction between methane and chlorine is photochemically initiated, which of the
following compounds cannot be formed through a termination reaction?
CH3Cl
HCl
CH3CH3
Cl2
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.
(a)
Write a detailed, stepwise mechanism for the following reaction.
The monochlorination of butane with chlorine gas under photolysis will give two products.
Draw both of them and then predict the product ratio.
(a)
For the compound below, the number of 1°, 2° and 3° hydrogens, respectively is ________.
1, 3, and 1
3, 6 and 2
3, 6 and 1
1, 6 and 0
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%
0.43; 30%
1.0; 50%
2.3; 70%
5.4; 84%
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 ________.
0.18; 15%
0.43; 30%
1.0; 50%
2.3; 70%
5.4; 84%
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 ________.
0.18; 15%
0.43; 30%
1.0; 50%
2.3; 70%
5.4; 84%
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?
(a)
Given a K of 2.2 at 25°C, calculate the corresponding ΔG° in kJ/mol. [R = 8.314 J/K∙ mol]
(a)
Given a K of 0.45 at 25°C, calculate the corresponding ΔG° in kJ/mol. [R = 8.314 J/K∙ mol]
(a)
Given a ΔG° of -8.0 kJ/mol at 25°C, calculate the corresponding K. [R = 8.314 J/K∙ mol]
(a)
If the percentage of the axial onformer (on the product side of the reaction) is 42%, what is
the ΔG° (in kJ/mol) for the reaction at 25°C? [R = 8.314 J/K∙ mol]
(a)
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° kcal/mol) for this reaction. R = 1.987 cal/mol∙K.
(a)
If the equilibrium constant (Keq) of a reaction is 0.5 then which of the following that must be
true?
The reaction will have an early transition state.
Reaction equilibrium will favor the products.
Gibbs free energy (G) is positive.
Gibbs free energy (G) is negative.
Given the pKa values for the two acids below, what would you expect the Keq for this
reaction to be?
Keq > 1
Keq = 1
Keq < 1
You can't tell from the information given.
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 △G°
for the reaction is 1.0 kcal/mol. R=1.987 cal/mol*K
0.40M
0.60M
1.00M
1.40M
1.60M
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.
The Keq at 25°C for this reaction is very small (<1), in other words this reaction does not
proceed from left to right but rather is favored from right to left under standard conditions .
At 250°C the equilibrium concentration is shifted right in favor of the products (CH3OH and
Cl-). In other words there is more product than at 25°C
At 250°C the equilibrium concentration of products and reactants is nearly the same.
Both A and C are correct.
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.
(a)
Which is a measure of the randomness of a system?
entropy
enthalpy
free energy
halogenation
stoichiometry
In an exothermic reaction, are stronger bonds broken and weaker bonds formed or are
weaker bonds broken and stronger bonds formed?
(a)
Which of the following statements correctly describes the contribution of ΔS° to ΔG°?
The entropy term makes a greater contribution to ΔG° at low temperatures.
The entropy term makes a greater contribution to ΔG° at high temperatures.
The entropy term makes a greater contribution to ΔG° in exothermic reactions.
The entropy term makes a greater contribution to ΔG° in endothermic reactions.
The entropy term always makes a more significant contribution to ΔG° than does the enthalpy
If a reaction is exothermic, then ________.
ΔS° < 0
ΔS° > 0
ΔH° < 0
ΔH° > 0
both B and D
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°
ΔG° = ΔH° + TΔS°
ΔG° = ΔS° - TΔH°
ΔG° = ΔS° + TΔH°
none of the above
Which of the following is true for the initiation step of a free radical chlorination reaction?
ΔH° > 0 and ΔS° > 0
ΔH° > 0 and ΔS° < 0
ΔH° < 0 and ΔS° > 0
ΔH° < 0 and ΔS° < 0
ΔH° = 0 and ΔS° = 0
Which of the following is true for the termination step of a free radical chlorination reaction?
ΔH° > 0 and ΔS° > 0
ΔH° > 0 and ΔS° < 0
ΔH° < 0 and ΔS° > 0
ΔH° < 0 and ΔS° < 0
ΔH° = 0 and ΔS° = 0
If stronger bonds are formed and weaker bonds are broken, then the reaction is (a) .
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.
(a)
Predict the signs of ΔH° and ΔS° in the reaction of cyclohexene with H2 to form
cyclohexane, shown below.
(a)
Will the sign of ΔS° in the combustion of propane be positive, negative or zero?
(a)
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)
(a)
The bond dissociation energy is the amount of energy required to break a bond ________.
homolytically
heterolytically
so as to produce the more stable pair of ions
via hydrogenation
none of the above
Draw the transition state for the hydrogen abstraction reaction shown below. (Hint: this is an
endothermic reaction.)
(CH3)3C—H + ∙Br → (CH3)3C∙ + H—Br
[(H₃C)₃C------H--Br]
[(H₃C)₂CH—H—Br]
[(H₃C)₃C—Br—H]
[(H₃C)₃CH + Br]
[(H₃C)₂C—H—Br₂]
What reactive species is produced in the initiation step of the free radical chlorination of 2,2-
dimethylpropane?
a chlorine atom
a chlorine radical anion
a carbon radical
a carbocation
Do you expect the initiation step in the free radical chlorination of 2,2-dimethylpropane to
have a positive or negative ΔS? Explain briefly.
(a)
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.
+8 Kcal/mol, endothermic
-8 Kcal/mol, exothermic
+16 Kcal/mol, endothermic
+8 Kcal/mol, exothermic
Energy is ________ when bonds are formed and is ________ when bonds are broken;
therefore, bond dissociation energies are always ________.
released; consumed; exothermic
released; consumed; endothermic
consumed; released; exothermic
consumed; released; endothermic
consumed; released; isothermic
Which compound has the smaller bond dissociation energy for its carbon-chlorine bond,
CH3Cl or (CH3)3CCl? Explain your reasoning.
(a)
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)
H2 + Cl2 ---> 2 HCl
H2 --> H∙ + H∙
H∙ + Cl2 → Cl∙ + HCl
H∙ + Cl∙ → HCl
Cl2 --> Cl∙ + Cl∙
Cl∙ + H2 → H∙ + HCl
H∙ + Cl2 → HCl + Cl∙
H2 --> H∙ + H∙
H∙ + Cl2 → Cl∙ + HCl
Cl∙ + H2 → HCl + H∙
Cl2 --> Cl∙ + Cl∙
Cl∙ + H2 → H∙ + HCl
H∙ + Cl∙ → HCl
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 ________.
methyl carbon
primary carbon
secondary carbon
tertiary carbon
quaternary carbon
Given the bond dissociation energies below (in kcal/mol), estimate the ΔH° for the
propagation step (CH3)2CH. +Cl2 --> (CH3)2CHCl +Cl.
CH3CH2CH2—H 98
(CH3)2CH—H 95
Cl—Cl 58
H—Cl 103
CH3CH2CH2—Cl 81
(CH3)2CH—Cl 80
-22 kcal/mol
+22 kcal/mol
-40 kcal/mol
+45 kcal/mol
Given the chlorination of acetone shown below, choose the correct rate law.
CH3COCH3 + Cl2 → CH3COCH2Cl + HCl
rate = [CH3COCH3]
rate = [Cl2]
rate = [CH3COCH3][Cl2]
rate = [CH3COCH3][Cl2]1/2
cannot be determined from stoichiometry; must be determined experimentally
Given the bond dissociation energies below (in kcal/mol), calculate the overall ΔH° for the
following reaction:
(CH3)3CH + Br2 → (CH3)3CBr + HBr
(CH3)3C—H 91
(CH3)3C—Br 65
Br—Br 46
H—Br 88
CH3—Br 70
(a)
Of the two C-H bonds shown, which has the smaller bond dissociation energy? Explain your
choice.
(a)
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-
(a)
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 third order in (CH3)3CBr.
The reaction is first order in CH3CH2OH.
The reaction is second order overall.
The reaction is first order overall.
none of the above
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
(a)
The following reaction occurs readily: CH3Br + I- --> CH3I + Br-
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?
(a)
(a) is the minimum kinetic energy reacting molecules must possess to overcome the
repulsions between their electron clouds when they collide.
The Arrhenius equation mathematically models which of the following statements?
The rate of a chemical reaction increases exponentially with increasing concentration of
reactants.
The rate of a chemical reaction is directly related to the Ea and that increasing the temperature
will alter the Ea for that reaction.
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.
both C and D
What term describes the highest-energy structure in a molecular collision which leads to
reaction?
(a)
Consider the reaction energy diagram shown below. Label the axes.
a. Which points on the curve indicate transition states? Label them with A, B, etc.
b. Is the overall reaction exothermic or endothermic?
A - exothermic
B - endothermic
A - endothermic
B- exothermic
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.
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.
Provide the structure of the transition state in the first propagation step of the free radical
chlorination of ethane.
(a)
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.
(a)
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.
The reaction of B to C.
The reaction of C to D.
All three steps occur at the same rate; there is no rate-limiting step.
The most exothermic step is rate-limiting
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?
(a)
The difference in energy between reactants and the transition state is known as (a) .
The rate of a reaction typically increases as the temperature increases because ________.
the A term in the Arrhenius equation increases
the fraction of molecules with kinetic energy greater than Ea increases
the activation energy decreases
the activation energy increases
the molecules make more collisions with the wall of the reaction vessel
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° = e-K/RT
ΔG° = eK/RT
ΔG° = RTlnK
ΔG° = -RTlnK
none of the above
Explain the significance of the frequency factor A in the Arrhenius equation.
(a)
Rank the following carbocations in order of stability. (The most stable is first.)
I > III > II
II > I > III
III > I > II
I > II > III
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
(a)
In the hydrocarbon shown below, how many tertiary hydrogens are present?
0
1
2
3
4
How many distinct dichlorination products can result when isobutane is subjected to free
radical chlorination?
1
2
3
4
6
How many distinct monochlorinated products can result when isobutane is subjected to free
radical chlorination?
1
2
3
4
5
How many distinct monochlorinated products can result when cyclopentane is subjected to
free radical chlorination?
1
2
3
4
5
What C5H12 isomer will give only a single monochlorination product?
(a)
The major monobrominated product which results when ethylcyclohexane is subjected to
free radical bromination is ________.
a primary bromide
a secondary bromide
a tertiary bromide
a quaternary bromide
bromomethane
Which of the halogens below undergoes free radical halogenation with ethane most rapidly?
fluorine
chlorine
iodine
bromine
pyridine
How many secondary hydrogens are present in the hydrocarbon below?
2
6
7
8
16
Write the structures of all of the monobromination products of 1,1,3,3-
tetramethylcyclobutane.
Provide the major organic product that results when 2,2,4-trimethylpentane is subjected to
free radical bromination.
Predict the major monobromination product in the following reaction.
(CH3)3CCH2CH3 + Br2 --->
(a)
Predict the major monobromination product in the following reaction.
What is the name of the major monobrominated product which results when 3-
methylpentane is subjected to Br2/hν conditions?
(a)
What is the name of the major monobrominated product which results when
methylcyclohexane is subjected to Br2/hν conditions?
(a)
Provide the major organic product(s) in the reaction below.
Butylated hydroxytoluene, or BHT, is a potent radical scavenger that is used as a food
preservative. It reacts preferentially with free radicals in food to form a new, less reactive
resonance stabilized species. The reaction of BHT with a damaging free radical is shown below.
Draw two additional resonance structures for the radical form of BHT where the free electron is
delocalized
Rank the free radicals (I-III) shown below in order of decreasing stability (i.e., from most
stable to least stable).
I > III > II
II > III > I
I > II > III
II > I > III
III > II > I
Which H atom in the molecule shown will be most readily abstracted by a bromine radical?
Ha
Hb
Hc
Hd
He
List the following radicals in order of increasing stability (i.e., from least stable to most
stable).
(CH3)3C∙, CH2 CHCH2∙, CH3CH2∙, CH3∙, (CH3)2CH∙
(a)
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.
(a)
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.
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.
(a)
When the free radical halogenation of methylpropane is done with chlorine, the major
product is 1-chloro-2-methylpropane, but if bromine is used, the major product is 2-bromo-2-
methylpropane. Explain the difference in the regiochemistry of these reactions.
(a)
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
(a)
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.
(a)
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?
(a)
Draw all monochlorination products expected from reaction of chlorine with 3,4-
dimethylhexane. Circle the major product.
Which of the following statements is the best statement of the Hammond Postulate?
In an endothermic reaction, the transition state is closer to the reactants in structure.
In an exothermic reaction, the transition state is closer in energy to the products.
Related species that are similar in energy are also similar in structure.
The structure of the transition state in an organic reaction is always modeled on the structure
of the reactants leading to that transition state.
Transition states are molecular species of finite lifetime whose properties can be probed
through
Use the Hammond Postulate to explain why free radical brominations are more selective
than free radical chlorinations.
(a)
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.
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.
(a)
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.
(a)
Which of the following depictions most closely resembles the structure of the transition
state for the following acid-base reaction?
Which of the following is a carbene?
CH2 CHO-
CH3CH2∙
:CCl2
CH3CH2+
NCO-
Which of the following reactive intermediates can best be described as both nucleophilic
and strongly basic?
carbanions
carbocations
carbenes
free radicals
alkanes
How do alkyl substituents stabilize a carbocationic center to which they are attached?
through an inductive donation of electron density to the cationic center
through an inductive removal of electron density from the cationic center
through hyperconjugation
both A and C
both B and C
Which of the following reactive intermediate species maintains sp3 hybridization?
methyl carbanion
dibromocarbene
tertiary carbocation
secondary alkyl radical
both B and C
When two carbenes collide, they immediately dimerize to give ________.
an alkane
an alkene
an alkyne
an aromatic ring
a carbanion
Provide the structure of the carbene that results when diazomethane (shown below)
decomposes.
Which of the two 2° radicals below is more stable and why?
(a)
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.
(a)
When acetaldehyde (CH3CHO) is deprotonated, the resulting anion is stabilized by
resonance. Draw the major resonance contributing forms of this anion.
What is the hybridization of the negatively charged carbon in (CH3)3C:-?
(a)
What is the CCC bond angle in (CH3)3C:-?
(a)
What is the hybridization of the positively charged carbon in (CH3)3C+?
(a)
What is the CCC bond angle in (CH3)3C+?
(a)
What reactive intermediate is formed when CHBr3 is treated with hydroxide or when
diazomethane is irradiated?
carbene
carbanion
carbocation
carbon radical
Remove an H+ from the following structure to create the most reactive (least stable)
carbanion.
Describe the hybridization of the cationic center and predict the CCC bond angle in
(CH3)3C+
(a)
