Chapter 5, Polar Addition and Elimination Reactions Video Solutions, Advanced Organic Chemistry. Part A. Structure and Mechanisms

Problem 1

Which compound of each pair will react faster with the specified reagent? Explain your answer.
a. 1-hexene or $E-3$-hexene with bromine in acetic acid.
b. cis- or trans-4-( $t$-butyl)cyclohexylmethyl bromide with $\mathrm{KOC}\left(\mathrm{CH}_{3}\right)_{3}$ in $t$-butyl alcohol.
c. 2-phenylpropene or 4-(1-methylethenyl)benzoic acid with sulfuric acid in water.

Lottie Adams

Numerade Educator

03:35

Problem 2

Predict the structure, including stereochemistry, of the product(s) expected for the following reactions. If more than one product is shown, indicate which is major and which is minor.

Madi Sousa

Numerade Educator

03:12

Problem 3

The reaction of the cis and trans isomers of $N, N, N$-trimethyl-(4-t-
butylcyclohexyl)ammonium chloride with $\mathrm{K}^{+-} \mathrm{O}-t$-Bu in $t$-butyl alcohol have
been compared. The cis isomer gives $90 \% 4-t$-butylcyclohexene and $10 \% N, N-$
dimethyl-(4-t-butylcyclohexyl)amine, whereas the trans isomer gives only the
latter product in quantitative yield. Explain the different behavior of the two
isomers.

Lottie Adams

Numerade Educator

01:01

Problem 4

For E2 eliminations in 2-phenylethyl systems with several different leaving groups, both the primary kinetic isotope effect and Hammett $\rho$ have been determined. Deduce information about the nature and location (early, late) of the TS in the variable E2 spectrum. How does the identity of the leaving group affect the nature and location of the TS?

Ajay Singhal

Numerade Educator

02:59

Problem 5

Predict the effect on the 1-butene, $Z-2$-butene, and $E-2$-butene product ratio when the E2 elimination (KOEt, EtOH) of erythro-3-deuterio-2-bromobutane is compared with 2-bromobutane. Which alkene(s) will increase in relative amount and which will decrease in relative amount? Explain the basis of your answer.

Narayan Hari

Numerade Educator

08:26

Problem 6

Arrange the following compounds in order of increasing rate of acidcatalyzed hydration: ethene, propene, 2-cyclopropylpropene, 2-methylpropene, 1 -cyclopropyl-1-methoxyethene. Explain the basis of your answer.

Zaida Minjares

Numerade Educator

05:33

Problem 7

Discuss the factors that are responsible for the regiochemistry and stereochemistry observed for the following reactions.

Niamat Khuda

Numerade Educator

11:40

Problem 8

Explain the mechanistic basis of the following observations and discuss how the observation provides information about the reaction mechanism.
a. When 1-aryl-2-methyl-2-propyl chlorides (8-A) react with $\mathrm{NaOCH}_{3}$, roughly 1:1 mixtures of internal (8-B) and terminal alkene (8-C) are formed. By using the product ratios, the overall reaction rate can be dissected into the rates for formation of 8-B and 8-C. The rates are found to be substituent dependent for 8-B $(\rho=+1.4)$ but not for $8-C(\rho=-0.1 \pm 0.1)$. All the reactions are second order, first order in reactant and first order in base.
b. When 1,3-pentadiene reacts with DCl, more $E-4$-chloro-5-deuterio-2-pentene $(60-75 \%)$ is formed than $E-4$-chloro-1-deuterio-2-pentene $(40-25 \%)$.
c. When indene (8-D) is brominated in $\mathrm{CCl}_{4}$, it gives some $15 \%$ syn addition, but indenone (8-E) gives only anti addition under these conditions. When the halogenation of indenone is carried out using $\mathrm{Br}-\mathrm{Cl}$, the product is trans-2bromo-3-chloroindenone.
d. The acid-catalyzed hydration of allene gives acetone, not allyl alcohol or propanal.
e. In the addition of HCl to cyclohexene in acetic acid, the ratio of cyclohexyl acetate to cyclohexyl chloride drops significantly when tetramethylammonium chloride is added in increasing concentrations. The rate of the reaction is also accelerated. These effects are not observed with styrene.
f. The $\rho$ value for elimination of $\mathrm{HF}$ using $\mathrm{K}^{+-} \mathrm{O}-t-\mathrm{Bu}$ from a series of 1 -aryl2-fluoroethanes increases from the mono- to di- and trifluoro derivatives, as indicated below.

Nicholas Sacco

Numerade Educator

13:47

Problem 9

$\text { Suggest mechanisms that account for the outcome of the following reactions: }$

Tom Rutherford

Numerade Educator

01:56

Problem 10

The rates of bromination of internal alkynes are roughly 100 times greater that the corresponding terminal alkynes. For hydration, however, the rates are less than 10 times greater for the disubstituted compounds. Account for this difference by comparison of the mechanisms for bromination and hydration.

Dr. Satish Ingale

Numerade Educator

03:16

Problem 11

The bromination of 3-aroyloxycyclohexenes gives rise to a mixture of stereoisomeric and regioisomeric products. The product composition for $\mathrm{Ar}=$ phenyl is shown. Account for the formation of each of these products.

Ian Kaigh

Numerade Educator

02:12

Problem 12

The Hammett correlation of the acid-catalyzed dehydration of 1,2 -diaryl ethanols has been studied. The correlation resulting from substitution on both the 1 - and 2-aryl rings is: $\log k=-3.78\left(\sigma_{\mathrm{Ar}}^{+}+0.23 \sigma_{\mathrm{Ar}^{\prime}}\right)-3.18$. Rationalize the form of this correlation equation. What information does it give about the involvement of the $\mathrm{Ar}^{\prime}$ ring in the rate-determining step of the reaction?

Banhishikha Sinha

Numerade Educator

03:48

Problem 13

The addition of $\mathrm{HCl}$ to alkenes such as 2-methyl-1-butene and 2-methyl-2-butene in nitromethane follow a third-order rate expression:
$$
\text { Rate }=k[\mathrm{HCl}]^{2}[\text { alkene }]
$$
It has also been established that there is no incorporation of deuterium into the reactant at $50 \%$ completion when $\mathrm{DCl}$ is used. Added tetraalkylammonium chloride retards the reaction, but the corresponding perchlorate salt does not. Propose a reaction mechanism that is consistent with these observations.

Niamat Khuda

Numerade Educator

05:35

Problem 14

In the bromination of substituted styrenes, a $\rho \sigma^{+}$plot is noticeably curved. If
the extremes of the curve are taken to represent straight lines, the curve can be
resolved into two Hammett relationships with $\rho=-2.8$ for EWG substituents
and $\rho=-4.4$ for ERG substituents. The corresponding $\beta$-methylstyrenes
give a similarly curved plot. The stereoselectivity of the reaction of the $\beta-$
methylstyrenes is also dependent on the substituents. The reaction is stere-
ospecifically anti for strong EWGs, but is only weakly stereoselective, e.g., $63 \%$
anti:37\% syn, for methoxy. Discuss a possible mechanistic basis for the curved
Hammett plots and the relationship to the observed stereochemistry.

Henry R

Numerade Educator

01:43

Problem 15

The second-order rate constants and solvent kinetic isotope effects for acidcatalyzed hydration are given below for several 2-substituted 1,3-butadienes. The products are a mixture of 1,2 - and 1,4-addition. What information do these data provide about the mechanism of the reaction?

George Bennett

Numerade Educator

01:17

Problem 16

The reaction of both $E$ - and $Z$-2-butene with acetic acid to give 2-butyl acetate is catalyzed by various strong acids. With $\mathrm{DBr}, \mathrm{DCl}$, and $\mathrm{CH}_{3} \mathrm{SO}_{3} \mathrm{H}$ in $\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{D}$, the reaction proceeds with largely $(84 \pm 2 \%)$ anti addition. If the reaction is stopped short of completion, there is no incorporation of deuterium into unreacted alkene, nor any interconversion of the $E=$ and $Z$-isomers. When the catalyst is changed to $\mathrm{CF}_{3} \mathrm{SO}_{3} \mathrm{H}$, the recovered butene shows small amounts of 1-butene and interconversion of the 2 -butene stereoisomers. The stereoselectivity of the reaction drops to 60-70\% anti addition. How can you account for the changes that occur when $\mathrm{CF}_{3} \mathrm{SO}_{3} \mathrm{H}$ is used as the catalyst, as compared with the other acids?

Nikhil Choudhary

Numerade Educator

01:48

Problem 17

A comparison of rate and product composition of the products from reaction of $t$-butyl chloride with $\mathrm{NaOCH}_{3}$ in methanol and methanol-DMSO mixtures has been reported. Some of the data are shown below. Interpret the changes in rates and product composition as the amount of DMSO in the solvent mixture is increased.

Lottie Adams

Numerade Educator

04:50

Problem 18

a. The gas phase basicity of substituted $\alpha$-methyl styrenes follows the YukawaTsuno equation with $r^{+}=1.0$. The corresponding $r^{+}$for 1 -phenylpropyne is $1.12$ and for phenylacetylene it is $1.21$. How are these values related to the relative stability of the carbocations formed by protonation?

Julia G.

Numerade Educator

01:13

Problem 19

Crown ethers have been found to catalyze the ring opening of epoxides by $\mathrm{I}_{2}$ and $\mathrm{Br}_{2}$. The catalysts also improve the regioselectivity, favoring addition of the halide at the less-substituted position. A related structure (shown on the right) is an even better catalyst. Indicate a mechanism by which these catalytic effects can occur.

Anatole Borisov

Numerade Educator

07:10

Problem 20

The chart below shows the regio- and stereoselectivity observed for oxymercuration reduction of some 3- and 4-alkylcyclohexenes. Provide an explanation for the product ratios in terms of the general mechanism for oxymercuration discussed in Section 5.6.1.

Rajesh Singh

Numerade Educator

06:01

Problem 21

Solvohalogenation can be used to achieve both regio- and stereochemical control for synthetic purposes in alkene addition reactions. Some examples are shown below. Discuss the factors that lead to the observed regio- or stereochemical outcome.
a. Control of the stereochemistry of an epoxide:
b. Formation of cis-diols:
c. Chemoselective functionalization of polyalkenes:

Ian Kaigh

Numerade Educator