7. Substitution Reactions

The allylic bromide below gives two Sₙ1 products. Justify the formation of each.

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Provide a mechanism for the following S_N1 reaction. What roles (acid/base/nucleophile/electrophile) does ethanol play in the reaction? What functional group is made in this reaction?

<IMAGE>

(••) Predict the product(s) that would result when molecules (a)–(p) are allowed to react under the following conditions: (vii) HCl; (viii) HBr; If no reaction occurs, write 'no reaction.'

(f) <IMAGE>

(••) Predict the product(s) that would result when molecules (a)–(p) are allowed to react under the following conditions: (i) SOCl₂ ; (ii) PBr₃ ; (iii) SOCl₂ , NEt₃ (iv) 1. TsCl, Et₃N 2. NaCN; (v) 1. TsCl, Et₃N 2. NaOt-Bu (vi) H₂SO₄ (vii) HCl; (viii) HBr; (ix) PCC; (x) H₂CrO₄ , H₂O (xi) HOCl, H₂O (xii) HIO₄ If no reaction occurs, write 'no reaction.'

(f) <IMAGE>

Which compound is more reactive in an SN1 reaction? In each case, you can assume that both alkyl halides have the same stability.

a. <IMAGE>

b. <IMAGE>

a. Assuming that the two compounds shown below have the same stability, which one would you expect to be more reactive in an SN1 reaction?

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b. Draw the products that each would form when the solvent is ethanol.

A reluctant first-order substrate can be forced to ionize by adding some silver nitrate (one of the few soluble silver salts) to the reaction. Silver ion reacts with the halogen to form a silver halide (a highly exothermic reaction), generating the cation of the alkyl group.

R—X + Ag+ —> R+ + AgX↓

Give mechanisms for the following silver-promoted rearrangements.

b. <IMAGE> AgNO3, H2O/CH3CH2OH—> <IMAGE>

Propose a mechanism involving a hydride shift or an alkyl shift for each solvolysis reaction. 

Explain how each rearrangement forms a more stable intermediate.

Hint: Most rearrangements convert 2° (or incipient 1°) carbocations to 3° or resonance-stabilized carbocations.

c. <IMAGE> + CH3C=O(OH) heat—> <IMAGE> + <IMAGE> 

Choose the member of each pair that will react faster by the SN1 mechanism.

c. n-propyl bromide or allyl bromide

d. 1-bromo-2,2-dimethylpropane or 2-bromopropane

Predict the compound in each pair that will undergo solvolysis (in aqueous ethanol) more rapidly.

c. <IMAGE> or <IMAGE> 

d. <IMAGE> or <IMAGE> 

Allylic halides have the structure

H2C=CHC(X)H2 <IMAGE>

a. Show how the first-order ionization of an allylic halide leads to a resonance-stabilized cation.

Often, compounds can be synthesized by more than one method. Show how this 3° alcohol can be made from the

following:

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(d) a 3° alkyl bromide

Give the substitution products expected from solvolysis of each compound by heating in ethanol.

(c) <IMAGE> 

(d) <IMAGE> 

<IMAGE>

Propose a mechanism for the reaction of benzyl bromide with ethanol to give benzyl ethyl ether (shown above).

Propose a mechanism involving a hydride shift or an alkyl shift for each solvolysis reaction.

Explain how each rearrangement forms a more stable intermediate.

Hint: Most rearrangements convert 2° (or incipient 1°) carbocations to 3° or resonance-stabilized carbocations.

d. <IMAGE> CH3CH2OH heat—> <IMAGE> + <IMAGE>

Predict the products of the following SN2 reactions.

c. <IMAGE> Na+ + Ch3CH2Br —>

d. CH3(CH2)8CH2Cl + Na+ -C≡CH —>

Choose the member of each pair that will react faster by the SN1 mechanism.

e. 2-iodo-2-methylbutane or tert-butyl chloride

f. 2-bromo-2-methylbutane or ethyl iodide

When tert-butyl bromide is heated with an equal amount of ethanol in an inert solvent, one of the products is ethyl tert-butyl ether.

What happens to the rate if the concentration of tert-butyl bromide is tripled and the concentration of ethanol is doubled?

What happens to the rate if the temperature is raised?

tert-Butyl chloride undergoes solvolysis in both acetic acid and formic acid. Solvolysis occurs 5000 times faster in one of these two solvents than in the other. In which solvent is solvolysis faster? Explain your answer. (Hint: Formic acid is more polar than acetic acid.)

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Rank the following alkyl halides from most reactive to least reactive in an SN1 reaction: 2-bromo-2-methylpentane, 2-chloro-2-methylpentane, 3-chloropentane, and 2-iodo-2-methylpentane.

Propose a mechanism for each reaction, showing explicitly how the observed mixtures of products are formed.

(e) 3-chlorobut-1-ene + AgNO3, H2O —> but-2-en-1-ol + but-3-en-2-ol

Provide the mechanism and final products for the following reactions.