Ch 6 Problem Solving

Question 1

What determines the overall order of reaction?

Answer 1

The strength of the base or nucleophile determines the order of the reaction. If a strong nucleophile (or base) is present, it will force second-order kinetics, either S_N2 or E2. A strong nucleophile attacks the electrophilic carbon atom or abstracts a proton faster than the molecule can ionize for first-order reactions.

Question 2

Explain the expected reaction(s) of a primary alkyl halide

Answer 2

Primary halides usually undergo the S_N2 reaction, occasionally the E2 reaction. Unless they are resonance-stabilized, primary halides rarely undergo first-order reactions, because primary carbocations are relatively unstable. With good nucleophiles, S_N2 sub- stitution is usually observed. With a strong base, E2 elimination may also be observed.

Question 3

Explain the expected reaction(s) of a tertiary alkyl halide

Answer 3

Tertiary halides usually undergo the E2 reaction (strong base) or a mixture of S_N1 and E1 (weak base). Tertiary halides cannot undergo the SN2 reaction. A strong base forces second-order kinet- ics, resulting in elimination by the E2 mechanism. In the absence of a strong base, terti- ary halides react by first-order processes, usually a mixture of S_N1 and E1. The specific reaction conditions determine the ratio of substitution to elimination.

Question 4

Explain the expected reaction(s) of a secondary alkyl halide

Answer 4

The reactions of secondary halides are the most difficult to predict. With a strong base, either the S_N2 or the E2 reaction is possible. With a weak base and a good ionizing solvent, either the S_N1 or the E1 reaction is possible. Mixtures of products are common. Shown are these possibilities with a secondary halide under second-order and first-order conditions.

Question 5

Explain the specificity of some nucleophiles and bases

Answer 5

Some nucleophiles and bases favor substitution or elimination. To promote elimination, the base should readily abstract a proton but not readily attack a carbon atom. A bulky strong base, such as tert-butoxide [^-OC(CH₃)₃], enhances elimination. Higher temperatures also favor elimination in most cases, because more molecules are formed, and ∆S < 0. As the temperature increases, the free energy term, -T∆S, becomes more negative and more favorable for elimination. To promote substitution, we need a good nucleophile with limited basicity: a highly polarizable species that is the con-jugate base of a strong acid. Bromide (Br^-) and iodide (I^-) are examples of good nucleophiles that are weak bases and favor substitution.

Question 6

Explain the effect of the strong nucleophile or base on the order of the reaction

Answer 6

The strength of the base or nucleophile determines the order of the reaction. If a strong nucleophile (or base) is present, it will force second-order kinetics, either S_N2 or E2. A strong nucleophile attacks the electrophilic carbon atom or abstracts a proton faster than the molecule can ionize for first-order reactions.

Question 7

Explain the effect of the strong nucleophile or base on the order of the reaction

Answer 7

The strength of the base or nucleophile determines the order of the reaction. If no strong base or nucleophile is present, the fastest reaction will probably be a first- order reaction, either S_N1 or E1. Addition of silver salts to the reaction can force some difficult ionizations.

Question 8

Describe the first step of the E1 reaction

Answer 8

E1 Step 1: Unimolecular ionization to give a carbocation (rate-limiting).

Question 9

Describe the rate-determing step of the E1 reaction

Answer 9

E1 Step 1: Unimolecular ionization to give a carbocation (rate-limiting).

Question 10

Describe the order of reaction of the E1 mechanism

Answer 10

The E1 reaction requires ionization to a carbocation intermediate like the S_N1, so it follows the same order of reactivity: 3° > 2° >> 1°.

Question 11

Describe the second step in the E1 mechanism

Answer 11

E1 Step 2: Deprotonation by a weak base (often the solvent) gives the alkene (fast).

Question 12

Describe the fast step of the E1 mechanism

Answer 12

E1 Step 2: Deprotonation by a weak base (often the solvent) gives the alkene (fast).

Question 13

Describe the strereochemistry of the S_N1 reaction

Answer 13

In the S_N1 mechanism, the carbocation intermediate is sp2 hybridized, planar and achiral; attack from both faces gives both enantiomers of the product. Such a process, giving both enantiomers of the product (whether or not the two enantiomers are produced in equal amounts), is called racemization. The product is either racemic or at least less optically pure than the starting material.

Question 14

Amongst SN1, SN2, E1 and E2, explain the effect of the leaving group

Answer 14

all require a good leaving group

Question 15

Describe the first step of the S_N1 reaction

Answer 15

S_N1 Step 1. Formation of the carbocation (rate-limiting).

Question 16

Describe the rate-determining step of the S_N1 reaction

Answer 16

S_N1 Step 1. Formation of the carbocation (rate-limiting).

Question 17

Describe the second step of the S_N1 reaction

Answer 17

S_N1 Step 2. Nucleophilic attack on the carbocation (fast).

Question 18

Describe the fast step of the S_N1 reaction

Answer 18

S_N1 Step 2. Nucleophilic attack on the carbocation (fast).

Question 19

Describe the strereochemistry of the S_N2 reaction

Answer 19

Back-side attack inverts the configuration of the carbon atom.

Question 20

Answer 20

strong nucleophile