CHEM 247 F18 - WS15 KEY - Competition in Substitution and Elimination Reactions PDF

Preview

Summary

Substitution and elimination key...

Description

WORKSHEET 15A KEY: Competition in Substitution and Elimination Reactions 1. Organize each of the following species by basicity and nucleophilicity. For example, HO- is negatively charged and has a pKaH = 15.7. This means it is both a good nucleophile and a strong base. NaI, MeOK, NaNH2, EtOH, KBr, NaNHCH3, KCN, EtSH, NaCl, KF, EtONa, CH3CH2–, HC≡C–, CH3COONa, H2O, t-BuOK H2S, ROH, DBU, DBN. a. First circle all the anions or salts e.g. KBr → K+ Br-. Species with negative charges are generally both good nucleophiles and strong bases, but there are two exceptions. • •

Stabilized anions—these are good nucleophiles but weak bases. Sterically hindered anions—these are too bulky to be good nucleophiles.

Stabilized anions include those with highly electronegative and/or large atoms sustaining the negative charge (i.e. the halides), resonance, or those attached to an electronegative atom through a highly polarizable triple bond are the conjugate bases of molecules that are considered to be acids e.g. HI, HCN, CH3COOH. b. Of the species with negative anions identify those that are stabilized and put them into the lower left quadrant of the table below, i.e. Good Nucleophile / Weak Base. c. One of the circled species is very sterically hindered this is a strong base but a poor nucleophile, place it in the correct quadrant. d. All the remaining species with negative anions are good nucleophiles and strong bases. This means they should go into the top left quadrant. e. The only other species listed above are neutral. Write these in the bottom right of the table. f.

The two molecules shown on the right of the table are neutral strong bases that are sterically hindered. Write the acronyms for these two molecules in the correct quadrant Good Nucleophile

Strong Base

Weak Base

MeO– – NH2 EtO– – NHCH3 CH3CH2– HC≡C–

Cl– F– Br–, I– CH3COO– – CN

Poor Nucleophile

DBU DBN t-BuO–

H2O, H2S, ROH EtSH,

2. In deciding whether E1, E2, SN1 or SN2 will be favored the most important thing is to look at the substrate. Use the table below to answer the following questions. Methyl SN2 E2 SN1 E1

✓

1° >

✓

 

✓ 





> <

2°

3°

✓

 ✓ ✓ ✓

<

✓ ✓ ✓

< <

✓ = Possible and Observed,  = Not Observed, and  = Not Possible a. For Methyl substrates, only SN2 reaction will be observed. i. Explain why E1 and E2 are not possible No β-H’s ii. Why SN1 is not observed T.S. to methylium cation is too high in energy, so carbocation nor formed. b. For Primary substrates, E1 and SN1 are not observed. Explain. T.S. to primary carbocation is too high in energy, so carbocation nor formed. c. For Tertiary substrates the bimolecular SN2 reaction is not observed but the bimolecular E2 reaction is observed. Explain why the steric hindrance of a 3° substrate does not prevent reaction via the E2 mechanism. Steric hindrance blocks the nucleophile from accessing the δ+ carbon on the substrate. The β-h’s are not however as sterically hindered. d. Tertiary substrates react faster than secondary substrates for both E1 and E2 mechanisms. What factor is most important in determining whether E1 or E2 is more favorable for secondary and tertiary substrates? Substrate / Nucleophilicity / Basicity / Solvent / Temperature. Explain. 3. Once you have narrowed down the options using the substrate type, the next thing to look at is the nucleophile/base. Good nucleophiles and strong bases can be considered impatient and will not allow a carbocation time to form. Patient nucleophiles and/or bases will ‘hold off’ and allow a carbocation time to form. Write in the mechanism(s) favored by species that exhibit the qualities shown for each quadrant, the upper left quadrant is done for you. Good Nucleophile

Poor Nucleophile

Strong Base

SN2 / E2

E2

Weak Base

SN2

SN1 / E1

WORKSHEET 15B KEY: More Competition in Substitution and Elimination Reactions 1.

Consider the following reagents and reaction conditions and answer the following questions. a. Circle the type of Substrate and use this rule out and delete mechanisms are not possible or will not be observed for that substrate. b. Write ‘’ through mechanisms that are not possible OR improbable based on the substrate. c. Determine if the Nucleophile is Good or Poor. Circle appropriately. d. Determine if the Base is strong or weak. Circle appropriately. e. Circle the mechanism(s) that will be favored or write ‘No RXN’ Polar Aprotic speeds up Elimn not possible heat speeds up rxn Low T / Aprotic favors SN2 High T / Protic Favors E2 Low T / Aprotic Less E2 and More SN2 High T/Protic More E2 Low T/Protic favors SN1 High T/Protic favors E1

2.

Once you have used nucleophilicity and basicity to determine the major mechanism(s) that will be observed. Solvent and heat can be used to ‘tweak’ the ratios as described below. a. b.

Protic solvents increase the amount of SN1 or E1 produced. Solvent is only useful in tweaking the ratios if unimolecular process is in competition with a bimolecular process e.g. SN1vs SN2. Otherwise it will just speed up or slow down the reaction. Heating the reaction will push the reaction to produce more elimination product. This is only useful if a substitution mechanism is in competition with an elimination mechanism., e.g. SN1 vs E1 or SN2 vs E2.

3.

In the space to the right of the table, for each of the reactions above state the effect of: a. Polar Protic vs. Polar Aprotic Solvent b. High vs. Low Temperature

4.

Suggest a synthesis for the following compounds. Give the substrate and nucleophile/base combination, as well as the reaction conditions i.e. protic/aprotic solvent and low/high temperature.

a. trans-2-butene

b. 2,3-dimethyl-2-hexene

c. 2-methoxy-2-methylbutane

d. R-2-pentanol

5. Complete the following reactions by adding the products, substrates, or nucleophile/base and necessary reaction conditions. Be sure to analyze all components of the reaction to determine which mechanism is best and make sure the favored mechanism is reflected by the conditions....