Mcmurry Oc8e Ev Ch09

Chapter 9—Alkynes: An Introduction to Organic Synthesis SHORT ANSWER Drawing Instructions: Draw structures corresponding to each of the given names. 1.

Draw: ethynylcyclopropane

ANS:

2.

Draw: 3,10-dimethyl-6-sec-butylcyclodecyne

ANS:

3.

Draw: 4-bromo-3,3-dimethyl-1-hexen-5-yne

ANS:

4.

Draw: acetylene

ANS:

IUPAC Naming Instructions: Provide proper IUPAC names. 5.

Name:

ANS: 4-methyl-2-heptyne

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Alkynes: An Introduction to Organic Synthesis

6.

Name:

ANS: 1-ethynyl-2-methylcyclopentane Exhibit 9-1 The compound below has been isolated from the safflower plant. Consider its structure to answer the following question(s).

7.

Refer to Exhibit 9-1. What is the molecular formula for this natural product?

ANS: C13H10 8.

Refer to Exhibit 9-1. What is the degree of unsaturation for this compound?

ANS: We can arrive at the degree of unsaturation for a structure in two ways. Since we know that the degree of unsaturation is the number of rings and/or multiple bonds in a compound, we can simply count them. There are three double bonds (3 degrees) and three triple bonds (six degrees), so the degree of unsaturation is 9. We can verify this by using the molecular formula, C13H10, to calculate a degree of unsaturation. The saturated 13-carbon compound should have the base formula C 13H28, so (28  10)  2 = 18  2 = 9.

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Chapter 9

9.

Refer to Exhibit 9-1. Assign E or Z configuration to each of the double bonds in the compound.

ANS:

10.

Refer to Exhibit 9-1. Provide the name for this unusual natural product.

ANS: (3E, 11E)-1,3,11-tridecatrien-5,7,9-triyne or (3E,11E)-trideca-1,3,11-trien-5,7,9-triyne Exhibit 9-2 To answer the following question(s), consider the information below: The heat of hydrogenation of 1,4-pentadiene is 254.4 kJ/mol. The heat of hydrogenation of 1-pentyne is approximately 292.9 kJ/mol. 11.

Refer to Exhibit 9-2. Which structure is more stable, 1,4-pentadiene or 1-pentyne? Briefly explain your answer.

ANS: 1,4-pentadiene The heat of hydration is the amount of energy released when hydrogen adds to the multiple carbon-tocarbon bonds in a compound and converts it to the corresponding saturated compound. The larger the value, the more energy that is released during the hydrogenation. Both of these substances, end up as pentane upon complete hydrogenation. That is, they end up at the same final energy level. Less energy is released by 1,4-pentadiene, therefore, this reactant has less energy to start with or is more stable than 1pentyne.

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Alkynes: An Introduction to Organic Synthesis

12.

Refer to Exhibit 9-2. Show the energy differences between the hydrogenation of 1,4-pentadiene and 1-pentyne on a reaction energy diagram.

ANS:

Exhibit 9-3 Consider an unknown with the molecular formula C 4H6. 13.

Refer to Exhibit 9-3. How many degrees of unsaturation are present?

ANS: A saturated four carbon compound has the formula C 4H10. Therefore this compound has (10  6)  2 = 2 degrees of unsaturation. 14. ANS:

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Refer to Exhibit 9-3. Draw six of the nine possible structures with this formula.

Chapter 9

Exhibit 9-4 Predict the product of each reaction below. Be sure to indicate stereochemistry when appropriate.

15. ANS:

16. ANS:

17. ANS:

18. ANS:

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Alkynes: An Introduction to Organic Synthesis

19. ANS:

20. ANS:

21. ANS:

Exhibit 9-5 To answer the following question(s) consider this reaction:

22.

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Refer to Exhibit 8-5. On the structures provided, draw arrows showing the electron flow for the reaction mechanism for the electrophilic addition of hydrogen bromide to 1-hexyne.

Chapter 9

ANS:

23.

Refer to Exhibit 8-5. The key intermediate in the electrophilic addition reaction shown above is called: an acetylenic carbocation a vinylic bromide a secondary carbocation a vinylic carbocation

ANS: d Exhibit 9-6 Select the best reagent or sequence of reagents from the list provided which would best accomplish each transformation below. Place the letter corresponding to the reagent(s) in the blank to the left of each reaction. Each transformation requires only one answer. H2SO4, H2O, HgSO4 H2, Lindlar BH3, THF H2O2 NaNH2, NH3 CH3CH2I

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Br2, CCl4 2 NaNH2, NH3 HCl NaNH2, NH3 Li/NH3 NaNH2, NH3 CH3CH2CH2I

_____

ANS: g

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Alkynes: An Introduction to Organic Synthesis

25.

_____

ANS: a

26.

_____

ANS: h

27.

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ANS: e Exhibit 9-7 Show all reagents and intermediates necessary to carry out the following conversions.

28. ANS:

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Chapter 9

29. ANS:

Exhibit 9-8 Using acetylene and any alkyl halides as starting materials, synthesize the following compounds. More than one step may be required. Show all reagents and all intermediate compounds in your synthetic scheme.

30. ANS:

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Alkynes: An Introduction to Organic Synthesis

31. ANS:

32. ANS:

33. ANS:

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Chapter 9

34. ANS:

35.

Draw the structure(s) of the product(s) formed when the following substance is treated with NaNH2/NH3 and 1-bromo-3-methylhexane.

ANS: No reaction occurs since this species is an internal alkyne. A terminal alkyne is needed to react under the given conditions. 36.

Consider the three structures shown below.

Explain why Structure C is unstable

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Alkynes: An Introduction to Organic Synthesis

ANS: The hybridization on the carbon atoms of the triple bond is sp and produces a bond angle of 180°. This dictates a linear geometry about both carbon atoms of the triple bond. In Structure C the internal bond angles between the sp hybridized carbon atoms and the adjacent sp3 carbon atoms of the ring is much less than 180° (approximately 120°). This compression of the bond angle produces too much strain for the ring to be stable. Puckering of the ring cannot relieve the strain to the degree that is does for Structures A and B. The linear geometry is required to allow for the appropriate orbital overlap to form the sigma bonds between the sp carbon atoms and the sp3 carbon atoms. 37.

ANS:

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Draw the skeletal formula for the reactant and product that correspond to the enol formed in a hydroboration/oxidation reaction. Atoms other than carbon and hydrogen are labeled.

Chapter 9

MULTIPLE CHOICE 1.

Specify the hybridization on the carbons shown in the following structure.

1: sp3 2: sp2 1: sp2 2: sp

1: sp3 2: sp 1: sp2 2: sp

ANS: C 2.

Consider the following compound.

This compound is weakly acidic. could produce a single carboxylic acid upon oxidation. would produce a high yield of product upon oxidative cleavage. yields an alkene upon treatment with hydrogen in the presence of a Lindlar catalyst. None of the above characterize this compound ANS: D

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Alkynes: An Introduction to Organic Synthesis

3.

What is the term given to the following species?

carbene carbanion

acetylide anion carbocation

ANS: C 4.

The following species

formed when ethyne is treated with a strong base. is the conjugate base of acetylene. could react with bromoethane to form 1-butyne. could react with bromocyclopentane to form an internal alkyne. a and b a, b, and c a, b, and d all of the above ANS: F 5.

The term retrosynthesis refers to syntheses developed: starting with simple molecules. devised by examining the product. working forward and backward from intermediates. beginning by examining the reactant available.

ANS: B 6.

An acetylide anion is more stable than either a vinylic or alkyl anion due to the stabilization of the negative charge in orbitals with higher s character. the greater distance of the negative charge from the nucleus of the carbon atom. the greater stability of an sp hybrid orbital. the larger number of  electrons.

ANS: A

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Chapter 9

7.

What is the skeletal formula for the product of the reaction when the following is treated with BH 3 in THF followed by H2O2?

ANS: D 8.

Which of the following alkynes will produce a single product upon mercury(II)-catalyzed hydration? 2-methyl-5-ethyl-3-heptyne 3,3-dimethyl-1-heptyne 3-hexyne 2-hexyne Both 3,3-dimethyl-1-heptyne and 3-hexyne

ANS: C 9.

When the following substance is treated with excess HBr,

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Alkynes: An Introduction to Organic Synthesis

The product of the reaction is

A mixture of a and b would be produced. A mixture of a, b, and c would be produced. ANS: C 10.

When the following substance is treated with 1 equivalent of Br 2,

The product of the reaction is

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Chapter 9

A mixture of a and b would be produced. ANS: A

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