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Chapter 9: Alkynes: An Introduction to Organic Synthesis

Every problem in the chapter beside the book’s answer, a chapter at a time as you scroll.

Chapter 9: Alkynes: An Introduction to Organic Synthesis

13 problems · 20 figures

9-1 §9.1 source ↗

Name the following alkynes:
(a)
(b)
(c)
(d)
(e)

Answer (5 parts)

(a)
2,5-Dimethyl-3-hexyne
(b)
3,3-Dimethyl-1-butyne
(c)
3,3-Dimethyl-4-octyne
(d)
2,5,5-Trimethyl-3-heptyne
(e)
2,4-Octadiene-6-yne

9-2 §9.1 source ↗

There are seven isomeric alkynes with the formula C6H10. Draw and name them.

Answer

1-Hexyne, 2-hexyne, 3-hexyne, 3-methyl-1-pentyne, 4-methyl-1-pentyne, 4-methyl-2-pentyne, 3,3-dimethyl-1-butyne

9-3 §9.3 source ↗

What products would you expect from the following reactions?
(a)
(b)
(c)

Answer (3 parts)

(a)
1,1,2,2-Tetrachloropentane
(b)
1-Bromo-1-cyclopentylethylene
(c)
2-Bromo-2-heptene and 3-bromo-2-heptene

9-4 §9.4 source ↗

What products would you obtain by mercury-catalyzed hydration of the following alkynes?
(a)
(b)

Answer (2 parts)

(a)
4-Octanone
(b)
2-Methyl-4-octanone and 7-methyl-4-octanone

9-5 §9.4 source ↗

What alkynes would you start with to prepare the following ketones?
(a)
(b)

Answer (2 parts)

(a)
1-Pentyne
(b)
2-Pentyne

9-6 §9.4 source ↗

What alkyne would you start with to prepare each of the following compounds by a hydroboration–oxidation reaction?
(a)
(b)

Answer (2 parts)

(a)
C6H5C ≡ CH
(b)
2,5-Dimethyl-3-hexyne

9-7 §9.4 source ↗

How would you prepare the following carbonyl compounds starting from an alkyne (reddish brown = Br)?
(a)
(b)

Answer (2 parts)

(a)
Mercuric sulfate–catalyzed hydration of phenylacetylene
(b)
Hydroboration/oxidation of cyclopentylacetylene

9-8 §9.5 source ↗

Using any alkyne needed, how would you prepare the following alkenes?
(a)
trans-2-Octene
(b)
cis-3-Heptene
(c)
3-Methyl-1-pentene

Answer (3 parts)

(a)
Reduce 2-octyne with Li/NH3.
(b)
Reduce 3-heptyne with H2/Lindlar catalyst.
(c)
Reduce 3-methyl-1-pentyne.

9-9 §9.7 source ↗

The pKa of acetone, CH3COCH3, is 19.3. Which of the following bases is strong enough to deprotonate acetone?

(a) KOH (pKa of H2O = 15.7) (b) Na+ −C ≡ CH (pKa of C2H2 = 25) (c) NaHCO3 (pKa of H2CO3 = 6.4)
(d) NaOCH3 (pKa of CH3OH = 15.6)

Answer

No: (a), (c), (d); yes: (b)

9-10 §9.8 source ↗

Show the terminal alkyne and alkyl halide from which the following products can be obtained. If two routes look feasible, list both.
(a)
(b)
(c)

Answer (3 parts)

(a)
1-Pentyne + CH3I, or propyne + CH3CH2CH2I
(b)
3-Methyl-1-butyne + CH3CH2I
(c)
Cyclohexylacetylene + CH3I

9-11 §9.8 source ↗

How would you prepare cis-2-butene starting from propyne, an alkyl halide, and any other reagents needed? This problem can’t be worked in a single step. You’ll have to carry out more than one reaction.

Answer

9-12 §9.9 source ↗

Beginning with 4-octyne as your only source of carbon, and using any inorganic reagents necessary, how would you synthesize the following compounds?
(a)
cis-4-Octene
(b)
Butanal
(c)
4-Bromooctane
(d)
4-Octanol
(e)
4,5-Dichlorooctane
(f)
Butanoic acid

Answer (6 parts)

(a)
H2, Lindlar catalyst
(b)
1. H2, Lindlar catalyst; 2. O3; 3. Zn CH3CO2H
(c)
1. H2/Lindlar; 2. HBr
(d)
1. H2/Lindlar; 2. BH3; 3. NaOH, H2O2
(e)
1. H2/Lindlar; 2. Cl2
(f)
KMnO4, H3O+

9-13 §9.9 source ↗

Beginning with acetylene and any alkyl halide needed, how would you synthesize the following compounds?
(a)
Decane
(b)
2,2-Dimethylhexane
(c)
Hexanal
(d)
2-Heptanone

Answer (4 parts)

(a)
1. HC ≡ CH + NaNH2 ; 2. CH3(CH2)6CH2Br; 3. 2 H2/Pd
(b)
HC ≡ CH + NaNH2 ; 2. (CH3)3CCH2CH2I; 3. 2 H2/Pd
(c)
1. HC ≡ CH + NaNH2 ; 2. CH3CH2CH2CH2I; 3. BH3; 4. H2O2
(d)
1. HC ≡ CH + NaNH2 ; 2. CH3CH2CH2CH2CH2I; 3. HgSO4, H3O+

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Problems and answers are adapted from Organic Chemistry: A Tenth Edition by John McMurry, OpenStax, and are used under the CC BY-NC-SA 4.0 licence. Access for free at https://openstax.org/books/organic-chemistry/pages/1-why-this-chapter. Illustrations: Copyright Rice University, OpenStax, under CC BY-NC-SA 4.0 license. This adaptation is released under the same licence, is non-commercial, and is neither affiliated with nor endorsed by OpenStax or Rice University.