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Organic Chemistry

Janice Gorzynski Smith

Chapter 11

Alkynes

Educators


Problem 1

Draw structures for the three alkynes having molecular formula $C_5H_8$ and classify each as an
internal or terminal alkyne.

Ian K.
Numerade Educator

Problem 2

Santalbic acid, a fatty acid isolated from the seeds of the sandalwood tree, is an unusual fatty
acid that contains a carbon-carbon triple bond. What orbitals are used to form each of the three
indicated single bonds in santalbic acid? Rank these cr bonds in order of increasing bond strength.

Nima G.
Numerade Educator

Problem 3

Give the IUPAC name for each compound.

Ian K.
Numerade Educator

Problem 4

Give the structure corresponding to each of the following names.
a. trans-2-ethynylcyclopentanol b. 4-tert-butyl-5-decyne c. 3-methylcyclononyne

Nima G.
Numerade Educator

Problem 5

Explain why an alkyne often has a slightly higher boiling point than an alkene of similar molecular
weight. For example, the bp of 1-pentyne is 39$^\circ$C, and the bp of 1-pentene is 30$^\circ$C.

Ian K.
Numerade Educator

Problem 6

Convert each compound to 1-hexyne, $HC\equiv CCH_2CH_2CH_2CH_3$.
a. $Br_2CH(CH_2)_4CH_3$ b. $CH_2=CCl(CH_2)_3CH_3$ c. $CH_2=CH(CH_2)_3CH_3$

Nima G.
Numerade Educator

Problem 7

Which bases can deprotonate acetylene? The $pK_a$ values of the conjugate acids are given in
parentheses.
a. $CH_3NH^- (PK_a = 40)$ b. $Co_3^2- (PK_a = 10.2)$ c. $CH_2 = CH^- (PK_ a = 44)$ d. $(CH_3)_3CO^- (pK_a = 18)$

Ian K.
Numerade Educator

Problem 8

Draw the organic products formed when each alkyne is treated with two equivalents of HBr.

Nima G.
Numerade Educator

Problem 9

Draw an additional resonance structure for each cation.

Ian K.
Numerade Educator

Problem 10

Draw the products formed when $CH_3CH_2C= CCH_2CH_3$ is treated with each reagent:
(a) $Br_2 (2 equiv); (b) Cl_2$ (1 equiv).

Nima G.
Numerade Educator

Problem 11

Explain the following result. Although alkenes are generally more reactive than alkynes towards
electrophiles, the reaction of $Cl_2$ with 2-butyne can be stopped after one equivalent of $Cl_2$ has
been added.

Ian K.
Numerade Educator

Problem 12

Draw the keto tautomer of each enol.

Nima G.
Numerade Educator

Problem 13

What two enols are formed when 2-pentyne is treated with $H_20, H_2S0_4, and\, HgS0_4$? Draw the
ketones formed from these enols after tautomerization.

Ian K.
Numerade Educator

Problem 14

(a) Draw two different enol tautomers of 2-methylcyclohexanone. (b) Draw two constitutional
isomers that are not tautomers, but contain a C = C and an OH group.

Nima G.
Numerade Educator

Problem 15

Draw the products formed when the following alkynes are treated with each set of reagents:
[1) $H_20, H_2S0_4, HgS0_4; or [2] R_2BH$ followed by $H_20_2, ^-OH$.

Ian K.
Numerade Educator

Problem 16

Draw the organic products formed in each reaction.

Nima G.
Numerade Educator

Problem 17

What acetylide anion and alkyl halide can be used to prepare each alkyne? Indicate all possibilities
when more than one route will work.
a. $(CH_3)_2CHCH_2C \equiv CH$ b. $CH_3C\equiv CCH_2CH_2CH_2CH_3$ c. $(CH_3)_2CC\equiv CCH_2CH_3$

Ian K.
Numerade Educator

Problem 18

Show how $HC\equiv CH, CH_3CH_2Br, and (CH_3)_2CHCH_2CH_2Br$ can be used to prepare
$CH_3CH_2C\equiv CCH_2CH_2CH(CH_3)_2$. Show all reagents, and use curved arrows to show
movement of electron pairs.

Nima G.
Numerade Educator

Problem 19

Explain why 2,2,5,5-tetramethyl-3-hexyne can't be made using acetylide anions.

Ian K.
Numerade Educator

Problem 20

Draw the products of each reaction.

Nima G.
Numerade Educator

Problem 21

Draw the products formed when $CH_3CH_2C\equiv c^-Na^+$ reacts with each compound.
a. $CH_3CH_2CH_2Br$
b. $(CH_3)_2CHCH_2CH_2CI$
c. $(CH_3CH_2)_3CCI$
d. $BrCH_2CH_2CH_2CH_20H$
e. ethylene oxide followed by $H_20$
f. propene oxide followed by $H_20$

Ian K.
Numerade Educator

Problem 22

Use retrosynthetic analysis to show how 3-hexyne can be prepared from acetylene and any other
organic and inorganic compounds. Then draw the synthesis in the synthetic direction, showing all
needed reagents.

Nima G.
Numerade Educator

Problem 23

Devise a synthesis of $CH_3CH_2CH_2CHO$ from two-carbon starting materials.

Ian K.
Numerade Educator

Problem 24

Give the IUPAC name for each compound.

Nima G.
Numerade Educator

Problem 25

Draw the enol tautomer of (a) and the keto tautomer of (b).

Ian K.
Numerade Educator

Problem 26

Answer the following questions about erlotinib and phomallenic acid C. Erlotinib, sold under the trade name Tarceva, was introduced in 2004 for the treatment of lung cancer. Phomallenic acid C is an inhibitor of bacterial fatty acid synthesis.
a. Which C- H bond in erlotinib is most acidic?
b. What orbitals are used to form the shortest C- C single bond in erlotinib?
c. Which H atom in phomallenic acid C is most acidic?
d. How many sp hybridized carbons are contained in phomallenic acid C?
e. Rank the labeled bonds in phomallenic acid C in order of increasing bond strength.

Nima G.
Numerade Educator

Problem 27

Draw the seven isomeric alkynes having molecular formula $C_6H_10$, and give the IUPAC name for each compound. Consider constitutional isomers only.

Ian K.
Numerade Educator

Problem 28

Give the IUPAC name for each alkyne.

Nima G.
Numerade Educator

Problem 29

Give the structure corresponding to each name.
a. 5,6-dimethyl-2-heptyne c. (4S)-4-chloro-2-pentyne e. 3,4-dimethyl-1,5-octadiyne
b. 5-tert-butyl-6,6-dimethyl-3-nonyne d. cis-1-ethynyl-2-methylcyclopentane f. (6Z)-6-methyl-6-octen-1-yne

Ian K.
Numerade Educator

Problem 30

Which of the following pairs of compounds represent keto-enol tautomers?

Nima G.
Numerade Educator

Problem 31

Draw the enol form of each keto tautomer in parts (a) and (b), and the keto form of each enol tautomer in parts (c) and (d).

Ian K.
Numerade Educator

Problem 32

How is each compound related to A? Choose from tautomers, constitutional isomers but not tautomers, or neither.

Nima G.
Numerade Educator

Problem 33

Ignoring stereoisomers, draw the two possible enols for 2-butanone (CH3COCH2CH3), and predict which one is more stable.

Ian K.
Numerade Educator

Problem 34

Conversion of an enol to a ketone also occurs in the presence of base. Draw a stepwise mechanism for the following tautomerization.

Nima G.
Numerade Educator

Problem 35

Enamines and imines are tautomers that contain N atoms. Draw a stepwise mechanism for the acid-catalyzed conversion of enamine X to imine Y.

Ian K.
Numerade Educator

Problem 36

Draw the products formed when 1-hexyne is treated with each reagent.
a. HCI (2 equiv) c. Cl2 (2 equiv) e. [1] R2BH; [2] H2O2, Hob. HBr (2 equiv) d. $H_2O + H_2SO_4 + HgSO-4$ f. NaH
g. [1] $-NH_2; [2] CH_3CH_2Br$
h. [1]-$NH_2; [2] (3] H_2O $

Nima G.
Numerade Educator

Problem 37

Draw the products formed when 3-hexyne is treated with each reagent.
a. HBr (2 equiv} c. $H_2O, H_2SO_4$
b. $Br_2$ (2 equiv} d. [1] $R_2BH; [2] H_2O_2, HO^-$

Ian K.
Numerade Educator

Problem 38

What reagents are needed to convert $(CH_3CH_2)_3CC\Delta CH$ to each compound?
a. $(CH_3CH_2)_3CCOCH_3$ c. $(CH_3CH_2)CCCl_2CH_3$
b. $(CH_3CH_2)CCH_2CHO$ d. $(CH_3CH_2)CC\Delta CCH_2CH-3$

Nima G.
Numerade Educator

Problem 39

Explain the apparent paradox. Although the addition of one equivalent of HX to an alkyne is more exothermic than the addition of HX to an alkene, an alkene reacts faster with HX.

Ian K.
Numerade Educator

Problem 40

What alkyne gives each of the following ketones as the only product after hydration with $H_2O, H_2SO_4$ , and $HgSO_4$?

Nima G.
Numerade Educator

Problem 41

What alkyne gives each of the following ketones as the only product after hydration with $H_2O, H_2SO_4$ , and $HgSO_4$?

Ian K.
Numerade Educator

Problem 42

What alkyne gives each compound as the only product after hydroboration-oxidation?

Nima G.
Numerade Educator

Problem 43

Draw the organic products formed in each reaction.

Ian K.
Numerade Educator

Problem 44

Draw the structure of compounds A-E in the following reaction scheme.

Nima G.
Numerade Educator

Problem 45

When alkyne A is treated with $NaNH_2$ followed by $CH_3I$, a product having molecular formula $C_6H_{10}O$ is formed, but it is not compound B. What is the structure of the product and why is it formed?

Ian K.
Numerade Educator

Problem 46

Draw the products formed in each reaction and indicate stereochemistry.

Nima G.
Numerade Educator

Problem 47

What reactions are needed to convert alcohol A into either alkyne B or alkyne C?

Ian K.
Numerade Educator

Problem 48

Identify the lettered compounds in the following reaction schemes. Each reaction sequence was used in the synthesis of a natural product.

Nima G.
Numerade Educator

Problem 49

Treatment of 2,2-dibromobutane with two equivalents of strong base affords 1-butyne and 2-butyne, as well as a small amount of 1,2-butadiene. Draw a mechanism showing how each compound is formed. Which alkyne should be the major product?

Ian K.
Numerade Educator

Problem 50

Explain the following statement. Although HC= c- is more stable than $CH_2= CH^-$, $HC\Delta c+$ is less stable than $CH_2= CH^+$.

Nima G.
Numerade Educator

Problem 51

Draw a stepwise mechanism for the following reaction and explain why a mixture of E and Z isomers is formed.

Ian K.
Numerade Educator

Problem 52

Draw a stepwise mechanism for each reaction.

Nima G.
Numerade Educator

Problem 53

From what you have learned about enols and the hydration of alkynes, predict what product is formed by the acid-catalyzed hydration of $CH_3CH_2CH_2C\Delta COCH_3$. Draw a stepwise mechanism that illustrates how it is formed.

Ian K.
Numerade Educator

Problem 54

What steps are needed to prepare phenylacetylene, $C_6H_5C= CH$, from each compound: (a) $C_6H_5CH_2CHBr_2$; (b) $C_6H_5CHBrCH_3$;
(c) $C_6H_5CH_2CH_2OH$?

Nima G.
Numerade Educator

Problem 55

What acetylide anion and alkyl halide are needed to synthesize each alkyne?

Ian K.
Numerade Educator

Problem 56

Synthesize each compound from acetylene. You may use any other organic or inorganic reagents.

Nima G.
Numerade Educator

Problem 57

Devise a synthesis of each compound using $CH_3CH_2CH = CH_2$ as the starting material. You may use any other organic compounds or inorganic reagents.

Ian K.
Numerade Educator

Problem 58

Devise a synthesis of each compound. You may use HC$CH$, ethylene oxide, and alkyl halides as organic starting materials and any inorganic reagents.
a. $CH_3CH_2CH_2CH_2CH_2CH_2C=CH$ c.$CH_3CH_2CH_2CH_2CH_2CH_2C= CCH_2CH_2OH$
b. $CH_3CH_2CH_2CH_2CH_2CH_2C= CCH_2CH_3 d.$CH_3CH_2CH_2CH_2CH_2CH_2C$ = $CCH_2CH_2OCH_2CH_3$

Nima G.
Numerade Educator

Problem 59

Devise a synthesis of the ketone 3-hexanone, $CH_3CH_2COCH_2CH_2CH_3$, from $CH_3CH-2Br$ as the only organic starting material; that is, all the carbon atoms in 3-hexanone must come from $CH_3CH_2Br$. You may use any other needed reagents.

Ian K.
Numerade Educator

Problem 60

Devise a synthesis of each compound using CH3CH2CH20H as the only organic starting material: (a) $CH_3C$ = $CCH_2CH_2CH_3$ ; (b) $CH_3C$ = $CCH-2CH(OH)CH_3$. You may use any other needed inorganic reagents.

Nima G.
Numerade Educator

Problem 61

Devise a synthesis of each compound from $CH_3CH_2OH$ as the only organic starting material: (a) $CH_3CH_2C$= $CCH_2CH_2OH$; (b) $CH_3CH_2C$ = $CCH_2CH_20CH_2CH3$. You may use any other needed reagents.

Ian K.
Numerade Educator

Problem 62

Explain why the C = C of an enol is more nucleophilic than the C = C of an alkene, despite the fact that the electronegative oxygen atom of the enol inductively withdraws electron density from the carbon-carbon double bond.

Nima G.
Numerade Educator

Problem 63

Draw a stepwise mechanism for the following reaction.

Ian K.
Numerade Educator

Problem 64

Draw a stepwise mechanism for the following reaction.

Nima G.
Numerade Educator

Problem 65

Why is compound X formed in the following reaction, instead of its constitutional isomer Y?

Ian K.
Numerade Educator

Problem 66

Write a stepwise mechanism for each of the following reactions. Explain why a more stable alkyne (2-butyne) is isomerized to a less stable alkyne (1-butyne), but under similar conditions, 2,5-dimethyl-3-hexyne forms 2,5-dimethyl-2,3-hexadiene.

Nima G.
Numerade Educator

Problem 67

Draw a stepwise mechanism for the following intramolecular reaction.

Ian K.
Numerade Educator

Problem 68

Explain why an optically active solution of (R)-a-methylbutyrophenone loses its optical activity when dilute acid is added to the solution.

Nima G.
Numerade Educator