Organic Chemistry

Marc Loudon, Jim Parise

Chapter 2

Alkanes - all with Video Answers

Educators

Chapter Questions

Problem 1

(a) How many hydrogen atoms are in the unbranched alkane with 18 carbon atoms?
(b) Is there an unbranched alkane containing 23 hydrogen atoms? If so, give its structural formula; if not, explain why not.

Aadit Sharma

Aadit Sharma

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Problem 2

Give the structural formula and estimate the boiling point of tridecane, $\mathrm{C}_{13} \mathrm{H}_{28}$

Aadit Sharma

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Problem 3

(a) Draw a Newman projection for each staggered and eclipsed conformation about the C2-C3 bond of isopentane, a compound containing a branched carbon chain.
<smiles>CCC(C)C</smiles>
isopentane
Show all staggered and eclipsed conformations.
(b) Sketch a curve of potential energy versus dihedral angle for isopentane, similar to that of butane in Fig. 2.5. Label each energy maximum and minimum with one of the conformations you drew in part (a).
(c) Which of the conformations you drew in part
(a) are likely to be present in greatest amount in a sample of isopentane? Explain.

Aadit Sharma

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Problem 4

Repeat the analysis in Problem 2.3 for either one of the terminal $\mathrm{C}-\mathrm{C}$ bonds of butane.

Aadit Sharma

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Problem 5

(a) Use the procedures described in this section to draw the line-and-wedge and sawhorse conformations of both gauchebutane conformations projected about the central carbon-carbon bond. Don't hesitate to use models.
(b) Which of the following line-and-wedge structures of dibromomethane $\left(\mathrm{H}_{2} \mathrm{CBr}_{2}\right)$ are correct? For the incorrect structures, tell why they are incorrect.

Aadit Sharma

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Problem 6

(a) Using models when necessary, draw a Newman projection and a sawhorse projection corresponding to the following lineand-wedge structure. Project the C2-C3 bond, with carbon-2 nearer the observer, and simplify your structures by abbreviating large groups.
(b) In your Newman projection, rotate carbon- 2 clockwise by $120^{\circ}$ about the C $2-\mathrm{C} 3$ bond, and draw both sawhorse and lineand-wedge structures for the resulting conformation.
(c) In your Newman projection of part (a), rotate carbon- $2180^{\circ}$ about the C2-C3 bond, and draw both sawhorse and line-andwedge structures for the resulting conformation.

Aadit Sharma

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Problem 7

Name the following compounds.
(a)
<smiles>CCCC(C)C(C)CC(C)C</smiles>
(b) compound $B$ in Study Problem 2.4 on p. 63
(c)
<smiles>CCCC(CC)C(CC)CC</smiles>
(d)
<smiles>CCCCC(C)(CC)CCC</smiles>

Aadit Sharma

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Problem 8

Draw structures for all isomers of (a) hexane and (b) heptane. Give their systematic names.

Aadit Sharma

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Problem 9

Name the following compounds. Be sure to designate the principal chain properly before constructing the name.

Aadit Sharma

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Problem 10

Draw a structure for $\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2}\right)_{2} \mathrm{CHCH}\left(\mathrm{CH}_{2} \mathrm{CH}_{3}\right)_{2}$ in which all carbon-carbon bonds are shown explicitly; then name the compound.

Aadit Sharma

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Problem 11

Draw the structure of 4 -isopropyl- 2,4,5 -trimethylheptane.

Aadit Sharma

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Problem 12

In the structure of 4 -isopropyl- 2,4,5 -trimethylheptane (Problem 2.11 )
(a) Identify the primary, secondary, tertiary, and quaternary carbons.
(b) Identify the primary, secondary, and tertiary hydrogens.
(c) Circle one example of each of the following groups: a methyl group; an ethyl group; an isopropyl group; a sec-butyl group; an isobutyl group.

Aadit Sharma

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Problem 13

Identify the ethyl groups and the methyl groups in the structure of $4-$ sec-butyl-5-ethyl-3-methyloctane, the compound discussed in Study Problem 2.5 (p. 66). Note that these groups are not necessarily confined to those specifically mentioned in the name, and a particular carbon might appear in more than one group.

Aadit Sharma

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Problem 14

Represent each of the following compounds with a skeletal structure.

Aadit Sharma

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Problem 15

Redraw the structures in Problem 2.14 using abbreviations for substituent groups. For structure (a), draw one structure that shows as many abbreviated methyl groups as possible. (Hint: There are six methyl groups.) Then draw another that shows a five-carbon skeleton, two abbreviated methyl groups, and one abbreviated tert-butyl group.

Aadit Sharma

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Problem 16

Name the following compounds.

Nicholas Sacco

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Problem 17

How many hydrogens are in an alkane of $n$ carbons containing (a) two rings? (b) three rings? (c) $m$ rings?

Grigoriy Sereda

Grigoriy Sereda

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Problem 18

How many rings does an alkane have if its formula is (a) $\mathrm{C}_{8} \mathrm{H}_{10} ?$ (b) $\mathrm{C}_{7} \mathrm{H}_{12}$ ? Explain how you know.

Aadit Sharma

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Problem 19

In each part, match the melting point with the compound.
(a) $-109^{\circ} \mathrm{C},-56^{\circ} \mathrm{C},+100.7^{\circ} \mathrm{C}:$ octane, 2 -methylheptane, and 2,2,3,3 -tetramethylbutane
(b) $-93^{\circ} \mathrm{C},+5.5^{\circ} \mathrm{C}:$ benzene and toluene

Aadit Sharma

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Problem 20

Gasoline consists mostly of alkanes. Explain why water is not usually very effective in extinguishing a gasoline fire.

Aadit Sharma

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Problem 21

Give a general balanced reaction for
(a) the complete combustion of an alkane (formula $\mathrm{C}_{n} \mathrm{H}_{2 n+2}$ ).
(b) the complete combustion of a cycloalkane containing one ring (formula $\mathrm{C}_{n} \mathrm{H}_{2 n}$ ).

Vishal Sharma

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Problem 22

Calculate the number of pounds of $\mathrm{CO}_{2}$ released into the atmosphere when 15 gallons of gasoline is burned in an automobile engine. Assume complete combustion. Also assume that gasoline is a mixture of octane isomers and that the density of gasoline is $0.692 \mathrm{~g} \mathrm{~mL}^{-1}$. (This assumption ignores about 10 volume percent of oxygenated additives.) Useful conversion factors:
1 gallon $=3.785 \mathrm{~L} ; 1 \mathrm{~kg}=2.204 \mathrm{lb}$

Aadit Sharma

Numerade Educator

Problem 23

Draw a structural formula for each of the following compounds. (Several formulas may be possible in each case.)
(a) a carboxylic acid with the molecular formula $\mathrm{C}_{2} \mathrm{H}_{4} \mathrm{O}_{2}$
(b) an alcohol with the molecular formula $\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}$

Aadit Sharma

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Problem 24

A certain compound was found to have the molecular formula $\mathrm{C}_{5} \mathrm{H}_{12} \mathrm{O}_{2}$. To which of the following compound classes could the compound belong? Give one example for each positive answer, and explain any negative responses.
an amide an ether a carboxylic acid a phenol an alcohol an ester

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Problem 25

Given the boiling point of the first compound in each set, estimate the boiling point of the second.

Aadit Sharma

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Problem 26

Draw the structures and give the names of all isomers of octane with
(a) five carbons
(b) six carbons in their principal chains.

Aadit Sharma

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Problem 27

Label each carbon in the following molecules as primary, secondary, tertiary, or quaternary.

Aadit Sharma

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Problem 28

Draw the structure of an alkane or cycloalkane that meets each of the following criteria.
(a) a compound that has more than three carbons and only primary hydrogens.
(b) a compound that has five carbons and only secondary hydrogens.
(c) a compound that has only tertiary hydrogens.
(d) a compound that has a molecular mass of 84.2 .

Aadit Sharma

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Problem 29

Name each of the following compounds using IUPAC substitutive nomenclature.

Aadit Sharma

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Problem 30

Draw structures that correspond to the following names.
(a) 4 -isobutyl- 2,5 -dimethylheptane
(b) 2,3,5 -trimethyl-4-propylheptane (skeletal structure)
(c) $5-$ sec-butyl-6-tert-butyl-2,2-dimethylnonane

Aadit Sharma

Numerade Educator

Problem 31

The following labels were found on bottles of liquid hydrocarbons in the laboratory of Dr. Ima Turkey following his disappearance under mysterious circumstances. Although each name defines a structure unambiguously, some are not correct IUPAC substitutive names. Give the correct name for any compounds that are not named correctly.
(a) 2 -ethyl- 2,4,6 -trimethylheptane
(b) 5-neopentyldecane
(c) 1 -cyclopropyl- 3,4 -dimethylcyclohexane
(d) 3 -butyl- 2,2 -dimethylhexane

Aadit Sharma

Numerade Educator

Problem 32

Although compounds are indexed by their IUPAC substitutive names, sometimes chemists give whimsical names to compounds that they discover. Assist these two chemists by providing substitutive names for their compounds.
(a) Chemist Val Losipede isolated an alkane with the following skeletal structure from asphalt scrapings following a bicycle race and named it "Tourdefrançane."
(b) Chemist Slim Pickins isolated a compound with the following structure from the floor of a henhouse and dubbed it "pullane" (pullus, Latin for chick).

Aadit Sharma

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Problem 33

Within each set, which two structures represent the same compound?

Crystal Wang

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Problem 34

(a) Draw a skeletal structure of the compound in part (a) of Problem 2.33 that is different from the other two compounds, and name the compound.
(b) Draw a Newman projection for the most stable conformation of the compound in part (b) of Problem 2.33 that is different from the other two compounds. Draw your Newman projection about the bond between carbons 3 and 4 in the IUPAC standard numbering system, with the projection viewed from the direction of carbon- 3 . Describe any ambiguity you encounter in drawing this structure. Name the compound.

Crystal Wang

Numerade Educator

Problem 35

Sketch a diagram of potential energy versus angle of rotation about the carbon-carbon bond of chloroethane, $\mathrm{H}_{3} \mathrm{C}-\mathrm{CH}_{2}-\mathrm{Cl}$. The magnitude of the energy barrier to internal rotation is $15.5 \mathrm{~kJ} \mathrm{~mol}^{-1}\left(3.7 \mathrm{kcal} \mathrm{mol}^{-1}\right)$. Label this
barrier on your diagram.

Aadit Sharma

Numerade Educator

Problem 36

Explain how you would expect the diagram of potential energy versus dihedral angle about the $\mathrm{C} 2-\mathrm{C} 3$ (central) carbon-carbon bond of 2,2,3,3 -tetramethylbutane to differ from that for ethane (Fig. 2.3 ), if at all.

Aadit Sharma

Numerade Educator

Problem 37

The anti conformation of 1,2 -dichloroethane, $\mathrm{Cl}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{Cl}$, is $4.81 \mathrm{~kJ} \mathrm{~mol}^{-1}\left(1.15 \mathrm{kcal} \mathrm{mol}^{-1}\right)$
more stable than the gauche conformation. The two energy barriers (measured relative to the energy of the gauche conformation) for carbon-carbon bond rotation are $21.5 \mathrm{~kJ} \mathrm{~mol}^{-1}\left(5.15 \mathrm{kcal} \mathrm{mol}^{-1}\right)$ and $38.9 \mathrm{~kJ} \mathrm{~mol}^{-1}$
$\left(9.3 \mathrm{kcal} \mathrm{mol}^{-1}\right)$
(a) Sketch a graph of potential energy versus dihedral angle about the carbon-carbon bond. Show the energy differences on your graph and label each minimum and maximum with the appropriate conformation of 1,2-dichloroethane.
(b) Which conformation of this compound is present in greatest amount? Explain.

Crystal Wang

Numerade Educator

Problem 38

(a) Draw Newman projections of the most stable conformations about each of the carbon-carbon bonds in the principal chain of 2,2 -dimethylpentane. Use models!
(b) Combine these to predict the most stable conformation of 2,2 -dimethylpentane.
(c) Draw a line-and-wedge structure of the conformation you derived in (b) with the carbon-carbon bonds of the principal chain shown as lines, the bonds to all hydrogens of the principal chain shown as wedges or dashed wedges, and methyl substituents shown either as $\mathrm{CH}_{3}$ or Me.

Lottie Adams

Numerade Educator

Problem 39

When the structure of compound $A$ was determined in 1972 , it was found to have an unusually long $\mathrm{C}-\mathrm{C}$ bond and unusually large $\mathrm{C}-\mathrm{C}-\mathrm{C}$ bond angles, compared with the similar parameters for compound $B$ (isobutane).
Explain why the indicated bond length and bond angle are larger for compound $A$.

Aadit Sharma

Numerade Educator

Problem 40

Which of the following compounds should have the larger energy barrier to internal rotation about the indicated bond? Explain your reasoning carefully.

Aadit Sharma

Numerade Educator

Problem 41

From what you learned in Sec. $1.3 \mathrm{~B}$ about the relative lengths of $\mathrm{C}-\mathrm{C}$ and $\mathrm{C}-\mathrm{O}$ bonds, predict which of the following compounds should have the larger energy difference between gauche and anti conformations about the indicated bond. Explain.

Crystal Wang

Numerade Educator

Problem 42

(a) What value is expected for the dipole moment of the anti conformation of 1,2 -dibromoethane, $\mathrm{Br}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{Br}$ ? Explain.
(b) The dipole moment $\boldsymbol{\mu}$ of any compound that undergoes internal rotation can be expressed as a weighted average of the dipole moments of each of its conformations by the following equation:
$$
\boldsymbol{\mu}=\boldsymbol{\mu}_{1} N_{1}+\boldsymbol{\mu}_{2} N_{2}+\boldsymbol{\mu}_{3} N_{3}
$$
in which $\boldsymbol{\mu}_{i}$ is the dipole moment of conformation $i$, and $N_{i}$ is the mole fraction of conformation $i$. (The mole fraction of any conformation $i$ is the number of moles of $i$ divided by the total moles of all conformations.) There are about 82 mole percent of anti conformation and about 9 mole percent of each gauche conformation present at equilibrium in 1,2 -dibromoethane, and the observed dipole moment $\boldsymbol{\mu}$ of 1,2 dibromoethane is 1.0 D. Using the preceding equation and the answer to part (a), calculate the dipole moment of a gauche conformation of 1,2 -dibromoethane.

Raghvendra Singh

Raghvendra Singh

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Problem 43

Carv and Di Oxhide drive their family car about 12,000 miles per year with an average mileage of about 25 miles per gallon of gasoline. What is the "carbon footprint" (pounds of $\mathrm{CO}_{2}$ released into the atmosphere) of the Oxhide family car over one year? Ignoring the oxygenates present, take the density of gasoline as $0.692 \mathrm{~g} \mathrm{~mL}^{-1}$. (Useful conversion factors: 1 gallon $=3.785 \mathrm{~L} ; 1 \mathrm{~kg}=2.204 \mathrm{lb}$.)

Aadit Sharma

Numerade Educator

Problem 44

This problem illustrates how combustion can be used to determine the molecular formula of an unknown compound. A compound $X(8.00 \mathrm{mg})$ undergoes combustion in a stream of oxygen to give $24.60 \mathrm{mg}$ of $\mathrm{CO}_{2}$ and $11.51 \mathrm{mg}$ of $\mathrm{H}_{2} \mathrm{O}$
(a) Calculate the mass of carbon and hydrogen in $X$.
(b) How many moles of $\mathrm{H}$ are present in $X$ per mole of $\mathrm{C} ?$ Express this as a formula $\mathrm{C}_{1} \mathrm{H}_{x}$
(c) Multiply this formula by successive integers until the amount of $\mathrm{H}$ is also an integer. This is the molecular formula of $X$.

Crystal Wang

Numerade Educator

Problem 45

A hydrocarbon $Y$ is found by combustion analysis to contain $87.17 \%$ carbon and $12.83 \%$ hydrogen by mass.
(a) Use the procedure in Problem $2.44(\mathrm{~b})$ and (c) to determine the molecular formula of $Y$. (Hint: Remember that all alkanes and cycloalkanes must contain even $\mathrm{num}-$ bers of hydrogens.)
(b) Draw the structure of an alkane (which may contain one or more rings) consistent with the analysis given in part (a) that has two tertiary carbons and all other carbons secondary. (More than one correct answer is possible.)
(c) Draw the structure of an alkane (which may contain one or more rings) consistent with the analysis given in part
(a) that has no primary hydrogens, no tertiary carbon atoms, and one quaternary carbon atom. (More than one correct answer is possible.)

Crystal Wang

Numerade Educator

Problem 46

Imagine a reaction that can replace one hydrogen atom of an alkane at random with a chlorine atom.
<smiles>CC(C)(C)CBr</smiles>
(a) If pentane were subjected to such a reaction, how many different compounds with the formula $\mathrm{C}_{5} \mathrm{H}_{11} \mathrm{Cl}$ would be obtained? Give their Lewis structures. Then build models (or draw line-and-wedge formulas) for each compound. Does this alter your answer in any way? Explain.
(b) Provide the same analysis as in part (a) for the same reaction carried out on 2,2 -dimethylbutane.

Crystal Wang

Numerade Educator

Problem 47

To which compound class does each of the following compounds belong?

Crystal Wang

Numerade Educator

Problem 48

The $\alpha$ -amino acids are the building blocks of proteins. Most have the following general structure:
<smiles>NC(CO)C(=O)[O-]</smiles>
general structure of the $a$ -amino acids
These amino acids differ only in their side chains $-\mathrm{R}$. What functional groups are present in the side chains of each of the following amino acids? (Hint: See inside front cover.)

Aadit Sharma

Numerade Educator

Problem 49

Organic compounds can contain many different functional groups. Identify the functional groups (aside from the alkane carbons) present in acebutolol (Fig. P2.49), a drug that blocks a certain part of the nervous system. Name the compound class to which each group belongs.

Aadit Sharma

Numerade Educator

Problem 50

(a) Two amides are constitutional isomers and have the formula $\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{NO},$ and each contains an isopropyl group as part of its structure. Give structures for these two isomeric amides.
(b) Draw the structure of two other amides with the formula $\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{NO}$ that do not contain isopropyl groups.
(c) Draw the structure of a compound $X$ that is a constitutional isomer of the amides in parts (a) and (b), but is not an amide, and contains both an amine and an alcohol functional group.
(d) Could a compound with the formula $\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{NO}$ contain a nitrile functional group? Explain.

Aadit Sharma

Numerade Educator