Introduction to General, Organic and Biochemistry

Frederick A. Bettelheim, William H. Brown, Mary K. Campbell

Chapter 11

Alkanes - all with Video Answers

Educators

Chapter Questions

Problem 1

Following is a line-angle formula for an alkane. What is the name and the molecular formula of this alkane?

Vasu Makani

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Problem 2

Do the line-angle formulas in each of the following sets represent the same compound or constitutional isomers?

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Problem 3

Draw structural formulas for the three constitutional isomers with the molecular formula $\mathrm{C}_{5} \mathrm{H}_{12}$.

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Problem 4

Write the molecular formula and IUPAC name for each alkane.

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Problem 5

Write the molecular formula and IUPAC name for each cycloalkane.

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Problem 6

Following is a chair conformation of cyclohexane with carbon atoms numbered 1 through 6. Draw methyl groups that are equatorial on carbons $1,2,$ and 4.

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Problem 7

Which of the following cycloalkanes show cis-trans isomerism? For each that does, draw both isomers.
(a) 1,3 -Dimethylcyclopentane
(b) Ethylcyclopentane
(c) 1,3 -Dimethylcyclohexane

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Problem 8

Arrange the alkanes in each set in order of increasing boiling point.
(a) $2-$ Methylbutane, pentane, and 2,2 -dimethylpropane
(b) 3,3 -Dimethylheptane, nonane, and 2,2,4 -trimethylhexane

Vasu Makani

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Problem 9

Reaction of propane with chlorine gives two products, each with the molecular formula $\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{Cl}$. Draw structural formulas for these two compounds and give each an IUPAC name and a common name.

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Problem 10

Answer true or false.
(a) A hydrocarbon is composed only of the elements carbon and hydrogen.
(b) Alkanes are saturated hydrocarbons.
(c) The general formula of an alkane is $\mathrm{C}_{n} \mathrm{H}_{2 n+2}$ where $n$ is the number of carbons in the alkane.
(d) Alkenes and alkynes are unsaturated hydrocarbons.

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Problem 11

Define:
(a) Hydrocarbon
(b) Alkane
(c) Saturated hydrocarbon

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Problem 12

Why is it not accurate to describe an unbranched alkane as a "straight-chain" hydrocarbon?

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Problem 13

What is meant by the term line-angle formula as applied to alkanes and cycloalkanes?

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Problem 14

For each condensed structural formula, write a line-angle formula.

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Problem 15

Write the molecular formula for each alkane.

Lottie Adams

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Problem 16

Answer true or false.
(a) Constitutional isomers have the same molecular formulas and the same connectivity of their atoms.
(b) There are two constitutional isomers with the molecular formula $\mathrm{C}_{3} \mathrm{H}_{8}$
(c) There are four constitutional isomers with the molecular formula $\mathrm{C}_{4} \mathrm{H}_{10}$
(d) There are five constitutional isomers with the molecular formula $\mathrm{C}_{5} \mathrm{H}_{12}$

Lottie Adams

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Problem 17

Which statements are true about constitutional isomers?
(a) They have the same molecular formula.
(b) They have the same molecular weight.
(c) They have the same connectivity of their atoms.
(d) They have the same physical properties.

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Problem 18

Each member of the following set of compounds is an alcohol; that is, each contains an $-\mathrm{OH}$ (hydroxyl group; see Section $10-4$ A). Which structural formulas represent the same compound, and which represent constitutional isomers?

David Collins

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Problem 19

Each member of the following set of compounds is an amine; that is, each contains a nitrogen atom bonded to one, two, or three carbon groups (Section 10-4B). Which structural formulas represent the same compound, and which represent constitutional isomers?

David Collins

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Problem 20

Each member of the following set of compounds is either an aldehyde or a ketone (Section $10-4 \mathrm{C}$ ). Which structural formulas represent the same compound, and which represent constitutional isomers?

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Problem 21

In the six following sets, which pairs of structural formulas represent constitutional isomers?

Lottie Adams

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Problem 22

Draw line-angle formulas for the nine constitutional isomers with the molecular formula $\mathrm{C}_{7} \mathrm{H}_{16}$.

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Problem 23

Answer true or false.
(a) The parent name of an alkane is the name of the longest chain of carbon atoms in the alkane.
(b) Propyl and isopropyl groups are constitutional isomers.
(c) There are four alkyl groups with the molecular formula $\mathrm{C}_{4} \mathrm{H}_{9}$

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Problem 24

Name these alkyl groups:

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Problem 25

Among the ingredients listed in one commercial foam shaving gel are isobutane and isopentane.
(a) Write the IUPAC name of each hydrocarbon.
(b) Why are these two hydrocarbons added to the formulation of the shaving gel?

Lottie Adams

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Problem 26

Answer true or false.
(a) The two major sources of alkanes the world over are petroleum and natural gas.
(b) The octane number of a particular gasoline is the number of grams of octane per liter of the fuel.
(c) Octane and 2,2,4 -trimethylpentane are constitutional isomers and have the same octane number.

Lottie Adams

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Problem 27

Answer true or false.
(a) Cycloalkanes are saturated hydrocarbons.
(b) Hexane and cyclohexane are constitutional isomers.
(c) The parent name of a cycloalkane is the name of the unbranched alkane with the same number of carbon atoms as are in the cycloalkane ring.

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Problem 28

Write the IUPAC names for these alkanes and cycloalkanes.

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Problem 29

Write line-angle formulas for these alkanes and cycloalkanes.
(a) 2,2,4 -Trimethylhexane
(b) 2,2 -Dimethylpropane
(c) 3 -Ethyl-2,4,5-trimethyloctane
(d) 5 -Butyl- 2,2 -dimethylnonane
(e) 4 -Isopropyloctane
(f) 3,3 -Dimethylpentane
(g) trans-1,3 -Dimethylcyclopentane
(h) cis-1,2-Diethylcyclobutane

Vasu Makani

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Problem 30

Answer true or false.
(a) Conformations have the same molecular formula and the same connectivity but differ in the three-dimensional arrangement of their atoms in space.
(b) In all conformations of ethane, propane, butane, and higher alkanes, all $\mathrm{C}-\mathrm{C}-\mathrm{C}$ and $\mathrm{H}-\mathrm{C}-\mathrm{H}$ bond angles are approximately $109.5^{\circ}$
(c) In a cyclohexane ring, if an axial bond is above the plane of the ring on a particular carbon, axial bonds on the two adjacent carbons are below the plane of the ring.
(d) In a cyclohexane ring, if an equatorial bond is above the plane of the ring on a particular carbon, equatorial bonds on the two adjacent carbons are below the plane of the ring.
(e) The more stable chair conformation of a cyclohexane ring has more substituent groups in equatorial positions.

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Problem 31

The condensed structural formula of butane is $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} .$ Explain why this formula does not show the geometry of the real molecule.

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Problem 32

Calculate the actual $\mathrm{C}-\mathrm{C}-\mathrm{C}$ bond angles in planar (a) cyclopropane and (b) cyclopentane and compare them with optimal bond angles.

Lottie Adams

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Problem 33

Answer true or false.
(a) Cis- and trans-cycloalkanes have the same molecular formula but a different connectivity of their atoms.
(b) A cis isomer of a cycloalkane can be converted to its trans isomer by rotation about an appropriate carbon-carbon single bond.
(c) A cis isomer of a cycloalkane can be converted to its trans isomer by exchange of two groups at a stereocenter in the cis-cycloalkane.
(d) Configuration refers to the arrangement in space of the atoms or groups of atoms at a stereocenter.
(e) Cis$-1,$ 4-dimethylcyclohexane and trans-1,4dimethylcyclohexane are classified as conformations.

David Collins

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Problem 34

What structural feature of cycloalkanes makes cistrans isomerism in them possible?

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Problem 35

Is cis-trans isomerism possible in alkanes?

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Problem 36

Name and draw structural formulas for the cis and trans isomers of 1,2 -dimethylcyclopropane.

Henry R

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Problem 37

Name and draw structural formulas for the six cycloalkanes with the molecular formula $\mathrm{C}_{5} \mathrm{H}_{10}$. Include cis-trans isomers as well as constitutional isomers.

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Problem 38

Why is equatorial methylcyclohexane more stable than axial methylcyclohexane?

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Problem 39

Following is a structural formula and a ball-and-stick model of cholestanol, a close relative of cholesterol.
(a) Describe the conformation of each six-membered ring and the one five-membered ring.
(b) Is the $-$ OH group on ring $\mathbf{A}$ in an axial or equatorial position?
(c) Is the $-\mathrm{CH}_{3}$ group between rings $\mathbf{A}$ and $\mathbf{B}$ in an axial or an equatorial position?

Lottie Adams

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Problem 40

Consider a cyclohexane ring substituted with one methyl group and one hydroxyl group. Draw the structural formula for a compound with this composition that:
(a) Does not show cis/trans isomerism.
(b) Does show cis/trans isomerism.

Lottie Adams

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Problem 41

Answer true or false.
(a) Boiling points among alkanes with unbranched chains increase as the number of carbons in the chain increases.
(b) Alkanes that are liquid at room temperature are more dense than water.
(c) Cis and trans isomers have the same molecular formula, the same connectivity, and the same physical properties.
(d) Among alkane constitutional isomers, the least branched isomer generally has the lowest boiling point.
(e) Alkanes and cycloalkanes are insoluble in water.
(f) Liquid alkanes are soluble in each other.

Lottie Adams

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Problem 42

In Problem $11-22,$ you drew structural formulas for the nine constitutional isomers with molecular formula $\mathrm{C}_{7} \mathrm{H}_{16}$. Predict which isomer has the lowest boiling point and which has the highest boiling point.

David Collins

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Problem 43

Which unbranched alkane (Table $11-4$ ) has about the same boiling point as water? Calculate the molecular weight of this alkane and compare it with the molecular weight of water.

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Problem 44

What generalizations can you make about the densities of alkanes relative to the density of water?

Lottie Adams

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Problem 45

What generalization can you make about the solubility of alkanes in water?

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Problem 46

Suppose that you have samples of hexane and octane. Could you tell the difference by looking at them? What color would each be? How could you tell which is which?

Lottie Adams

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Problem 47

As you can see from Table $11-4$, each $\mathrm{CH}_{2}$ group added to the carbon chain of an alkane increases its boiling point. This increase is greater going from $\mathrm{CH}_{4}$ to $\mathrm{C}_{2} \mathrm{H}_{6}$ and from $\mathrm{C}_{2} \mathrm{H}_{6}$ to $\mathrm{C}_{3} \mathrm{H}_{8}$ than it is going from $\mathrm{C}_{8} \mathrm{H}_{18}$ to $\mathrm{C}_{9} \mathrm{H}_{20}$ or from $\mathrm{C}_{9} \mathrm{H}_{20}$ to $\mathrm{C}_{10} \mathrm{H}_{22}$ What do you think is the reason for this difference?

David Collins

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Problem 48

How are the boiling points of hydrocarbons during petroleum refining related to their molecular weight?

Lottie Adams

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Problem 49

Answer true or false.
(a) Combustion of alkanes is an endothermic reaction.
(b) The products of complete combustion of an alkane are carbon dioxide and water.
(c) Halogenation of an alkane converts it to a haloalkane.

Lottie Adams

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Problem 50

Write balanced equations for the combustion of each of the following hydrocarbons. Assume that each is converted completely to carbon dioxide and water.
(a) Hexane
(b) Cyclohexane
(c) 2 -Methylpentane

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Problem 51

The heat of combustion of methane, a component of natural gas, is 212 kcal/mol. That of propane, a component of LP gas, is $530 \mathrm{kcal} / \mathrm{mol}$. On a gramfor-gram basis, which hydrocarbon is the better source of heat energy?

David Collins

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Problem 52

Draw structural formulas for these haloalkanes.
(a) Bromomethane
(b) Chlorocyclohexane
(c) 1,2 -Dibromoethane
(d) 2 -Chloro-2-methylpropane
(e) Dichlorodifluoromethane (Freon-12)

Lottie Adams

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Problem 53

The reaction of chlorine with pentane gives a mixture of three chloroalkanes, each with the molecular formula $\mathrm{C}_{5} \mathrm{H}_{11} \mathrm{Cl}$. Write a line-angle formula and the IUPAC name for each chloroalkane.

Lottie Adams

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Problem 54

Complete and balance the equation for the complete combustion of each hydrocarbon.
(a) Hexane
(b) Cyclohexane
(c) $2-$ Methylpentane

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Problem 55

Name and draw structural formulas for all possible monochlorination products that might be formed in each reaction.

Lottie Adams

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Problem 56

There are three constitutional isomers with the molecular formula $\mathrm{C}_{5} \mathrm{H}_{12}$. When heated with chlorine at $300^{\circ} \mathrm{C},$ isomer A gives a mixture of four monochlorination products. Under the same experimental conditions, isomer B gives a mixture of three monochlorination products and isomer $C$ gives only one monochlorination product. From this information, deduce the structural formulas of isomers $\mathrm{A}, \mathrm{B},$ and $\mathrm{C}$

Lottie Adams

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Problem 57

Answer true or false.
(a) Freons are members of a class of organic compounds called chlorofluorocarbons (CFCs).
(b) An advantage of Freons as heat-transfer agents in refrigeration systems, propellants in aerosol sprays, and solvents for industrial cleaning is that they are nontoxic, nonflammable, odorless, and noncorrosive
(c) Freons in the stratosphere interact with ultraviolet radiation and thereby set up chemical reactions that lead to the destruction of the stratospheric ozone layer
(d) Alternative names for the important laboratory and industrial solvent $\mathrm{CH}_{2} \mathrm{Cl}_{2}$ are dichloromethane, methylene chloride, and chloroform.

Lottie Adams

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Problem 58

(Chemical Connections 11 A) How many rings in tetrodotoxin contain only carbon atoms? How many contain nitrogen atoms? How many contain two oxygen atoms?

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Problem 59

(Chemical Connections $11 \mathrm{B}$ ) What is an "octane rating"? What two reference hydrocarbons are used for setting the scale of octane ratings?

Lottie Adams

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Problem 60

(Chemical Connections $11 \mathrm{B}$ ) Octane has an octane rating of $-20 .$ Will it produce more or less engine knocking than heptane does?

Lottie Adams

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Problem 61

(Chemical Connections $11 \mathrm{B}$ ) Ethanol is added to gasoline to produce $\mathrm{E}-15$ and $\mathrm{E}-85 .$ It promotes more complete combustion of the gasoline and is an octane booster. Compare the heats of combustion of 2,2,4 -trimethylpentane $(1304 \mathrm{kcal} / \mathrm{mol})$ and ethanol $(327 \mathrm{kcal} / \mathrm{mol})$. Which has the higher heat of combustion in kcal/mol? In kcal/g?

Lottie Adams

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Problem 62

(Chemical Connections $11 \mathrm{C}$ ) What are Freons? Why were they considered ideal compounds to use as heat-transfer agents in refrigeration systems? Give structural formulas of two Freons used for this purpose.

Lottie Adams

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Problem 63

(Chemical Connections $11 \mathrm{C}$ ) In what way do Freons negatively affect the environment?

Lottie Adams

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Problem 64

(Chemical Connections $11 \mathrm{C}$ ) What are HFCs and HCFCs? How does their use in refrigeration systems prevent the environmental problems associated with the use of Freons?

Lottie Adams

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Problem 65

Tell whether the compounds in each set are constitutional isomers.

Lottie Adams

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Problem 66

Explain why each of the following is an incorrect IUPAC name. Write the correct IUPAC name for the compound.
(a) 1,3 -Dimethylbutane
(b) $4-$ Methylpentane
(c) 2,2 -Diethylbutane
(d) 2 -Ethyl-3-methylpentane
(e) 2 -Propylpentane
(f) 2,2 -Diethylheptane
(g) 2,2 -Dimethylcyclopropane
(h) 1 -Ethyl-5-methylcyclohexane

Pronoy Sinha

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Problem 67

Which of the following compounds can exist as cis-trans isomers? For each that can, draw both isomers using solid and dashed wedges to show the orientation in space of the $-\mathrm{OH}$ and $-\mathrm{CH}_{3}$ groups.

Lottie Adams

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Problem 68

Tetradecane, $\mathrm{C}_{14} \mathrm{H}_{30}$ is an unbranched alkane with a melting point of $5.9^{\circ} \mathrm{C}$ and a boiling point of $254^{\circ} \mathrm{C} .$ Is tetradecane a solid, liquid, or gas at room temperature?

Lottie Adams

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Problem 69

Dodecane, $\mathrm{C}_{12} \mathrm{H}_{26},$ is an unbranched alkane. Predict the following:
(a) Will it dissolve in water?
(b) Will it dissolve in hexane?
(c) Will it burn when ignited?
(d) Is it a liquid, solid, or gas at room temperature and atmospheric pressure?
(e) Is it more or less dense than water?

Shalini Tyagi

Numerade Educator

Problem 70

Following is a structural formula for 2-isopropyl-5-methylcyclohexanol:
Using a planar hexagon representation for the cyclohexane ring, draw a structural formula for the cis-trans isomer with isopropyl trans to $-\mathrm{OH}$ and methyl cis to - OH. If you answered this part correctly, you have drawn the isomer found in nature and given the name menthol.

Lottie Adams

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Problem 71

On the left is a representation of the glucose molecule. Convert this representation to the alternative representations using the rings on the right. (We discuss the structure and chemistry of glucose in Chapter $20 .$ )

David Collins

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Problem 72

On the left is a representation for 2 -deoxy-D-ribose. This molecule is the "D" of DNA. Convert this representation to the alternative representation using the ring on the right. (We discuss the structure and chemistry of this compound in more detail in Chapter $20 .$)

Lottie Adams

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Problem 73

As stated in Section $11-9$, the wax found in apple skins is an unbranched alkane with the molecular formula $\mathrm{C}_{27} \mathrm{H}_{56}$. Explain how the presence of this alkane in apple skins prevents the loss of moisture from within the apple.

Vasu Makani

Numerade Educator

Problem 74

Following are structural formulas for dimethyl ether and ethanol. These compounds are constitutional isomers with the molecular formula $\mathrm{C}_{2} \mathrm{H}_{6} \mathrm{O}$
\[
\begin{array}{cc}
\mathrm{CH}_{3} \mathrm{OCH}_{3} & \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} \\
\text { Dimethyl ether } & \text { Ethanol }
\end{array}
\]
One of these compounds has a boiling point of $78^{\circ} \mathrm{C}$ the other has a boiling point of $-24^{\circ} \mathrm{C}$
(a) Which compound has which boiling point? Explain your reasoning. (Chapter 5 )
(b) One compound is soluble in water in all proportions, that is, it is infinitely soluble. The other has a water solubility of only $7.8 \mathrm{g} / 100 \mathrm{mL}$ Which compound has which water solubility? How do you account for this difference in water solubility? We discuss the physical properties of alcohols and ethers in detail in Chapter 14.

David Collins

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