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7 Focus on Health & Medicine: Medical Imaging Without Radioactivity

7 Focus on Health & Medicine: Medical Imaging Without Radioactivity

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318



NUCLEAR CHEMISTRY



CHAPTER HIGHLIGHTS

KEY TERMS

Alpha (α) particle (10.1)

Becquerel (10.4)

Beta (β) particle (10.1)

Chain reaction (10.6)

Critical mass (10.6)

Curie (10.4)

Gamma (γ) ray (10.1)

Geiger counter (10.4)



Gray (10.4)

Half-life (10.3)

LD50 (10.4)

Nuclear fission (10.6)

Nuclear fusion (10.6)

Nuclear reaction (10.1)

Positron (10.1)

Rad (10.4)



Radioactive decay (10.2)

Radioactive isotope (10.1)

Radioactivity (10.1)

Radiocarbon dating (10.3)

Rem (10.4)

Sievert (10.4)

X-ray (10.7)



KEY CONCEPTS

❶ Describe the different types of radiation emitted by a

radioactive nucleus. (10.1)

• A radioactive nucleus can emit α particles, β particles,

positrons, or γ rays.

• An α particle is a high-energy nucleus that contains two

protons and two neutrons.

• A β particle is a high-energy electron.

• A positron is an antiparticle of a β particle. A positron has a

+1 charge and negligible mass.

• A γ ray is high-energy radiation with no mass or charge.

❷ How are equations for nuclear reactions written? (10.2)

• In an equation for a nuclear reaction, the sum of the mass

numbers (A

(A) must be equal on both sides of the equation.

The sum of the atomic numbers (Z

(Z) must be equal on both

sides of the equation as well.

❸ What is the half-life of a radioactive isotope? (10.3)

• The half-life (t

(t1/2) is the time it takes for one-half of a

radioactive sample to decay. Knowing the half-life and the

amount of a radioactive substance, one can calculate how

much sample remains after a period of time.

• The half-life of radioactive C-14 can be used to date

archaeological artifacts.

❹ What units are used to measure radioactivity? (10.4)

• Radiation in a sample is measured by the number of

disintegrations per second, most often using the curie (Ci);

1 Ci = 3.7 × 1010 disintegrations/s. The becquerel (Bq) is

also used; 1 Bq = 1 disintegration/s; 1 Ci = 3.7 ì 1010 Bq.

The exposure of a substance to radioactivity is measured

with the rad (radiation absorbed dose) or the rem (radiation

equivalent for man).



smi26573_ch10.indd 318



❺ Give examples of common radioisotopes used in medicine.

(10.5)

• Iodine-131 is used to diagnose and treat thyroid disease.

• Technetium-99m is used to evaluate the functioning of the

gall bladder and bile ducts, and in bone scans to evaluate

the spread of cancer.

• Red blood cells tagged with technetium-99m are used to

find the site of a gastrointestinal bleed.

• Thallium-201 is used to diagnose coronary artery disease.

• Cobalt-60 is used as an external source of radiation for

cancer treatment.

• Iodine-125 and iridium-192 are used in internal radiation

treatment of prostate cancer and breast cancer, respectively.

• Carbon-11, oxygen-15, nitrogen-13, and fluorine-18 are

used in positron emission tomography.

❻ What are nuclear fission and nuclear fusion? (10.6)

• Nuclear fission is the splitting apart of a heavy nucleus into

lighter nuclei and neutrons.

• Nuclear fusion is the joining together of two light nuclei to

form a larger nucleus.

• Both nuclear fission and nuclear fusion release a great deal

of energy. Nuclear fission is used in nuclear power plants to

generate electricity. Nuclear fusion occurs in stars.

❼ What medical imaging techniques do not use

radioactivity? (10.7)

• X-rays and CT scans both use X-rays, a high-energy form

of electromagnetic radiation.

• MRIs use low-energy radio waves to image soft tissue.



12/4/08 10:57:50 AM



PROBLEMS



319



PROBLEMS

Selected in-chapter and end-of-chapter problems have brief answers provided in Appendix B.



Isotopes and Radiation

10.23



10.24



10.25



Compare fluorine-18 and fluorine-19 with regard to

each of the following: (a) atomic number; (b) number

of protons; (c) number of neutrons; (d) mass number.

Give the isotope symbol for each isotope. F-19 is a stable

nucleus and F-18 is used in PET scans.

Compare nitrogen-13 and nitrogen-14 with regard to

each of the following: (a) atomic number; (b) number of

protons; (c) number of neutrons; (d) mass number. Give

the isotope symbol for each isotope. N-14 is a stable

nucleus and N-13 is used in PET scans.

Complete the table of isotopes, each of which has found

use in medicine.



type of radiation. (The blue spheres represent protons and

the red spheres represent neutrons.)



+

positron



10.32



Complete the nuclear equation by drawing the nucleus

of the missing atom. Give the symbol for each atom and

type of radiation. (The blue spheres represent protons and

the red spheres represent neutrons.)



Atomic

Mass

Number

Number Isotope

number number of protons of neutrons symbol



a. Chromium-51

b.



46



c.



19



d.

10.26



+



103



133



23



54



Complete the table of isotopes, each of which has found

use in medicine.

Atomic

Mass

Number

Number Isotope

number number of protons of neutrons symbol



a. Sodium-24

b.



89



c.



59



10.33



10.34



51

26



10.35



d. Samarium-153

10.27



10.28



10.29

10.30



How much does the mass and charge of a nucleus change

when each type of radiation is emitted: (a) α particle;

(b) β particle; (c) γ ray; (d) positron?

Compare α particles, β particles, and γ rays with regard

to each of the following: (a) speed the radiation travels;

(b) penetrating power; (c) protective equipment that must

be worn when handling.

What is the mass and charge of radiation that has each of

the following symbols: (a) α; (b) n; (c) γ; (d) β?

What is the mass and charge of radiation that has each of

the following symbols?

a. –10 e

b. +10 e

c. 42He

d. β+



Nuclear Reactions

10.31



smi26573_ch10.indd 319



Complete the nuclear equation by drawing the nucleus

of the missing atom. Give the symbol for each atom and



10.36



Complete each nuclear equation.

a. 59

? + –10e

c. 178

26Fe

80Hg



10.38



10.39



0

+1e



b. 190

? + 42He

78Pt

Complete each nuclear equation.

a. 77

? + +10 e

c. 66

37Rb

29Cu



? +



b. 251

? + 42He

102No

Complete each nuclear equation.

90

a. 90

c. 210

39Y

40 Zr + ?

83Bi



? + 42He



135

0

b. ?

59Pr + +1e

Complete each nuclear equation.

90

0

a. ?

c. 214

39Y + –1e

84Po



? + 42He



b.

10.37



? +



29

15P



29

14Si



0

–1e



+ ?



Bismuth-214 can decay to form either polonium-214

or thallium-210, depending on what type of radiation

is emitted. Write a balanced nuclear equation for each

process.

Lead-210 can be formed by the decay of either thallium210 or polonium-214, depending on what type of

radiation is emitted. Write a balanced nuclear equation

for each process.

Write a balanced nuclear equation for each reaction.

a. decay of thorium-232 by α emission

b. decay of sodium-25 by β emission

c. decay of xenon-118 by positron emission

d. decay of curium-243 by α emission



12/4/08 10:57:50 AM



320



10.40



NUCLEAR CHEMISTRY



Write a balanced nuclear equation for each reaction.

a. decay of sulfur-35 by β emission

b. decay of thorium-225 by α emission

c. decay of rhodium-93 by positron emission

d. decay of silver-114 by β emission



10.56



Half-Life

10.41

10.42

10.43



10.44



10.45

10.46

10.47

10.48

10.49



10.50



If the amount of a radioactive element decreases from

2.4 g to 0.30 g in 12 days, what is its half-life?

If the amount of a radioactive element decreases from

0.36 g to 90. mg in 22 min, what is its half-life?

Radioactive iodine-131 (t1/2 = 8.0 days) decays to form

xenon-131 by emission of a β particle. How much of

each isotope is present after each time interval if 64 mg of

iodine-131 was present initially: (a) 8.0 days; (b) 16 days;

(c) 24 days; (d) 32 days?

Radioactive phosphorus-32 decays to form sulfur-32 by

emission of a β particle. Estimating the half-life to be

14 days, how much of each isotope is present after each

time interval if 124 mg of phosphorus-32 was present

initially: (a) 14 days; (b) 28 days; (c) 42 days; (d) 56 days?

If the half-life of an isotope is 24 hours, has all the

isotope decayed in 48 hours?

Explain how the half-life of carbon-14 is used to date

objects.

Why can’t radiocarbon dating be used to determine the

age of an artifact that is over 50,000 years old?

Why can’t radiocarbon dating be used to estimate the age

of rocks?

A patient is injected with a sample of technetium-99m

(t1/2 = 6.0 h), which has an activity of 20 mCi. What

activity is observed after each interval: (a) 6 h; (b) 12 h;

(c) 24 h?

A sample of iodine-131 (t1/2 = 8.0 days) has an activity of

200. mCi. What activity is observed after each interval:

(a) 8.0 days; (b) 24 days; (c) 48 days?



10.57



10.58



10.59



10.60



Nuclear Fission and Nuclear Fusion

10.61

10.62



10.63



10.64



Measuring Radioactivity

10.51

10.52

10.53



10.54



10.55



If a radioactive sample had an activity of 5.0 mCi, how

many disintegrations per second does this correspond to?

Why is the average amount of background radiation

generally higher at higher elevations?

A patient must be administered a 28-mCi dose of

technetium-99m, which is supplied in a vial containing

a solution with an activity of 12 mCi/mL. What volume

of solution must be given?

A radioactive isotope used for imaging is supplied in

an 8.0-mL vial containing a solution with an activity of

108 mCi. What volume must be given to a patient who

needs a 12-mCi dose?

Radioactive sodium-24, administered as 24NaCl, is given

to treat leukemia. If a patient must receive 190 µCi/kg



smi26573_ch10.indd 320



and the isotope is supplied as a solution that contains

5.0 mCi/mL, what volume is needed for a 68-kg patient?

Radioactive phosphorus-32, administered as sodium

phosphate (Na332PO4), is used to treat chronic leukemia.

The activity of an intravenous solution is 670 µCi/mL.

What volume of solution must be used to supply a dose

of 15 mCi?

The units chosen to report radiation amounts give us

different information. What is measured using the curie

compared to the rad?

The units chosen to report radiation amounts give us

different information. What is measured using the

millicurie compared to the rem?

The initial responders to the Chernobyl nuclear disaster

were exposed to 20 Sv of radiation. Convert this value to

rem. Did these individuals receive a fatal dose of radiation?

Many individuals who fought fires at the Chernobyl

nuclear disaster site were exposed to 0.25 Sv of radiation.

Convert this value to rem. Did these individuals receive

a fatal dose of radiation? Would you expect any of these

individuals to have shown ill health effects?



10.65



10.66



10.67



What is the difference between nuclear fission and

nuclear fusion?

What is the difference between the nuclear fission process

that takes place in a nuclear reactor and the nuclear

fission that occurs in an atomic bomb?

For which process does each statement apply—nuclear

fission, nuclear fusion, both fission and fusion?

a. The reaction occurs in the sun.

b. A neutron is used to bombard a nucleus.

c. A large amount of energy is released.

d. Very high temperatures are required.

For which process does each statement apply—nuclear

fission, nuclear fusion, both fission and fusion?

a. The reaction splits a nucleus into lighter nuclei.

b. The reaction joins two lighter nuclei into a heavier

nucleus.

c. The reaction is used to generate energy in a nuclear

power plant.

d. The reaction generates radioactive waste with a long

half-life.

Complete each nuclear fission equation.

1

1

a. 235

? + 97

92U + 0n

42Mo + 2 0n

1

1

b. 235

? + 140

92U + 0n

56Ba + 3 0n

Complete each nuclear fission equation.

1

1

a. 235

? + 139

92U + 0n

57La + 2 0n

1

1

0

b. 235

? + 140

92U + 0n

58Ce + 2 0n + 6 –1e

The fusion of two deuterium nuclei (hydrogen-2) forms

a hydrogen nucleus (hydrogen-1) as one product. What

other product is formed?



12/4/08 10:57:51 AM



PROBLEMS



10.68



321



Fill in the missing product in the following nuclear fusion

reaction.

3

2He



10.69

10.70

10.71



+ 32He



10.72



? + 2 11H



Discuss two problems that surround the generation of

electricity from a nuclear power plant.

Why are there as yet no nuclear power plants that use

nuclear fusion to generate electricity?

All nuclei with atomic numbers around 100 or larger do

not exist naturally; rather they have been synthesized by

fusing two lighter-weight nuclei together. Complete the

following nuclear equation by giving the name, atomic

number, and mass number of the element made by this

reaction.

209

83Bi



+



58

26Fe



+



14

7N



10.76



? + 10n



Complete the following nuclear equation, and give the

name, atomic number, and mass number of the element

made by this reaction.

235

92U



10.75



? + 5 10n



Applications

10.77



General Questions

10.73



10.74



Arsenic-74 is a radioisotope used for locating brain

tumors.

a. Write a balanced nuclear equation for the positron

emission of arsenic-74.

b. If t1/2 for As-74 is 18 days, how much of a 120-mg

sample remains after 90 days?

c. If the radioactivity of a 2.0-mL vial of arsenic-74 is

10.0 mCi, what volume must be administered to give a

7.5-mCi dose?

Sodium-24 is a radioisotope used for examining

circulation.

a. Write a balanced nuclear equation for the β decay of

sodium-24.

b. If t1/2 for Na-24 is 15 h, how much of an 84-mg

sample remains after 2.5 days?

c. If the radioactivity of a 5.0-mL vial of sodium-24 is

10.0 mCi, what volume must be administered to give a

6.5-mCi dose?



Answer the following questions about radioactive

iridium-192.

a. Write a balanced nuclear equation for the decay of

iridium-192, which emits both a β particle and a γ ray.

b. If t1/2 for Ir-192 is 74 days, estimate how much of a

120-mg sample remains after five months.

c. If a sample of Ir-192 had an initial activity of 36 Ci,

estimate how much activity remained in the sample

after 10 months.

Answer the following questions about radioactive

samarium-153.

a. Write a balanced nuclear equation for the decay of

samarium-153, which emits both a β particle and a γ ray.

b. If t1/2 for Sm-153 is 46 h, estimate how much of a

160-mg sample remains after four days.

c. If a sample of Sm-153 had an initial activity of 48 Ci,

estimate how much activity remained in the sample

after six days.



10.78

10.79



10.80

10.81



10.82

10.83



10.84



Explain how each isotope is used in medicine.

a. iodine-131

b. iridium-192

c. thallium-201

Explain how each isotope is used in medicine.

a. iodine-125

b. technetium-99m

c. cobalt-60

How does the half-life of each of the following isotopes

of iodine affect the manner in which it is administered to

a patient: (a) iodine-125, t1/2 = 60 days; (b) iodine-131,

t1/2 = 8 days?

Explain why food is irradiated with γ rays.

A mammogram is an X-ray of the breast. Why does an

X-ray technician leave the room or go behind a shield

when a mammogram is performed on a patient?

Why is a lead apron placed over a patient’s body when

dental X-rays are taken?

One of the radioactive isotopes that contaminated the

area around Chernobyl after the nuclear accident in 1986

was iodine-131. Suggest a reason why individuals in the

affected region were given doses of NaI that contained

the stable iodine-127 isotope.

The element strontium has similar properties to calcium.

Suggest a reason why exposure to strontium-90, a

product of nuclear testing in the atmosphere, is especially

hazardous for children.



CHALLENGE QUESTIONS

10.85



smi26573_ch10.indd 321



An article states that the fission of 1.0 g of uranium-235

releases 3.4 × 108 kcal, the same amount of energy

as burning one ton (2,000 lb) of coal. If this report is

accurate, how much energy is released when 1.0 g of coal

is burned?



10.86



Radioactive isotopes with high atomic numbers often

decay to form isotopes that are themselves radioactive,

and once formed, decay to form new isotopes. Sometimes

a series of such decays occurs over many steps until a

stable nucleus is formed. The following series of decays

occurs: Polonium-218 decays with emission of an α

particle to form X, which emits a β particle to form Y,

which emits an α particle to form Z. Identify X, Y, and Z.



12/4/08 10:57:51 AM



11

CHAPTER OUTLINE

11.1



Introduction to Organic Chemistry



11.2



Characteristic Features of Organic

Compounds



11.3



Shapes of Organic Molecules



11.4



Drawing Organic Molecules



11.5



Functional Groups



11.6



Properties of Organic Compounds



11.7



FOCUS ON HEALTH & MEDICINE:

Vitamins



CHAPTER GOALS

In this chapter you will learn how to:

➊ Recognize the characteristic features

of organic compounds

➋ Predict the shape around atoms in

organic molecules

➌ Use shorthand methods to draw

organic molecules

➍ Recognize the common functional

groups and understand their

importance

➎ Distinguish organic compounds from

ionic inorganic compounds

➏ Determine whether an organic

compound is polar or nonpolar

➐ Determine solubility properties of

organic compounds

➑ Determine whether a vitamin is fat

soluble or water soluble



Vitamin A, a key component of the vision receptors in the eye, is synthesized in the body from

β-carotene, the orange pigment found in carrots.



INTRODUCTION TO

ORGANIC MOLECULES

AND FUNCTIONAL GROUPS

CONSIDER for a moment the activities that occupied your past 24 hours. You

likely showered with soap, drank a caffeinated beverage, ate at least one form of

starch, took some medication, read a newspaper, listened to a CD, and traveled in

a vehicle that had rubber tires and was powered by fossil fuels. If you did any one

of these, your life was touched by organic chemistry. In Chapter 11, we learn about

the characteristic features of organic molecules.



322



smi26573_ch11.indd 322



12/5/08 11:40:21 AM



CHARACTERISTIC FEATURES OF ORGANIC COMPOUNDS



323



11.1 INTRODUCTION TO ORGANIC CHEMISTRY

What is organic chemistry?

• Organic chemistry is the study of compounds that contain the element carbon.



While it may seem odd that an entire discipline is devoted to the study of a single element in the

periodic table, millions of organic compounds are known, far more than the inorganic compounds

discussed in Chapters 1–10. These organic chemicals affect virtually every facet of our lives,

and for this reason, it is important and useful to know something about them.

Clothes, foods, medicines, gasoline, refrigerants, and soaps are composed almost solely of organic

compounds. Some, like cotton, wool, or silk are naturally occurring; that is, they can be isolated

directly from natural sources. Others, such as nylon and polyester are synthetic, meaning they are

produced by chemists in the laboratory. By studying the principles and concepts of organic chemistry, you can learn more about compounds such as these and how they affect the world around

you. Figure 11.1 illustrates some common products of organic chemistry used in medicine.







FIGURE 11.1



Some Common Products of Organic Chemistry Used in Medicine



a. Oral contraceptives



b. Plastic syringes



c. Antibiotics



d. Synthetic heart valves



Organic chemistry has given us contraceptives, plastics, antibiotics, synthetic heart valves, and a myriad of other materials. Our lives

would be vastly different today without these products of organic chemistry.



PROBLEM 11.1



Which molecular formulas represent organic compounds and which represent inorganic

compounds?

a. C6H12

b. H2O



c. KI

d. MgSO4



e. CH4O

f. NaOH



11.2 CHARACTERISTIC FEATURES OF ORGANIC COMPOUNDS

Perhaps the best way to appreciate the variety of organic molecules is to look at a few. Simple

organic compounds that contain just one or two carbon atoms, respectively, are methane and

ethanol.



smi26573_ch11.indd 323



12/5/08 11:40:32 AM



324



INTRODUCTION TO ORGANIC MOLECULES AND FUNCTIONAL GROUPS



H

H



C



H



H



H

methane



H



H



C



C



H



H



OH



ethanol



Methane, the main component of natural gas, burns in the presence of oxygen. The natural gas we

use today was formed by the decomposition of organic material millions of years ago. Ethanol,

the alcohol present in wine and other alcoholic beverages, is formed by the fermentation of sugar.

Ethanol can also be made in the lab by a totally different process. Ethanol produced in the lab

is identical to the ethanol produced by fermentation.

Two more complex organic molecules are capsaicin and caffeine. Capsaicin, the compound

responsible for the characteristic spiciness of hot peppers, is the active ingredient in several

topical creams for pain relief. Caffeine is the bitter-tasting stimulant found in coffee, tea, cola

beverages, and chocolate.

H

H



C



H



H

O



H



H



O

C



H



C

C



C

C



C



H



C



H

H



H



H



C



C



C



C



H H H

capsaicin



H



C



H

H



H



N



O



H H



C



H H



C



C



C



H



H



C



H



H



H

H

H



C



O H



C



C



N



N



C



H C

N

O



C



H



H



C



H



C



H



N



H

caffeine



What are the common features of these organic compounds?

[1] All organic compounds contain carbon atoms and most contain hydrogen atoms. Carbon

always forms four covalent bonds, and hydrogen forms one covalent bond.



Carbon is located in group 4A of the periodic table, so a carbon atom has four valence electrons

available for bonding (Section 4.1). Since hydrogen has a single valence electron, methane (CH4)

consists of four single bonds, each formed from one electron from a hydrogen atom and one

electron from carbon.

Remember that each solid line

represents one two-electron bond.



H



H

H



C

H



H



=



H



C



H

two-electron bond



H

methane



[2] Carbon forms single, double, and triple bonds to other carbon atoms.



smi26573_ch11.indd 324



12/5/08 11:40:41 AM



CHARACTERISTIC FEATURES OF ORGANIC COMPOUNDS



325



When a compound contains two or more carbon atoms, the type of bonding is determined by the

number of atoms around carbon. Consider the three compounds drawn below:



H



H



H



C



C



H



H



H



H



Each C forms

four single bonds.

H

H



C



H



H



C



H



H



H



H



A double bond contains

four electrons.

H



H



C



C



H



H



H



ethane



C



A triple bond contains

six electrons.



H



C



C



H



C



C



C



H



H



ethylene



acetylene



• A C atom surrounded by four atoms forms four single bonds. In ethane (C2H6), each

carbon atom is bonded to three hydrogen atoms and one carbon atom. All bonds are

single bonds.

• A C atom surrounded by three atoms forms one double bond. In ethylene (C2H4),

each carbon atom is surrounded by three atoms (two hydrogens and one carbon); thus,

each C forms a single bond to each hydrogen atom and a double bond to carbon.

• A C atom surrounded by two atoms generally forms one triple bond. In acetylene

(C2H2), each carbon atom is surrounded by two atoms (one hydrogen and one carbon);

thus, each C forms a single bond to hydrogen and a triple bond to carbon.



[3] Some compounds have chains of atoms and some compounds have rings.



For example, three carbon atoms can bond in a row to form propane, or form a ring called cyclopropane. Propane is the fuel burned in gas grills, and cyclopropane is an anesthetic.

H



H

H



H



H



H



C



C



C



H



H



H



propane

C3H8



C

H



H

H



C



H



C



H



cyclopropane

C3H6



[4] Organic compounds may also contain elements other than carbon and hydrogen.

Any atom that is not carbon or hydrogen is called a heteroatom.



The most common heteroatoms are nitrogen, oxygen, and the halogens (F, Cl, Br, and I).

• Each heteroatom forms a characteristic number of bonds, determined by its location in

the periodic table.

• The common heteroatoms also have nonbonding, lone pairs of electrons, so that each

atom is surrounded by eight electrons.

The number of bonds formed

by common elements was first

discussed in Section 4.1.



Thus, nitrogen forms three bonds and has one lone pair of electrons, while oxygen forms two bonds

and has two additional lone pairs. The halogens form one bond and have three additional lone pairs.

Common bonding patterns for atoms in organic compounds are summarized in Table 11.1. Except

for hydrogen, these common elements in organic compounds follow one rule in bonding:

Number of bonds



smi26573_ch11.indd 325



+



Number of lone pairs



=



4



12/5/08 11:40:49 AM



326



INTRODUCTION TO ORGANIC MOLECULES AND FUNCTIONAL GROUPS



TABLE 11.1



Common Bonding Patterns for Atoms

in Organic Compounds

lone pairs of electrons

H



C



N



O



X



hydrogen



carbon



nitrogen



oxygen



halogen



Number of bonds



1



4



3



2



1



Number of nonbonded

electron pairs



0



0



1



2



3



X = F, Cl, Br, I



Oxygen and nitrogen form both single and multiple bonds to carbon. The most common multiple bond between carbon and a heteroatom is a carbon–oxygen double bond (C O). The

bonding patterns remain the same even when an atom is part of a multiple bond, as shown with

methanol (CH3OH) and formaldehyde (H2C O, a preservative).

two lone pairs on O



H

H



C



two lone pairs on O

H



O



C



H



H

four bonds to C



O



H

two bonds to O



four bonds to C



methanol



two bonds to O



formaldehyde



These features explain why there are so many organic compounds: Carbon forms four strong

bonds with itself and other elements. Carbon atoms combine together to form rings and

chains.



SAMPLE PROBLEM 11.1



Draw in all H’s and lone pairs in each compound.

a. C C Cl



ANALYSIS



b. C C O



c. C C N



Each C and heteroatom must be surrounded by eight electrons. Use the common bonding

patterns in Table 11.1 to fill in the needed H’s and lone pairs. C needs four bonds; Cl needs one

bond and three lone pairs; O needs two bonds and two lone pairs; N needs three bonds and one

lone pair.



SOLUTION



Cl needs three lone pairs.



a.



H



H



H



C



C



H



H



N needs one lone pair.

H



Cl



c.



H



C



C



N



H



C needs 3 H’s to

have four bonds.



C already has four bonds.



O needs two lone pairs.

H



b.



H



C



C



H



H



O



C needs 1 H to

have four bonds.



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SHAPES OF ORGANIC MOLECULES



PROBLEM 11.2



327



Fill in all H’s and lone pairs in each compound.

O



a.



C



C



C



C



b.



C



O



C



c.



C



C



O



C



d.



C



C



C



N



C



e.



C

O

C



11.3 SHAPES OF ORGANIC MOLECULES

The shape around atoms in organic molecules is determined by counting groups using the principles of VSEPR theory (Section 4.6). Recall that a group is either another atom or a lone pair

of electrons. The most stable arrangement keeps these groups as far away from each other

as possible.

• An atom surrounded by two groups is linear and has a bond angle of 180°.

Because acetylene produces a very

hot flame on burning, it is often used

in welding torches.



Each carbon in acetylene (HC CH) is surrounded by two atoms and no lone pairs. Thus, each

H C C bond angle is 180°, making all four atoms of acetylene linear.

H



180°

C C H

180°



=

acetylene

ball-and-stick model



two atoms around each C



• An atom surrounded by three groups is trigonal planar and has a bond angle of 120°.



Each carbon in ethylene (CH2 CH2) is surrounded by three atoms and no lone pairs. Thus, each

H C C bond angle is 120°, and all six atoms of ethylene lie in one plane.

H



H

C



H



=



C 120°

H



three atoms around each C



Ethylene is an important starting

material in the preparation of the

plastic polyethylene.



ethylene



• An atom surrounded by four groups places these four groups at the corners of a

tetrahedron, giving bond angles of approximately 109.5°.



The carbon atom in methane (CH4) is bonded to four hydrogen atoms, pointing to the corners of

a tetrahedron.

H

109.5°

H



C



H



H

tetrahedral carbon



To draw the three-dimensional tetrahedron on a two-dimensional page, place two of the bonds in

the plane of the page, one bond in front, and one bond behind. Then, use the drawing conventions

first presented in Section 4.6:

• A solid line is used for bonds in the plane.

• A wedge is used for a bond in front of the plane.

• A dashed line is used for a bond behind the plane.



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328



INTRODUCTION TO ORGANIC MOLECULES AND FUNCTIONAL GROUPS



bond behind



H

bonds in the plane



=



C

H



H

H

methane

ball-and-stick model



bond in front



All carbons having four single bonds are tetrahedral. For example, each carbon of ethane

is tetrahedral and can be drawn with two bonds in the plane, one in front on a wedge, and one

behind on a dashed line.



H



H



H



C



C



H



H



ethane



Ethane is a constituent of natural

gas.



H H

H



C

H



H



=



C



H H

tetrahedral C’s



Don’t forget about the lone pairs of electrons on nitrogen and oxygen when determining the

shape around these atoms. The N atom in methylamine (CH3NH2) is surrounded by three atoms

and one lone pair—four groups. To keep these four groups as far apart as possible, each of these

groups occupies the corner of a tetrahedron. In this way, the N atom of methylamine resembles

the N atom of ammonia (NH3) discussed in Section 4.6. The H N C bond angle of 112° is

close to the theoretical tetrahedral bond angle of 109.5°. This molecular shape is referred to as a

trigonal pyramid, because one of the groups around the N is a lone pair, not another atom.

One corner of the tetrahedron

has an electron pair, not a bond.



C



four groups around N

• three atoms

• one lone pair

N H



H



H



H

H



N



H3C



H



=



112°



H



methylamine



trigonal pyramid



Similarly, the O atom in methanol (CH3OH) is surrounded by two atoms and two lone pairs—four

groups. To keep these groups as far apart as possible, each of these groups occupies the corner of a

tetrahedron. In this way the O atom of methanol resembles the O atom of water (H2O) discussed in

Section 4.6. The C O H bond angle of 109° is close to the theoretical tetrahedral bond angle of

109.5°. Methanol has a bent molecular shape around O because two of the groups around oxygen

are lone pairs. Table 11.2 summarizes the possible shapes around atoms in organic compounds.

Two corners of the tetrahedron

have electron pairs, not bonds.



H

H



C



four groups around O

• two atoms

• two lone pairs

O H



H3C



H



H

methanol



SAMPLE PROBLEM 11.2



=



O



109°



bent shape



Determine the shape around each atom in dimethyl ether.

H

H



C

H



H

O



C



H



H



dimethyl ether



smi26573_ch11.indd 328



12/5/08 11:40:55 AM



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