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8 Focus on the Human Body: The Sense of Smell

8 Focus on the Human Body: The Sense of Smell

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466



THE THREE-DIMENSIONAL SHAPE OF MOLECULES







FIGURE 15.4 The Shape of Molecules and the Sense of Smell



brain

olfactory

nerve cell

airflow



mucus

receptor on an

olfactory hair



lining of the olfactory

bulb in the nasal passage



olfactory

hairs



nasal passage



cyclooctane bound

to a receptor site



Cyclooctane and other molecules similar in shape bind to a particular olfactory receptor on

the nerve cells that lie at the top of the nasal passage. Binding results in a nerve impulse that

travels to the brain, which interprets impulses from particular receptors as specific odors.



Since enantiomers interact with chiral smell receptors, some enantiomers have different odors.

There are a few well-characterized examples of this phenomenon in nature. For example, one

enantiomer of carvone is responsible for the odor of caraway, whereas the other carvone enantiomer is responsible for the odor of spearmint.



CH3



CH3



O



O



C



H



CH3



CH3



H 2C

caraway seeds



C



H

CH2



carvone

enantiomer A



carvone

enantiomer B



A has the odor of caraway.



spearmint leaves



B has the odor of spearmint.



Thus, the three-dimensional structure of a molecule is important in determining its odor.



PROBLEM 15.18



Limonene is similar to carvone in that each enantiomer has a different odor; one enantiomer

occurs in lemons and the other occurs in oranges. Identify the chirality center in limonene and

draw both enantiomers of limonene.

CH2

CH3



C

CH3

limonene



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PROBLEMS



467



CHAPTER HIGHLIGHTS

KEY TERMS

Achiral (15.2)

Chiral (15.2)

Chirality center (15.2)

Constitutional isomer (15.1)



Cross formula (15.6)

Diastereomer (15.7)

Enantiomer (15.2)

Fischer projection formula (15.6)



Racemic mixture (15.5)

Stereochemistry (15.1)

Stereoisomer (15.1)



KEY CONCEPTS

❶ When is a molecule chiral or achiral? (15.2)

• A chiral molecule is not superimposable on its mirror

image.

• An achiral molecule is superimposable on its mirror image.

• To see whether a molecule is chiral, draw it and its mirror

image and see if all of the atoms and bonds align.



❺ What is a Fischer projection? (15.6)

• A Fischer projection is a specific way of depicting a

chirality center. The chirality center is located at the

intersection of a cross. The horizontal lines represent bonds

that come out of the plane on wedges, and the vertical lines

represent bonds that go back on dashed lines.



❷ What is a chirality center? (15.2, 15.3)

• A chirality center is a carbon with four different groups

around it.

• A molecule with one chirality center is chiral.



❻ What is the difference between an enantiomer and a

diastereomer? (15.7)

• Enantiomers are stereoisomers that are nonsuperimposable

mirror images of each other.

• Diastereomers are stereoisomers that are not mirror images

of each other.



❸ What are enantiomers? (15.2, 15.3)

• Enantiomers are mirror images that are not superimposable

on each other. To draw two enantiomers, draw one molecule

in three dimensions around the chirality center. Then draw

the mirror image so that the substituents are a reflection of

the groups in the first molecule.

❹ Why do some chiral drugs have different properties from

their mirror-image isomers? (15.5)

• When a chiral drug must interact with a chiral receptor,

only one enantiomer fits the receptor properly and evokes

a specific response. Ibuprofen, naproxen, and l-dopa are

examples of chiral drugs in which the two enantiomers have

very different properties.



❼ How is the shape of a molecule related to its odor? (15.8)

• It is thought that the odor of a molecule is determined more

by its shape than the presence of a particular functional

group, so compounds of similar shape have similar odors.

For an odor to be perceived, a molecule must bind to an

olfactory receptor, resulting in a nerve impulse that travels

to the brain. Enantiomers may have different odors because

they bind with chiral receptors and each enantiomer fits the

chiral receptors in a different way.



PROBLEMS

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



Chiral Compounds, Chirality Centers,

and Enantiomers

15.19

15.20

15.21



15.22



Label each of the following objects as chiral or achiral:

(a) chalk; (b) shoe; (c) baseball glove; (d) soccer ball.

Label each of the following objects as chiral or achiral:

(a) boot; (b) index card; (c) scissors; (d) drinking glass.

Explain the following statement. Butane

(CH3CH2CH2CH3) has a mirror image but it does not

have an enantiomer.

Explain why the human body is chiral.



15.23



Draw a mirror image for each compound. Label each

compound as chiral or achiral.

CF3



a.



C

Br



H



c.



Cl

H



Cl



C



H



Cl



halothane

(general anesthetic)



dichloromethane

(common solvent)



COOH



b.



H

(CH3)2CH



C

NH2



valine

(naturally occurring amino acid)



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468



15.24



THE THREE-DIMENSIONAL SHAPE OF MOLECULES



Draw a mirror image for each compound. Label each

compound as chiral or achiral.



15.28



CH2OH



a.



O



CH3CH2



C



c.



CH2CH3



diethyl ether

(general anesthetic)



HOCH2



Label the chirality center (if one exists) in each cyclic

compound. Some compounds contain no chirality centers.

a.



C



H



glycerol

(triol used in lotions)



b.

15.29



erythrulose

(a carbohydrate)



Label the chirality center (if one exists) in each compound.

Compounds contain zero or one chirality center.

a. CH3CH2CHCl

c. CH3CHCH3



15.30



Cl



Cl



CH3 Cl



b.



CH3CHCHCl2



d.



CH3CCH2CHCH3



Cl



15.26



H



Label the chirality center (if one exists) in each

compound. Compounds contain zero or one chirality

center.



15.31

15.32



O



a.



15.33



C

CH3



c.



CH2CH3



CH3CHCHO

CH3



O



b.



d.



CHCH2CH3

CH3



15.27



a.

N

CH3



CH3CHCHO



nicotine

(addictive stimulant from tobacco)



Cl



O



Label the chirality center (if one exists) in each cyclic

compound. Some compounds contain no chirality centers.

a.



OH



O



Draw the structure of a compound that fits each

description:

a. an alkane of molecular formula C7H16 that contains

one chirality center

b. an alcohol of molecular formula C6H14O that contains

one chirality center

Draw the structure of a compound that fits each

description:

a. an alkyl bromide of molecular formula C4H9Br that

contains one chirality center

b. a diol of molecular formula C5H12O2 that contains two

chirality centers

Explain why a carbonyl carbon can never be a chirality

center.

Explain why a carbon atom that is part of a triple bond

can never be a chirality center.

Locate the chirality center(s) in each biologically active

compound.



N



C

CH3



d.



CHCH3

OH



OH



CH2OH



15.25



O

CH3



COCH2OH



b.



c.



CH3



OH

H



c.



NHCH3



b.

CH3

Cl



b.



OH



d.



ketamine

(anesthetic)



CH3



NH2



c.



CO2CH3



HO2CCH2CHCNHCHCH2

O

aspartame

Trade name: Equal

(synthetic sweetener)



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PROBLEMS



15.34



469



Isomers



Locate the chirality center(s) in each drug.

CH3



O



15.37



CHCO2H



How are the compounds in each pair related? Are they

identical molecules or enantiomers?



a.



CH3



a.



ketoprofen

(anti-inflammatory agent)



H



C



CH



b.



and



Cl



Cl

H



H



a.



C

CH3CH2



CH3



Br



COOH

and



C



Br



H



b.



methamphetamine



Dobutamine is a heart stimulant used in stress tests to

measure cardiac fitness. Identify the chirality center in

dobutamine, and then draw both enantiomers in three

dimensions around the chirality center.



OH



COOH

and



HO



CH3



C



c.



C

CH3O



OCH3



CHO

CH3



and



OHC

CH3



C



CH3



b.



CH3

OH



and



HO



Br



H

OH



OCH3



How are the molecules in each pair related? Choose from:

identical molecules, constitutional isomers, enantiomers,

diastereomers, or not isomers of each other.

a. CH3CH2CH2OCH3 and CH3CH2OCH2CH3



HO

CH2CH2NCHCH2CH2



CH2OH



CH3



OCH3



15.39



CH3



C



HOCH2



CH2CH3



H



COOH



CH2CHNCH3



CH3



C



COOH



N



Methamphetamine is an addictive stimulant sold illegally

as “speed,” “meth,” or “crystal meth.” Identify the

chirality center in methamphetamine, and then draw both

enantiomers in three dimensions around the chirality

center.



C



HO2C

H



How are the compounds in each pair related? Are they

identical molecules or enantiomers?



H



c.



CH3CH2CH2



C



Br

CH3



and



CH3



Br



CH3

Cl



d.



H



CH2CH3



CH2CH2CH3



CH3



H

Cl



C

Br



CH3



dobutamine



smi26573_ch15.indd 469



H



Br



C

CH3



meperidine

Trade name: Demerol

(narcotic pain reliever)



HO



and



CO2H



H



c.



CO2CH2CH3



15.36



C



NH2



Br



15.38



15.35



CH3



OH



C

CH3



methylphenidate

Trade name: Ritalin

(used for attention deficit

hyperactivity disorder)



CH3



and



NH2

CO2CH3



c.



CH3



OH



NH



b.



CH3



and



Cl



H



Cl



H

CH2CH3



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470



15.40



THE THREE-DIMENSIONAL SHAPE OF MOLECULES



How are the molecules in each pair related? Choose from:

identical molecules, constitutional isomers, enantiomers,

diastereomers, or not isomers of each other.

C

CH3CH2



b.



H



15.45



OH



OH



a.



Fischer Projections

Convert each three-dimensional representation into a

Fischer projection.

OCH3



CHO

CO2H



CH3CH2CH2CHOH



and



and



HO2C

H



C



a.



CH2CH3



C



H



OH



c.



CH3



CH3CH2OCH(CH3)2



CH3



C



H



CH3



C



OH



CH2CH3



CH3

COCH3



O



c.



CH3CH2OCH2CH3



and



b.



C

CH3



H



C



CH2CH3



CH2CH3



15.46



Convert each three-dimensional representation into a

Fischer projection.



d.

H



OH



H



OH

CH2CH3



15.41



15.42



15.43



15.44



and



HO



H



HO



H



smi26573_ch15.indd 470



COCH2CH3



OCH2CH3



a.



CH3



C



H



c.



N(CH3)2



CH2CH3



Answer each question with a compound of molecular

formula C5H10O2.

a. Draw the structure of a compound that contains a

COOH group and one chirality center.

b. Draw the structure of a carboxylic acid that is a

constitutional isomer of the compound drawn in

part (a).

c. Draw the structure of a constitutional isomer of

the compound drawn in part (a), which contains a

different functional group.

Answer each question with a compound of molecular

formula C5H12O.

a. Draw the structure of a compound that contains an

ether group and one chirality center.

b. Draw the structure of an ether that is a constitutional

isomer of the compound drawn in part (a).

c. Draw the structure of a constitutional isomer of

the compound drawn in part (a), which contains a

different functional group.

Draw the structure of the four constitutional isomers

of molecular formula C5H10O that contain an aldehyde

( CHO) as a functional group. Label any chirality center

present in each compound. Some compounds have no

chirality centers.

Although there are four constitutional isomers of

molecular formula C4H9Cl, only one contains a chirality

center and can therefore exist as a pair of enantiomers.

Draw both enantiomers of this alkyl chloride in three

dimensions around the chirality center.



NH2



H



C



Br



H



C



OH



CH3

CHO



b.



HO



C



CH3



CH(CH3)2



15.47



Convert each Fischer projection into a three-dimensional

representation with wedges and dashed bonds.

COOH



CH3



a.



H



c.



F



H



NH2



CH3



CH(CH3)2



H

CH2CH3



CH2Cl



b.



15.48



H



NH2



Convert each Fischer projection into a three-dimensional

representation with wedges and dashed bonds.

CH3



CH2CH3



a.



HO



CH3



c.



H



Cl



H



Cl

CH2Cl



CO2H



b.



H



NH2

CH2SH



12/9/08 5:26:44 PM



PROBLEMS



15.49



471



How are the Fischer projection formulas in each pair

related to each other? Are they identical or are they

enantiomers?

CH3



a.



Cl



CH3



H



and



H



Cl



CH3



CH3



b.



CH3



OH



HO



and



CH3

H



H

CO2H



c.



and



H



H2N



CH2OH



CH2OH



C(CH3)3



d.



(CH3)3C



OH



and



H



and



H



H



15.55



NH2

Cl



Cl

CH3CH2



CH3



and



CH3



CH2CH3



CH2OH



CH2OH



OH



CH3



and



H



15.57



CH2OCH3

CH3CH2



CH3



CH2OCH3

and



CH3



CH2CH3



Cl



Cl



Enantiomers and Diastereomers

15.51

15.52

15.53



In what way are two enantiomers different from two

diastereomers?

What are the two major types of stereoisomers? Give an

example of each type.

Consider the stereoisomers (A–D) drawn below. These

compounds are four-carbon sugars, as we will learn in

Chapter 20.

CHO



CHO



H



C



OH



HO



C



H



H



C



OH



H



C



OH



CH2OH

A



O



C



O



C



O



C



O



H



C



OH



HO



C



H



HO



C



H



H



C



OH



H



C



OH



H



C



OH



HO



C



H



HO



C



H



CH2OH

B



CHO



CHO



H



C



OH



HO



C



H



HO



C



H



HO



C



H



CH2OH

C



CH2OH



CH2OH



F



G



CH2OH

H



a. How are the compounds in each pair related? Are the

compounds enantiomers or diastereomers: [1] E and

F; [2] E and G; [3] E and H; [4] F and G; [5] F and

H; [6] G and H?

b. Draw a constitutional isomer of E that contains an

aldehyde, not a ketone.

In Chapter 13 we learned that cis and trans alkenes are

stereoisomers. Are cis-2-butene and trans-2-butene

enantiomers or diastereomers? Explain your choice.

Are cis-2-butene and trans-2-butene chiral or achiral

compounds? Explain your choice.



Applications



CH3



HO



H



d.



15.56



Cl



Cl



c.



C



E



COOH



NH2



b.



CH2OH



CH2OH



H



COOH

H



CH2OH



C(CH3)3



HO



How are the Fischer projection formulas in each pair

related to each other? Are they identical or are they

enantiomers?

a.



CH2OH



C(CH3)3



H



15.50



CH2OH



CO2H



NH2



H



15.54



COOH



COOH



a. How are the compounds in each pair related? Are the

compounds enantiomers or diastereomers: [1] A and

B; [2] A and C; [3] A and D; [4] B and C; [5] B and

D; [6] C and D?

b. Draw a constitutional isomer of A that contains a

ketone, not an aldehyde.

Consider the stereoisomers (E–H) drawn below. These

compounds are five-carbon sugars, as we will learn in

Chapter 20.



15.58



Lactic acid is a product of glucose metabolism. During

periods of strenuous exercise, lactic acid forms faster than

it can be oxidized, resulting in the aching feeling of tired

muscles.

CH3CHCO2H a. Locate the chirality center in lactic

acid.

OH

b. Draw both enantiomers of lactic acid in

lactic acid

three dimensions around the chirality

center.

c. Draw Fischer projection formulas for

both enantiomers of lactic acid.

Locate the chirality centers in each vitamin.

CH3

HO



a.



(CH2)3CH(CH2)3CH(CH2)3CH(CH3)2

CH3



O

CH3



CH3



CH3



CH3



vitamin E



CH2OH

OH



D



b.



HOCH2CH



O



O



HO

OH

vitamin C



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472



15.59



THE THREE-DIMENSIONAL SHAPE OF MOLECULES



Hydroxydihydrocitronellal is another example of a

compound that has two enantiomers that smell differently.

One enantiomer smells minty, and the other smells like

lily of the valley.

OH



CH3



CH3CCH2CH2CH2CHCH2



O

CH2

H



a. Locate the chirality center in this compound.

b. Draw both enantiomers in three dimensions around the

chirality center.

c. Draw Fischer projection formulas for both

enantiomers.

Two enantiomers can sometimes taste very differently.

l-Leucine, a naturally occurring amino acid used in

protein synthesis, tastes bitter, but its enantiomer,

d-leucine, tastes sweet.



CH2CH(CH3)2

leucine



a. Locate the chirality center in leucine.

b. Draw both enantiomers in three dimensions around the

chirality center.

c. Draw Fischer projection formulas for both

enantiomers.



C



CH3



C



H



CH2N(CH3)2



H



C



CH3CH2CO2



Darvon



COOH

H2N



Answer the following questions about the analgesic

propoxyphene (trade name: Darvon).



C



CH3

hydroxydihydrocitronellal



15.60



15.61



15.62



a. Label the chirality centers.

b. Draw the enantiomer. Like many of the drugs

discussed in Section 15.5, the enantiomer,

levopropoxyphene (trade name: Novrad) has different

biological properties. Levopropoxyphene is an

antitussive agent, meaning it relieves coughs. (Do you

notice anything about the trade names Darvon and

Novrad?)

c. Draw the structure of one diastereomer.

d. Draw the structure of one constitutional isomer.

e. Convert Darvon to a Fischer projection formula.

Plavix, the trade name for the generic drug clopidogrel,

is used in the treatment of coronary artery disease. Like

many newer drugs, Plavix is sold as a single enantiomer.

CO2CH3

S



N



C

H

Cl



clopidogrel

Trade name: Plavix



a. Locate the chirality center.

b. Draw both enantiomers in three dimensions around

the chirality center.

c. Convert both enantiomers to Fischer projection

formulas.



CHALLENGE QUESTIONS

15.63



Identify the nine chirality centers in sucrose, the sweettasting carbohydrate we use as table sugar.

OH



HOCH2

O

HO



OH



HOCH2



15.64



Identify the six chirality centers in the anabolic steroid

nandrolone. Nandrolone is used to increase muscle mass

in body builders and athletes, although it is not permitted

in competitive sports.

CH3 OH



O

O

HO



OH

sucrose



CH2OH



O

nandrolone



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16

CHAPTER OUTLINE

16.1



Structure and Bonding



16.2



Nomenclature



16.3



Physical Properties



16.4



FOCUS ON HEALTH & MEDICINE:

Interesting Aldehydes and Ketones



16.5



Reactions of Aldehydes and Ketones



16.6



Reduction of Aldehydes and Ketones



16.7



FOCUS ON THE HUMAN BODY:

The Chemistry of Vision



16.8



Acetal Formation



CHAPTER GOALS

In this chapter you will learn how to:

➊ Identify the characteristics of

aldehydes and ketones

➋ Name aldehydes and ketones

➌ Give examples of useful aldehydes

and ketones

➍ Draw the products of oxidation

reactions of aldehydes

➎ Draw the products of reduction

reactions of aldehydes and ketones

➏ Understand the basic reactions

involved in vision

➐ Identify and prepare hemiacetals

and acetals



11-cis-Retinal is the light-sensitive aldehyde that plays a key role in the chemistry of vision for all

vertebrates, arthropods, and mollusks.



ALDEHYDES AND KETONES

CHAPTER 16 is the first of two chapters that concentrate on compounds that contain a

carbonyl group (C O), perhaps the most important functional group in organic chemistry. In this chapter we examine aldehydes and ketones, compounds that contain a

carbonyl carbon bonded to hydrogen or carbon atoms. Aldehydes and ketones occur

widely in nature, and also serve as useful starting materials and solvents in industrial

processes. All simple carbohydrates contain a carbonyl group, and more complex carbohydrates are derived from reactions discussed in this chapter.



473



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474



ALDEHYDES AND KETONES



16.1 STRUCTURE AND BONDING

Two broad classes of compounds contain a carbonyl group.

O

=



C

carbonyl group



1. Compounds that have only carbon and hydrogen atoms bonded to the carbonyl group

O



O



at least one H



C

R



C

H



R



aldehyde



R'



two R groups



ketone



• An aldehyde has at least one H atom bonded to the carbonyl group.

• A ketone has two alkyl groups bonded to the carbonyl group.

2. Compounds that contain an electronegative atom bonded to the carbonyl group

O



O



C

R



O



C

R



OH



C

OR'



R



N



H (or R')



H (or R')

carboxylic acid



ester



amide



These compounds include carboxylic acids, esters, and amides, which contain an electronegative oxygen or nitrogen atom bonded directly to the carbonyl carbon. Esters and amides are called

carboxylic acid derivatives, since they can be prepared from carboxylic acids. Carboxylic acids

and their derivatives are discussed in Chapter 17, while aldehydes and ketones are the subject of

this chapter.

Two structural features dominate the properties and chemistry of the carbonyl group.

δ−



O

120°



C



120°



polar bond

=



δ+



trigonal planar



• The carbonyl carbon atom is trigonal planar, and all bond angles are 120°. In this way the

carbonyl carbon resembles the carbons of a carbon–carbon double bond.

• Since oxygen is more electronegative than carbon, a carbonyl group is polar. The

carbonyl carbon is electron poor (𝛅+) and the oxygen is electron rich (𝛅–). In this way

the carbonyl carbon is very different from the carbons of a nonpolar carbon–carbon

double bond.



As mentioned in Chapter 11, the double bond of a carbonyl group is usually omitted in drawing

shorthand structures. An aldehyde is often written as RCHO. Remember that the H atom is

bonded to the carbon atom, not the oxygen. Likewise, a ketone is written as RCOR, or if both

alkyl groups are the same, R2CO.



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STRUCTURE AND BONDING



475



O

=



C

CH3



H



=



CH3CHO



=



CH3COCH3



• C is singly bonded to H.

• C is doubly bonded to O.



acetaldehyde

O

=



C

CH3



CH3



acetone



or



(CH3)2CO



• C is singly bonded to 2 C’s.

• C is doubly bonded to O.



Many simple aldehydes and ketones are naturally occurring. For example, octanal, decanal,

and piperitone are among the 70 organic compounds that contribute to the flavor and odor of

an orange.



O

CH(CH3)2



CH3

piperitone

O



O



C

CH3(CH2)6



C

H



CH3(CH2)8



octanal



PROBLEM 16.1



H



decanal



Draw out each compound to clearly show what groups are bonded to the carbonyl carbon.

Label each compound as a ketone or aldehyde.

a. CH3CH2CHO



b. CH3CH2COCH3



c. (CH3)3CCOCH3



d. (CH3CH2)2CHCHO



PROBLEM 16.2



Draw the structure of the three constitutional isomers of molecular formula C4H8O that contain

a carbonyl group. Label each compound as a ketone or aldehyde.



PROBLEM 16.3



Label each trigonal planar carbon in 11-cis-retinal, the chapter-opening molecule whose

chemistry is discussed in Section 16.6.

CH3



CH3 H

C



C



H

CH3



CH3



H



C



C



C

H



H

C

H



C

CH3



C

C

H



O



11-cis-retinal



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476



ALDEHYDES AND KETONES



16.2 NOMENCLATURE

Both IUPAC and common names are used for aldehydes and ketones.



16.2A



NAMING ALDEHYDES



• In IUPAC nomenclature, aldehydes are identified by the suffix -al.



To name an aldehyde using the IUPAC system:

1. Find the longest chain containing the CHO group, and change the -e ending of the parent

alkane to the suffix -al.

2. Number the chain or ring to put the CHO group at C1, but omit this number from the

name. Apply all of the other usual rules of nomenclature.



Simple aldehydes have common names that are widely used. In fact, the common names formaldehyde, acetaldehyde, and benzaldehyde are virtually always used instead of their IUPAC

names. Common names all contain the suffix -aldehyde.

O



O



C



C



H



H



CH3



formaldehyde

(methanal)



O

C

H



H



acetaldehyde

(ethanal)



benzaldehyde

(benzenecarbaldehyde)



(IUPAC names are in parentheses.)



SAMPLE PROBLEM 16.1



Give the IUPAC name for each aldehyde.

CH3



a.



O



CH3CHCH



C



H



b.



H



CH3CH2CH2



CH3



ANALYSIS AND SOLUTION



a.



C



CH2CH3



CHO



[1] Find and name the longest chain

containing the CHO.

CH3



[2] Number and name substituents, making

sure the CHO group is at C1.



O



CH3CHCH



O



CH3



C



H



CH3CHCH

3 2



CH3



C



H



CH3

1



butane

(4 C’s)

b.



butanal



[1] Find and name the longest chain

containing the CHO.



Answer: 2,3-dimethylbutanal



[2] Number and name substituents, making

sure the CHO group is at C1.



H

CH3CH2CH2



H



C



CH2CH3



C



O



H

pentane

(5 C’s)



smi26573_ch16.indd 476



CH3CH2CH2

1



2



C



CH2CH3



C



O



H

pentanal



Answer: 2-ethylpentanal



12/10/08 11:25:11 AM



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