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7 Focus on the Human Body: The Chemistry of Vision

7 Focus on the Human Body: The Chemistry of Vision

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488



ALDEHYDES AND KETONES







FIGURE 16.2 Vision

optic nerve

CH3



CH3 H

C



retina



CH3



H



C



C



C



C



H

CH3



H



11-cis-retinal

pupil



H

C



CH3



C



H

C

C



H



O



cross-section

of the eye

rhodopsin



hydrophobic

region



disc

membrane

rod cell in the retina



rhodopsin in a rod cell

Rhodopsin contains 11-cis-retinal

bound to the protein opsin.



In the rod cells of the eye, the 11-cis-retinal bonded to opsin absorbs light, and the crowded

11-cis double bond is isomerized to the trans isomer. This process generates a nerve impulse

that is converted to a visual image by the brain.



opsin, forming rhodopsin or visual purple. When light hits the retina, the 11-cis double bond is

isomerized to its more stable trans isomer, and all-trans-retinal is formed. This process sends a

nerve impulse to the brain, which is then converted into a visual image.

In order for the process to continue, the all-trans-retinal must be converted back to 11-cis-retinal.

This occurs by a series of reactions that involve biological oxidation (Section 14.5) and reduction

(Section 16.6). As shown in Figure 16.3, NADH is the coenzyme that reduces the aldehyde in alltrans-retinal to all-trans-retinol, vitamin A (Reaction [1]). NAD+ is the coenzyme that oxidizes

11-cis-retinol back to 11-cis-retinal (Reaction [3]), so the chemical cycle of vision can continue.

This scheme explains the role of vitamin A (Section 11.7) in vision. Vitamin A can be obtained

either directly in the diet or from the orange pigment β-carotene in carrots. Vitamin A, also called

all-trans-retinol, is converted in two steps to 11-cis-retinal, the aldehyde essential for vision in

the rod cells. Since the rod cells are responsible for vision in dim light, a deficiency of vitamin A

causes night blindness.



PROBLEM 16.17



How are the compounds in each pair related? Choose from stereoisomers, constitutional

isomers, or not isomers of each other.

a. all-trans-retinal and 11-cis-retinal

b. all-trans-retinal and vitamin A

c. vitamin A and 11-cis-retinol



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ACETAL FORMATION







FIGURE 16.3



489



Vitamin A and the Chemistry of Vision

needed for vision



CH3



CH3 H

C



CH3



H



C



C



CH3



H



C



C



C



C



C



C



H

CH3



H



H



H



CH3 H



CH3



C



[1] Reduction

CH3



CH3 H

C



H



C



C



C



C

C



H

CH3



H



C



C



C



C



C



H

CH3



H



H



H



CH3 H



CH3

CH2OH



C

[2] Isomerization



all-trans-retinol

vitamin A



H



C

CH3



NAD+

CH3



H

C

C

C



11-cis-retinal



NADH



CH3



H



C



O



all-trans-retinal



CH3



H



O



[3] Oxidation

CH3



H



C



C



C



C



H

CH3



H



H

C

H



C

CH3



C



11-cis-retinol



CH2OH



obtained from the diet



• Reaction [1] (reduction): The aldehyde in all-trans-retinal (labeled in blue) is reduced to a 1° alcohol using the coenzyme NADH.

The product is all-trans-retinol.

• Reaction [2] (isomerization): The trans double bond (labeled in red) is isomerized to a cis double bond, forming 11-cis-retinol.

• Reaction [3] (oxidation): The 1° alcohol in 11-cis-retinol (labeled in blue) is oxidized with the coenzyme NAD+ to an aldehyde. This

re-forms 11-cis-retinal and the chemical cycle of vision continues.



16.8 ACETAL FORMATION

Aldehydes and ketones undergo addition reactions with alcohols (ROH) to form hemiacetals and acetals. Acetal formation is done in the presence of sulfuric acid (H2SO4).

R'OH

H2SO4



O



Acetal

formation



R



C

R



O



H



C



OR'



OR'

R



C



OR'



H (or R)

H (or R)



aldehyde

or

ketone



16.8A



R'OH

H2SO4



hemiacetal



+



H2 O



H (or R)

acetal



ACETALS AND HEMIACETALS



Addition of one molecule of alcohol (ROH) to an aldehyde or ketone forms a hemiacetal.

Like other addition reactions, one bond of the C O is broken and two new single bonds are

formed. Acyclic hemiacetals are unstable. They react with a second molecule of alcohol to

form acetals.



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490



ALDEHYDES AND KETONES



• A hemiacetal contains an OH (hydroxyl) group and an OR (alkoxy) group bonded to the

same carbon.

• Acetals are compounds that contain two OR groups (alkoxy groups) bonded to the same

carbon.

O



H



OR



C



OR



C



OR



hemiacetal



acetal



One C is bonded to:

• one OH group

• one OR group



One C is bonded to:

• two OR groups



Two examples of acetal formation using ethanol (CH3CH2OH) as the alcohol component are

given.

CH3CH2OH

H2SO4



O



CH3CH2



C

CH3CH2



H



CH3CH2OH

H2SO4



O



H



C



OCH2CH3



OCH2CH3

CH3CH2



hemiacetal

CH3CH2OH

H2SO4



C

CH3



SAMPLE PROBLEM 16.6



CH3



CH3



H2O



+



H2O



acetal

CH3CH2OH

H2SO4



O



H



C



OCH2CH3



OCH2CH3

CH3



OCH2CH3



C



CH3



CH3



hemiacetal



acetal



Draw the hemiacetal and acetal formed when cyclohexanone is treated with methanol (CH3OH)

in the presence of H2SO4.

O



cyclohexanone



ANALYSIS



+



H



H



O



OCH2CH3



C



+



H2SO4

CH3OH



methanol

(2 equivalents)



To form a hemiacetal and acetal from a carbonyl compound:

• Locate the C O in the starting material.

• Break one C O bond and add one equivalent of CH3OH across the double bond, placing

the OCH3 group on the carbonyl carbon. This forms the hemiacetal.

• Replace the OH group of the hemiacetal by OCH3 to form the acetal.



SOLUTION



new bond



Break one bond.

H2SO4



O



H



CH3OH

H2SO4



new bond

OCH3



O



CH3O



H



OCH3

new bond

hemiacetal



OCH3

acetal



Break the single bond.



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ACETAL FORMATION



491



PROBLEM 16.18



Draw the hemiacetal and acetal formed when each carbonyl compound is treated with two

equivalents of the given alcohol in the presence of H2SO4.

O

O



a.



C

CH3



b.



C

H



(CH3CH2)2C



+



CH3OH



O



+



H



c.



+



CH3CH2OH



CH3OH



Acetals are not ethers, even though both compounds contain a C OR bond.

OR

C



OR



C



O



C



acetal



ether



2 OR groups on one carbon



2 C’s on a single O atom



• An acetal contains one carbon atom bonded to two OR groups.

• An ether contains only one oxygen atom bonded to two carbons.



SAMPLE PROBLEM 16.7



Identify each compound as an ether, hemiacetal, or acetal.



a.

ANALYSIS



CH3CH2



O



b.



CH2CH3



O



O



CH3



CH3



Recall the definitions to identify the functional groups:

• An ether has the general structure ROR.

• A hemiacetal has one C bonded to OH and OR.

• An acetal has one C bonded to two OR groups.



SOLUTION

a.



CH3CH2



O



O



b.



CH2CH3



O



CH3



2 C’s on one O atom



CH3



This C is bonded to 2 O’s.



ether



acetal



Note that the acetal in part (b) is part of a ring. It contains one ring carbon bonded to two

oxygens, making it an acetal.



PROBLEM 16.19



Identify each compound as an ether, hemiacetal, or acetal.

OH



OCH3



a.



c.



CH3CH2CH2CH2



C



OCH3



H

OCH3



b.



OCH3



O



d.

O



smi26573_ch16.indd 491



CH3

CH3



12/10/08 11:25:40 AM



492



ALDEHYDES AND KETONES



16.8B



CYCLIC HEMIACETALS



Although acyclic hemiacetals are generally unstable, cyclic hemiacetals containing five- and

six-membered rings are stable compounds that are readily isolated.

Hemiacetal

General structure



Cyclic hemiacetals

OH



OH

C



OR



OH

O



O



Each indicated C is bonded to:

• an OH group

• an OR group that is part of a ring



One C is bonded to:

• one OH group

• one OR group



These cyclic hemiacetals are formed by an intramolecular reaction of a compound that contains

both a hydroxyl group and an aldehyde or ketone.

The C

O



O

C

HOCH2CH2CH2CH2



O and OH groups react.



re-draw



H



2



1



OH

H



2



OH



3



5-hydroxypentanal



4



hemiacetal



1 O



3



5

4



5



Cyclic hemiacetals are especially important in carbohydrate chemistry. The most common

simple carbohydrate, glucose, exists predominantly as a cyclic hemiacetal. Glucose has five

OH groups, but only one OH group is bonded to a carbon that is also bonded to another oxygen

atom, making it a hemiacetal. All of the other OH groups in glucose are “regular” alcohols. We

will learn much more about these stable cyclic hemiacetals in Chapter 20.

HOCH2

hemiacetal



O

HO



OH

HO



OH



glucose

(cyclic hemiacetal form)



Cyclic hemiacetals are converted to acetals by reaction with another alcohol (ROH). To

draw the structure of the resulting acetal, replace the OH group of the hemiacetal by the OR

group of the alcohol.

OH

Replace OH

by OCH3.



OCH3

O



+



CH3O



H



H2SO4



O



acetal

+



H2O



Many biologically active molecules contain acetals and cyclic hemiacetals. For example, lactose,

the principal carbohydrate in milk, contains one acetal and one hemiacetal. Many individuals,

mainly of Asian and African descent, lack adequate amounts of lactase, the enzyme needed

to digest and absorb lactose. This condition, lactose intolerance, is associated with abdominal

cramping and recurrent diarrhea.



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ACETAL FORMATION



493



HOCH2

acetal



O

O



HO

HO



CH2OH

O



HO

HO



hemiacetal



lactose HO



SAMPLE PROBLEM 16.8



OH



Identify the three acetals in digoxin, a naturally occurring drug prescribed for patients with

congestive heart failure and other heart ailments.

O



O



HEALTH NOTE

OH

CH3



CH3



CH3

O



HO



O



O



CH3



CH3



O

OH



HO



HO



O



O



OH

digoxin



ANALYSIS Although the structure of digoxin is complicated, look at each O atom

individually and recall that an acetal has one C bonded to two OR groups.

SOLUTION The acetal carbons in digoxin are indicated with arrows and the oxygen atoms of

the acetals are drawn in red.

O

OH

acetal



CH3



The leaves of the woolly foxglove

plant are the source of the potent

cardiac drug digoxin.



HO



CH3

O



HO



O



O



acetal



CH3



O



O



CH3



CH3



O

OH



HO



acetal



O



OH

digoxin



PROBLEM 16.20



Label the hemiacetal or acetal in each compound. Glucosamine [part (a)] is available in many

over-the-counter remedies for the treatment of osteoarthritis, although recent studies cast doubt

on its efficacy. Salicin [part (b)] is a naturally occurring pain reliever isolated from the bark of

the willow tree.

CH2OH



HOCH2



HOCH2



O



a.



OH



HO



O



b.

HO



HO



NH2



glucosamine



smi26573_ch16.indd 493



O



OH

OH

salicin



12/10/08 12:02:02 PM



494



ALDEHYDES AND KETONES



PROBLEM 16.21



What acetal is formed in each reaction?

O



a.



+



OH



CH3CH2OH



H2SO4



O



b.



OH



+



H2SO4



OH



16.8C ACETAL HYDROLYSIS

Acetals are stable molecules, but they can be converted back to aldehydes and ketones by treatment with acid and water. This reaction is a hydrolysis, since bonds are cleaved by reaction

with water.

OR'



Acetal

hydrolysis



R



C



OR'



+



O



H2SO4



C



H2O



R



H (or R)



+



2 R'O



H



H (or R)

aldehyde

or

ketone



SAMPLE PROBLEM 16.9

ANALYSIS



What hydrolysis products are formed when the acetal (CH3CH2)2C(OCH3)2 is treated with

H2SO4 in H2O?

To draw the products of hydrolysis:

• Locate the two C OR bonds on the same carbon.

• Replace the two C O single bonds with a carbonyl group (C O).

• Each OR group then becomes a molecule of alcohol (ROH) product.



SOLUTION



Break two bonds.



Form a carbonyl group.



OCH3

CH3CH2



C



OCH3



+



H2O



CH2CH3

acetal



PROBLEM 16.22



O



H2SO4



C

CH3CH2



CH2CH3



2 CH3O



H



Two molecules of

alcohol are also formed.



3-pentanone



What products are formed by hydrolysis of each acetal?

CH3CH2O



OCH2CH3



OCH3



a.



CH3



C



OCH3



CH2CH2CH3



smi26573_ch16.indd 494



+



OCH3



b.



c.



C



H



OCH3



12/10/08 11:25:48 AM



CHAPTER HIGHLIGHTS



495



CHAPTER HIGHLIGHTS

KEY TERMS

Acetal (16.8)

Aldehyde (16.1)

Carbonyl group (16.1)

Cyclic hemiacetal (16.8)

Hemiacetal (16.8)



Hydrogenation (16.6)

Hydrolysis (16.8)

Ketone (16.1)

NAD+ (16.6)



NADH (16.6)

Oxidation (16.5)

Reduction (16.6)

Tollens reagent (16.5)



KEY REACTIONS

[1]



Oxidation of aldehydes (16.5)

RCHO



K2Cr2O7



RCOOH



or

Tollens reagent



carboxylic acid



[2] Reduction (16.6)

a.



Aldehydes



H2



RCHO



RCH2OH



Pd



1° alcohol

b.



Ketones



H2



R2CO



R2CHOH



Pd



2° alcohol

O

c.



Biological reduction



OH

NADH



C



NAD+



+



C



enzyme



H



[3] Acetals and hemiacetals (16.8)

R'OH

H2SO4



O

a.



General reaction



R



C

R



R'OH

H2SO4



O



H



C



OR'



OR'

R



C



H (or R)



acetal

O



O

C

Cyclic hemiacetal



HOCH2CH2CH2CH2



H



OH

H



re-draw



O



OH



5-hydroxypentanal



cyclic hemiacetal



OH



OR

O



c.



Cyclic acetals



+



ROH



R



C



OR'



+



+



H2 O



cyclic acetal



OR'

Acetal hydrolysis



O



H2SO4



cyclic hemiacetal



d.



H2O



H (or R)



hemiacetal



b.



+



OR'



H (or R)



O



H2SO4

H2O



C

R



H (or R)



+



2 R'O



H



H (or R)



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496



ALDEHYDES AND KETONES



KEY CONCEPTS

❶ What are the characteristics of aldehydes and ketones?

• An aldehyde has the general structure RCHO, and contains

a carbonyl group (C O) bonded to at least one hydrogen

atom. (16.1)

• A ketone has the general structure RCOR', and contains a

carbonyl group (C O) bonded to two carbon atoms. (16.1)

• The carbonyl carbon is trigonal planar, with bond angles of

approximately 120°. (16.1)

• The carbonyl group is polar, giving an aldehyde or ketone

stronger intermolecular forces than hydrocarbons. (16.3)

• Aldehydes and ketones have lower boiling points than

alcohols, but higher boiling points than hydrocarbons of

comparable size. (16.3)

• Aldehydes and ketones are soluble in organic solvents, but

those having less than six C’s are water soluble, too. (16.3)

❷ How are aldehydes and ketones named? (16.2)

• Aldehydes are identified by the suffix --al,

al, and the carbon

chain is numbered to put the carbonyl group at C1.

• Ketones are identified by the suffix -one, and the carbon chain

is numbered to give the carbonyl group the lower number.

❸ Give examples of useful aldehydes and ketones. (16.4)

• Formaldehyde (CH2 O) is an irritant in smoggy air, a

preservative, and a disinfectant, and it is a starting material

for synthesizing polymers.

• Acetone [(CH3)2C O] is an industrial solvent and polymer

starting material.

• Several aldehydes—cinnamaldehyde, vanillin, citral, and

citronellal—have characteristic odors and occur naturally in

fruits and plants.

• Dihydroxyacetone [(HOCH2)2C O] is the active ingredient

in artificial tanning agents, and many other ketones are

useful sunscreens.

• Amygdalin, a naturally occurring carbonyl derivative, forms

toxic HCN on hydrolysis.



❹ What products are formed when aldehydes are oxidized?

(16.5)

• Aldehydes are oxidized to carboxylic acids (RCOOH) with

K2Cr2O7 or Tollens reagent.

• Ketones are not oxidized since they contain no H atom on

the carbonyl carbon.

❺ What products are formed when aldehydes and ketones

are reduced? (16.6)

• Aldehydes are reduced to 1° alcohols with H2 and a Pd

catalyst.

• Ketones are reduced to 2° alcohols with H2 and a Pd

catalyst.

• Biological reduction occurs with the coenzyme NADH.

❻ What reactions occur during vision? (16.7)

11-cis-• When light hits the retina, the crowded C C in 11-cis

retinal is isomerized to a more stable trans double bond, and

a nerve impulse is generated. All-trans

All-trans-retinal

-retinal is converted

back to the cis isomer by a three-step sequence—reduction,

isomerization, and oxidation.

❼ What are hemiacetals and acetals, and how are they

prepared? (16.8)

• Hemiacetals contain an OH group and an OR group bonded

to the same carbon.

• Acetals contain two OR groups bonded to the same carbon.

• Treatment of an aldehyde or ketone with an alcohol (ROH)

first forms an unstable hemiacetal, which reacts with more

alcohol to form an acetal.

• Cyclic hemiacetals are stable compounds present in

carbohydrates and some drugs.

• Acetals are converted back to aldehydes and ketones by

hydrolysis with water and acid.



PROBLEMS

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



Structure and Bonding

16.23



Draw the structure of a compound that fits each

description:

a. an aldehyde of molecular formula C8H16O that has six

carbons in its longest chain

b. a ketone of molecular formula C6H12O that contains

five carbons in its longest chain

c. a ketone of molecular formula C5H8O that contains a

ring

d. an aldehyde of molecular formula C6H10O that has the

CHO group bonded to a ring



16.24



16.25

16.26

16.27

16.28



smi26573_ch16.indd 496



Draw the structure of a constitutional isomer of

2-heptanone (CH3COCH2CH2CH2CH2CH3) that:

a. contains an aldehyde

b. contains a ketone

c. contains a hydroxyl group (OH)

How do a C O and a C C compare with regards to the

following: (a) geometry; (b) polarity; (c) type of reaction?

How do an aldehyde and a ketone compare in

(a) structure; (b) geometry; (c) polarity?

Can an aldehyde have molecular formula C5H12O?

Explain why or why not.

Can a ketone have molecular formula C4H10O? Explain

why or why not.



12/10/08 11:25:49 AM



PROBLEMS



497



Nomenclature

16.29



16.33



Give an acceptable name for each ketone.

O



Give an acceptable name for each aldehyde.

a. CH3CH2CH2CHCH2CHO



a.



C

(CH3)2CHCH2



CH3

CH3



CH3



b.



CH3



O

CH3



b.



CH3CH2CHCH2CHCH2CHO

CH3



O



CH2CHO



c.



C



CH3CH2CH2CHCH2CH2CH3

CH3



d.



(CH3CH2)3C(CH2)3



C



CH3



c.



CH2CH2CH2CH3



CHO



O

CH3



d.



CH2CH3



e.



CHCH2CH3

CH3



CH3(CH2)3CHCHCH2CH2CHCHO

CH3



f.



C

(CH3)2CH



O



CH3



CI



e.



CHO



C

(CH3CH2)2CHCH2CH



CH3



CH2CH3



16.30



Give an acceptable name for each aldehyde.

a. (CH3)3CCH2CHO

b.



f.



O



(CH3CH2)2CHCH2CH2CHO



16.34



CH3



c.



CI



CH3CH2CH2CH2CHCHCH3



Give an acceptable name for each ketone.

a. CH3CH2CH2CH2CHCH2CH3

C



CH2CHO



O



CH3



d.



CH3CH2CH2



C



e.



O



CH2CH2CHCH2CHO



CH3



CH2CH3



b.



CI

CI



(CH3CH2CH2CH2)2CHCH2CHO



O



CH3CH2



c.

f.

16.31



16.32



CH3



C

CH3CH2



(CH2)5CH(CH3)2



CHO



Draw the structure corresponding to each name.

a. 3,3-dichloropentanal

c. o-bromobenzaldehyde

b. 3,4-dimethylhexanal

d. 4-hydroxyheptanal

Draw the structure corresponding to each name.

a. 2-bromooctanal

c. 3,4-dimethoxybenzaldehyde

b. 2-propylheptanal

d. 3,4-dihydroxynonanal



O



d.



C

CH3CH2CH



CHCH2CH3



CH3



CH3



CH2CH3



e.



O

CH3

O

CH3



f.



(CH3CH2)2CH



C



C

CH2



CH2CH2CH3



CH3



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498



16.35



16.36



16.37



16.38



16.39



16.40



ALDEHYDES AND KETONES



Draw the structure corresponding to each name.

a. 3,3-dimethyl-2-hexanone

b. methyl propyl ketone

c. m-ethylacetophenone

d. 2,4,5-triethylcyclohexanone

Draw the structure corresponding to each name.

a. dibutyl ketone

b. 1-chloro-3-pentanone

c. p-bromoacetophenone

d. 3-hydroxycyclopentanone

Draw the structure of the four constitutional isomers of

molecular formula C6H12O that contain an aldehyde and

four carbons in the longest chain. Give the IUPAC name

for each aldehyde.

Draw the structure of the three isomeric ketones of

molecular formula C5H10O. Give the IUPAC name for

each ketone.

Each of the following names is incorrect. Draw the

structure of the compound and name it correctly.

a. 1-pentanone

c. 3-propyl-2-butanone

b. 4-hexanone

d. 2-methyl-1-octanal

Each of the following names is incorrect. Draw the

structure of the compound and name it correctly.

a. 6-octanone

c. 3-propyl-1-cyclopentanone

b. 1-heptanone

d. 5-methylcyclohexanone



Physical Properties and Intermolecular Forces

16.41



16.42



16.43



Draw out the structure of benzaldehyde, including any

lone pairs on oxygen. Then, show the hydrogen bonding

interactions between benzaldehyde and water.

Can hydrogen bonding occur between the compounds in

each pair? If hydrogen bonding is possible, draw out the

compounds and indicate how that interaction would take

place.

a. two molecules of acetaldehyde (CH3CHO)

b. CH3CHO and H2O

c. CH3CHO and CH3OH

Which compound in each pair has the higher boiling point?

a. (CH3)3CCH2CH2CH3 or (CH3)3CCH2CHO



16.45



Rank the following compounds in order of increasing

melting point.

O



16.46



COCH3



or



O



CH3



16.44



CH3(CH2)6CHO



16.47



menthol



Label each compound as water soluble or water insoluble.

CHO



a.



c.



CH3CH2CH2CH3



O



b.

16.48



C

CH3



CH2CH3



Label each compound as water soluble or water insoluble.

O



a.



c.



CH3CH2CH2OH



CH3



b.

16.49



16.50



CH3CH2CH2CHO



Explain why 2,3-butanedione (CH3COCOCH3) is more

soluble in water than acetone [(CH3)2CO] even though it

has more carbon atoms. Would you expect 2,3-butanedione

to be soluble in an organic solvent like diethyl ether?

Explain why the simplest ketone, acetone, has a

much higher boiling point than the simplest aldehyde,

formaldehyde.



Oxidation and Reduction

16.51



What product is formed when each compound is treated

with K2Cr2O7? With some compounds, no reaction occurs.

O



16.52

CH3



or



CH(CH3)2



menthone



b.



Which compound in each pair has the higher boiling

point?

a. CH3(CH2)6CHO or CH3(CH2)7OH

b.



CH3(CH2)4CHO



c.



CH2CH3



CH2CHO



d.



CH3(CH2)4CH2OH



What product is formed when each compound is treated

with K2Cr2O7? With some compounds, no reaction occurs.

OH



CI



a.



c.



C



CH3CHCH2CH2CH3



CHO



CH3(CH2)2CHO



O



O



c.



OH



CH2CH2OH



or

CH3



CH3



O



CH(CH3)2



CH3



c.



CH3



Menthone and menthol are both isolated from mint.

Explain why menthol is a solid at room temperature but

menthone is a liquid.



a.

b.



OH



C

or



CH(CH3)2



b.



CH3(CH2)8CHO



d.



CH3



CH3



smi26573_ch16.indd 498



12/10/08 11:25:52 AM



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