CHAPTER 4
Aldehydes and Ketones
Section 1 Section 2 Section 3 Section 4
1. The carbonyl functional group is an oxygen atom double bonded to a carbon which is then single bonded to two other atoms. The carbonyl group is characteristic of both aldehydes and ketones.
Name the following functional group:
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4.1 Nomenclature of Aldehydes and Ketones |
2. When a carbonyl group is bonded to at least one hydrogen, the compound is an aldehyde. Ketones have two carbons bonded directly to the carbonyl group.
Identify each of the following as an aldehyde or ketone:
a)
b)
c)
3. Naming aldehydes is very similar to naming alcohols. First, find the longest carbon chain that includes the carbonyl carbon. The ending –e is dropped from the alkane name and –al is put in its place. Name and give the location of any branches as usual. It is also important to remember that the carbonyl carbon is designated as number one (see Example 4.1).
Name the following aldehydes:
a)
b)
4. When naming ketones, locate the longest chain that includes the carbonyl group. Giving it the name of the corresponding alkane, drop the –e ending and replace it with –one. Begin numbering carbons at the end closest to the carbonyl group. The carbonyl group position is indicated by a number. Finally, name and locate any branches before combining all parts of the name (see Example 4.2).
Name the following ketones:
a)
b)
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4.2 Physical Properties |
5. The lack of a hydrogen on the oxygen atom in aldehydes and ketones prevents hydrogen bonding between molecules. Therefore, the boiling points of aldehydes and ketones are lower than alcohols with similar molecular weights.
a) Aldehydes and ketones have boiling points lower than those of alcohols because of the lack of ________. As pure compounds, aldehydes and ketones (can/cannot) exhibit hydrogen bonding.
b) Arrange the following in order of increasing boiling points:
1) 1, 5-pentanediol
2) 3-pentanol
3) pentane
4) pentanal
6. The difference in electronegativity between carbon and oxygen atoms gives the carbonyl group a polar nature. The attraction between dipoles of different molecules causes aldehydes and ketones to have higher boiling points than similar molecular weight alkanes. The polarity of the carbonyl group also means that hydrogen bonding with water molecules is possible. Therefore, low molecular weight aldehydes and ketones are water soluble.
a) In a carbonyl group, which atom carries a partial positive charge? A partial negative charge?
b) The hydrogens of water molecules are attracted to which atom of the carbonyl group?
c) Diagram how water can hydrogen bond with
d) Arrange the following inorder of increasing water solubility:
1) 1-pentanol
2) 3-pentanone
3) 2-pentene
Self Test Questions:
1. 1)
2) 
3) 
a) Identify the functional group in each of the compounds above.
b) Draw how each of the compounds can hydrogen bond to a molecule identical to itself (if possible).
c) Draw how each compound can hydrogen bond to a water molecule (if possible)
d) Rank the ability of each of the compounds to dissolve in water, from least soluble to most soluble.
e) Rank the boiling points of the compounds, from lowest boiling point to highest boiling point.
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4.3 Chemical Properties |
7. Tollen’s reagent is a mild oxidizing solution that is used to test for the presence of aldehydes. The reagent contains silver ions which are reduced to metallic silver when reacted with an aldehyde and precipitate out, coating clear glass to form a mirror.
In testing for aldehydes, ________ reagent will precipitate silver atoms coating clear glass to form a mirror if the test is positive.
8. Benedict’s reagent is another mild oxidizing solution used to test for aldehydes. Benedict’s reagent contains Cu2+ ions, giving it a bright blue color. When aldehyde oxidation takes place, a red Cu2O precipitate is formed.
What result indicates a positive Benedict's test for the presence of an aldehyde?
9. Hydrogenation reactions for aldehydes and ketones are the reverse of primary and secondary alcohol oxidation reactions. Therefore, by adding hydrogen to a ketone a secondary alcohol is produced and adding hydrogen to an aldehyde produces a primary alcohol (see Reactions 4.7 and 4.8).
Draw the structural formula for the product of each of the following reactions and identify it as a primary, secondary, or tertiary alcohol:
a)
b)
10. The addition of alcohols to aldehydes is a two step process. The first step produces hemiacetals, which are unstable and difficult to isolate. The second step produces stable acetal products. Hemiacetals are important in carbohydrate chemistry.
The addition of alcohols to aldehydes produces unstable (a) ________ and stable (b) ________.
11. Hemiacetals have an –OH, hydrogen and an –OR group on the same carbon. Acetals have had the –OH group of a hemiacetal replaced by an –OR group. Acetal carbons have two –OR groups and a hydrogen attached (see Reaction 4.11).
Identify each of the following as an acetal, hemiacetal, or neither.
a)
b)
c)
12. Ketones also undergo addition reactions with alcohol in a similar manner similar to that of aldehydes. With ketones, a hemiketal intermediate compound is produced which is then changed to a ketal (see Reaction 4.15).
Identify each of the following as a ketal, hemiketal, or neither:
a)
b)
c)
13. Reaction 4.11 illustrates the formation of hemiacetals and acetals.
Draw the structural formula for the hemiacetal and hemiketal intermediates and for the acetal and ketal products of the following reactions. Label each structure as a hemiacetal, hemiketal, acetal, or ketal.
a)
b)
Please refer to the "Study Skills Map for Aldehydes and Ketones" on page 115 of your text when answering the following questions.
2. Complete the following reactions:
a)
b)
14. The following flow chart can help you distinguish between hemiacetals, hemiketals, acetals, ketals, or none of the above.
a)
b)
c)
d)
Self Test Question:
3. Identify each of the following as a hemiacetal, acetal, hemiketal, ketal, or neither.
a)
b)
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4.4 Important Aldehydes and Ketones |
15. The simplest aldehyde, formaldehyde, is probably one of the most widely used aldehydes. Formaldehyde is a gas at room temperature, but in a 37% aqueous solution called formalin, it is used as a sterilizer for surgical instruments and as an embalming fluid for cadavers. Formaldehyde is also used in the production of plastics such as Bakelite and Formica.
________ is a key aldehyde in the production of plastics.
16. Acetone is a particularly useful solvent because it dissolves most organic compounds and yet it is soluble in water. Acetone is widely used as a solvent for coatings, ranging from nail polish remover to exterior enamel paints.
Because it readily dissolves organic compounds and is water soluble, ________ is a widely used ketone.