Chapter 10. Organic Chemistry

10.6 Nomenclature of Aldehydes, Ketones, Carboxylic Acids, Esters, and Amides

Learning Objectives

By the end of this section, you will be able to:

  • Describe the structure and properties of aldehydes, ketones, carboxylic acids, esters and amides
  • Name and draw structures for aldehydes, ketones, carboxylic acids, esters and amides

Another class of organic molecules contains a carbon atom connected to an oxygen atom by a double bond, commonly called a carbonyl group. The carbonyl group can attach to two other substituents leading to several subfamilies, some of which are: aldehydes, ketones, carboxylic acids, esters and amides.

Naming of Aldehydes and Ketones

Both aldehydes and ketones contain a carbonyl group. In an aldehyde, the carbonyl group is bonded to at least one hydrogen atom. In a ketone, the carbonyl group is bonded to two carbon atoms.  The names for aldehyde and ketone compounds are derived using similar nomenclature rules as for alkanes and alcohols, and include the class-identifying suffixes -al and -one, respectively:

Therefore when naming aldehydes following IUPAC, you follow these rules:

Rule 1. Identify the longest chain of carbons which contains the carbonyl group (PREFIX-ANE+AL).  And when numbering the parent chain, the carbonyl group gets the lowest possible number, therefore it is always 1 and therefore is not included in the name.

Rule 2. Names and position of the substituents.

aldehydes

Methanal has a common name with which you may be familiar: formaldehyde.

When naming ketones following IUPAC, you follow these rules:

Rule 1. Identify the longest chain of carbons which contains the carbonyl group (PREFIX-#-ANE+ONE).  And when numbering the parent chain, the carbonyl group gets the lowest possible number.  In the smaller ketones (propanone and butanone), the locant number is not used because there is no alternative placement in these smaller ketones.

Rule 2. Names and position of the substituents.

propanonepentan-3-one

The common name for propanone is acetone. There is a non-IUPAC way to name ketones that is commonly used as well: name the alkyl groups that are attached to the carbonyl group and add the word ketone to the name. So propanone can also be called dimethyl ketone, while butan-2-one is called methyl ethyl ketone.

In condensed structure, an aldehyde group is represented as –CHO; a ketone is represented as –C(O)– or –CO–.

Example 1

Draw the structures of:  a) pentan-2-one;   b) hexan-2-one;    c) butane

 

Solution

a) This molecule has five C atoms in a chain, with the carbonyl group on the second C atom. Its structure is:

pentan-2-one

b) This molecule has six C atoms in a chain, with the carbonyl group on the second C atom. Its structure is:

methyl_butyl_ketone

c) This molecule has four C atoms in a chain, with the carbonyl group on the first C atom since it is an aldehyde (ends with -al). Its structure is:

 

Test Yourself
Give the condensed structure of the following amines:

a) propanone   b) propanal   c) heptan-3-one   d) octanal

 

Answer

a) CH3COCH3    b) CH3CH2CHO   

c) CH3CH2CH2CH2COCH2CH3   or  CH3(CH2)3COCH2CH3

d) CH3CH2CH2CH2CH2CH2CH2CHO   or  CH3(CH2)6CHO

Naming Carboxylic Acids and Esters

Both carboxylic acids and esters contain a carbonyl group with a second oxygen atom bonded to the carbon atom in the carbonyl group by a single bond. In a carboxylic acid, the second oxygen atom also bonds to a hydrogen atom. In an ester, the second oxygen atom bonds to another carbon atom. The names for carboxylic acids and esters include prefixes that denote the lengths of the carbon chains in the molecules.

The names for carboxylic acid and ester compounds are derived using similar nomenclature rules as seen previously with aldehydes, and include the class-identifying suffixes -oic acid and -oate, respectively:

Therefore when naming carboxylic acids following IUPAC, you follow these rules:

Rule 1. Identify the longest chain of carbons which contains the carbonyl group (PREFIX-ANE+OIC ACID).  And when numbering the parent chain, the carbonyl group gets the lowest possible number, therefore it is always 1 and therefore is not included in the name.

Rule 2. Names and position of the substituents.

When naming esters following IUPAC, you follow these rules:

Rule 1. Identify the longest chain of carbons which contains the carbonyl group (PREFIX-ANE+OATE).  And when numbering the parent chain, the carbonyl group gets the lowest possible number, therefore it is always 1 and therefore is not included in the name.  AND then name the other carbon chain (PREFIX+YL).

Rule 2. Names and position of the substituents.


Two structures are shown. The first structure is labeled, “ethanoic acid,” and, “acetic acid.” This structure indicates a C atom to which H atoms are bonded above, below and to the left. To the right of this in red is a bonded group comprised of a C atom to which an O atom is double bonded above. To the right of the red C atom, an O atom is bonded which has an H atom bonded to its right. Both O atoms have two sets of electron dots. The second structure is labeled, “methyl ethanoate,” and, “methyl acetate.” This structure indicates a C atom to which H atoms are bonded above, below and to the left. In red, bonded to the right is a C atom with a double bonded O atom above and a single bonded O atom to the right. To the right of this last O atom in black is another C atom to which H atoms are bonded above, below and to the right. Both O atoms have two pairs of electron dots.

The functional groups for an acid and for an ester are shown in red in these formulas.  In brackets you have the common names for ethanoic acid and methyl ethanoate.

The hydrogen atom in the functional group of a carboxylic acid will react with a base to form an ionic salt:
A chemical reaction is shown. On the left, a structure of propionic acid is indicated. This structure includes a 2 carbon hydrocarbon group on the left end in black. Above, below, and to the left, H atoms are bonded. This group is bonded to a red group comprised of a C atom to which an O atom is double bonded above. To the right of the red C atom, an O atom is connected with a single bond. To the right of the O atom, an H atom is bonded. To the right of this structure appears a plus and N a O H. Following the reaction arrow, the propionate ion is shown. This structure is in brackets. Appearing inside the brackets, is a 2 carbon hydrocarbon group on the left end. Above, below, and to the left, H atoms are bonded. To the right of this group, a group in red is attached comprised of a C atom to which an O atom is double bonded above and a second O atom is single bonded to the right. Outside the brackets appears a superscript minus symbol. This is followed by a plus sign, N a superscript plus another plus sign and H subscript 2 O. The singly bonded O atom in the propionate ion structure has 3 pairs of electron dots. All other O atoms have two pairs of electron dots.

Carboxylic acids are weak acids (see the chapter on acids and bases), meaning they are not 100% ionized in water. Generally only about 1% of the molecules of a carboxylic acid dissolved in water are ionized at any given time. The remaining molecules are undissociated in solution.

In condensed structure, the carboxylic acid group is represented as –COOH; an ester is represented as –COO– .

Example 2

Draw the structures of:  a) 3-methylpentanoic acid;   b) ethyl ethanoate;    c) propyl 2-chlorobutanoate

 

Solution

a) Its structure is:

b)Its structure is:

c) Its structure is:

 

Test Yourself
Name the following compounds:

a) CH3CH2COOH  b) CH3CH2CH2CH2COOCH2CH3  c) BrCH2(CH2)2COCH3  d) (CH3)2CH(CH2)6COOH

 

Answer

a) propanoic acid   b) ethyl pentanoate   c) methyl 4-bromobutanoate    d) 9-methylnonanoic acid   

Example 3

Complete the chemical reaction.

example_10a

 

Solution

The OH– ion removes the H atom that is part of the carboxyl group:

example_10_solution

The carboxylate ion, which has the condensed structural formula CH3CO2−, is the ethanoate ion, but it is commonly called the acetate ion.

 

Test Yourself

Complete the chemical reaction.

example_10_test_yourself_a

 

Answer

example_10_test_yourself_solution

The ion is the methanoate ion, which is commonly called the formate ion.

Naming Simple Amides

Amides are molecules that contain nitrogen atoms connected to the carbon atom of a carbonyl group.  The names for amide compounds are derived using similar nomenclature rules as seen previously with aldehydes and carboxylic acids, and include the class-identifying suffixes -amide:

Therefore when naming amides following IUPAC, you follow these rules:

Rule 1. Identify the longest chain of carbons which contains the carbonyl group (PREFIX-ANE+AMIDE).  And when numbering the parent chain, the carbonyl group gets the lowest possible number, therefore it is always 1 and therefore is not included in the name.

Rule 2. Names and position of the substituents.

In condensed structure, the amide group is represented as –CONH2 or –CONHR or –CONR2.

Example 4

Give the condensed structures of:  a) decanamide;   b) hexanamide;    c) 2-chloroethanamide

 

Solution

a) Its condensed structure is: CH3(CH2)8CONH2

b) Its condensed structure is: CH3(CH2)4CONH2

c) Its condensed structure is: ClCH2CONH2

 

Test Yourself
Name the following compounds:

a) CH3(CH2)2CONH2  b) BrCH2(CH2)3CONH2  d) (CH3CH2)2CH(CH2)5CONH2

 

Answer

a) butanamide  b) 5-bromopentanamide   c) 7-ethylnonanamide

Key Concepts and Summary

Functional groups related to the carbonyl group include the –CHO group of an aldehyde, the –CO– group of a ketone, the –CO2H group of a carboxylic acid, the –CO2R group of an ester and the –CONH2 group of an amide.

The systematic methods of naming these carbonyl containing functional groups follow a similar procedure and the names have three main parts:

1) specifying the information about the substituents,

2) specifying the information about the parent chain, and

3) the ending which specifies what functional group is present in the structure being named.

Aldehydes:    #-substituents–PREFIX–ANE+AL

Ketones:   #-substituents–PREFIX–ANE+ONE

Carboxylic Acids:   #-substituents–PREFIX–ANE+OIC ACID

Esters:   ALKYL #-substituents–PREFIX–ANE+OATE

Amides:   #-substituents–PREFIX–ANE+AMIDE

Exercises

1. Write a condensed structural formula of the following compounds.

a) 2-propanol

b) acetone

c) dimethyl ether

d) acetic acid

e) 3-methyl-1-hexene

2. A peptide is a short chain of amino acids connected by amide bonds. How many amide bonds are present in this peptide?

ex_9
3. How many amide bonds are present in this peptide? (See Exercise 2 for the definition of a peptide.)
ex_10

4. Name a similarity between the functional groups found in aldehydes and ketones. Can you name a difference between them?

5. Name each molecule.

a)  ex_3a     b)  ex_3b

6. Name each molecule.

a)  ex_4a    b)  ex_4b

7. Name each molecule.

a)  ex_5a    b)  ex_5b

8. Name each molecule.

a)  ex_6a    b)  ex_6b

9. Name this molecule.

ex_7

10. The drug known as aspirin is shown here.  Identify the functional group(s) in this molecule.

ex_13

11. The drug known as naproxen sodium is the sodium salt of the molecule shown here. Identify the functional group(s) in this molecule.

ex_14
Answers
1. a) CH3CH(OH)CH3
b) [latex]\text{CH}_3\text{COCH}_3[/latex]:
c) CH3OCH3
d) CH3COOH
e) CH3CH2CH2CH(CH3)CHCH2
2. two amide bonds
3. one amide bond

4. They both have a carbonyl group, but an aldehyde has the carbonyl group at the end of a carbon chain, and a ketone’s carbonyl carbon is surrounded by two other carbons.

5. a)  proposal b)  butanone

6. a) 3-chloro-3-methylbutanal  b) heptan-4-one

7. a)  3-methylbutanoic acid      b)  ethyl propionate

8. a) 2,2,2-trichlroethanoic acid    b) butyl ethanoate

9. ethyl propyl ether

10. carboxylic acid, arene and ester

11. carboxylic acid, arene and ether

Glossary

aldehyde: organic compound containing a carbonyl group bonded to two hydrogen atoms or a hydrogen atom and a carbon substituent

carbonyl group: carbon atom double bonded to an oxygen atom

carboxylic acid: organic compound containing a carbonyl group with an attached hydroxyl group

ester: organic compound containing a carbonyl group with an attached oxygen atom that is bonded to a carbon substituent

ketone: organic compound containing a carbonyl group with two carbon substituents attached to it

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