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Example 9.1 Kinds of Hydrocarbon Formulas
Without referring to Table 9.2, give the molecular formula, the structural formula, and the
condensed structural formula for heptane.
Solution
The stem hept- means seven carbon atoms, and the ending -ane indicates an alkane. For
the structural formula, we can write out a string of seven carbon atoms:
C—C—C—C—C—C—C
Then we attach enough hydrogen atoms to the carbon atoms to give each carbon four
bonds. This requires three hydrogen atoms on each end carbon and two each on the
others.
H
H
H
H H
H
H
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H—C—C—C—C—C—C—C—H
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H H
H H
H H
H
For the condensed form, simply write each carbon atom’s set of hydrogen atoms next to
the carbon:
CH3CH2CH2CH2CH2CH2CH3
For the molecular formula, we can simply count the carbon and hydrogen atoms to arrive
at C7H16. Alternatively, we could use the general formula CnH2n+2 with n = 7 to get C7H16.
Exercise 9.1
Give the molecular, structural, and condensed structural formulas for octane.
Example 9.2 Cyclic Hydrocarbons: Structural Formulas
Give the structural formula for cyclobutane. What geometric figure is used to represent
cyclobutane?
Solution
Cyclobutane has four carbon atoms arranged in cyclic fashion:
C
C
C
C
Each carbon atom needs two hydrogen atoms to complete its set of four bonds:
CH2
CH2
CH2
CH2
Cyclobutane is represented by a square:
Exercise 9.2A
Give the structural formula and geometric representation for cyclopentane.
Exercise 9.2B
What is the general formula for cycloalkanes?
Example 9.3 Molecular Formulas of Hydrocarbons
What is the molecular formula for 1-octene? (The “1-” is a part of a systematic name that
indicates the location of the double bond; it follows the first carbon atom of the chain.)
Solution
The stem oct- indicates eight carbon atoms, the ending -ene tells us that the compound is
an alkene. Using the general formula CnH2n for an alkene, with n = 8, we see that an
alkene with eight carbon atoms has the molecular formula C 8H16.
Exercise 9.3
What is the molecular formula for 1-heptyne?
Example 9.4 Formulas of Chlorinated Hydrocarbons
What is the formula for dichloromethane (also known as methylene chloride)?
Solution
The “dichloro” indicates two chlorine atoms. The “methane” part of the name indicates
that the compound is derived from methane. We conclude that two hydrogen atoms of
methane (CH4) have been replaced by chlorine atoms; the formula for dichloromethane is
therefore CH2Cl2.
Exercise 9.4
What is the formula for (a) trichloromethane (also known as chloroform) and (b)
tetrachloromethane (carbon tetrachloride)?
Example 9.5 Structural Formulas of Alcohols
Write the structural formula for tert-butyl alcohol, sometimes used as an octane booster in
gasoline.
Solution
An alcohol can be considered an alkyl group joined to a hydroxyl group. From Table 9.5,
we see that the tert-butyl group is
CH3
CH3
C
CH3
Connecting this group to an OH group gives tert-butyl alcohol:
CH3
CH3
C
OH
CH3
Exercise 9.5
Write the structural formulas for (a) butyl alcohol and (b) sec-butyl alcohol.
Conceptual Example 9.6 The Functional Group
Classify each of the following as an alcohol, an ether, or a phenol.
Solution
a. The functional group, an OH, is attached directly to the benzene ring; the compound is
a phenol.
b. The O atom is between two alkyl groups; the compound is an ether.
c. The OH group is attached to an alkyl group; the compound is an alcohol.
d. An alcohol; the OH is not attached directly to the benzene ring.
e. An ether; the O atom is between two C atoms.
Exercise 9.6
Classify the following as alcohols, ethers, or phenols.
Example 9.7 Formulas for Ethers
Give the formula for isopropyl methyl ether.
Solution
Isopropyl methyl ether has an oxygen atom joined to an isopropyl group (three carbons
joined to oxygen by the middle carbon) and a methyl group. The formula is
Exercise 9.7A
Give the formula for methyl propyl ether.
Exercise 9.7B
Give the formula for ethyl tert-butyl ether.
Conceptual Example 9.8 Aldehyde or Ketone?
Identify each of the following compounds as an aldehyde or a ketone.
Solution
a. A hydrogen atom is attached to the carbonyl carbon atom; the compound is an
aldehyde.
b. The carbonyl group is between two other carbon atoms; the compound is a ketone.
c. A ketone. (Remember that the corner of the hexagon stands for a carbon atom.)
Exercise 9.8
Identify each of the following compounds as an aldehyde or a ketone.
Example 9.9 Structural Formulas of Oxygen-Containing Organic Compounds
Give the structural formula for each of the following compounds:
a. propionaldehyde
b. acetic acid
c. ethyl methyl ketone
Solution
a.
Propionaldehyde has three carbon atoms with an aldehyde function:
O
||
C—C—C—H
Adding the proper number of hydrogen atoms to the other two carbon atoms gives the structure
H H
O
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H—C—C—C—H
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H H
b.
or
CH3CH2CHO
Acetic acid has two carbon atoms with a carboxylic acid function:
H
O
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H — C — C — OH
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H
or
CH3COOH
Example 9.9 (cont.)
c.
Ethyl methyl ketone has a ketone function between an ethyl and a methyl group:
H H
O H
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H—C—C—C—C—H
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H H
H
or
CH3CH2COCH3
Exercise 9.9A
Give the structural formula for each of the following compounds:
a. butyric acid
b. acetaldehyde
c. diethyl ketone
Exercise 9.9B
Give the structural formula for each of the following compounds:
a. hexanoic acid
b. 3-octanone
c. heptanal
Conceptual Example 9.10 Amine Isomers: Structures and Names
Give structures and names for the other three-carbon amines.
Solution
Three-carbon atoms can be in one alkyl group, and there are two such propyl groups:
CH3CH2CH2NH2
CH3CHCH3
|
NH2
Propylamine
Isopropylamine
Three-carbon atoms can also be split into one methyl group and one ethyl group:
CH3CH2NHCH3
Ethylmethylamine
Exercise 9.10
Give structures for the following amines:
a. butylamine
b. diethylamine
c. methylpropylamine
d. isopropylmethylamine
Conceptual Example 9.11 Amine or Amide?
Which of the following are amides and which are amines? Identify the functional groups.
a. CH3CH2CH2NH2
b. CH3CONH2
c. CH3CH2NHCH3
d. CH3COCH2CH2NH2
Solution
a. amine; the NH2 is the amine function.
b. amide; the CONH2 is the amide function.
c. amine; the NH is the amine function.
d. amine; the NH2 is an amine function; there is a carbonyl group, but the NH2 is not
attached to it.
Exercise 9.11
Which of the following are amides and which are amines? Identify the functional
groups.
a. CH3NHCH3
b. CH3CONHCH3
c. CH3CH2N(CH3)2
d. CH3NHCH2CONH2
Conceptual Example 9.12 Heterocyclic Compounds
Which of the following structures represent heterocyclic compounds?
Solution
Compounds a, b, and d have oxygen, sulfur, and nitrogen atoms, respectively, in a ring
structure; these represent heterocyclic compounds.
Exercise 9.12
Which of the following structures represent heterocyclic compounds?