CONCEPT MAP: ACIDS AND BASES
12.10 Drawing and Naming Cycloalkanes
Even condensed structures become awkward when we work with large molecules that contain rings. Thus, line structures are used almost exclusively in drawing cycloal- kanes, with polygons used for the cyclic parts of the molecules. A triangle represents cyclopropane, a square represents cyclobutane, a pentagon represents cyclopentane, and so on.
Cyclopropane Cyclobutane Cyclopentane Cyclohexane Cycloheptane
Methylcyclohexane, for example, looks like this in a line structure:
is the same as
CH3
This three-way intersection is a CH group.
CH3 H2 C H2C H2C
CH2 CH2
CH
These intersections represent CH2 groups.
Cycloalkanes are named by a straightforward extension of the rules for naming open-chain alkanes. In most cases, only two steps are needed:
STEP 1: Use the cycloalkane name as the parent. That is, compounds are named as alkyl-substituted cycloalkanes rather than as cycloalkyl-substituted alkanes. If there is only one substituent on the ring, it is not even necessary to assign a number because all ring positions are identical.
Parent compound:
Name:
(not cyclohexylmethane)
CH3 Cyclohexane
Methylcyclohexane
STEP 2: Identify and number the substituents. Start numbering at the group that has alphabetical priority, and proceed around the ring in the direction that gives the sec- ond substituent the lower possible number.
(not 1-ethyl-5-methylcyclohexane or 1-methyl-3-ethylcyclohexane or 1-methyl-5-ethylcyclohexane) 2
5
3 1
4 6
CH2CH3 1-ethyl-3-methylcyclohexane H3C
Worked Example 12.12 Naming Organic Compounds: Cycloalkanes What is the IUPAC name of the following cycloalkane?
CH H3C
CH3
CH3
ANALYSIS First identify the parent cycloalkane, then add the positions and identity of any substituents.
S E C T I O N 1 2 . 1 0 Drawing and Naming Cycloalkanes 387
SOLUTION
STEP 1: The parent cycloalkane contains 6 carbons (hexane); hence, cyclohexane.
STEP 2: There are two substituents; a methyl 1iCH32 and an isopropyl
1CH3CHCH32. Alphabetically, the isopropyl group is given priority (number 1);
the methyl group is then found on the third carbon in the ring.
3 4
5 6
2 1 H3C
1-isopropyl-3-methylcyclohexane CH
CH3
CH3
Worked Example 12.13 Molecular Structures: Drawing Line Structures for Cycloalkanes Draw a line structure for 1,4-dimethylcyclohexane.
ANALYSIS This structure consists of a 6-carbon ring with two methyl groups attached at positions 1 and 4. Draw a hexagon to represent a cyclohexane ring, and attach a iCH3 group at an arbitrary position that becomes the first carbon in the chain, designated as C1. Then count around the ring to the fourth carbon (C4), and attach another iCH3 group.
SOLUTION
Note that the second methyl group is written here as H3Ci because it is attached on the left side of the ring.
1,4-dimethylcyclohexane
3 2
5 6
1
4 CH3
H3C
PROBLEM 12.18
What are the IUPAC names of the following cycloalkanes?
CH2CH3 (a) H3C
CH3CH2 CH(CH3)2 (b)
PROBLEM 12.19
Draw line structures that represent the following IUPAC names:
(a) 1,1-Diethylcyclohexane (b) 1,3,5-Trimethylcycloheptane PROBLEM 12.20
In the box Chemistry in Action: Surprising Uses of Petroleum, three alkenes were mentioned as being important materials obtained from the refining of petroleum:
ethylene, propylene, and butadiene.
(a) What consumer products are manufactured with ethylene and propylene?
(b) Butadiene is used in the manufacture of synthetic rubber. Why is this more desired than natural rubber?
KEY CONCEPT PROBLEM 12.21
What is the IUPAC name of the following cycloalkane?
SUMMARY: REVISITING THE CHAPTER GOALS
1. What are the basic properties of organic compounds?
Compounds made up primarily of carbon atoms are classified as organic. Many organic compounds contain carbon atoms that are joined in long chains by a combination of single 1CiC2, double 1C“C2, or triple 1C‚C2 bonds. In this chapter, we focused primarily on alkanes, hydrocarbon compounds that contain only single bonds between all C atoms (see Problems 29, 31, 32).
2. What are functional groups, and how are they used to classify organic molecules? Organic compounds can be classi- fied into various families according to the functional groups they contain. A functional group is a part of a larger molecule and is composed of a group of atoms that has characteristic structure and chemical reactivity. A given functional group undergoes nearly the same chemical reactions in every molecule where it occurs (see Problems 25, 34–37, 66, 73).
3. What are isomers? Isomers are compounds that have the same formula but different structures. Isomers that differ in their connections among atoms are called constitutional isomers.
When atoms other than carbon and hydrogen are present, the ability to have functional group isomers arises; these are mol- ecules that, due to the differences in their connections, have not only different structures but also belong to different families of organic molecules (see Problems 28, 38–51).
4. How are organic molecules drawn? Organic compounds can be represented by structural formulas in which all atoms and bonds are shown, by condensed structures in which not all bonds are drawn, or by line structures in which the carbon skeleton
is represented by lines and the locations of C and H atoms are understood (see Problems 22–24, 44, 45, 48, 49–51).
5. What are alkanes and cycloalkanes, and how are they named? Compounds that contain only carbon and hydro- gen are called hydrocarbons, and hydrocarbons that have only single bonds are called alkanes. A straight-chain alkane has all its carbons connected in a row, a branched-chain alkane has a branching connection of atoms somewhere along its chain, and a cycloalkane has a ring of carbon atoms. Alkanes have the general formula CnH2n+2, whereas cycloalkanes have the formula CnH2n. Straight-chain alkanes are named by adding the family ending -ane to a parent; this tells how many carbon atoms are present.
Branched-chain alkanes are named by using the longest continu- ous chain of carbon atoms for the parent and then identifying the alkyl groups present as branches off the main chain. The positions of the substituent groups on the main chain are identi- fied by numbering the carbons in the chain so that the substitu- ents have the lowest number. Cycloalkanes are named by adding cyclo- as a prefix to the name of the alkane (see Problems 25, 27, 52, 61, 67).
6. What are the general properties and chemical reac- tions of alkanes? Alkanes are generally soluble only in nonpo- lar organic solvents, have weak intermolecular forces, and are nontoxic. Their principal chemical reactions are combustion, a reaction with oxygen that gives carbon dioxide and water, and halogenation, a reaction in which hydrogen atoms are replaced by chlorine or bromine (see Problems 62–65, 70, 76, 77).
Key Words 389
O, N, S present
C O present (carbonyl)
Single bonds only
Only C and H
Halogen present
Double bonds only
Alkyne (Chapter 13)
Alkane (Chapter 12)
No OH present OH
present
Alkyl halide (Chapter 14)
Amine (Chapter 15, 18)
Ester (Chapter 17)
Anhydride (Chapter 17)
Ketone (Chapter 16)
Aldehyde (Chapter 16)
Carboxylic Acid (Chapter 17, 18)
Amide (Chapter 17, 18) No ring or
fewer than three double bonds in ring
Three double bonds in ring, alternating with single
bonds
O
SH S S
S N OH
attached
OR attached
N attached Multiple
bonds present
Triple bond
Single bonds only
No single bond O or N connected to C O
Single bond O or N connected to
C O Only C, H, or
halogen present
Organic Functional
Groups
Disulfide (Chapter 14) Ether
(Chapter 14)
Thiol (Chapter 14) Alcohol
(Chapter 14)
Alkene (Chapter 13)
Aromatic (Chapter 13)