(BQ) Part 2 book General chemistry The essential concepts has contents: Introduction to organic chemistry, intermolecular forces and liquids and solids, physical properties of solutions, chemical kinetics, chemical equilibrium, thermodynamics, redox reactions and electrochemistry,...and other contents.
Trang 1HO
O
O N H
CH3
ESSENTIAL CONCEPTS
Organic Compounds Organic compounds contain primarily carbon and hydrogen atoms, plus nitrogen, oxygen, sulfur, and atoms of other elements The parent compounds of all organic compounds are the hydrocarbons—the alkanes (containing only single bonds), the alkenes (containing carbon-carbon double bonds), the alkynes (containing carbon-carbon triple bonds), and the aromatic hydrocarbons (containing the benzene ring).
Functional Groups The reactivity of organic compounds can
be reliably predicted by the presence of functional groups, which are groups of atoms that are largely responsible for the chemical behavior of the compounds.
Chirality Certain organic compounds can exist as nonsuperim-posable mirror-image twins These compounds are said to be chiral The pure enantiomer of a compound can rotate plane-polarized light Enantiomers have identical physical properties but exhibit different chemical properties toward another chiral substance.
STUDENT INTERACTIVE ACTIVITIES
Animations
Chirality (11.5)
Electronic Homework
Example Practice Problems End of Chapter Problems
CHAPTER OUTLINE
11.1 Classes of Organic Compounds 364
11.2 Aliphatic Hydrocarbons 364
Alkanes • Cycloalkanes • Alkenes • Alkynes
11.3 Aromatic Hydrocarbons 379
Nomenclature of Aromatic Compounds •
Properties and Reactions of Aromatic Compounds
11.4 Chemistry of the Functional Groups 382
Alcohols • Ethers • Aldehydes and Ketones • Carboxylic Acids
• Esters • Amines • Summary of Functional Groups
11.5 Chirality—Th e Handedness of Molecules 389
Introduction to Organic Chemistry
11
C H A P T E R
The burning sensation of chili peppers such as habaneros is mostly
due to the organic compound capsaicin (illustrated by its skeletal
structure).
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11.1 Classes of Organic Compounds
Carbon can form more compounds than most other elements because carbon atoms are able not only to form single, double, and triple carbon-carbon bonds, but also to
link up with each other in chains and ring structures The branch of chemistry that
deals with carbon compounds is organic chemistry.
Classes of organic compounds can be distinguished according to functional groups
they contain A functional group is a group of atoms that is largely responsible for the
chemical behavior of the parent molecule Different molecules containing the same kind
of functional group or groups undergo similar reactions Thus, by learning the charac-teristic properties of a few functional groups, we can study and understand the proper-ties of many organic compounds In the second half of this chapter we will discuss the functional groups known as alcohols, ethers, aldehydes and ketones, carboxylic acids, and amines
All organic compounds are derived from a group of compounds known as
hydrocarbons because they are made up of only hydrogen and carbon On the basis
of structure, hydrocarbons are divided into two main classes—aliphatic and aromatic
Aliphatic hydrocarbons do not contain the benzene group, or the benzene ring, whereas aromatic hydrocarbons contain one or more benzene rings.
11.2 Aliphatic Hydrocarbons
Aliphatic hydrocarbons are divided into alkanes, alkenes, and alkynes, discussed in this section (Figure 11.1)
Alkanes
Alkanes are hydrocarbons that have the general formula C n H 2n 12 , where n 5 1,
2, The essential characteristic of alkanes is that only single covalent bonds are
present The alkanes are known as saturated hydrocarbons because they contain the
maximum number of hydrogen atoms that can bond with the number of carbon atoms present.
The simplest alkane (that is, with n 5 1) is methane CH4, which is a natural product
of the anaerobic bacterial decomposition of vegetable matter under water Because it was
fi rst collected in marshes, methane became known as “marsh gas.” A rather improbable
Common elements in organic
compounds.
Cl F S O P N Si C B H
Br I
1A
3A
8A
For a given number of carbon atoms, the
saturated hydrocarbon contains the
larg-est number of hydrogen atoms.
Hydrocarbons
Figure 11.1
Classifi cation of hydrocarbons.
Trang 311.2 Aliphatic Hydrocarbons 365
but proven source of methane is termites When these voracious insects consume wood,
the microorganisms that inhabit their digestive system break down cellulose (the major
component of wood) into methane, carbon dioxide, and other compounds An estimated
170 million tons of methane are produced annually by termites! It is also produced in
some sewage treatment processes Commercially, methane is obtained from natural gas
Figure 11.2 shows the structures of the fi rst four alkanes (n 5 1 to n 5 4)
Natu-ral gas is a mixture of methane, ethane, and a small amount of propane We discussed
the bonding scheme of methane in Chapter 10 The carbon atoms in all the alkanes can
be assumed to be sp3-hybridized The structures of ethane and propane are
straightfor-ward, for there is only one way to join the carbon atoms in these molecules Butane,
however, has two possible bonding schemes resulting in different compounds called
n-butane (n stands for normal) and isobutane n-Butane is a straight-chain alkane because
the carbon atoms are joined in a continuous chain In a branched-chain alkane like
isobutane, one or more carbon atoms are bonded to a nonterminal carbon atom Isomers
that differ in the order in which atoms are connected are called structural isomers.
In the alkane series, as the number of carbon atoms increases, the number of structural isomers increases rapidly For example, C4H10 has two isomers; C10H22 has 75 isomers;
and C30H62 has over 400 million possible isomers! Obviously, most of these isomers do
not exist in nature nor have they been synthesized Nevertheless, the numbers help to
explain why carbon is found in so many more compounds than any other element
Termites are a natural source of methane.
H
C
Isobutane
n-Butane
A
O O O
A H
H
H
H C
AO O A H H
O A
A H
H H
C
C A
O O O A H H
A A
H H
H C
O A A H
H
H
A
O O
O
A H
H
A A H H
H
C O
A A
H
H C A
A
H H H C
H C A
O O O A H
H
A A
H
C O A A H
H C
O OC H H
A
A A
Figure 11.2
Structures of the fi rst four alkanes Note that butane can exist in two structurally different forms, called structural isomers.
EXAMPLE 11.1
How many structural isomers can be identifi ed for pentane, C5H12?
Strategy For small hydrocarbon molecules (eight or fewer C atoms), it is relatively
easy to determine the number of structural isomers by trial and error.
Solution The fi rst step is to write the straight-chain structure:
HOCOCOCOCOCOH
H A A H
H A A H
H A A H
H A A H
H A A H
n-pentane (b.p 36.1 ° C)
n-pentane (Continued)
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The second structure, by necessity, must be a branched chain:
HOCOCOOOCOCOH
H A A H
CH 3
A C A H
H A A H
H A A H
2-methylbutane (b.p 27.9 ° C)
Yet another branched-chain structure is possible:
HOCOCOOOCOH
H A A H
CH3 A A
CH 3
H A A H
2,2-dimethylpropane (b.p 9.5 ° C)
We can draw no other structure for an alkane having the molecular formula C5H12 Thus, pentane has three structural isomers, in which the numbers of carbon and hydrogen atoms remain unchanged despite the differences in structure.
Practice Exercise How many structural isomers are there in the alkane C6H14?
2-methylbutane
2,2-dimethylpropane
Similar problems: 11.11, 11.12.
Table 11.1 shows the melting and boiling points of the straight-chain isomers of the fi rst 10 alkanes The fi rst four are gases at room temperature; and pentane through decane are liquids As molecular size increases, so does the boiling point
Drawing Chemical Structures
Before proceeding further, it is useful to learn different ways of drawing the structure
of organic compounds Consider the alkane 2-methylbutane (C5H12) To see how atoms are connected in this molecule, we need to fi rst write a more detailed molecular formula,
Shortly we will discuss the nomenclature
of alkanes.
Crude oil is the source of many
hydrocarbons.
Trang 511.2 Aliphatic Hydrocarbons 367
CH3CH(CH3)CH2CH3, and then draw its structural formula, shown in Figure 11.3(a)
While informative, this structure is time-consuming to draw Therefore, chemists have
devised ways to simplify the representation Figure 11.3(b) is an abbreviated version
and the structure shown in Figure 11.3(c) is called the skeletal structure in which all
the C and H letters are omitted A carbon atom is assumed to be at each intersection
of two lines (bonds) and at the end of each line Because every C atom forms four
bonds, we can always deduce the number of H atoms bonded to any C atom One of
the two end CH3 groups is represented by a vertical line What is lacking in these
structures, however, is the three-dimensionality of the molecule, which is shown by the
molecular model in Figure 11.3(d) Depending on the purpose of discussion, any of
these representations can be used to describe the properties of the molecule
Conformation of Ethane
Molecular geometry gives the spatial arrangement of atoms in a molecule However,
atoms are not held rigidly in position because of internal molecular motions For this
reason, even a simple molecule like ethane may be structurally more complicated than
we think
The two C atoms in ethane are sp3-hybridized and they are joined by a sigma bond (see Section 10.5) Sigma bonds have cylindrical symmetry, that is, the
over-lap of the sp3 orbitals is the same regardless of the rotation of the COC bond Yet
this bond rotation is not totally free because of the interactions between the H atoms
on different C atoms Figure 11.4 shows the two extreme conformations of ethane
Conformations are different spatial arrangements of a molecule that are generated
by rotation about single bonds In the staggered conformation, the three H atoms
on one C atom are pointing away from the three H atoms on the other C atom,
whereas in the eclipsed conformation the two groups of H atoms are aligned
paral-lel to one another
A simpler and effective way of viewing these two conformations is by using the Newman projection, also shown in Figure 11.4 Look at the COC bond end-on The
two C atoms are represented by a circle The COH bonds attached to the front C
atom are the lines going to the center of the circle, and the COH bonds attached to
the rear C atom appear as lines going to the edge of the circle The eclipsed form of
ethane is less stable than the staggered form Figure 11.5 shows the variation in the
potential energy of ethane as a function of rotation The rotation of one CH3 group
relative to the other is described in terms of the angle between the COH bonds on
Skeletal structure is the simplest struc-ture Atoms other than C and H must be shown explicitly in a skeletal structure.
CH 3
A
H
A
C
A
H
H
A
C
A
A
A
C
A
H
H
A
C
A
H
H
A
C
A
H
H3CD
CH
G
CH2D
CH 3
HOCOCOCOCOH
HOCOH
Figure 11.3
Different representations of 2-methylbutane (a) Structural formula (b) Abbreviated formula (c) Skeletal formula
(d) Molecular model.
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12 kJ/mol
Dihedral angle
Figure 11.5
Potential energy diagram for the
internal rotation in ethane Here
the dihedral angle is defi ned by
the angle between the two COH
bonds (with the red spheres
representing the H atoms)
Dihedral angles of 0°, 120°, 240°,
and 360° represent the eclipsed
conformation, while those of 60°,
180°, and 300° represent the
staggered conformation Thus,
a rotation of 60° changes the
eclipsed conformation to the
staggered one and vice versa
The staggered conformation is
more stable than the eclipsed
conformation by 12 kJ/mol
However, these two forms
interconvert rapidly and cannot
be separated from each other.
H
H H
H
H
H
Molecular models
Newman projections
Staggered conformation
Eclipsed conformation
H H
Figure 11.4
Molecular models and Newman
projections of the staggered and
eclipsed conformations of ethane
The dihedral angle in the
staggered form is 60° and that
in the eclipsed form is 0° The
COC bond is rotated slightly
in the Newman projection of the
eclipsed form in order to show
the H atoms attached to the back
C atom The proximity of the H
atoms on the two C atoms in the
eclipsed form results in a greater
repulsion, and hence its instability
relative to the staggered form.
front and back carbons, called the dihedral angle The dihedral angle for the fi rst
eclipsed conformation is zero A clockwise rotation of 60° about the COC bond generates a staggered conformation, which is converted to another eclipsed conforma-tion by a similar rotaconforma-tion and so on
Conformational analysis of molecules is of great importance in understanding the details of reactions ranging from simple hydrocarbons to proteins and DNAs
Alkane Nomenclature
The nomenclature of alkanes and all other organic compounds is based on the rec-ommendations of the International Union of Pure and Applied Chemistry (IUPAC)
The fi rst four alkanes (methane, ethane, propane, and butane) have nonsystematic names As Table 11.1 shows, the number of carbon atoms is refl ected in the Greek
Trang 711.2 Aliphatic Hydrocarbons 369
prefi xes for the alkanes containing 5 to 10 carbons We now apply the IUPAC rules
to the following examples:
1 The parent name of the hydrocarbon is that given to the longest continuous chain
of carbon atoms in the molecule Thus, the parent name of the following com-pound is heptane because there are seven carbon atoms in the longest chain
CH1 3OCH2 2OCH3 2OCHOCH4 5 2OCH6 2OCH7 3
CH3 A
2 An alkane less one hydrogen atom is an alkyl group For example, when a hydrogen
atom is removed from methane, we are left with the CH3 fragment, which is called
a methyl group Similarly, removing a hydrogen atom from the ethane molecule gives
an ethyl group, or C2H5 Table 11.2 lists the names of several common alkyl groups
Any chain branching off the longest chain is named as an alkyl group
3 When one or more hydrogen atoms are replaced by other groups, the name of
the compound must indicate the locations of carbon atoms where replacements are made The procedure is to number each carbon atom on the longest chain in the direction that gives the smaller numbers for the locations of all branches
Consider the two different systems for the same compound shown below:
4-methylpentane 2-methylpentane
CH3OCH2OCH2OCHOCH3
CH3OCHOCH2OCH2OCH3
CH3
A
The compound on the left is numbered correctly because the methyl group is
located at carbon 2 of the pentane chain; in the compound on the right, the methyl group is located at carbon 4 Thus, the name of the compound is 2-methylpentane, and not 4-methylpentane Note that the branch name and the parent name are written as a single word, and a hyphen follows the number
4 When there is more than one alkyl branch of the same kind present, we use a
prefi x such as di-, tri-, or tetra- with the name of the alkyl group Consider the
following examples:
CH3OCHOCHOCH2OCH2OCH3 CH3OCH2OCOCH2OCH2OCH3
CH3 A A
CH3
3,3-dimethylhexane 2,3-dimethylhexane
CH3
A CH3 A
When there are two or more different alkyl groups, the names of the groups are
listed alphabetically For example,
CH3OCH2OCHOCHOCH2OCH2OCH3
CH3
A C2H5 A
4-ethyl-3-methylheptane
5 Of course, alkanes can have many different types of substituents Table 11.3 lists
the names of some substituents, including bromo and nitro Thus, the compound
CH 3 OCHOCHOCH 2 OCH 2 OCH 3
NO 2
Br A
Table 11.2
Common Alkyl Groups
Name Formula
Methyl OCH3 Ethyl OCH 2 OCH 3
n-Propyl O(CH2)2OCH3
n-Butyl O(CH 2 ) 3 OCH 3
Isopropyl OCOH
CH 3
A A
CH3
t-Butyl* OCOCH3
CH3 A A
CH 3
*The letter t stands for tertiary.
Table 11.3
Names of Common Substituent Groups
Functional Group Name
OF Fluoro OCl Chloro OBr Bromo
OI Iodo
OCHPCH 2 Vinyl
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is called 3-bromo-2-nitrohexane Note that the substituent groups are listed alpha-betically in the name, and the chain is numbered in the direction that gives the lowest number to the fi rst substituted carbon atom
EXAMPLE 11.2
Give the IUPAC name of the following compound:
CH 3 OCOCH 2 OCHOCH 2 OCH 3
CH 3
A A
CH 3
CH 3
A
Strategy We follow the IUPAC rules and use the information in Table 11.2 to name the compound How many C atoms are there in the longest chain?
Solution The longest chain has six C atoms so the parent compound is called hexane
Note that there are two methyl groups attached to carbon number 2 and one methyl group attached to carbon number 4.
CH3OCOCH2OCHOCH2OCH3
CH3 A A
CH3
CH3 A
Therefore, we call the compound 2,2,4-trimethylhexane
Practice Exercise Give the IUPAC name of the following compound:
CH 3 OCHOCH 2 OCHOCH 2 OCHOCH 2 OCH 3
CH 3
A C 2 H 5
A C 2 H 5
A
Similar problems: 11.28(a), (b), (c).
EXAMPLE 11.3
Write the structural formula of 3-ethyl-2,2-dimethylpentane.
Strategy We follow the preceding procedure and the information in Table 11.2 to write the structural formula of the compound How many C atoms are there in the longest chain?
Solution The parent compound is pentane, so the longest chain has fi ve C atoms
There are two methyl groups attached to carbon number 2 and one ethyl group attached
to carbon number 3 Therefore, the structure of the compound is
CH3OCOOCHOCH2OCH3
CH3 A A
CH 3
C2H5 A 3
Practice Exercise Write the structural formula of 5-ethyl-2,6-dimethyloctane.
Similar problems: 11.27(a), (c), (e)
Example 11.3 shows that prefi xes such as di-, tri-, and tetra- are used as needed, but are ignored when alphabetizing
Trang 911.2 Aliphatic Hydrocarbons 371
Reactions of Alkanes
Alkanes are generally not considered to be very reactive substances However, under
suitable conditions they do react For example, natural gas, gasoline, and fuel oil are
alkanes that undergo highly exothermic combustion reactions:
CH4(g)1 2O2(g) ¡ CO2(g)1 2H2O(l) DH° 5 2890.4 kJ/mol
2C2H6(g)1 7O2(g) ¡ 4CO2(g)1 6H2O(l) DH° 5 23119 kJ/mol
These, and similar combustion reactions, have long been utilized in industrial
pro-cesses and in domestic heating and cooking
Halogenation of alkanes—that is, the replacement of one or more hydrogen atoms
by halogen atoms—is another type of reaction that alkanes undergo When a mixture
of methane and chlorine is heated above 100°C or irradiated with light of a suitable
wavelength, methyl chloride is produced:
CH4(g)1 Cl2(g) ¡ CH3Cl(g) 1 HCl(g)
If an excess of chlorine gas is present, the reaction can proceed further:
CH3Cl(g)1 Cl2(g) ¡ CH2Cl2(l) 1 HCl(g)
CH2Cl2(l)1 Cl2(g) ¡ CHCl3(l) 1 HCl(g)
chloroform
CHCl3(l)1 Cl2(g) ¡ CCl4(l) 1 HCl(g)
A great deal of experimental evidence suggests that the initial step of the fi rst
halo-genation reaction occurs as follows:
Cl21 energy ¡ Cl ? 1 Cl ?
Thus, the covalent bond in Cl2 breaks and two chlorine atoms form We know it is
the ClOCl bond that breaks when the mixture is heated or irradiated because the bond
enthalpy of Cl2 is 242.7 kJ/mol, whereas about 414 kJ/mol are needed to break COH
bonds in CH4
A chlorine atom is a radical, which contains an unpaired electron (shown by a
single dot) Chlorine atoms are highly reactive and attack methane molecules
accord-ing to the equation
CH41 Cl ? ¡ ? CH31 HCl
This reaction produces hydrogen chloride and the methyl radical ? CH3 The methyl
radical is another reactive species; it combines with molecular chlorine to give methyl
chloride and a chlorine atom:
? CH31 Cl2 ¡ CH3Cl1 Cl ? The production of methylene chloride from methyl chloride and any further reactions
can be explained in the same way The actual mechanism is more complex than the
scheme we have shown because “side reactions” that do not lead to the desired
prod-ucts often take place, such as
Cl? 1 Cl ? ¡ Cl2
? CH 1 ? CH ¡ CH
Trang 10372 CHAPTER 11 Introduction to Organic Chemistry
Alkanes in which one or more hydrogen atoms have been replaced by a halogen atom
are called alkyl halides Among the large number of alkyl halides, the best known are
chloroform (CHCl3), carbon tetrachloride (CCl4), methylene chloride (CH2Cl2), and the chlorofl uorohydrocarbons
Chloroform is a volatile, sweet-tasting liquid that was used for many years as an anesthetic However, because of its toxicity (it can severely damage the liver, kidneys, and heart) it has been replaced by other compounds Carbon tetrachloride, also a toxic substance, serves as a cleaning liquid, for it removes grease stains from clothing
Methylene chloride is used as a solvent to decaffeinate coffee and as a paint remover
Cycloalkanes
Alkanes whose carbon atoms are joined in rings are known as cycloalkanes They
have the general formula CnH2n,where n 5 3, 4, The simplest cycloalkane is cyclopropane, C3H6 (Figure 11.6) Many biologically signifi cant substances such as antibiotics, sugars, cholesterol, and hormones contain one or more such ring systems
Cyclohexane can assume two different conformations called the chair and boat that are relatively free of angle strain (Figure 11.7) By “angle strain” we mean that the bond angles at each carbon atom deviate from the tetrahedral value of 109.5° required
for sp3 hybridization
Alkenes
The alkenes (also called olefi ns) contain at least one carbon-carbon double bond
Alkenes have the general formula C n H 2n , where n 5 2, 3, The simplest alkene
The systematic names of methyl chloride,
methylene chloride, and chloroform are
chloromethane, dichloromethane, and
trichloromethane, respectively.
In addition to C, atoms such as N, O, and
S may also occupy the ring positions in
these compounds.
H C
C C
H H H
H
H
C C
C C H H H
H H
H C
C C C
H H H H H H H H H
C C
C C C
H H H H H H H H
Figure 11.6
Structures of the fi rst four
cycloalkanes and their
simplifi ed forms.
Figure 11.7
The cyclohexane molecule can
exist in various shapes The
most stable shape is the chair
conformation and a less stable
one is the boat conformation
Two types of H atoms are
labeled axial and equatorial,
respectively.
Axial
Equatorial