2 2-1 organic chemistry william h. brown & christopher s. foote william h. brown &...
TRANSCRIPT
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Organic Organic ChemistryChemistry
William H. Brown &William H. Brown &
Christopher S. FooteChristopher S. Foote
William H. Brown &William H. Brown &
Christopher S. FooteChristopher S. Foote
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Chapter TwoChapter Two
AlkanesAlkanes andand
CycloalkanesCycloalkanes
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StructureStructure Hydrocarbon:Hydrocarbon: a compound composed only of
carbon and hydrogen Saturated hydrocarbon:Saturated hydrocarbon: a hydrocarbon
containing only single bonds Alkane:Alkane: a saturated hydrocarbon whose carbons
are arranged in a open chain Aliphatic hydrocarbon:Aliphatic hydrocarbon: another name for an
alkane
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StructureStructure Shape
• tetrahedral about carbon• all bond angles are approximately 109.5°
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NomenclatureNomenclature Alkanes have the general formula CnH2n+2
CondensedStructural Formula
MolecularFormulaName
heptane
hexane
pentane
butane
propaneethanemethane CH4 CH4
C2H6 CH3CH3
C3H8 CH3CH2CH3C4H10 CH3(CH2)2CH3
C5H12 CH3(CH2)3CH3
C6H14 CH3(CH2)4CH3
C7H16 CH3(CH2)5CH3
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NomenclatureNomenclature Alkanes (contd.)
tetradecane
hexadecane
octadecaneeicosane
decane
nonane
octane C8H18 CH3(CH2)6CH3
C9H20 CH3(CH2)7CH3C10 H22 CH3(CH2)8CH3
C14 H30 CH3(CH2)12 CH3
C16 H34 CH3(CH2)14 CH3
C18 H38 CH3(CH2)16 CH3C20 H42 CH3(CH2)18 CH3
dodecane C12 H26 CH3(CH2)10 CH3
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Constitutional IsomerismConstitutional Isomerism Constitutional isomers:Constitutional isomers: compounds with the
same molecular formula but a different connectivity (order of attachment of their atoms)• example: C4H10
CH3CH2CH2CH3 CH3CHCH3
CH3
Butane 2-Methylpropane
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Constitutional IsomerismConstitutional Isomerism
World populationWorld populationis aboutis about6,000,000,0006,000,000,000
4,111,846,763
4,347
75
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Constitutional Isomers
MolecularFormula
CH4
C5H12
C10H22
C15H32
C30H62
36,797,588C25H52
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Nomenclature - IUPACNomenclature - IUPAC Suffix -aneane specifies an alkane Prefix tells the number of carbon atoms
undec-dodec-
tetradec-pentadec-hexadec-heptadec-
nonadec-eicos-
tridec-
11121314151617
octadec- 181920
Prefixmeth-eth-prop-but-pent-hex-
oct-non-dec-
1234567hept-89
10
Carbons CarbonsPrefix
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Nomenclature - IUPACNomenclature - IUPAC Parent name: the longest carbon chain Substituent: a group attached to the parent chain
• alkyl group: a substituent derived by removal of a hydrogen from an alkane; given the symbol R-
Alkane Alkyl group
CH4
Name Name
Methane CH3- Methyl group
CH3CH3 Ethane CH3CH2- Ethyl group
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NomenclatureNomenclature1.The name of a saturated hydrocarbon with an
unbranched chain consists of a prefix and suffix
2. For branched alkanes, the parent chain is the longest chain of carbon atoms
3. Each substituent is given a name and a number
4. If there is one substituent, number the chain from the end that gives it the lower number
CH3CHCH3
CH3
2-Methylpropane1
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2-Methylpentane
CH3
CH3CH2CH2CHCH3 14 3 2
5
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NomenclatureNomenclature6. If there are two or more different substituents,• list them in alphabetical order• number from the end of the chain that gives the
substituent encountered first the lower number
CH3CH2CHCH2CHCH2CH3
3-Ethyl-5-methylheptane
CH3
CH2CH3
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43
21 7
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NomenclatureNomenclature7. The prefixes di-, tri-, tetra-, etc. are not included in alphabetization
8. Substituents are named by the same set of rules.
CH3CCH2CHCH2CH3
CH3 CH2CH3
CH3
4-Ethyl-2,2-dimethylhexane
1 2 34 5
6
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NomenclatureNomenclature Alkyl groups
1-methylethyl (isopropyl)
propyl
ethyl
methyl
CondensedStructural FormulaName
CH3
-CH2CH3
-CH3
-CH2CH2CH3
-CHCH3
1,1-dimethylethyl (tert-butyl)
1-methylpropyl (sec-butyl)
2-methylpropyl (isobutyl)
butyl
CH3
CH3
CH3
CH3
-CH2CH2CH2 CH3
-CH2CHCH3
-CHCH2CH3
-CCH3
CondensedStructural FormulaName
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Classification of C & HClassification of C & H Primary (1°) C:Primary (1°) C: a carbon bonded to one other
carbon• 1° H: a hydrogen bonded to a 1° carbon
Secondary (2°) C:Secondary (2°) C: a carbon bonded to two other carbons• 2° H: a hydrogen bonded to a 2° carbon
Tertiary (3°) C:Tertiary (3°) C: a carbon bonded to three other carbons• 3° H: a hydrogen bonded to a 3° carbon
Quaternary (4°) CQuaternary (4°) C: a carbon bonded to four other carbons
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CycloalkanesCycloalkanes General formula CCnnHH2n2n
• five- and six-membered rings are the most common
Structure and nomenclature• to name, prefix the name of the corresponding open-
chain alkane with cyclo-cyclo-,, and name each substituent on the ring
• if only one substituent, no need to give it a number• if two substituents, number from the substituent of
lower alphabetical order• if three or more substituents, number to give them the
lowest set of numbers and then list substituents in alphabetical order
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CycloalkanesCycloalkanes Line-angle drawings
• each line represents a C-C bond• each angle represents a C
C
C CC
CC
C
C H2C
H2CCH2
CH
CH2
CH
CH3
CH3
C8H16
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CycloalkanesCycloalkanesExample:Example: name these cycloalkanes
(a) (b)
(c) (d)
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BicycloalkanesBicycloalkanes Bicycloalkane:Bicycloalkane: an alkane that contains two rings
that share two carbons
Bicyclo[4.4.0]decane(Decalin)
Bicyclo[4.3.0]nonane(Hydrindane)
Bicyclo[2.2.1]heptane(Norbornane)
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BicycloalkanesBicycloalkanes Nomenclature
• parent is the alkane of the same number of carbons as are in the rings
• number from a bridgehead, along longest bridge back to the bridgehead, then along the next longest bridge, etc.
• show the lengths of bridges in brackets, from longest to shortest
bicyclo[2.2.1]heptane
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IUPAC - GeneralIUPAC - General prefix-infix-suffix
• prefix tells the number of carbon atoms in the parent • infix tells the nature of the carbon-carbon bonds• suffix tells the class of compound
one or more triple bonds
one or more double bonds
all single bonds
-yn-
-en--an-
Nature of Carbon-Carbon Bonds in the Parent ChainInfix
Suffix Class
-e
-ol
-al
-one
-oic acid
hydrocarbon
alcohol
aldehyde
ketonecarboxylic acid
-amine amine
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IUPAC - GeneralIUPAC - Generalprop-en-e = propene
eth-an-ol = ethanol
but-an-one = butanone
but-an-al = butanal
pent-an-oic acid = pentanoic acid
cyclohex-an-ol = cyclohexanol
eth-yn-e = ethyne
eth-an-amine = ethanamine
CH3CH=CH2
CH3CH2OHCH3CH2CH2CH2COH
O
HC CH
OH
CH3CH2CH2CH
O
CH3CCH2CH3
O
CH3CH2NH2
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ConformationsConformations Conformation:Conformation: any three-dimensional
arrangement of atoms in a molecule that results from rotation about a single bond
Newman projection:Newman projection: a way to view a molecule by looking along a carbon-carbon bond
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ConformationsConformations Staggered conformation:Staggered conformation: a conformation about a
carbon-carbon single bond in which the atoms on one carbon are as far apart as possible from the atoms on an adjacent carbon
H
H H
H H
H
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ConformationsConformations Eclipsed conformation:Eclipsed conformation: a conformation about a
carbon-carbon single bond in which the atoms on one carbon are as close as possible to the atoms on an adjacent carbon
H
H H
H
HH
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ConformationsConformations Torsional strain:Torsional strain: the force that opposes the
rotation of one part of a molecule about a bond while the other part of the molecule is held fixed• the torsional strain between eclipsed and staggered
ethane is approximately 12.6 kJ (3.0 kcal)/mol
+12.6 kJ/mol
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ConformationsConformations
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ConformationsConformations anti conformation
• a conformation about a single bond in which the groups lie at a dihedral angle of 180°
CH3
H H
H H
CH3
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ConformationsConformations Nonbonded interaction strain:
• the strain that arises when atoms not bonded to each other are forced abnormally close to one another
• butane - gauche conformation; nonbonded interaction strain is approx. 3.8 kJ (0.9 kcal)/mol
CH3
H H
H CH3
H
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ConformationsConformations
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CyclopropaneCyclopropane• angle strain: the C-C-C bond angles are compressed
from 109.5° to 60°• torsional strain: there are 6 sets of eclipsed hydrogen
interactions• strain energy is about 116 kJ (27.7 kcal)/mol
H
H
H
H
H
H
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CyclobutaneCyclobutane• puckering from planar cyclobutane reduces
torsional strain but increases angle strain• the conformation of minimum energy is a puckered
“butterfly” conformation• strain energy is about 110 kJ (26.3 kcal)/mol
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CyclopentaneCyclopentane• puckering from planar cyclopentane reduces torsional
strain, but increases angle stain• the conformation of minimum energy is a puckered
“envelope” conformation• strain energy is about 42 kJ (6.5 kcal)/mol
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CyclohexaneCyclohexane Chair conformation:Chair conformation: the most stable puckered
conformation of a cyclohexane ring• all bond angles are approx. 109.5° • all bonds on adjacent carbons are staggered
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CyclohexaneCyclohexane In a chair conformation, six H are equatorial and
six are axial
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CyclohexaneCyclohexane For cyclohexane, there are two equivalent chair
conformations• all C-H bonds equatorial in one chair are axial in the
alternative chair, and vice versa
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CyclohexaneCyclohexane Boat conformation:Boat conformation: a puckered conformation of a
cyclohexane ring in which carbons 1 and 4 are bent toward each other• less stable than a chair conformation by 27 kJ (6.5
kcal)/mol
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CyclohexaneCyclohexane Twist-boat conformationTwist-boat conformation
• approx. 41.8 kJ (5.5 kcal)/mol less stable than a chair conformation
• approx. 6.3 kJ (1.5 kcal)/mol more stable than a boat conformation
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CyclohexaneCyclohexane
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MethylcyclohexaneMethylcyclohexane Equatorial and axial methyl conformations
CH3
CH3
+7.28 kJ/mol
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G° axial ---> equatorialG° axial ---> equatorial
F
Cl
Br
I
OH
CH3
CH2CH3
CH(CH3)2
C(CH3)3
2.2
2.4
Group− °G
1.9
3.9
7.287.3
9.021.0
1.0
( / )kJ mol
C
CCOOHNH2
CH=CH2
0.85.95.9
7.11.7CH
N
Group ( / )kJ mol− °G
--> axial equatorial
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Cis,trans IsomerismCis,trans Isomerism Cis,trans isomers have:
• the same molecular formula• the same connectivity• a different arrangement of their atoms in space due to
the presence of either a ring or a carbon-carbon double bond (see Chapter 5)
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Cis,trans IsomerismCis,trans Isomerism 1,2-dimethylcyclopentane
trans-1,2-Dimethyl-cyclopentane
cis-1,2-Dimethyl-cyclopentane
CH3
H
CH3
H
H
HH
H
HH
H
H
CH3
CH3
H
HH
HH
H
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Cis,trans IsomerismCis,trans Isomerism 1,4-dimethylcyclohexane
• planar hexagon representations
trans-1,4-Dimethyl-cyclohexane
cis-1,4-Dimethyl-cyclohexane
H
H3C
CH3
H
H
H3C
H
CH3
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Cis,trans IsomerismCis,trans Isomerism trans-1,4-dimethylcyclohexane
• the diequatorial-methyl chair conformation is more stable by approximately 2 x (7.28) = 14.56 kJ/mol
CH3
H
H
CH3
HH3C
CH3
H(more stable)(less stable)
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Cis,trans IsomerismCis,trans Isomerism cis-1,4-dimethylcyclohexane
conformations are of equal stability
H
CH3
H
CH3
H
H3C
H
CH3
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Cis,trans IsomerismCis,trans IsomerismH
H
trans-decalin
H
H
cis-decalin
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Physical PropertiesPhysical Properties Intermolecular forces of attraction
• ion-ion (Na+ and Cl- in NaCl)• ion-dipole (Na+ and Cl- solvated in aqueous solution)• dipole-dipole and hydrogen bonding• dispersion forces (very weak electrostatic attraction
between temporary dipoles)
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Physical PropertiesPhysical Properties Low-molecular-weight alkanes
(methane....butane) are gases at room temperature
Higher molecular-weight alkanes (pentane, decane, gasoline, kerosene) are liquids at room temperature
High-molecular-weight alkanes (paraffin wax) are semisolids or solids at room temperature
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Physical PropertiesPhysical Properties Constitutional isomers have different physical
properties
NameDensity (g/mL)
hexane2-methylpentane
3-methylpentane
2,3-dimethylbutane
2,2-dimethylbutane
68.7
60.3
63.358.0
49.7
-95
-154
-118-129
-98
0.659
0.653
0.6640.661
0.649
bp(°C)
mp(°C)
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Oxidation of AlkanesOxidation of Alkanes Oxidation is the basis for their use as energy
sources for heat and power• heat of combustion:heat of combustion: heat released when one mole of a
substance in its standard state is oxidized to carbon dioxide and water
Methane
++CH4 2O2 CO2 2H2O -890 (-212)
++
Propane
CH3CH2CH3 3CO25O2 4H2O -2220 (-530)
°H ( )/kJ kcal mol
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Heat of CombustionHeat of Combustion Heat of combustion for constitutional isomers
Hydrocarbon
Octane
2-Methylheptane
2,2-Dimethylhexane
2,2,3,3-Tetramethylbutane
H0
[ ( )/ ]kJ kcal mol Structural formula
-5470.6 (-1307.5)
-5465.6 (-1306.3)
-5458.4 (1304.6)
-5451.8 (1303.0)
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Heats of CombustionHeats of Combustion For constitutional isomers [kJ (kcal)/mol]
-5470.6 (-1307.5)-5465.6 (-1306.3)-5458.4 (1304.6)-5451.8 (1303.0)
8CO2 +9H2O
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Heat of CombustionHeat of Combustion• ring strain as determined by heats of combustion
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Sources of AlkanesSources of Alkanes Natural gas
• 90-95% methane
Petroleum• gases (bp below 20°C)• naphthas, including gasoline (bp 20 - 200°C)• kerosene (bp 175 - 275°C)• fuel oil (bp 250 - 400°C)• lubricating oils (bp above 350°C)• asphalt (residue after distillation)
Coal
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GasolineGasoline Octane rating:Octane rating: the percent 2,2,4-trimethylpentane
(isooctane) in a mixture of isooctane and heptane that has equivalent antiknock properties
CH3(CH2)5CH3 CH3CCH2CHCH3
CH3
CH3 CH3
Heptane(octane rating 0)
2,2,4-Trimethylpentane(octane rating 100)
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Synthesis GasSynthesis Gas A mixture of carbon monoxide and hydrogen in
varying proportions which depend on the means by which it is produced
C + H2O heat CO + H2Coal
1+catalyst +
2CH4 2H2COO2
Methane
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Synthesis GasSynthesis Gas Synthesis gas is a feedstock for the industrial
production of methanol and acetic acid
• it is likely that industrial routes to other organic chemicals from coal via methanol will also be developed
CH3OH + COcatalyst
CH3COHO
Methanol Acetic acid
+CO 2H2 CH3OHcatalyst
Methanol
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Prob 2.19Prob 2.19 Which sets represent pairs of constitutional isomers?
(a) (b)CH3CH2OH andCH3OCH3 CH3CCH3 andCH3CH2CHO O
(c) (d)CH3COCH3 andCH3CH2COH CH3CHCH2CH3 andCH3CCH2CH3
O O OH O
(e) (f)andCH3CH2CH2CH2CH3 and CH2=CHCH2CH2CH3
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Prob 2.22Prob 2.22 Write the IUPAC name of each compound.
(a) (b) (c)
(d) (e)
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Prob 2.24Prob 2.24 Explain why each is an incorrect IUPAC name, and write
the correct IUPAC name.
(a) 1,3-dimethylbutane
(b) 4-methylpentane
(c) 2,2-dimethylbutane
(d) 2-ethyl-3-methylpentane
(e) 2-propylpentane
(f) 2,2-diethylheptane
(g) 2,2-dimethylcyclopropane
(h) 1-ethyl-5-methylcyclohexane
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Prob 2.26Prob 2.26 Write the IUPAC name of each compound.
(a) (b)
(c)
CH3CH2CCH3 CH3CH2CH
CH3CH2CH2CH2CH2COH
O O
O(d)
(e) (f)
CH3CHCH3
O OH
OH
(g) (h)CH3CH=CH2
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Prob 2.40Prob 2.40 Complete the table for cis,trans and equatorial,axial
substituted cyclohexanes.
Position ofSubstitution cis trans
1,4-1,3-
1,2-
a,e or e,a e,e or a,a____ or ____ ____ or ____
____ or ____ ____ or ____
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Prob 2.43Prob 2.43 Draw alternative chair conformations for each.
(a)
(d)(c)
(b)HO
H
OH
HOH
OH
H
OH
H
H
HO
CH3H
CH2OH
OHHHO
OH
H
H
H
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Prob 2.44Prob 2.44 Draw the more stable chair conformation of glucose.
OHO
HO
OH
OH
OH
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Prob 2.51Prob 2.51 Given these heats of combustion, which constitutional
isomer is the more stable?
CH3-CH H2C CH2
O
Acetaldehyde-1164 kJ (-278.8 kcal)/mol
Ethylene oxide-1264 kJ (-302.1 kcal)/mol
O
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Alkanes Alkanes
and and CycloalkanesCycloalkanes
End Chapter 2End Chapter 2