Chapter 2: Alkanes

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Chapter 2 Alkanes
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I. Classes of Organic Compounds
Hydrocarbons (C H only)
Heteroatomic compounds
alcohols
aliphatic
aromatic
alkanes
ethers
aldehydes
alkenes
ketones
carboxylic acids
alkynes
esters
amines
cyclic compounds
amides
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II. Alkanes
A. General formula CnH2n 2
CH4 methane (natural gas, swamp
gas) CH3CH3 (C2H6) ethane CH3CH2CH3 (C3H8)
propane (LPG) CH3CH2CH2CH3 (C4H10) butane
75-85 CH4 5-10 C2H6 1-5 C3H8
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II. Alkanes
B. Isomers normal and branched alkanes
butane lighters 5 n-butane 95 isobutane
C4H10
n-butane
straight chain
constitutional isomers
Chem3D
isobutane
branched chain
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II. Alkanes
B. Isomers normal and branched alkanes
C5H12
n-pentane
isopentane
neopentane
C6H14 5 isomers C10H22 75 isomers C20H42 366,319
isomers
obviously need system of nomenclature
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II. Alkanes
B. Isomers normal and branched alkanes
Question 2-1. Draw the constitutional isomers of
the following. Click on the arrow to check your
answers. C6H14 (5) C7H16 (9)
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II. Alkanes
B. Isomers normal and branched alkanes
Answer 2-1. Draw the constitutional isomers of
the following. Click on the arrow to check your
answers. C6H14 (5) C7H16 (9)
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II. Alkanes
B. Isomers normal and branched alkanes
Types of carbons
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II. Alkanes
C. Structure-property relationships
Intermolecular attractions London (or van der
Waals) forces (weak)
b.p. CH4 -160ºC C2H6 -89 C3H8 -42 n-C4H10 -0.
4 n-C5H10 36
vs
Branching lowers b.p.
greater surface area for attraction
Chem3D
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III. IUPAC Nomenclature
A. Parent chains normal alkanes
systematic name side groupsparent chainsuffix
Parent names CH4 methane n-C11H24 undecane CH3
CH3 ethane n-C12H26 dodecane CH3CH2CH3 propane
n-C13H28 tridecane CH3(CH2)2CH3 butane
n-C14H30 tetradecane CH3(CH2)3CH3 pentane CH3(
CH2)4CH3 hexane n-C20H42 icosane n-C7H16 heptane
n-C30H62 triacontane n-C8H18 octane
n-C40H82 tetracontane n-C9H20 nonane
n-C10H22 decane etc.
know to here (teens)
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III. IUPAC Nomenclature
B. Substituents alkyl groups
methyl group
ethyl group
propyl
isopropyl
butyl
sec-butyl
tert-butyl
isobutyl
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III. IUPAC Nomenclature
C. IUPAC rules
1. Find the longest straight chain parent
chain 2. Number the parent chain in the
direction that gives the lowest number to the
substituents at the first point of
difference. 3. Two or more identical groups
are indicated by di, tri, tetra, etc.
Longest chain is 7 (not 6). This is
3-methylheptane.
.
2,3,6-trimethylheptane (not 2,5,6-trimethyl heptan
e)
2,2,6,6,7-pentamethyloctane (not 2,3,3,7,7-pentame
thyloctane)
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III. IUPAC Nomenclature
C. IUPAC rules
4. Different groups are listed in alphabetical
order (di, tri, tetra, etc. dont count n, sec,
tert dont count iso does). 5. If the
numbering is the same in both directions, choose
the numbering to follow the alphabetical order.
6-ethyl-5-isopropyl-2,2-dimethyloctane
5-ethyl-6-methyldecane
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III. IUPAC Nomenclature
C. IUPAC rules
6. When two or more chains compete for the
longest, the choice goes to the one with the
greater number of side groups. 7. Complex
side groups (usually gt 4 carbons) are named
systematically a. start numbering at the carbon
attached to the parent chain b. enclose name in
parentheses c. first letter counts
alphabetically, regardless of what it is
2,4,6-trimethyl-5-propyloctane
4-(1,2-dimethylpropyl)-3-(1-methylethyl)- decane
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III. IUPAC Nomenclature
C. IUPAC rules
Question 2-3. Name the following compounds.
Click the arrow to check your answers.
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III. IUPAC Nomenclature
C. IUPAC rules
Answer 2-3. The names are here.
6-ethyl-2,2,5,7-tetramethylnonane
(not 6,7-diethyl-2,2,5-trimethyloctane. Did we
catch you? )
8-sec-butyl-7-tert-butyl-4-isopropyldodecane (hyph
enated words arent alphabetized).
OR 7-(2,2-dimethylethyl)-4-(1-methylethyl)- 8-(2-m
ethylpropyl)dodecane
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III. IUPAC Nomenclature
C. IUPAC rules
8. Cycloalkanes
general formula CnH2n
cyclopropane
ethylcyclobutane
1-isopropyl-3-methylcyclopentane
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III. IUPAC Nomenclature
C. IUPAC rules
Question 2-4. Name the following cycloalkanes.
Check your answers by clicking on the arrow.
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III. IUPAC Nomenclature
C. IUPAC rules
Answer 2-4. Name the following cycloalkanes.
Check your answers by clicking on the arrow.
1,1,2-trimethycyclohexane
4-cyclopentyl-2,3-dimethylhexane
1,2-dicyclopropylethane
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IV. Chemical Properties of Alkanes
A. Combustion
C8H18 ?
Heats (enthalpies) of combustion DHcomb




1307.5 kcal
1306.3 kcal
1304.6 kcal
1303.0 kcal




CO2 H2O
More branched isomers have lower DHcomb, are more
stable.
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IV. Chemical Properties of Alkanes
B. Oxidation and reduction in organic molecules
CH4 Br2 ? CH3Br HBr
oxidation more C-O bonds, fewer C-H
bonds reduction fewer C-O bonds, more C-H bonds
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IV. Chemical Properties of Alkanes
B. Oxidation and reduction in organic molecules
oxidation fewer C-H bonds reduction more C-H
bonds
H2CCH2 H2O2 ? HOCH2CH2OH H2CCH2 H2O ?
CH3CH2OH H2CCH2 H2 ? CH3CH3
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IV. Chemical Properties of Alkanes
B. Assigning oxidation numbers 1. For each
atom in a bond, assign a 1 to the
more electropositive atom and a -1 to the more
electronegative atom. (If the atoms are the
same, each atom gets a 0.) 2. Total each
atom.
Oxidation number of carbon is 1 1 1 -1 2
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IV. Chemical Properties of Alkanes
Question 2-5. Determine oxidation numbers for
the carbons in each of the following molecules.
Click on the arrow to check your answers.
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IV. Chemical Properties of Alkanes
Answer 2-5. Determine oxidation numbers for the
carbons in each of the following molecules.
Click on the arrow to check your answers.
C 2
C1 3 C2 -1
C 0
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IV. Chemical Properties of Alkanes
Question 2-6. Determine whether the following
reactions will be oxidation, reduction, or
neither. Check your answers by clicking on the
arrow.
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IV. Chemical Properties of Alkanes
Answer 2-6. Determine whether the following
reactions will be oxidation, reduction, or
neither. Check your answers by clicking on the
arrow.
Oxidation Each carbon changes from a -2 to a -1.
Neither. One carbon changes from a -2 to a -3
(gets reduced), but the other carbon goes from a
-2 to a -1 (gets oxidized.) No net change.