alkanes(??): have only single bond;

1
Hydrocarbons(?) compounds composed entirely of
carbon and hydrogen

• alkanes(??) have only single bond
• saturated (??) hydrocarbon
• have no reactive functional
  groups
• simplest and least reactive.
• alkenes(??) have carbon-carbon double bond
  (CC)
• alkynes(??) have carbon-carbon triple bond
  (CC)
• aromatic hydrocarbons(???) have aromatic rings
  (resembling benzene)

2
Chapter 2

• Alkanes and cycloalkanes
• (??????)
• Reading materials
• Text 1 Chapter 3, 4
• Text 2 ?????

3
Contents

• 2.1 Alkanes
• 1. Molecular formulas of alkanes
• 2. Nomenclature of alkanes
• 3. Physical properties of alkanes
• 4. Uses and sources of alkanes
• 5. Reactions of alkanes
• 6. The free-radical chain reactions
• 7. Structure and conformations of alkanes
• 2.2 Cycloalkanes
• 1. Molecular formulas
• 2. Nomenclature
• 3. Physical and chemical properties
• 4. cis-trans isomerism in cycloalkanes
• 5. Stabilities of cycloalkanes ring strain
• 6. Cyclohexane conformations
• 7. Conformations of substituted cyclohexanes

4
2.1 Alkanes (??)

• 1). Molecular formulas of alkanes
• CH4 HCH2H
• CH3CH3 H(CH2)2H
• CH3CH2CH3 H(CH2)3H
• CH3CH2CH2CH3 H(CH2)4H
  
• General formula (??) for alkanes
  CnH2n2
• CH2, methylene group (???)

5

• Homologous series (???) , CH2 ????
• Homolog(ue), homologous compound (???)
• Isomerism (????) n gt 3
• Isomer (???)
• Constitutional isomerism(structural isomerism,
  ???? See p 54, 2-8)
• Branched alkanes (????), unbranched (straight
  chain, ??) alkanes

6
2). Nomenclature (??) of alkanes (See 3-3)

• Common names (?????)

7

• IUPAC or systematical names (?????)

What is IUPAC?

• The formal system of nomenclature used today is
  one proposed by the International Union of Pure
  and Applied Chemistry (IUPAC, ????????????). This
  system was first developed in 1892.
• Each different compound should have a different
  name and make a systematic set of rules.

8
Nomenclature of the unbranched alkanes
-ane
?? ?? ?? ?? ?? ?? ?? ?? ?? ??
Learn their name by heart !!!
(See p 81, Table 3-2)
9
Nomenclature of branched-Chain alkanes ???????

• IUPAC Nomenclature Rules
• Rule 1. Finding the main chain Find the longest
  continuous chain of carbon atoms, and use the
  name of this chain as the base name or the
  alkane.(??? ? ???, ??????????????)
• Rule 2. Numbering the main chain numbering the
  longest chain, beginning with the end of the
  chain nearest the substituent.(????, ????????)

10
Rule 3. Naming alkyl groups Name the substituent
groups attached to the longest chain as alkyl
groups. Give the location of each alkyl group by
the number of the main-chain carbon atom to which
it is attached. (????????)
Rule 4. Organizing multiple groups (???????????)
When two or more substituents are
present, list them in alphabetical order(?????).
(??!!!??????????????????) When two or
more the same alkyl substituents are present, use
the prefixes di-, tri-, tetra-, penta-,
hexa-,etc. (ignored in alphabetizing) to avoid
having to name the alkyl group twice.
11
Nomenclature of unbranched alkyl groups (??)
-ane to -yl
12
2-Methylhexane 2-????
3-Methylheptane 3-????
3,3-dimethylhexane 3,3- ?????
2,3,4-trimethylhexane 2,3,4-?????
3-Ethyl-3-methylhexane 3-??-3-????
13
2,3,5-trimethylhexane 2,3,5-????? not
2,4,5-trimethylhexane
3-ethyl-2,4,5-trimethylheptane 2,4,5-???-3-????
14
Nomenclature of branched alkyl groups
Degree of carbon atoms
a primary carbon 1carbon ?? ???
a tertairy carbon 3 carbon ?? ???
a quaternary carbon 4 carbon ?? ???
a secondary carbon 2carbon ?? ???
15
(No Transcript)
16
4-?????
4-?????
5-?????
5-?????
17
Which name is correct?
3-tert-butyl-2,3-dimethylpentane
4-isopropyl-2,2,5-trimethylheptane
3-ethyl-2,2,3,4-tetramethylpentane
4-isopropyl-3,6,6-trimethylheptane
18
Nomenclature of complex substituents
a (1-ethyl-2-methylpropyl) group 2-??-1-????
a (1,1,3-trimethylbutyl) group 1,1,3-?????
3-ethyl-5-(1-ethyl-2-methylpropyl)nonane 3-??-5-(2
-??-1-????)??
Chemdraw 3-ethyl-5-(2-methylpentan-3-yl)nonane
19
5-(2-ethylbutyl)-2,4,4-trimethyldecane 2,4,4-???-5
-(2-????)??
20
Problem Write structural formulas and
IUPAC names for the nine isomers of C7H16
Heptane ??
2-methylhexane 2-????
3-methylhexane
2,2-dimethylpentane
2,3-dimethylpentane 2,3-?????
3-ethylpentane
2,4-dimethylpentane
3,3-dimethylpentane
2,2,3-trimethylbutane
21
Summary of the IUPAC nomenclature

• ???,??(???)?,????
• CCS (Chinese chemical society) names
• ???????
• ????? ???? (??????????51?)
• Problems text 1 problem 3-3, 3-4, 3-7
• text 2 p 43 1, 2, 3
• ????????????????.

22
3. Physical properties of alkanes (Read 3-4)

• State (??)
• At ordinary pressure(???),
• C 14 gas
• C 516 liquid
• C gt17 solid
• Solubility (???) insoluble in water, hydrophobic
  (water hating, ???, ???)
• Density (??)0.7 g/mL, less than water.
• Boiling points (??)
• van der Waals attraction (?????),
• London force (dispersion force, ???)
• (1) For unbranched alkanes, the bp increase
  smoothly with increasing numbers of carbon atoms
  and increasing molecular weights.
• (2) with same carbons, unbranched alkane gt
  branched alkane

23
Melting points (??) (1) For unbranched
alkanes, the mp increase in a sawtoooth-shaped
(??? ) graph with increasing numbers of carbon
atoms and increasing molecular weights. Alkanes
with even numbers of carbon atoms have higher mp,
alkanes with odd numbers of carbon atoms have
lower mp. (2) With same carbons, branched
alkane gt unbranched alkane
?????????,??????????????????????????,??????,??????
??
bp / ? 60 58
50 mp / ? -154
-135 -98
24
4. Uses and Sources of alkanes (See 3-5)

• 1) Alkane sources petroleum refining
• Alkanes derive mostly from petroleum
  (often called crude oil) and petroleum
  by-products.
• Distillation (??)
• Catalytic cracking (????), thermal
  cracking (???)
• (Table 3-3 Typical fractions obtained from
  distillation of crude petroleum)

25
2) Major uses of alkanes Mixtures of
alkanes are perfectly suitable for uses as fuels,
solvents, and lubricants (???), the primary uses
of petroleum. Natural gas (???) C13,
about 70 methane, 10 ethane. 15 propane,
depending on the source of the gas.
Liquified petroleum gas (LPG, ?????) C3C4
Gasoline (??) C5C8 Kerosene (??)
C9C16 gtC16, lubricant (???), paraffin
(??) chemically raw materials (????)
26
2,2,4-trimethylpentane (isooctane ???)
Isooctane burns very smoothly without
knocking, but heptane burns to produce much
knocking. A mixture of 87 isooctane and 13
heptane would be rated as 87-octane gasoline
???????????????,???????,?????????????,????????
???,?????????????.

27
5. reactions of alkanes

• Alkanes, as a class, are characterized by a
  general inertness to many chemical reagents.
• Carbon-carbon and carbon-hydrogen bonds are quite
  strong.
• Carbon and hydrogen atoms have nearly the same
  electronegativity, the carbon-hydrogen bonds of
  alkanes are only slightly polarized
• It is of lower reactivity of alkanes toward many
  reagents.
• When heated, alkanes also react with chlorine and
  bromine, and they react explosively with fluorine

28
1) Combustion (??) rapid oxidation taking place
at high temperature to convert alkanes to carbon
dioxide and water.
2) Cracking and hydrocracking (???????)
catalytic cracking catalytic
hydrocracking
29
3) Halogenation (?????) Alkanes can
react with halogens (F2, Cl2, Br2, I2) to form
alkyl halides (???, ???) Reaction
conditions heat or light
Reactivity (????) X2 F2 gt Cl2 gt Br2 gt I2
? C-H
3gt2gt 1 ?
Reaction mechanism (????)
free-radical chain reaction (??????)
The mechanism is the complete, step-by-step
description of exactly which bonds breaks and
which bonds form in what order to give the
observed products.
30
6. The free-radical chain reactions (chaper 4)
?
Initiation step (???)
Initiation steps (???)
Propagation steps (???)
Rate determining step (????)
Termination steps (???)
31
Reaction-energy diagram for chlorination of
methane (??????????)
?
?
32
Transition state (???)???????,?????
Intermediate (???)???????,????????
Activation energy (???)??????????????
Reaction heat (???)???????????????????,??????
33
Which hydrogen is more easily substituted? ???????
???
???????????4??
???????????5?
Reactivity C-H 3gt2gt 1
34
Reactivity C-H 3gt2gt 1
Why?
Reason 1 bond dissociation energy (????)
1C-H 410.3Kj / mol
2C-H 397.7Kj / mol
3C-H 389.4Kj / mol
??????,???,?????
Reason 2 free-radical stability (???????)
????????,?????????????????,??????,????????
?????
gt
gt
gt
Why?
35
? ????? ??????,???,????,???????,???
????
? ??????????? (hyperconjugation)
??????????????????,??????????,??C-H??s??????????p?
?(?p??)??(????),???????,???????s??p?(?p??)????????
??,s??p?????s- p?????,s??p??????s- p ??????
36


CH2CH3 ?3?C-H s???P????
CH3
CH2(CH3)2 ?6?C-H ?s???P????
CH2(CH3)3 ?9?C-H ?s???P????
37
???????????(selectivity)
????????????,??????????
38
7. Structure and conformations of alkanes

• Constitution (??) ?????,??????????
• Conformation (??) The different arrangements
  formed by rotation about a single bond are called
  conformations. (??????,?????????,???????????????)
• Conformer (?????) Pure conformers cannot be
  isolated in most cases.
• For examples
• ethane molecule, have two important
  conformers (???????)

39
Dashes and wedges structure (???)
The staggered conformation of ethane
The eclipsed conformation of ethane
????????
????????
40
Sawhorse structure (?????)
41
Newman Projections (?????)
42
??
???
???
???
43
Conformational analysis of ethane
12.5 kJ/mol the torsional energy (???????)
Energy (kJ/mol)
Rotation/()
44
Conformation of propane
The staggered conformation of propaneane
????????
The eclipsed conformation of propane
????????
45
Conformational analysis of butane
46
the important conformations of butane(???????)
eclipsed ?????(???)
anti ?????(???)
gauche?????(???)
totally eclipsed????(???)
47
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48
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49
Summary of alkane

• 1. General molecular formulas
• 2. Nomenclature of alkanes
• 3. Physical properties of alkanes bp, mp
• 4. Reactions of alkanes halogenation
• 5. The free-radical chain reactions
• 6. Conformations of ethane and butane

Assignments text 2 p 44-45 7, 8, 10, 11, 12, 15
50
2.2 Cycloalkanes
1. Molecular formulas General formula
CnH2n 2. Nomenclature of cycloalkane
Monocyclic compounds (?????)

51
Monosubstituted cycloalkanes (??????)
Methylcyclohexane ?????
Isopropylcyclohexane ??????
polysubstituted cycloalkanes (??????)
1,3-dimethylcyclobutane 1,3-??????
1-ethyl-3-methylcyclohexane 1-??-3-?????
????!!
52
Cycloalkyl- ???
Cyclopropylcyclopentane ??????
1,4-dicyclohexylbutane 1,4-??????
Butylcyclopropane ????? 1-cyclopropylbutane 1-????
?
Pentylcyclobutane ????? 1-cyclobutylpentane 1-????
?
53
Bicyclic compounds (?????)
bridged bicycloalkanes ??

• fused bicycloalkanes
• ??

Spirocycloalkanes ??
54
Bridged ( and fused ) cycloalkanes (??????? )
bicyclo1.1.0butane (??1.1.0??)
bicyclo2.2.1heptane (??2.2.1??)
???,???
55
If substituents are present, we number the
bridged ring system beginning at one bridgehead
first
2-ethyl-1-methylbicyclo1.1.0butane (1-??-2-????
1.1.0??)
1,7-dimethylbicyclo2.2.1heptane (1,7-?????2.2.1
??)
1,5,6-trimethylbicyclo2.1.1hexane (1,5,6-?????2
.1.1??
56
Spiro4.4nonane ?4.4??
Spiro3.4 octane ?3.4??
57
3. Physical and chemical properties

• Physical properties
• Resemble the open-chain (noncyclic,acyclic)
  alkanes.
• nonpolar, bp and mp depending on their
  molecular weights, inert.
• Chemical properties (??104-106?,??)
• free-radical substituted(?????)
• oxidation(??)
• addition (??)??????????????,???????

58
4. Cis-tran isomerism (????)in cycloalkanes
cis-1,2-dimethylcyclopentane ?-1,2-??????
trans-1,2-dimethylcyclopentane ?-1,2-??????
59
5. Stabilities of cycloalkanes Ring strain
(???)
Why are five-membered and six-membered rings more
common than the other sizes?(?????????????)
Experimental facts Heats of combustion
(???)for cycloalkanes
60
(Table 3-5, p 104)Heats of combustion
(kcal/mole) for some simple cycloalkanes
Ring size cycloalkane Molar heat of combustion Heat of combustion per CH2 group Ring strain per CH2 group Total ring strain
3 cyclopropane 499.8 166.6 9.2 27.6
4 cyclobutane 655.9 164.0 6.6 26.4
5 cyclopentane 793.5 158.7 1.3 6.5
6 cyclohexane 944.5 157.4 0.0 0.0
7 cycloheptane 1108.3 158.3 0.9 6.3
8 cyclooctane 1268.9 158.6 1.2 9.6
Reference long-chain alkane Reference long-chain alkane Reference long-chain alkane 157.4 0.0 0.0
61
E
C
B
?Hc E(p) E(r)
A
reactant
Product, CO2 H2O
Stability AgtBgtCheat of combustion
AltBltCAnalyze Table 3-5, p 104Conclusion
stability 6gt5gt4gt3
62
The origin of ring strain in cyclopropane and
cyclobutane Angle strain(???) and torsional
strain(??)
Adolf von Baeyer first attempted to explain the
relative stabilities of cyclic molecules in the
late nineteenth century, and he was awarded a
Nobel Prize for this work in 1905.

• The normal tetrahedral bond angle of an
  sp3-hybridized atom is 109.5o.
• How about the angle of the cyclopropane and
  cyclobutane ?

63
Cyclopropane
Orbital overlap is less effective in cyclopropane
???????????????????????(angle strain)
64
eclipsed hydrogens torsional strain(??)
Angle strain gtgt torsioal strain
65
Cyclobutane also has considerable angle strain
Angle strain gtgt torsioal strain
Slightly folded conformation
66
The Bent or envelope form of cyclopentane
Pentagon ???
Envelope conformation ?????
Important!
67
6. Conformations of cyclohexane

• There are two important conformations for
  cyclhexane
• chair and boat
• The chair form is most stable conformation

1) Chair conformation ????
68
Axial bond???(a?)     6?(3??3?) Equatorial
???(e?)     6?(3??3?)
The chair conformation, stable ????, ??
??????? ????????
69
How to write the chair conformation?( see p 309 )
70
The inversion of chair conformation ????
71
2)The boat conformation, unstable ????, ???
??????? ????????
72
The inversion of conformations
Chair
Chair
Half-chair ???
Twist boat ???
Boat
73
7. Conformations of substituted cyclohexanes
74
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75
1,2-, 1,3-, and 1,4-Dimethylcyclohexane
Cis-1,2-Dimethylcyclohexane
trans-1,2-Dimethylcyclohexane
76
1,3-Dimethylcyclohexane
Cis-1,3-Dimethylcyclohexane
trans-1,3-Dimethylcyclohexane
77
1,4-Dimethylcyclohexane
trans-1,4-Dimethylcyclohexane
cis-1,4-Dimethylcyclohexane
78
Sample problem

• cis-1,2-Dichlorocyclohexane
• cis-1,3-Dichlorocyclohexane
• c. trans-1,2-Dichlorocyclohexane

79
Cis-1-tert-butyl-4-methylcyclohexane
(Stable)
(Less stable)
Remember!! The large group puts in the
equatorial position as soon as possible
80
But
stable
81
1,4-di-tert-butylcyclohexane
Chair, unstable
Twist boat, stable
1,3-di-tert-butylcyclohexane
boat, stable
Chair, unstable
82
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83
Decalin shows cis-trans conformations isomerism
Bicyclo and polycyclic alkanes
trans, stable
84
Adamantane (???) is a tricyclic system that
contains a three dimensional array of cyclohexane
85
Summary of cycloalkanes

• Nomenclature
• Stabilities of cycloalkanes ring strain angle
  strain, torsional strain
• Conformations of cyclohexane and substituted
  cyclohexanes
• chair(??), boat(??)
• a bond (a?,???), e bond(e?,???)

86
Assignments

• Text 1 3-37, 39,44 (p-123)