1. Chapter 2 Alkanes and Cycloalkanes Section 1 Alkanes
Section 2 Cycloalkanes

2. Definition and Classification of Hydrocarbons
Alicyclic
CH3CH3
Alkanes
Saturated
Unsaturated
Aliphatic ~
Alkenes
Alkynes
CH2=CH2
CH ≡CH
Hydrocarbon
is only composed of carbon and
hydrogen.
Aromatic ~

3. Section 1 Alkanes Structure Nomenclature Properties
Alkane is one kind of hydrocarbon in which all of the carbon are bound to each other and hydrogen by single (s) bonds.
Structure
Nomenclature
Properties

4. Ⅰ. Structure
1. sp3 hybridization
C: 1s2; 2s22p2
sp3
tetrahedral
shape

5. Staut model of methane CH4 takes a tetrahedron shape. Ball-stick model

6. sp3 hybridization and structure of methane
s-sp3 s bond by head to head overlap (see remark)
109.5o
sp3 hybridization and structure of methane
σbond:It is formed by the overlap of two atomic orbitals along the line joining the two nuclei of the atoms involved (bond-axis). It is symmetrically about the axis joining the nuclei of the atoms (bond-axis).
Nature ofσbond: ① symmetrical about the bond-axis;
② firm and not easy to break;
③σ bond can rotate around the bond-axis

7. C－Cσbond formed by head to head
bond : rotate freely ； bigger overlap degree; higher stability, lower polarity.
Ethane (CH3CH3)
C－Cσbond formed by head to head
near
109.5oC(tetrahedral)
110pm
154pm
s bond

8. Bonding of alkanes All carbons are hybridized by sp3
All bonds are σ-bonds
All bond angles are about 109°28’
Butane(C4H10)
Pentane(C5H12)

9. General formula: CnH2n+2 CH4, CH3CH3, CH3CH2CH3, CH3CH2CH2CH3……
Homolog has similar chemical properties with different reacting rate, but their physical properties vary with the number of carbon atoms. The first member of homologous series often has special property.

10. 3. Isomerism of alkanes Isomerism Carbon-chain ~ Constitutional
Functional ~
The atoms are linked together in different way
Isomerism
Positional ~
Cis-trans ~
Configurational ~
Stereo-
isomerism
Enantio- merism
Conformational ~

11. (1) Carbon-chain isomerism:
n-Pentane
iso-Pentane
neo-Pentane

12. Q 1 Write structural formulas or skeletal structures (simplified line structures) for the isomeric hexanes (C6H14).

13. A: (b) same, (c) isomer, and (d) same.
Q 2 Identify as same or isomer the relationship between structure (a) and structure (b), (c), and (d).
A: (b) same, (c) isomer, and (d) same.

14. Classification of C and H
C : be grouped by the number of carbon atoms it attached.
primary carbon，1°
secondary carbon，2°
tertiary carbon ，3°
quaternary carbon ，4° 1° 1° 2° 3° 2° 4° 1° 1° 1°

15. Notice: H: Primary H ( 1º H ): those attach to 1º carbon
Secondary H ( 2º H ): those attach to 2º carbon
Tertiary H ( 3º H ): those attach to 3º carbon H:
Notice:
No “ 4°” H

16. Which of the following compound contains 1°, 2°, 3°and 4°carbon atoms?
A、2，2，3-trimethylbutane
B、2，2，3-trimethylpentane
C、2，3，4-trimethylpentane
D、3，3-dimethylpentane
Answer: B

17. (2) Conformational isomerism
The bonding arrangements of atoms remains constant, but the relationship of the atoms in space differs as a result of rotation around a single bond are called conformations isomerism.
Conformational
isomers of ethane

19. (A) The characteristics of conformations
1. interconverting rapidly and easily at room temperature.
2. an infinite number of conformations of ethane.
3. very little difference in physical properties, but probably great difference of some molecules in biochemistry .

20. (B) Representation of conformational isomers
represented in three ways: perspective formula, sawhorse formula and Newman projections.
perspective formula

21. Sawhorse representations view the C—C bond from a C—C bond oblique angle.
sawhorse formula

22. Newman projection structures view the C-C bond end-on along the axis of connection.
Staggered
Eclipsed
Newman projection

23. (C) Energy analysis of conformations
249
306
227
289
< the sum of V’s radius
repulsion will increase greatly
＞≈ the sum of V’s radius
attract each other weakly
higher energy
lower energy

24. Energy curve of ethane conformations
The low energy conformations is termed as conformer.

25. (D) Conformations of butane
The conformation is more complex.

26. Q: Which one is the most stable, which one is the least?

27. Energy curve of butane conformation
(~70%)
(~30%)
Energy curve of butane conformation

28. At room temperature, butane has many conformational isomers
Notice:
At room temperature, butane has
many conformational isomers
The most stable one is staggered
The most unstable one is eclipsed
Conclusion
The energy of four typical conformations:
Anti-conformation
Gouche conformation
Eclipsed(partial) conformation
Eclipsed conformation
<
<
<

29. in zigzag chain

30. Q: How many conformations of butane are there?
A B C. >4 D. infinite

31. Nomenclature

32. 1. Common nomenclature CH3CH2CH2CH3 CH3CHCH3 CH3 n- iso Butane butane
carbon atoms < 6.
Straight chain : normal alkanes.
contains iso group (CH3)2CH-, : iso-alkane.
CH3CH2CH2CH3 CH3CHCH3
CH3 n-
Butane butane
iso
n-Pentane Isopentane Neopentane

33. A chemical name has 3 parts in the IUPAC system.
2. IUPAC nomenclature
A chemical name
has 3 parts in the IUPAC system.
Prefix
Parent
Suffix
Where; what; how many (are the substituents?)
What family?
How many
Carbons?

34. (1) Unbranched alkanes (1~10): Formula Name Formula Name
CH methane C6H hexane
C2H ethane C7H heptane
C3H propane C8H octane
C4H butane C9H nonane
C5H pentane C10H decane
C11H22 undecane C12H26 dodecane C13H28 tridecane
C14H30 tetradecane C15H32 pentadecane C20H42 eicosane

35. Alkyl group: one H is removed from an alkane.
(2) Alkyl groups: ( R— )
R—H An alkane
Alkyl group: one H is removed from an alkane.
Methyl
(Me)
Ethyl
(Et)
Propyl
(Pr)
Isopropyl
(i-Pr)

36. Butyl (Bu) (i-Bu) sec-Butyl (s-butyl,s-Bu) Isobutyl tert-Butyl
(t-butyl,t-Bu)
Neopentyl
tert-Pentyl, t-Pentyl
(t-amyl)

37. (3) Branched alkanes: Parent: octane
Step 1: Find the parent. Find the longest continuous chain of carbon atoms; this chain determines the base name for the alkane.
Parent: octane

38. When two chains of equal length compete for selection as the base chain, choose the one with more substituents. x 7 6 5
Parent: heptane

39. Step 2: Number each carbon atom in the parent chain from the end of the chain nearest the branching .
Parent: heptane

40. Step 3: Name the alkyl groups, locate their positions by the numbers and write the name as a single word, using hyphens to separate the various prefixes and commas to separate numbers. If two or more different side chains are present, list them in order of alphabet.
On C-5, CH3-
On C-3, CH3CH2-
3-Ethyl -
5-methyl
octane

41. 1 2 3 4 5
2,3,4-Trimethylpentane
If two or more identical side chains are present, use di-, tri-, tetra-, and so forth
Prefixes: di=2 tri= tetra= penta= hexa=6 hepta=7 octa=8 nona=9 deca=10

42. Exercise:
3-ethyl-2,5-dimethyl-4-propylheptane

43. 2,3,5-Trimethylhexane
Not 2,4,5-
3-Ethyl-6-methyloctane

44. Notice: ① Prefixes: n-, sec-, tert- and iso, neo, are common names;
② When alphabetizing the substitutents, some prefixes (di, tri, tetra and n-, sec-, tert- and so on) should be ignored; but some prefixes ( iso, neo, cyclo) shouldn’t be ignored;
③ Be careful not to mix the two nomenclature systems. E.g. 2-methylisobutane is wrong.

45. Exercise:
3-ethyl-4-isopropyl-3-methyl-5-propyloctane

46. Properties 1. Physical properties of alkanes
2. Chemical properties of alkanes

47. 1. Physical properties of alkanes
Simple alkanes are colorless gases, clear liquids, or white solids, depending on their molecular weight.
36.1℃

48. (4) Density and solubility. d≈ 0.8g.mL-1 . Alkanes are hydrophobic.
(1) C1～C4 gas,
C5～C17 liquid,
≥ C18 solid.
(2) molecular weight , bp.
(3) branched , bp.
bp(℃)
mp(℃)
(4) Density and solubility. d≈ 0.8g.mL-1 . Alkanes are hydrophobic.

49. 2. Chemical properties of alkanes
Stability: C－C and C－H bonds are quite strong, so alkanes are inert to many chemical reagents. Unaffected by bases, acids, reductants, or oxidants.
Q4 Why are alkanes so inert?
A reactive site in a molecule usually has one or more unshared pairs of electrons, a polar bond, an electron-deficient atom or an atom with an expandable octet. Alkanes have none of these.

50. Substitution reaction
—— Halogenation of alkanes
Methyl chloride
Substitution reactions

51. CH3-Cl + Cl2 ——> CH2Cl2 + HCl
Dichloromethane
hv or heat
CH2Cl2 + Cl2 ——> CHCl3 + HCl
Trichloromethane
hv or heat
CHCl3 + Cl ——> CCl4 + HCl
Tetrachloride
hv or heat
400~450℃, CH4:Cl2=10:1，main product: CH3Cl.
400℃~ , CH4:Cl2=0.263:1，main product: CCl4.

52. Free radical chain reaction
A. Reaction mechanism
mechanism : How does the reaction take place?
Free radical chain reaction

53. chain-initiating step chain-propagating step
hv or △
chain-propagating step
chain-terminating step

54. Other alkanes ~ methane
The reactivity of halogenation of alkanes:
F2 > Cl2 > Br2 > I2
Methane:
violent
not iodination
Other alkanes ~ methane

55. B. Reactivity of hydrogenQ:
For the above reaction, how many isomeric monochlorination products will produce? The ratio?
45%
55%
Ratio of activity for 1ºH , 2ºH :

56. 3°H > 2°H > 1°H > CH3-H Why？
CH3CH3 hv
+ Cl CH3Cl + CH3CH2Cl or
Ratio:
63%
37%
Ratio of activity
for 1ºH and 3ºH:
The order of reactivity for different hydrogen atoms is:
Activity Order of H:
3°H > 2°H > 1°H > CH3-H
Why？

57. DE. E CH3· RCH2· R2CH· R3C· CH3—H RCH2—H R2CH—H R3C—H ΔH=435kJ/mol

58. Q1: Which one is the most stable?（ ）
A、CH3· B、(CH3)3C·
C、CH3CH2CH2· D、(CH3)2CH·
Q2: What hybridization does the methyl free radical (CH3·) adopt? How many electrons are there in its 2p orbital? 0, 1 or 2?

59. Geometry of Alkyl Radicals

60. C. Selectivity
2°/1°=（97/2）÷（3/6）= 97/1

61. SUMMARY Structure sp3 hybridization，tetrahedron
Isomerism conformation: sawhorse formula & Newman projection
Nomenclature IUPAC name: 3 steps
4. Properties Free radical substitution
5. Mechanism, Reactivity and Selectivity
Reactivity: R3CH > R2CH2 > RCH3 ; F2> Cl2 > Br2 > I2