Structure And Stereochemistry Of Alkanes

Organic Chemistry, 6th Edition L. G. Wade, Jr.

Chapter 3 Structure and Stereochemistry of Alkanes

Jo Blackburn Richland College, Dallas, TX Dallas County Community College District  2006, Prentice Hall

Classification Review

Chapter 3

2

Alkane Formulas • All C-C single bonds • Saturated with hydrogens • Ratio: CnH2n+2 • Alkane homologs: CH3(CH2)nCH3 • Same ratio for branched alkanes H H H H H H C C C C H

H C H H H

H H H H

H C C C H H H H

=> Chapter 3

3

Common Names • Isobutane, “isomer of butane” • Isopentane, isohexane, etc., methyl branch on next-to-last carbon in chain. • Neopentane, most highly branched • Five possible isomers of hexane, 18 isomers of octane and 75 for decane!

=>

Chapter 3

4

Alkane Examples

=> Chapter 3

5

IUPAC Names • Find the longest continuous carbon chain. • Number the carbons, starting closest to the first branch. • Name the groups attached to the chain, using the carbon number as the locator. • Alphabetize substituents. • Use di-, tri-, etc., for multiples of same substituent. => Chapter 3

6

Longest Chain

• The number of carbons in the longest chain determines the base name: ethane, hexane. (Listed in Table 3.2, page 82.) • If there are two possible chains with the same number of carbons, use the chain with the most substituents. H3C

CH CH2

CH3

CH3 H3C CH2

C CH3 Chapter 3

CH

CH2

CH2

CH3

=> 7

Number the Carbons • Start at the end closest to the first attached group. • If two substituents are equidistant, look for the next closest group. 1

H3C

CH3

3

4

CH CH CH2 2

CH2CH3 Chapter 3

5

CH2

CH3 CH CH3 6

7 => 8

Name Alkyl Groups • CH3-, methyl CH3

• CH3CH2-, ethyl

CH3

• CH3CH2CH2-, n-propyl

isobutyl

• CH3CH2CH2CH2-, n-butyl CH3

CH CH2

CH CH2

CH3 CH3

sec-butyl

H3C

C

CH3 tert-butyl

=> Chapter 3

9

Propyl Groups H H H H C

C C

H H H H

H C

H H H

H

n-propyl A primary carbon

C C H

H

H

isopropyl A secondary carbon => Chapter 3

10

Butyl Groups H H H H H C

C C

C

H H H H

H

H C

H H H H

H

n-butyl A primary carbon

C C

H

C

H

H H

sec-butyl A secondary carbon => Chapter 3

11

Isobutyl Groups H

H

C H H H

C H H H H

C

C

C

H

H

H

H

C H

H H H isobutyl A primary carbon

C H

C H H

tert-butyl A tertiary carbon Chapter 3

12

=>

Alphabetize • Alphabetize substituents by name. • Ignore di-, tri-, etc. for alphabetizing. CH3 H3C

CH3

CH CH CH2

CH2

CH CH3

CH2CH3 3-ethyl-2,6-dimethylheptane => Chapter 3

13

Complex Substituents • If the branch has a branch, number the carbons from the point of attachment. • Name the branch off the branch using a locator number. • Parentheses are used around the complex branch name. 1

2

3 1-methyl-3-(1,2-dimethylpropyl)cyclohexane Chapter 3

14

=>

Physical Properties • Solubility: hydrophobic • Density: less than 1 g/mL • Boiling points increase with increasing carbons (little less for branched chains). • Melting points increase with increasing carbons (less for oddnumber of carbons). Chapter 3

15

Boiling Points of Alkanes Branched alkanes have less surface area contact, so weaker intermolecular forces.

=>

Chapter 3

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Melting Points of Alkanes Branched alkanes pack more efficiently into a crystalline structure, so have higher m.p.

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Chapter 3

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Branched Alkanes • Lower b.p. with increased branching • Higher m.p. with increased branching • Examples: CH3 CH3 CH3

CH CH2 CH2 CH3 bp 60°C mp -154°C

CH3 CH3

CH

CH

CH3 CH3

bp 58°C mp -135°C

Chapter 3

CH3 C CH2 CH3 CH3 bp 50°C mp -98°C 18

=>

Major Uses of Alkanes • C1-C2: gases (natural gas) • C3-C4: liquified petroleum (LPG) • C5-C8: gasoline • C9-C16: diesel, kerosene, jet fuel • C17-up: lubricating oils, heating oil • Origin: petroleum refining => Chapter 3

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Reactions of Alkanes • Combustion 2 CH3CH2CH2CH3

+ 13 O2

heat

8 CO2

+ 10 H2O

• Cracking and hydrocracking (industrial) • Halogenation CH4 + Cl2

heat or light

CH3Cl + CH2Cl2 + CHCl3 + CCl4

=> Chapter 3

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Conformers of Alkanes • Structures resulting from the free rotation of a C-C single bond • May differ in energy. The lowestenergy conformer is most prevalent. • Molecules constantly rotate through all the possible conformations. => Chapter 3

21

Ethane Conformers • Staggered conformer has lowest energy. • Dihedral angle = 60 degrees H H

H

H

model

H

H Newman projection Chapter 3

=> sawhorse 22

Ethane Conformers (2) • Eclipsed conformer has highest energy • Dihedral angle = 0 degrees

=> Chapter 3

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Conformational Analysis • Torsional strain: resistance to rotation. • For ethane, only 12.6 kJ/mol

=> Chapter 3

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Propane Conformers Note slight increase in torsional strain due to the more bulky methyl group.

=> Chapter 3

25

Butane Conformers C2-C3 • Highest energy has methyl groups eclipsed. • Steric hindrance • Dihedral angle = 0 degrees

totally eclipsed Chapter 3

=> 26

Butane Conformers (2) • Lowest energy has methyl groups anti. • Dihedral angle = 180 degrees

anti => Chapter 3

27

Butane Conformers (3) • Methyl groups eclipsed with hydrogens • Higher energy than staggered conformer • Dihedral angle = 120 degrees

=>

eclipsed Chapter 3

28

Butane Conformers (4) • Gauche, staggered conformer • Methyls closer than in anti conformer • Dihedral angle = 60 degrees

gauche

=>

Chapter 3

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Conformational Analysis

=> Chapter 3

30

Higher Alkanes • Anti conformation is lowest in energy. • “Straight chain” actually is zigzag.

H H H H H C C C C C H H H H H H H

Chapter 3

=> 31

Cycloalkanes • Rings of carbon atoms (-CH2- groups) • Formula: CnH2n • Nonpolar, insoluble in water • Compact shape • Melting and boiling points similar to branched alkanes with same number of carbons => Chapter 3

32

Naming Cycloalkanes • • • •

Cycloalkane usually base compound Number carbons in ring if >1 substituent. First in alphabet gets lowest number. May be cycloalkyl attachment to chain. CH2CH3 CH2CH3 CH3

=> Chapter 3

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Cis-Trans Isomerism

• Cis: like groups on same side of ring • Trans: like groups on opposite sides of ring => Chapter 3

34

Cycloalkane Stability • • • •

5- and 6-membered rings most stable Bond angle closest to 109.5° Angle (Baeyer) strain Measured by heats of combustion per -CH2 =>

Chapter 3

35

Heats of Combustion/CH2 Alkane + O2 → CO2 + H2O 697.1 686.1 658.6 kJ

Long-chain

664.0

663.6 kJ/mol 662.4 658.6

=> Chapter 3

36

Cyclopropane • Large ring strain due to angle compression • Very reactive, weak bonds

=> Chapter 3

37

Cyclopropane (2) Torsional strain because of eclipsed hydrogens

=> Chapter 3

38

Cyclobutane • Angle strain due to compression • Torsional strain partially relieved by ring-puckering

=> Chapter 3

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Utilizando  los  datos  de  la  tabla  dada  a  continuación.    Demuestre  cuantitativamente  que  la  tensión  total  en  ciclobutano  es  aproximadamente  26.4  kcal/mol.    Describa  los  factores  que  contribuyen  a  esta  tensión  en  ciclobutano.    •  o

cicloalcano

∆H comb  (kcal/mol)

Tensión Total  (kcal/mol)

ciclopropano

499.8

27.6

ciclobutano

655.9

26.4

ciclopentano

793.5

6.5

ciclohexano

944.5

0

cicloheptano

1108.3

6.3

ciclooctano

1268.9

9.6

Chapter 3

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Cyclopentane • If planar, angles would be 108°, but all hydrogens would be eclipsed. • Puckered conformer reduces torsional strain.

=> Chapter 3

41

Cyclohexane • Combustion data shows it’s unstrained. • Angles would be 120°, if planar. • The chair conformer has 109.5° bond angles and all hydrogens are staggered. • No angle strain and no torsional strain. => Chapter 3

42

Chair Conformer

=> Chapter 3

43

Boat Conformer

=> Chapter 3

44

Conformational Energy

=> Chapter 3

45

Axial and Equatorial Positions

=> Chapter 3

46

Monosubstituted Cyclohexanes

=> Chapter 3

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1,3-Diaxial Interactions

=> Chapter 3

48

Disubstituted Cyclohexanes

=> Chapter 3

49

Cis-Trans Isomers Bonds that are cis, alternate axialequatorial around the ring. CH3 CH3 One axial, one equatorial Chapter 3

=> 50

Bulky Groups • Groups like t-butyl cause a large energy difference between the axial and equatorial conformer. • Most stable conformer puts t-butyl equatorial regardless of other substituents.

=> Chapter 3

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• 3)  Considere el siguiente ciclohexano sustituído: H

C 3

CH(CH

) 3

2

OCH 3

• a)  Dibuje la conformación silla correspondiente.  • (b)  Haga  la  representación  Newman  correspondiente.    (Recuerde especificar la perspectiva que representa) • c)

Dibuje 

ambas 

sillas 

(interconversión 

de 

los 

confórmeros).  Señale la que considere más estable.          

                          

o  de  o • d) Utilizando  los  valores  de  la  tabla  Sust de ∆G∆G   kcal/mol de  sustituyentes,  calcule  el  ∆Go  para  el  proceso  interconversión. CH3 1.74 • e)  Calcule el por ciento de cada confórmero OCH3 0.75

CH(CH3)2 Chapter 3

2.61 52

Bicyclic Alkanes • Fused rings share two adjacent carbons. • Bridged rings share two nonadjacent C’s.

bicyclo[3.1.0]hexane Chapter 3

bicyclo[2.2.1]heptane => 53

Cis- and Trans-Decalin • Fused cyclohexane chair conformers • Bridgehead H’s cis, structure more flexible • Bridgehead H’s trans, no ring flip possible. H

H

=>

H

H

cis-decalin

trans-decalin Chapter 3

54

Bicyclo[4.4.0]decane

=> Chapter 3

55

End of Chapter 3

Chapter 3

56