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Organic Chemistry , 5 th Edition L. G. Wade, Jr. Chapter 3 Structure and Stereochemistry of Alkanes. ALKANE FORMULAS. Four carbon/or hydogen atoms bonded to each carbon atom. All C-C single bonds. NOTE: always an even number of hydrogen atoms in a hydrocarbon. Ratio: C n H 2n+2.

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alkane formulas
ALKANE FORMULAS
  • Four carbon/or hydogen atoms bonded to

each carbon atom

  • All C-C single bonds

NOTE: always an even number of

hydrogen atoms in a hydrocarbon

  • Ratio: CnH2n+2
  • Alkane homologs : each member in a

alkane series different from the next

member by a CH2 group

Chapter 3

slide3

CH3CH2CH2CH2CH2CH3

CH3CH2CH2CH2CH3

C6H14

C5H12

CH2

Chapter 3

lower membered alkanes trivial common names
Lower Membered Alkanestrivial (common) names

Condensed

Line-angle

  • METHANE

CH4

  • ETHANE
  • PROPANE

Chapter 3

butanes
BUTANES

n-BUTANE

iso-BUTANE

Note: 1 H here

Chapter 3

pentanes

=>

Pentanes

Chapter 3

writing isomers heptanes
Writing Isomers -Heptanes

1. Start with longest straight chain possible

2. Shorten chain by one carbon and add the "extra" at all possible positions, starting with left-handed side. 2-Methylhexane

Obviously you do not add the extra carbon to end carbon!!!

3-Methylhexane

2-Methylhexane

Chapter 3

3-Methylhexane

2-Methylhexane

slide8

3. Shorten chain by one more carbon. This gives a 5-carbon chain (pentane). Two carbons are to be added as two methyl groups or a single two-carbon group (ethyl group).

THIS is 3-methylhexane!!

Chapter 3

slide9

Shorten chain by yet one more carbon. This gives a 4-carbon chain

(butane). Three carbons are to be added as three methyl group.

NOTE: Can’t derive another butane chain by adding an ethyl group.

2,2,3-trimethylbutane

3,3-dimethylpentane

Chapter 3

common names
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

iupac names
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

longest chain

=>

Longest Chain
  • The number of carbons in the longest chain determines the base name: ethane, hexane. (Listed in Table 3.2, page 81.)
  • If there are two possible chains with the same number of carbons, use the chain with the most substituents.

Chapter 3

number the carbons

1

3

4

5

2

6

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.

=>

Chapter 3

name alkyl groups

=>

Name Alkyl Groups
  • CH3-, methyl
  • CH3CH2-, ethyl
  • CH3CH2CH2-, n-propyl
  • CH3CH2CH2CH2-, n-butyl

Chapter 3

propyl groups
Propyl Groups

H

H

n-propyl

isopropyl

A secondary carbon

=>

A primary carbon

Chapter 3

butyl groups
Butyl Groups

H

H

n-butyl

sec-butyl

A secondary carbon

=>

A primary carbon

Chapter 3

isobutyl groups
Isobutyl Groups

H

H

isobutyl

tert-butyl

A tertiary carbon

=>

A primary carbon

Chapter 3

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

3-ethyl-2,6-dimethylheptane

=>

Chapter 3

complex substituents

1

2

3

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-methyl-3-(1,2-dimethylpropyl)cyclohexane =>

Chapter 3

physical properties
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 odd- number of carbons).

Chapter 3

boiling points of alkanes
Boiling Points of Alkanes

Branched alkanes have less surface area contact,

so weaker intermolecular forces.

=>

Chapter 3

melting points of alkanes
Melting Points of Alkanes

Branched alkanes pack more efficiently into

a crystalline structure, so have higher m.p.

=>

Chapter 3

branched alkanes

C

H

3

C

H

C

C

H

C

H

3

2

3

C

H

C

H

C

H

3

3

3

C

H

C

H

C

H

C

H

C

H

C

H

C

H

2

2

3

3

C

H

C

H

C

H

3

3

3

bp 50°C

bp 60°C

bp 58°C

mp -98°C

mp -154°C

mp -135°C

=>

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

Chapter 3

major uses of alkanes
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

reactions of alkanes

=>

Reactions of Alkanes
  • Combustion
  • Cracking and hydrocracking (industrial)
  • Halogenation

Chapter 3

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

Chapter 3

ethane conformers

H

H

H

=>

H

H

Newman

projection

sawhorse

H

model

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

Chapter 3

ethane conformers 2

=>

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

Chapter 3

conformational analysis

=>

Conformational Analysis
  • Torsional strain: resistance to rotation.
  • For ethane, only 3.0 kcal/mol

Chapter 3

propane conformers
Propane Conformers

Note slight increase in torsional strain

due to the more bulky methyl group.

=>

Chapter 3

butane conformers c2 c3

totally eclipsed

=>

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

Chapter 3

butane conformers 2

anti

=>

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

Chapter 3

butane conformers 3

=>

eclipsed

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

Chapter 3

butane conformers 4

=>

gauche

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

Chapter 3

higher alkanes

=>

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

Chapter 3

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 =>

Cycloalkanes

Chapter 3

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.

=>

Naming Cycloalkanes

Chapter 3

cis trans isomerism
Cis: like groups on same side of ring

Trans: like groups on opposite sides of ring =>

Cis-Trans Isomerism

Chapter 3

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 - =>

Cycloalkane Stability

Chapter 3

heats of combustion alkane o 2 co 2 h 2 o

166.6

164.0

158.7

158.6

158.3

157.4

157.4

=>

Long-chain

Heats of Combustion Alkane + O2  CO2 + H2O

Chapter 3

cyclopropane

=>

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

Chapter 3

cyclopropane 2
Cyclopropane (2)

Torsional strain because of eclipsed hydrogens

=>

Chapter 3

cyclobutane

=>

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

Chapter 3

cyclopentane

=>

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

Chapter 3

cyclohexane
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

chair conformer
Chair Conformer

=>

Chapter 3

boat conformer
Boat Conformer

=>

Chapter 3

cis trans isomers
Cis-Trans Isomers

Bonds that are cis, alternate axial-equatorial around the ring.

=>

Chapter 3

bulky groups

=>

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

bicyclic alkanes

bicyclo[3.1.0]hexane

bicyclo[2.2.1]heptane

=>

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

Chapter 3

cis and trans decalin

=>

cis-decalin

trans-decalin

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

Chapter 3

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