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CHAPTER 4: ALKANES

ALKANES

EXAMPLES

Crude Oil Refinery Process

USA Energy Usage 20071

Fats and Oils:

1 Vollhardt, P., Schore, N., Organic Chemistry, 6th ed., Freeman, 2011, pp. 103

H C

H

H

H

Methane

H C

H

H

C

H

H

H

Ethane

H C

H

H

C

H

H

C

Propane

H

H

H

H C

H

H

C

H

H

C

Butane

H

H

C

H

H

H

O

OO O

O

R

O

GC Spectrum of 87 Octane Gasoline

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PHYSICAL PROPERTIES

CLASSES OF CARBON

Primary carbon

1˚ Secondary carbon

2˚ Tertiary carbon

3˚ Quaternary carbon

4˚ Find examples of 1˚, 2˚, 3˚ and 4˚ carbon atoms on this compound:

COMBUSTION

EQUATION

C C C

C

C C C

C

C C C

C

C

C

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MEASURING THE ENERGY OF ORGANIC MATERIAL

1)   INTERPRETING HEAT OF COMBUSTION DATA

Complete Reaction DH˚ (kcal/mol)2

CH3CH2CH3 + 5 O2 à 3 CO2 + 4 H2O -530.6

CH3CH2CH2CH3 + 13/2 O2 à 4 CO2 + 5 H2O -687.4

•   Does the combustion of butane release more heat than the combustion of an equal quantity of propane?

•   Is butane higher in energy than propane?

2)   COMPARING ISOMERS

Complete Reaction DH˚ (kcal/mol)2

+ 13/2 O2 à 4 CO2 + 5 H2O -687.4

+ 13/2 O2 à 4 CO2 + 5 H2O -685.4

2 Heat of combustion data from: Vollhardt, P., Schore, N., Organic Chemistry, 6th ed., Freeman, 2011, pp. 122

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CYCLOALKANES

HEAT OF COMBUSTION DATA

Complete Reaction DH˚ (kcal/mol)3

CH3CH2CH3 + 5 O2 à 3 CO2 + 4 H2O -530.6

CH3CH2CH2CH3 + 13/2 O2 à 4 CO2 + 5 H2O -687.4

CH3CH2CH2CH2CH3 + 8 O2 à 5 CO2 + 6 H2O -845.2

•   Using more alkane combustion data, every CH2 releases on average -157.4 kcal/mol of energy during combustion.

•   How much energy should cyclopropane release (kcal/mol) when it combusts?

RING STRAIN

SOURCES OF RING STRAIN Ring strain is referred to the increase in energy of the system caused by constraining atoms in a ring. Ring strain is caused by: Effect 1:

3 Heat of combustion data from: Vollhardt, P., Schore, N., Organic Chemistry, 6th ed., Freeman, 2011, pp. 122

CC

H

HHC

H

H H

HH

HC

H

HH

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Effect 2:

RING STRAIN VALUES The following ring strain values4 were calculated using an average value of -157.4 kcal/mol per CH2 combusted.

# Carbons Ring Strain (kcal/mol)

# Carbons Ring Strain (kcal/mol)

# Carbons Ring Strain (kcal/mol)

3 6 9

4 7 10

5 8 11

12

OVERVIEW OF SMALL RINGS

CYCLOBUTANE

1)   STRUCTURE

4 Wade, L.G., Organic Chemistry, 8th ed., Pearson, 2013, pp. 107

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2)   PENICILLIN ANTIBIOTICS

Penicillin targets a bacterial enzyme used to cross link and rigidify bacterial cell membranes.

CYCLOPENTANE

RHN

O N

S

O

HH

Penicillin FamilyO

OHNH

ONHO

αβ

αβ

γ

β-lactam γ-lactam

HN

O N

S

O

HH

Penicillin GO

OH

HN

O N

S

O

HH

AmpicillinO

OH

NH2HN

O N

S

O

HH

Amoxicillin

O

OHHO

NH2HN

O N

S

O

HH

MethicillinO

OH

OCH3

OCH3

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CYCLOHEXANE

LARGER RINGS, 14+ C

Some “musky” odors arise from molecules with rings 14-17 carbons.

Civetone (ring of 17) Muscone (ring of 15)

H

HH

HH

HH

H

O

CH3

O

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CYCLOHEXANE

EXAMPLES

CH3

CH3

Generic steroid H

CH3 H

H

CH3

O

CH3

CH3OH

Testosterone

HO

CH3OH

Estradiol

O

CH3

CH3O

Progesterone

O

HO

CH3OH

C CH

Ethynlestradiol (Birth Control Pills)

O

CH3OH

Nandrolone(muscle growth steroid)

CH3

CH3

O

O OH

OOH

Cortisone

CH3

CH3

HO

O OH

OOH

Cortisol (Hydrocortisone)

HO

CH3

CH3

Cholesterol

O

OH

OH

OH

OH

OH

Glucose

O

OH

OH

OH

OH

O

O

OH

OH

OH

OH

Sucrose (sugar)

Diamond Crystal Structure of Ice

O

O

O

O

O

O

H

H

H

H

HH

H

H

H H

H

H

O

OO

O

OO

H

H

H

H

H

H

HHH H

HH

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AXIAL AND EQUATORIAL POSITIONS

AXIAL AND EQUATORIAL

DRAWING CHAIR SKELETON

AXIAL POSITIONS

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EQUATORIAL POSITIONS

ENERGY OF AXIAL AND EQUATORIAL POSITIONS

Glucose Galactose (a carbohydrate in lactose, milk sugar)

OHOHO

OH

OH

HOO

HO

OH

OH

HOOH

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CHAIR TO CHAIR INTERCONVERSIONS

WAY TO VISUALIZE THE INTERCONVERSION

ACTUAL MECHANISM (FYI)

SUBSTITUENT ORIENTATION AFTER A RING FLIP

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PROBLEMS Perform a “ring flip” and draw the other chair conformation of each compound. Then decide which chair is lower in energy.

“A VALUES” “A values” refer to the difference in energy between the two chair conformations of mono-substituted cyclohexanes when the group is in the axial and equatorial conformations.

Substituent “A Value” (kcal/mol)5

Substituent “A Value” (kcal/mol)

Substituent “A Value” (kcal/mol)

-CH3 1.74 -CH=CH2 1.70 -F 0.25

-CH2CH3 1.75 -OH 0.94 -Cl 0.52

-CH(CH3)2 2.15 -OCH3 0.75 -Br 0.55

-C(CH3)3 4.9 -NH2 1.4 -I 0.46

5 Smith, M.B., March, J., March’s Advanced Organic Chemistry, 5th ed., Wiley, 2001, pp.174

CH3 O

OH

BrBr

CHCH3H3C

CH3

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MORE PROBLEMS Draw the other chair conformation of each compound. Then decide which chair is lower in energy.

AN INTERESTING COMPARISON

In 1,4-cyclohexanediol the diequatorial conformation is lower in energy, while in 1,3-cyclohexanediol the diaxial conformation is lower in energy.

OCH3

Br

I

CH3

CH2CH3

ClBr

OH

OH

OH

HOOH OH HO

OH

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STEREOISOMERS

ISOMER CATEGORIES

CONSTITUTIONAL (STRUCTURAL) ISOMERS

STEREOISOMERS

CIS-TRANS ISOMERS

Are each pair cis-trans isomers?

Br

H

Cl

H

Cl

H

Cl

Cl

H

H

Br Br

H Br

H H

Br H

Br H

Br Br

H Br

H

Cl Cl

H H

H H

Cl

HH

Cl

HH

CH3

Br

CH3

BrCl

Cl

CH3

H

Cl

Cl

H

CH3

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NAMING CIS-TRANS ISOMERS

FROM LINE STRUCTURE

FROM CHAIR CONFORMATIONS

ENERGY OF CIS-TRANS ISOMERS

CYCLOPROPANE, CYCLOBUTANE OR CYCLOPENTANES

CH3

Cl

Cl

Cl Br

Br

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CYCLOHEXANES

POLYCYCLICS

CIS AND TRANS DECALINS

Trans-decalin Cis-decalin

H

H

H

H

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CHOLESTEROL + BILE

Some of biological functions of cholesterol:

•   Precursor for biosynthesis of steroids and bile. •   Maintains integrity of cell membranes: makes membrane less deformable and decreases permeability.

Bile is synthesized in the liver from cholesterol, and is secreted from the gall bladder into the stomach and first segment of the small intestine to help emulsify fats. One of the most abundant bile salts in the human body is taurocholate. It form micelles that encapsulate lipids.

Taurocholate

HO

CH3

CH3

H H

H

cholesterol

H

H

CH3CH3

HHO

cholesterol

CH3

HHO

OH

NH

O

O3S

CH3

H

HH

OH

HNO

S OO O

CH3CH3

H

HH