chapter 3 mcmurry

7/28/2019 Chapter 3 Mcmurry

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Chapter 3: Alkanes and their Stereochemistry

Coverage:

1. Survey of Functional Groups

2. Alkane Isomers3. Alkyl Groups

4. Nomenclature of Alkanes

5. Properties of Alkanes

6. Conformations of Ethane, Propane, Butane

7. Special Topic: Refining of Petroleum

Goals:

1. Be able to recognize and identify the functional group(s) in a molecule.

2. Be able to predict the hybridization of an atom in a functional group.

3. Know the definition of constitutional and stereoisomers and be able to apply definitions

to alkanes.

4. Know the structures of alkyl groups up to 4 carbons.5. Know the IUPAC system of nomenclature for naming alkanes.

6. Know trends in boiling points and melting points of alkanes, both branched and straight

chain.

7. Know the meaning of gauche, anti, eclipsed, staggered in conformations.

8. Be able to draw Newman projections of ethane, propane, butane and others.

9. Know the sources of hydrocarbons, particularly petroleum refining.


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Classification of Organic Molecules

Functional Groups

1. Hydrocarbons - molecules that possess only hydrogen and carbon

a1. Alkanesnoncyclic hydrocarbons with only C-C single bonds.

Formula CnH2n+2 where n is an integer.

a2. Cycloalkanescyclic hydrocarbons with only C-C single bonds.

Formula CnH2n where n is an integer.

Example: Butane CH3CH2CH2CH3 C4H10

Example: Cyclobutane C4H8

b1. Alkenesnoncyclic hydrocarbons with C=C double bonds.

Formula CnH2n where n is an integer.

Example: 2-Butene CH3CH=CHCH3 C4H8


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c. Alkynesnoncyclic hydrocarbons with C=C triple bonds.

Formula CnH2n-2 where n is an integer.

Example: Butyne CH3C=CCH3 C4H6

b2. Cycloalkenescyclic hydrocarbons with C=C double bonds.

Formula CnH2n-2 where n is an integer.

Example: Cyclobutene C4H6

d. Aromatic Hydrocarbonsbenzene and its derivatives

C

CC

C

CC H

H

H

H

H

H

or C6H6


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2. Compounds containing Oxygen

a. Alcohols R-OH

Functional Group OH hydroxyl group

CH3OH methyl alcohol HOCH2CH2OH ethylene glycol (antifreeze)

b. Ethers R-O-R

CH3CH2-O-CH2CH3 Diethylether

c. Aldehydes and Ketones

R-C-H

O

R-C-R'

O

Functional Group C=O carbonyl group

O

O

O

H


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d. Carboxylic Acids

Functional Group -CO2H -COOH -COH Carboxyl Group

R-C-OH

O

O

CH3-C-OH

O

OH2

H3O+ CH

3CO-

O

+ +

Acetic Acid

e. Derivatives of Carboxylic Acids

R-C-OR'

O

R-C-Cl

O

R-C-NH2

O

Ester Acid Chloride Amide


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3. Nitrogen-Containing Molecules

a. Amines R-NH2

Functional Group -NH2 amino group

CH3NH

2

NH

N

Amines are Lewis and Bronsted basesbecause of the nonbonded pair of electrons

NH2

ClH NH3+ Cl+..

b. Amides (see above)

c. NitrilesR C N

Functional Group Cyano GroupC N


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Use of Wedges and Dashed Lines

CH

H

H

HSolid Wedge indicates coming toward you

Dashed Line indicates going awayNarrow Line indicates in the plane of paper


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Single versus Double Bonds

C C

H

H

HH

H

H

Ethane

The carbon-carbon single bond in ethane freely rotates at room temperature.

CC

HH

HH

Ethylene

The carbon-carbon double bond in ethylenedoes not freely rotate at room temperature.

Why not?

CC

HH

HH

C C

H H

H

H H

H

Answer: The Pi bond would have to be broken. It will not break at room temperature

Pi bond broken


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Isomersdifferent compounds with the same formula

a. Constitutional (Structural) Isomersisomers that differ in their bonding sequence

CH3CHCH

2CH

3\

CH3

CH3CH

2CH

2CH

2CH

3vs

C5H12 C5H12

C=C

Cl

H H

Br

C=CH

H Cl

Br

C2H2BrCl C2H2BrCl

b. Stereoisomersisomers that differ in their spatial orientation

C=CCl

H H

Br

C=CCl

H Br

H

C2H2BrCl C2H2BrCl

Cis isomer Trans isomer


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Alkyl Groups

Ris the general symbol for an alkyl group.

For example R-OH represents an alcohol where R is an alkyl group.R-Cl is an alkyl chloride.

Alkyl Groupsknow the following

-CH3

methyl

-CH2CH

3ethyl

-CH2CH

2CH

3propyl

-CH2CH

2CH

2CH

3butyl

-CH2CHCH3

CH3

-CHCH2CH

3

CH3

-C

CH3

CH3

CH3

isobutyl

sec-butyl

tert-butyl


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Carbon Substitution

a. Primary carbon (1o

) - carbon attached to one other carbon.

R-CH3 1o

b. Secondary carbon (2o) - carbon attached to two other carbons.

R-CH2-R

2o

c. Tertiary carbon (3o)carbon attached to three other carbons

3o

R-CH-R

R

d. Quaternary (4o)carbon attached to four other carbons.

C

R

R

R

R4o


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Alkane Nomenclature

1. Find the parent hydrocarbon

a. Find the longest continuous chain of carbon atoms and name it accordingly.

Number Carbons Name

1 methane

2 ethane

3 propane

4 butane

5 pentane

6 hexane

7 heptane

8 octane

9 nonane

10 decane

Named as a hexane


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b. If two different chains of equal length are present, choose the one with the

larger number of branch points.

Named as hexane with 2 substituents not hexane with 1 substituent

2. Number the atoms in the main chain

a. Begin at the end nearer the first branch point.

1

2

3

4

5

6

b. If there are branch points at equal distances away, begin at the end nearer the second

branch point.

7

6

5

4

3

2

1

8

9

Named as a nonane with branch points

at carbons 3, 4, 7.


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3. Identify and number the substituents.

7

6

5

4

3

2

1

8

9

3-ethyl

4-methyl

7-methyl

4. Write the name as a single word.

a. Use prefixes such asmono, di, tri, tetra, etc. to indicate multiple substituents.

b. List substituents alphabetically, ignoring prefixes.c. Use dashes to separate substitutents, use commas to separate numbers.

The above example would have the name

3-ethyl-4,7-dimethylnonane

Name this alkane: Answer: 5-sec-butyldecane

1

2

3

4

5

6

7

8

9

10


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Physical Properties of Alkanes

1. Combustion - Alkanes are inflammable, i.e. they burn.

CH4 O2 CO2 OH2+ +2 2

2. Boiling and melting points

a. Both bp and mp increase with increasing carbon number for straight-chainalkanes with formula CnH2n+2

Carbon Number Physical State

C1- C4 gases

C5C16 liquids

C17C30 oils and greases

C30C50 paraffin waxes>C50 plastics (polyethylene)

b. Branching tends to raise the melting point and lower the boiling point.


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b. Branching tends to lower the boiling point and raise the melting point

BP 60oC 58oC 50oC

MP -154oC -135oC -98oC

Explanation:

MP Branching reduces the flexibility of the molecule which reduces the entropy term

S in the equation Tmp = H/S. Since S is in the denominator, Tmp increases.

BP Branching reduces surface area (more compact structure), and therefore London

dispersion forces which control boiling point for these molecules.

3. Solubilityalkanes are nonpolar molecules and therefore insoluble in water, which

is polar. Alkanes are hydrophobic.

4. Densitiesalkanes are less dense than water, with densities near 0.7 g/mL. Therefore

they float on water, e.g. Exxon Valdez oil spill.


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1. Conformation of Ethane

CH3CH

3

C C

H

H

H

H

H

H

Rotate 60o

HH

HH

H

H HH

H

H

H

H

Staggered Conformation Eclipsed Conformation

Lower Energy Higher Energy

More Stable Less Stable

The staggered conformation is more stable by 3.0 kcal/mol

Sawhorse

Newman

projection

C1

C2

H

H

HH

H

H


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Eclipsed Staggered Eclipsed Staggered Eclipsed Staggered Eclipsed

0.0 kcal/mol

3.0 kcal/mol

Dihedral Anglethe angle defined by the C-H bond on the front carbon

and the C-H bond on the back carbon in a Newman projection.

HH

H

H

H

H

600

HH

H

H

H

H

00


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Why is the staggered conformation more stable than the eclipsed conformation?

In the eclipsed conformation, the C-H bonds are closer, resulting in arepulsion

of the electron clouds.

This effect is referred to torsional strain. Torsional strain exists anytime C-H

bonds are eclipsed.

HH

H

H

H

H

1.0 kcal/mol

1.0 kcal/mol

1.0 kcal/mol

Total Energy: 3.0 kcal/mol


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2. Conformations of Propane

Rotate 60

0

C2

C1

H

H

HH

H

CH3

C1

C2

H

H

H

H

H

CH3

HH

HH

CH3

HRotate 600

More stable Less stable

0.0 kcal/mol 3.3 kcal/mol

1.3 kcal/mol

1.0 kcal/mol1.0 kcal/mole The additional 0.3 kcal/mol energy is due

to steric strain.

Steric strainelectronic repulsion that

occurs when two atoms or groups are

forced together.


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3. Conformations of Butane

C3C2

H

H

CH3 H

H

CH3

C2

C3

CH3

H

H

HH

CH3

Anti Conformation

Staggered

180o dihedral

Gauche Conformation

Staggered

60o dihedral

HH

CH3

H

CH3

H CH3H

H

H

H

CH3

CH3

H

HH

CH3

H HH

CH3

H

H

CH3

HCH3

HH

CH3

H HCH3

H

H

H

CH3

Anti

60o

60o

Eclipsed


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There are two staggered conformations of

different energy.

There are two eclipsed conformations ofdifferent energy.

Remember, staggered conformations are

more stable than eclipsed conformatiions

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Petroleum Refining


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1. The first step in petroleum refining is fractional distillation.


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2. Upstream processing of the distillates.

a. Catalytic hydrocrackingproduces small alkanes from large alkanes by

adding hydrogen.

H2, heat

Si-Al catalyst

b. Catalytic crackingproduces small alkenes and alkanes by cracking in the absence

of hydrogen.

heat

Si-Al catalyst

c. Catalytic Reformingthe alkanes and cycloalkanes are upgraded to higher octane number

by conversion into aromatic compounds.

-3H2-H2

catalyst, heat catalyst, heat


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Octane Number

Straight-chain hydrocarbons have low octane and make poor fuels.

Octane Number = 0

Branched Alkanes burn more slowly and reduce the knocking in the engine.

heptane

2,2,4-trimethylpentane

Octane Number = 100

chapter 3 mcmurry

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