c341/spring 2010 chapter 2 chapter 2 outline:...

C341/Spring 2010 Chapter 2

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Chapter 2 Outline: Alkanes

1. Structure of Alkanes & Cycloalkanes

2. Nomenclature overview

3. Newman Projections - Conformations of Alkanes in 3-D space

4. Chair Conformations - Conformations of Cycloalkanes in 3-D space

5. Physical Properties of Alkanes & Cycloalkanes (to be covered on your own)

6. Sources & Importance of Alkanes (to be covered on your own)

7. Reactions of Alkanes (to be covered on your own)

Everyone should work at ALL the problems at the end, but below is a list of the minimum problems you should do at the end of the chapter: 2.16-2.18, 2.21-2.23, 2.25-2.31, 2.34, 2.35, 2.39, 2.43, 2.47-2.51, 2.53, 2.54, 2.57, 2.59, 2.60 – 2.62


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1. Structures of Alkanes

• Alkanes are hydrocarbons containing only single C—C bonds, with a

formula of CnH2n+2.

• Alkanes are also called saturated compounds = containing as much hydrogen as possible (i.e. all carbons have 4 bonds to hydrogens).

Identify types of carbons:


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Cycloalkanes Alkanes do not have to just straight chains, but they can also exist as rings and these are called “cyclic hydrocarbons”. Ring sizes range from 3-30, with rings of 5 and 6 carbons being especially common and stable. Compare hexane and cyclohexane. Once you make a ring, what would be its formula? This difference in the formula corresponds to one site of unsaturation since the structure has two fewer hydrogens than if it were open chain. Site of unsaturation (degree of unsaturation) = • • •

Ring strain =


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2. Nomenclature (i.e. naming compounds in a standard format) There is are two types of naming: common & IUPAC

Rules Established by IUPAC = International Union of Pure and Applied Chemistry 1. Determine the number of the longest chain. 2. Number the chain so that substituents get the lowest possible combination of

numbers. 3. List substituents in alphabetical order. Prefixes such as di-, tri-, tetra-, sec-,

and tert- are ignored in alphabetizing; n, sec, tert don’t count; iso does. 4. In the case that there are two hydrocarbon chains with the same number of

carbons, choose the one with the most substituents possible. 5. Numbers are used only for IUPAC names and never for common names. 6. A number and a word are separated by a hyphen; numbers are separated by a

comma. You will need to know the parent names and common names of substituents in the following tables.


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Constitutional Isomers = Draw several constitutional isomers of hexane & name them:


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More practice (work on the nomenclature worksheet posted online too):

a.

b. Br

c.

d.

e.


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f.

g.

h. i. 7,7-dimethyl-4-propylnon-2-ene j. 1,3-pentadiene k. 2-propanol (isopropanol)


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Naming cycloalkanes:

Br


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Bicycloalkanes

bicyclo[m.n.o]alkane where m, n and o are the numbers of the carbons in each bridge, in decreasing order of number. Name:


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Provide structures for the following: Bicyclo[3.2.1]octane Bicyclo[2.2.2]octane Bicyclo[4.3.0]nonane


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3. Newman Projections - conformations of alkanes in 3-D space • Line drawings do not show the three dimensional structure of the compound

as it really exists.

• σ-bonds rotate in most compounds at all moments above 0 Kelvin.

• When the bonds in an alkane rotate, the compounds is said to change its conformation (they are NOT constitutional isomers).

Due to this strain (3 kcal/mol), bond rotation occurs 1011 times/second instead of 1013 times/second if this strain did not exist!


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Newman Projection allows one to write on paper a three dimensional structure without models.

Rotation through an entire 360o bond rotation


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Potential Energy Diagram for Butane:


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Q: Look at the following set of Newman projections and determine which configurations have the highest and lowest stability.

Me Me

EtH H

Et

Me Me

EtEt H

H

Me Me

EtH Et

H

Me Me

Et

Et

H

H

Me Me

Et

H

H

EtMe Me

Et

H

Et

H

A B C

D E F

Which of the following structures above appears the most stable? Which is appears the least stable? What compound is this?? Can you draw the structure from what is given?


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4. Conformations of Cycloalkanes in 3-dimensional space Do you think cycloalkanes move freely like open chain alkanes?

Which ones above would you expect to experience ring strain and why? Stereoisomerism in cycloalkanes These are examples of stereoisomers (same connectivity, but different 3-dimensional orientation of substituents).

Can these compounds be interconverted through a simple bond rotation?


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Let’s focus mostly on cyclohexane: If cyclohexane lies flat, the molecule experiences both angle strain and torsional strain due to eclipsed atoms.

However, if the molecule can twist out of the flat configuration, these strains can be alleviated. This is called a chair conformation.

Now, let’s practice drawing them together:


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Cyclohexane chair flips • Cyclohexane undergoes a “chair flip” at room temperature

• Bonds rotate during the chair flip, but they never rotate a full 360o

• Axial hydrogens become equatorial hydrogens after a chair flip


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Energy Considerations during Chair Flips Which chair conformation is preferred?


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Let’s practice drawing chair flips together:

tBu


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Br

OH


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Applications of six membered rings


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For timing reasons, I reserve the right NOT to cover this next material during lecture. These notes are for your benefit, but you can read this section in the textbook easily on your own. Yes, you are still responsible for this material. 5. Physical Properties of Alkanes & Cycloalkanes • London dispersion forces are the most important IMF for alkanes.

• Hydrocarbon densities are always less than water, so they always float on water

when spilled.

• Molecular weight, IMF and surface area of molecules completely determine physical properties.

• Molecular weight has much to do with function & properties

1 carbon unit → methane gas 3 carbon units → propane gas used for outdoor grills 4 carbon units → butane liquid – used in cigarette lighters 8 carbon units → octane – gasoline 15-17 carbon units → mineral oil 22-30 carbon units → paraffin wax – candle 10,000 carbon units → usable, moldable plastic like a milk jug

Recall molecular structure & surface area: For example, the boiling points of octanes:

125.6 oC 117.7 oC 99.2 oC

??


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6. Sources & Importance of Alkanes

• Fossil fuels are hydrocarbons that are excavated from the ground. Fossil fuels encompass coal, oil & natural gas.

World reserves of fossil fuels (as of 2008; US government estimates):

• Coal 250 year supply • Petroleum 45 year supply • Natural gas 70 year supply

Natural Gas Consists of: methane 90-95% ethane 5-10% Petroleum - thick, viscous liquid

o Provides 90% of the raw starting materials for all organic compounds

o Crude petroleum is a crude mixture of >1000 of organic compounds. The majority of these are hydrocarbons.

o The proportion of compounds in petroleum varies with location.

o Crude petroleum must be refined (separated into its fractions) by fractional distillation.

Coal – Types of Coal Carbon

content BTUs per pound

Comments

anthracite 86-98% 15,500 Small segment of available coal

bituminous 45-86% 10,500-15,500 Most plentiful and most used in US for generating electricity

subbituminous 35-45% 8,300-10,500 Lower sulfur content

lignite 25-35% 4,000-8,300 Geologically young but available


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Fractional Distillation:


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What is gasoline?

• Commercial gasoline is actually a mixture of alkanes of C5 – C12 compounds. • MSDS (Material Safety Data Sheet) for unleaded gasoline shows at least 15

hazardous chemicals occurring in various amounts from 5% to 35% by volume of gasoline.

o benzene (up to 5% by volume), o toluene (up to 35% by volume), o naphthalene (up to 1% by volume), o trimethylbenzene (up to 7% by volume), o MTBE (up to 18% by volume) o and about 10 others.

• Different hydrocarbons burn differently:

o Branched alkanes and cycloalkanes burn more evenly than straight-chain alkanes.

o Short alkanes (C4H10) burn more evenly than long alkanes (C7H16).

o Alkenes burn more evenly than alkanes.

o Aromatic hydrocarbons burn more evenly than cycloalkanes.

What is octane rating? • octane rating – measure of how smoothly a fuel burns in an engine, lower

octane gasoline leads to knocking • Octane is measured relative to a mixture of 2,2,4-trimethylpentane (100) and n-

heptane (0). • E.g. 87-octane gasoline, for example, has the same knock resistance as a mixture

of 87 vol-% isooctane and 13 vol-% n-heptane.


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7. Reactions of Alkanes Oxidation is the basis for the use of alkanes as energy sources for heat and power.

• Heat of combustion = the heat released when one mole of a substance in its standard state is oxidized to carbon dioxide and water.

More branched isomers are lower in energy, i.e. more stable, and give off less energy. Reduction reactions reduce the number of hydrogens on a hydrocarbon.

Halogenation of alkanes will be covered later in chapter 7.

c341/spring 2010 chapter 2 chapter 2 outline:...

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