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Intro to Organic ChemistryPart 08: Alkanes and the Environment

Chapter 10

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Part 08 Alkanes & the Environment

• Fossil fuels– Decayed organic matter from plants and

animals – subjected to high temperatures and pressure

deep within the earth– Carboniferous Period of the Paleozoic Era

(300 – 350 million years ago)– Types: Coal, petroleum, natural gas

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Fossil Fuels

• Natural gas– Colorless, odorless, tasteless

• “Rotten egg” odor due to additive called mercaptan used to detect leaks

– Flammable mixture of primarily methane and ethane

– Usually found in wells near petroleum deposits

– Very efficient, clean burning

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Natural gas composition

• Methane CH4 70-90%• Ethane C2H6

Propane C3H8 0-20%

Butane C4H10

• Carbon dioxide CO2 0-8%• Oxygen O2 0-0.2%• Nitrogen N2 0-5%• Hydrogen sulfide H2S 0-5%• Rare gases Ar, He, Ne, Xe trace

http://www.naturalgas.org/overview/overview.asp

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Fossil Fuels

• Coal– Mixture of carbon & compounds

containing C, H, O and S– Grades of coal (classified based

on C content)• Anthracite• Bituminous• Lignite

Carbon content

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Fossil Fuels

• Coal– High sulfur content coal contributes to acid

rain

C(s) + O2(g) CO2(g)

S(s) + O2(g) SO2(g)

+ H2O(g) acid rain

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Fossil Fuels

• Petroleum– Mixture of saturated and unsaturated

hydrocarbons, both straight chain and cyclical– Mixture is separated into useful materials and

is purified by refining• Physical separation method involved in refining

process is fractional distillation– Based on boiling points of the components of the mixture

(lowest to highest)

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Petroleum

• Refining Petroleum– Gas fractions are called straight run gasolines

(bp 30-200ºC)• Poor fuels

– Branched chain alkanes are better fuels• More volatile• Burn less rapidly in cylinders • Produce less knocking• Produced by process called catalytic cracking

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Petroleum

• Catalytic cracking– Cracks or breaks C11-C24 chains into C3-C5

chains at high temperatures– These smaller molecules are then catalytically

combined to produce branched C7-C10 chains which are better fuels.

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Branched-chain alkanes

• Produce less knocking– “knocking” is the pinging sound heard in an

engine that is not burning gasoline efficiently– Anti-knock quality is measured by octane

number• Rating scale based on isooctane (2,2,4-

trimethylpentane) given a rating of 100 (very good fuel) and n-heptane a rating of 0 (very poor fuel)

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Octane rating

• An octane rating of 87 means the gasoline has the knocking properties of 87% isooctane and 13% n-heptane.

• As octane rating increases– Less knocking– Cost increases

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Air pollution

• From unburned gasoline– Evaporates from gas tanks– Producing hydrocarbon fragments due to

incomplete combustion

• From complete or incomplete combustion of gasoline– Greenhouse gas CO2 or CO

– Produces other gases, NO2, H2S and SO2, that react with water to make acid rain

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Air pollution

• CFCs (chlorofluorocarbons)– Chlorine and fluorine derivatives of CH4 and

C2H6

• Nonflammable, non-toxic, non-corrosive, unreactive, inexpensive to manufacture

– Manufactured by General Motors and DuPont in 1930s – called “Freon”

• Originally made to replace toxic gases that were used as refrigerants in refrigerators

• Used in air conditioners, as propellants in aerosol cans, for cleaning electronic equipment and producing expanded plastics

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CFC naming

• Can be named following IUPAC rules

• Also named by refrigerant naming rules

C F

F

F

Cl C

F

F

1-chloro-1,1,2,2,2-pentafluoroethane

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Refrigerant naming

CFC-115C F

F

F

Cl C

F

FThe “hundreds” place is the number of carbons minus 1. 2-1 = 1The “tens” place is the number of hydrogen atoms plus 1. 0+1 = 1The “units” place is the number of fluorine atoms. 5

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What is the IUPAC & refrigerant name for

1,1,2,2-tetrafluoroethane

C H

F

F

H C

F

F

CFC-134

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CFCs

• CFCs that contain bromine are called halons

• Halons were used as fire extinguishers where using water could cause damage– Computer installations– Museums– Aircraft– Military

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Environmental impact

• Freons and halons destroy ozone layer– Get into atmosphere by cans corroding,

leaking cooling devices and crumbling foams– Don’t decompose by ordinary chemical

reactions– Aren’t washed back to the ground by rain– Remain in stratosphere for decades or longer

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Environmental impact

• Freons and halons destroy ozone layer– Absorb sun’s UV rays and decompose

CCl2F2(g) CClF2(g) + Cl(g)

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Environmental impact

• Ozone layer in stratosphere– 20 miles above Earth’s surface– Very thin layer of triatomic form of oxygen– Pale blue gas with characteristic odor– Altitude, shape & dimensions vary depending

on latitude, seasons and intensity of solar radiation

– Ozone efficiently absorbs UV rays between 200 to 350 nm

– Cyclical decomposition and regeneration of ozone

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Environmental impact

• CFCs decompose forming free radicals which interfere with this cycle– React with ozone to make oxygen molecules

which don’t regenerate ozone as readily as the monatomic oxygen molecules.

– Depletes ozone.– Chlorine monoxide – major form of chlorine

that destroys ozone.• One molecule ClO can destroy 100,000 ozone

molecules

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Environmental impact

• “Hole” in the ozone layer getting larger– Develops in Antarctica each year in Sept and

Oct.– Similar hole over North Polar regions

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Environmental impact

• Mitigate damage to ozone– 1978 - Ban production and use of CFCs and

halons– 1987 – Montreal Protocol (international effort

to cut back on CFCs and Halons)– 1994 – US stopped making Halons– 1996 – US stopped making CFCs– Find substitutes (difluoroethanes – used in

automobile air conditioners).

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Environmental impact

• Mitigate damage to ozone– Still so many CFC and Halon molecules in

the stratosphere– Ozone damage expected for decades– Increase in skin cancers