1 Intro to Organic Chemistry Part 08: Alkanes and the Environment Chapter 10

2 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

3 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

4 Natural gas composition MethaneCH % EthaneC 2 H 6 PropaneC 3 H % ButaneC 4 H 10 Carbon dioxideCO 2 0-8% OxygenO % NitrogenN 2 0-5% Hydrogen sulfideH 2 S0-5% Rare gases Ar, He, Ne, Xetrace

5 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

6 Fossil Fuels Coal –High sulfur content coal contributes to acid rain C(s) + O 2 (g)  CO 2 (g) S(s) + O 2 (g)  SO 2 (g) + H 2 O(g)  acid rain

7 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)

8 Petroleum Refining Petroleum –Gas fractions are called straight run gasolines (bp ºC) Poor fuels –Branched chain alkanes are better fuels More volatile Burn less rapidly in cylinders Produce less knocking Produced by process called catalytic cracking

9 Petroleum Catalytic cracking –Cracks or breaks C 11 -C 24 chains into C 3 -C 5 chains at high temperatures –These smaller molecules are then catalytically combined to produce branched C 7 -C 10 chains which are better fuels.

10 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)

11 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

12 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 CO 2 or CO –Produces other gases, NO 2, H 2 S and SO 2, that react with water to make acid rain

13 Air pollution CFCs (chlorofluorocarbons) –Chlorine and fluorine derivatives of CH 4 and C 2 H 6 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

14 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

15 Refrigerant naming CFC-115 C F F F Cl C F F The “hundreds” place is the number of carbons minus = 1 The “tens” place is the number of hydrogen atoms plus = 1 The “units” place is the number of fluorine atoms. 5

16 What is the IUPAC & refrigerant name for 1,1,2,2-tetrafluoroethane C H F F H C F F CFC-134

17 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

18 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

19 Environmental impact Freons and halons destroy ozone layer –Absorb sun’s UV rays and decompose CCl 2 F 2 (g)  CClF 2 (g) + Cl(g)

20 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

21 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

22 Environmental impact “Hole” in the ozone layer getting larger –Develops in Antarctica each year in Sept and Oct. –Similar hole over North Polar regions

23 Environmental impact Mitigate damage to ozone – 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).

24 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