Chapter 12 Alkanes 12.4 Haloalkanes

Haloalkanes In a haloalkane, one or more H atoms in an alkane is replaced by a halogen atom. Simple haloalkanes are named alkyl halides. In IUPAC names, a halogen is named as fluoro, chloro, bromo, or iodo. CH3Br bromomethane (methyl bromide) Cl | CH3—CH—CH2—CH3 2-chlorobutane (sec-butyl chloride)

Substituents Two or more substituents are named in alphabetical order. Cl Br | | CH3—CH—CH2—CH—CH2—CH3 4-bromo-2-chlorohexane

Naming of Haloalkanes Methane compounds with two or more chlorine atoms are sometimes named by common names that do not reflect their structures.

Chlorofluorocarbons (CFCs) The ozone (O3) layer in the atmosphere absorbs most of the sun’s harmful radiation. Chlorofluorocarbons (CFCs) cause reactions that destroy the ozone layer. CFCs such as Freon-12 (CF2Cl2) have been used in refrigeration, air conditioning, and foam insulation. The use of CFCs in spray cans is no longer allowed.

Regions of Earth’s atmosphere Troposphere (the air we breathe and where our weather takes place) Stratosphere (where the ultraviolet (UV) – protective ozone layer is found) “Good” ozone is found in the stratosphere, where it forms a protective blanket, absorbing UV radiation “Bad” ozone is found in the air we breathe Exposures to concentrations of ozone near 0.12 ppm lower the volume of air a person breathes out The only effective way to limit ozone is to limit nitrogen oxides emissions

Nitrogen Oxides There are 8 oxides of nitrogen, three of which are in the atmosphere: N2O, NO, NO2. N2O is produced by bacteria N2 + O2  2 NO 2 NO + O2  2 NO2 97% of nitrogen oxides are naturally produced 3% result from human activity Normal concentration is a few parts per billion (ppb) Nitrogen oxides are formed during lightning storms and are washed out by rain This is one of the ways nitrogen is made available to plants Most oxides of nitrogen from sources other than nature are produces from fossil fuel combustion

Nitrogen Dioxide (NO2) Corrosive agent – 3 ppm NO2 for 1 hour causes bronchioconstriction; 150-220 ppm is fatal Forms secondary pollutant ozone Reacts with water to form nitric acid 2 NO2 + H2O  HNO3 + HNO2

Chlorofluorocarbons (CFCs) Unreactive, nontoxic, reside for many years in troposphere, mix with air in the stratosphere, which leads to depletion of protective ozone layer Trichlorofluoromethane (CFCl3) & dichlorofluoromethane (CF2Cl2)are broken by UV light; this photodissociation produces a chlorine atom that collides with ozone molecule to produce a chlorine oxide (ClO) free radical and an oxygen molecule CF2Cl2 + energy (UV light)  CF2Cl + Cl Cl· + O3  ClO + O2 ClO + O  O2 + Cl The result of this reaction cycle is that a single Cl atom may react up to 100,000 times before it eventually reacts with water to form HCl, which then mixes into the troposphere and washes out in acidic rainfall.

Hydrochlorofluorocarbons (HCFCs) CFCs (most common automotive refrigerants) have been replaced by HCFCs, which are more reactive in the lower atmosphere, thus have less chances of getting into the stratosphere

Chlorofluorocarbons and Ozone In the stratosphere, the CFCs are cleaved by the high-energy UV radiation from the sun. UV light CF2Cl2 CF2Cl + Cl The Cl reacts with ozone (O3). The ClO produced destroys another O3 forming additional Cl. Thus one Cl can destroy many O3 molecules. Cl + O3 ClO + O2 ClO + O3 Cl + 2O2

Impact of Loss of Ozone According to the National Academy of Sciences, each 1% loss of ozone (O3) increases the amount of UV radiation reaching the earth by 2%. More UV radiation means more skin cancer and cataracts in humans, more intense photochemical smog, and lower crop yields.