OZ8 - What is the state of the ozone layer now?

List 4 benefits of CFC

• Low boiling points

• low reactivity

• non-toxic

• non-flammable

Explain halogenoalkane breakdown in the ozone

• Halogenoalkane dissociates in UV light to form halogen radicals

• Halogen radicals then catalyse the breakdown of ozone

How was the ozone hole discovered

• Cl radicals were shown to breakdown O3 in a lab

• UV spectroscopy suggested ozone levels were much lower than expected

• Results so low that it was ignored for a number of year

• Results later confirmed and CFC were banned

Today what is used instead of CFC

HFC (hydrofluorocarbons) - has zero ozone depleting potential

• the strong C-F bond cannot be broken down in stratosphere

Definition of a nucleophile

molecule or negatively charged ion with a lone pair of electrons that it can donate to form a covalent bond e.g. OH-, NH3, H20 and -CN

Definition of substitution

reaction in which one atom or group in a molecule is replaced by another atom or group

Halogenoalkanes react with nucleophile in a what reaction

substitution reaction

Homolytic fission

• bonds break evenly - each atom gets one electron

• occurs under UV light

• produces free radicals

Heterolytic fission

• Bonds break unevenly - one atom takes both electrons

• produces a carbocation (positive) and a halide (negative)

What does the term SN2 tell us

• 2 step process with substitution by a nucleophile

  • a carbocation is formed

  • the carbocation is attacked

What are the two factors that effect reactivity

• Bond enthalpy (energy required to break a bond)

• Bond polarity

Bond polarity

Bond enthalpy

What matters more bond enthalpy or bond polarity

Explain why a iodoalkane is more reactive than a fluoroalkane

• C-I bond is the longest

• weaker attraction between the outer electron of one atom and the nucleus of the other