13 - Halogenoalkanes Flashcards
What is a photochemical reaction?
A reaction using the correct wavelength of light to start a reaction by breaking the bonds, instead of using heat.
State the electromagnetic spectrum in terms of colours?
Infrared (highest wavelength)
Red
Orange
Yellow
Green
Blue
Indigo
Violet
Ultraviolet (highest frequency)
What is a free radical?
A highly reactive species with a single unpaired electron.
What is photochlorination?
A reaction between methane and chlorine.
What are the steps in free radical substitution?
Initiation
Propagation
Termination
Why might free radical substitution not be useful?
You can get multiple different possible products formed, which are hard to predict.
What is homolytic bond fission?
Type of chemical bond breaking in which the shared pair of electrons is split equally between the two atoms, resulting in the formation of two free radicals.
What is free radical substitution an example of?
Chain reaction
Why are chain reactions important?
They release a large amount of energy compared to other chemical reactions.
What is the general formula of halogenoalkanes?
CnH2n+1X
What is the bond polarity of C-X?
C+-H-
Halogens are more electronegative than carbon.
What are the physical properties of halogenoalkanes?
Solubility
IMF
Not polar enough to be soluble in water.
Main intermolecular forces are dipole-dipole and vdw forces.
Mix with hydrocarbons.
What are the boiling point trends in halogenoalkanes?
Chain and halogens
Relative to alkanes
The boiling point increases with chain length. The boiling point increases going down the halogen group as the larger the molecule, the more electrons, the stronger the vdw forces.
They have a higher bp than alkanes due to having a greater mass and polarity.
What determines polarity?
The smaller the atom the stronger the attraction between the pair of electrons and the nucleus, making the bond more reactive by theory as it is more likely to be attacked by nucleophiles.
What determines enthalpy?
The larger the atom the weaker the bond, as the shared electron pair is further away from the nucleus, so the greater the enthalpy.
What is more important in terms of reactivity, bond polarity or enthalpy?
Enthalpy
What is a nucleophile?
A species with a lone pair used to form a covalent bond.
Electron pair donor, attracted to positive matter.
What does nucleophilic substitution with :OH- use?
And conditions.
Aqueous or alcoholic potassium/ sodium hydroxide.
Reflux
What does nucleophilic substitution with :OH- form?
Alcohol
What does nucleophilic substitution with :CN- use?
And conditions.
Aqueous or alcoholic solution of potassium cyanide.
Reflux.
What does nucleophilic substitution with :CN- form?
Nitrile
What is refluxing?
Heating up a solution to its boiling point, while making sure you don’t evaporate the solution.
What does nucleophilic substitution with :NH3 use?
And conditions.
Excess concentrated ammonia.
Under pressure
What does nucleophilic substitution with :NH3 form?
Amine
What is the issue with nucleophilic substitution with :NH3?
The primary amine formed can act as a nucleophile and go on to form secondary and tertiary amines.
What factors affect whether a nucleophilic substitution or elimination reaction occurs?
Reaction conditions (aq or ethanoic)
Type of halogenoalkane (1°, 2°, 3°)
What are the conditions determining a nucleophilic substitution reaction?
Room temperature aqueous OH
Primary halogenoalkane
What are the conditions determining a elimination reaction?
Hot ethanoic OH
Tertiary halogenoalkane
What are CFC’s?
Chlorofluorocarbons
What do CFC’s do?
Absorb heat when they evaporate in a closed system.
What are short chain CFC’s used for?
Aerosol propellants
Refrigerants
Foams
What are long chain CFC’s used for?
Dry cleaning
De-greasing solvents
What are the issues associated with CFC’s?
They are normally unreactive, however they end up in the atmosphere and break down to form free radicals which decompose ozone O3 into O2.
What does the ozone layer do?
Prevents UV from passing through the atmosphere.
UV causes skin cancer, cataracts and harms wildlife.
What are alternatives to CFC’s?
HCFC’s - hydrochlorofluorocarbons
HFC’s - hydrofluorocarbons
