Halogenalkanes Flashcards
what is a halogenoalkane
an alkane in which one or more of the H atoms is replaced with a halogen
nucleophile
a species with a lone pair of electrons that can be donated to an electron-deficient species
reflux
process of continuous evaporation or condensation
nucleophilic substitution
Halogenoalkanes have a carbon to halogen bond and since halogens are more electronegative than carbon this bond is polar
dipole means halogenoalkanes are susceptible to nucleophilic attack on delta + carbon. leading to substitution
reason reactant is heated under reflux
allows prolonged heating of volatile chemicals without loss.
continuous evaporation and condensation
means liquids can be boiled for as long as needed to achieve a reaction without loss of material.
effect of changing the halogen- electronegativity
The higher the electronegativity, the stronger the bond as electrons move closer to the halogen, the bond becomes harder to break
Elimination reaction
One that involves the loss of a small molecule to produce a double bond
Nucleophilic substitution
Used to prepare alcohols from halogenoalkanes
When the vapour reaches the condenser it condenses and liquid forms. This liquid drips back to the reaction flask
Test for halogenoalkanes
Aqueous sodium hydroxide often used to hydrolyze the halogenoalkane and reaction is heated
Before Aquia Silver nitrate is added, excess sodium hydroxide must be neutralised by adding dilute nitric acid because sodium hydroxide would interfere with the test
If dilute nitric acid wasn’t added
If wasn’t added before Aquia Silver Nitrate, Silver Hydroxide as a brown preticipate would form
test for halogenoalkanes results
Chlorine - white perticipate
Bromine - cream preticipate
Iodine - yellow
Elimination reaction
One that involves a loss of a small molecule to produce a double bond
Unsymmetrical elimination reaction
If the halogenoalkane is on symmetrical, more than one alkene is formed and hydrogen is removed on either side of the halogen
to make an alkene we must heat the halogenyoalkane with sodium hydroxide in pure ethanol
Use of halogenoalkanes- solvent
They contain a polar section due to the presence of the carbon to halogen bond but also contain non polar section due to presence of alkyl chain
Meaning halogenalalkanes can mix with a variety of polar and nonpolar organic substances and use that solvents
Also non-flammable used in dry cleaning clothes and degreasing
Use of halogenoalkanes- anaesthetics
eg. Trichloromethane CHCl3
Use of halogenoalkanes - refrigerants
Presence of permanent dipole - permanent dipole attractions mean boiling temperatures are close to room temperature. they are therefore liquids and can easily be evaporated or gases that can easily be liquefied at room temp.
Eg CFS ( Chlorofluorocarbons) Since the heat needed to change the liquid to gas is removed from the fridge, cooling fridge contents
Non flammability and non toxicity of halogenoalkanes make it suitable
Environmental effects of chlorofluorocarbons
Found to be toxic and can cause damage to the Earth’s ozone layer
Use have caused holes in the ozone layer especially Arctic
Ozone layer prevents harmful u v radiation reaching a surfaces so depletion of ozone layer results in penetration to earth of UV rays which could cause skin cancer
Damage process involves radical substitution reactions
CFCS initiation stage
In the upper atmosphere, UV radiation causes homomoletic bond vision to the C-Cl in the CFC and produces chlorine radicals
Eg. CCl2F2 —-> Cl* + CClF2*
CFCS propagation stage
Since these form a chain reaction, the presence of a small number of chlorine radicals can cause decrease of many ozone molecules
Eg.
Cl* + O3 —-> ClO* + O2
ClO* + O3 —> Cl* + 2O2
Alternatives to chlorofluorocarbons
To avoid formation of chlorine radicals alternative compounds that don’t contain chlorine are being used.
HFCs only contain Flourine- Radicals are not formed when exposed to uv radiation
Insufficient energy to break the stronger C- F bonds but enough energy can break C-Cl
