đŸ”´đŸ”¸2.6 -Halogenalkanes Flashcards
Define halogenoalkane
An alliance in which one or more hydrogen atoms have been replaced by a halogen
Define a nucleophile
A species with a lone pair of electrons that can be donated to an electron deficient species
Define a reflux
A process of continuous evaporation and condensation
Define hydrolysis
A reaction with water to produce a new product
What is the general formula of halogenoalkanes
CnH(2n+1)X (where X is a halogen)
Why is the carbon to halogen bond in halogenoalkanes polar?
Halogens are more electronegative than carbon (carbon = đŸ”º+)
Halogen = đŸ”º-
Why are halogenoalkanes susceptible to nucleophilic attack?
Where?
What does this lead to?
Susceptible to attack due to dipole
On the đŸ”º+ carbon atom
Leads to substitution
LEARN THE NUCLEOPHILIC SUBSTITUTION MECHANISM
LEARN THE NUCLEOPHILIC SUBSTITUTION MECHANISM
What can nucleotide substitution be used for?
Where is the nucleophile OH- needed for reaction provided by?
Preparation of alcohols from halogenoalkanes
Nucleophile provides by aqueous sodium hydroxide
What precautions do you make to aid slow reaction in the preparation of alcohols
-reflux the mixture of halogenoalkane and aqueous sodium hydroxide, to stop liquid evaporation, as the reaction is slow
Why must the tip of the container by open when liquids are refluxed?
Because if not the air will expand within the apparatus and blow the stopper off violently
Describe what occurs in a reflux reaction
- liquids evaporate when heated
- vapour reaches condenser and condensed back into liquid form
- liquid drips back into reaction flask
Name a benefit of reflux reactions
Allows continuous evaporation and condensing so liquid can be boiled for as long as needed to achieve reaction without any loss of material
What are the two main factors affecting the rate of reaction when changing a halogen in a halogenoalkane
Electronegativity
Bond strength
How does electronegativity affect rate of reaction of halogenoalkane
Electronegativity decreases as the size of the halogen increases
C-Cl is the most polar :: carbon most đŸ”º+
How does bond strength affect rate of reaction of halogenoalkane
Substitution reaction involves breaking the carbon to halogen bond :: strongest for C-Cl bond most difficult to break decreasing rate of reaction.
Name the order of rate of hydrolysis for the halogenoalkanes
Fastest: iodo > bromo > chloro :slowest
How do you measure the rate of reaction of nucleophilic substitution?
After hydrolysis halide ion is produced in solution, can be detected by adding Ag+ (aq), usually as aqueous silver nitrate and timing how long precipitate takes to form
Describe the test for halogens
- Hydrolyse halogenoalkane with aqueous sodium hydroxide
RX + NaOH(aq) —> ROH + Na+(aq) + X-(aq)
2.neutralise excess sodium hydroxide with dilute nitric acid
3.add aqueous silver nitrate
X-(aq) + Ag+(aq) —> AgX(s)
Colour change of halogens with Ag+
Chlorine = white ppt Bromine = cream ppt Iodine = yellow ppt
Results when adding NH3(aq) to precipitate formed with Ag+
Chlorine = dissolved in dilute NH3(aq) Bromine= dissolves in concentrated NH3(aq) Iodine= does not dissolve in NH3(aq)
Define elimination reaction
A reaction that involves the loss of a small molecule to produce a double bond
Why can halides be removed from a halogenoalkane in an elimination reaction?
Because they are acidic :: can be removed using an alkali
Why does the alkali added to a halogenoalkane in an elimination reaction need to be dissolved in ethanol?
To avoid a substitution reaction
LEARN ELIMINATION REACTION MECHANISM
LEARN ELIMINATION REACTION MECHANISM
How many products form is the halogenoalkane is not symmetrical under an elimination reaction?
2
Define a CFC
Halogenoalkane containing both chlorine and fluorine
Define Ozone layer
A layer surrounding the earth that contains O3 molecules
Define HFCs
Halogenoalkanes containing fluorine as the only halogen
Name some uses of halogenoalkanes
- Solvents
- As anaesthetics
- As refrigerants
Why are halogenoalkanes good solvents
Polar sections (carbon halogen bond) and non polar sections (alkyl chain) :: interact with polar and non polar organic substances
Why are halogenoalkanes used in anaesthetics
Many act as general anasthetics such as halothane
Why are halogenoalkanes used as refrigerants such as CFC’s
Small halogenoalkanes are gases at room temperature and :: heat needed to change the liquid to a gas is removed from the fridge to cool its contents
-non flammable
Why are the uses of halogenoalkanes now regulated?
Many are toxic such as polychloroalkanes and others (CFC’s) damage ozone layer, creating holes
Why are holes in ozone layer bad?
Allows UV rays to reach earths surface and cause skin cancer
Describe initiation stage of UV reaction.
UV radiation causes homolytic bond fission of the C-Cl bond in the CFC
Example:
CCl2F2 —> Cl• + CCl•F2
Show propagation stage of UV reaction
Cl• + O3 —> ClO• + O2
Causes ozone 03 to turn to 02
Why is a small value of chlorine radicals very damaging to ozone layer?
Because it’s a chain reaction :: small chlorine = decrease of many ozone molecules
Name an alternative the CFC’s
HFC’s as the only halogen they contain is fluorine who’s radicals are not formed when they are exposed to UV radiation
