2.6 - halogenoalkanes Flashcards
What is a halogenalkane?
An alkane in which one or more hydrogen atoms have been replaced by a halogen.
Name the halogens.
Group 7, forms salts
Fluorine, Chlorine, Bromine, Iodine, Astatine
They are the triggers for a nucleophilic subsititution reaction
What is the formation of a halogenalkane?
CnH2n+1X (where x is the halogen)
RX (shortened)
What bond does a halogenalkane contain?
Carbon to halogen bond
Halogens are more electronegative than carbon, so delta positive carbon and delta negative halogen creates permant dipole interaction.
The dipole means halogenalkanes can be nucleophillically attacked on the delta positive carbon, which leads to substitution
What is a nucleophile?
A species with a lone pair of electrons that can be donated to an electron deficient species (shown with a positive charge)
Describe nucleophilic substitution.
The nucleophile OH- (from aqueous NaOH) comes into contact with a halogenalkane, which has a permanent dipole. The negative OH- donates electrons to +carbon, which then breaks the bond of the halogen on the end, becoming a halide ion (negative charge)
How can alcohol be prepared from nucleophilic substitution?
A halogenalkane comes into contact with NaOH, and takes the OH onto the alkyl group, turning it into an alcohol.
What are the conditions needed for nucleophilic substitution?
Reaction is slow so NaOH and halogenalkane must be heated for a significant amount of time
If done in a flask, evaporation would occur and yield of product would be low so it is done under reflux.
What is meant by ‘reflux’?
Process of continuous evaporation and condensation.
Why can nucleophilic subsitution be classed as hydrolysis?
The change can occur by using OH- in water
Using water is much slower, NaOH is faster
What effect does changing the halogen have on haloalkanes?
The rate in which the halogen occurs depends on the nature of the halogen.
What are the 2 factors when considering different halogens?
Electronegativity
Bond strength
How does electronegativity effect the halogens?
Electronegativity decreases as the size of the halogen increases, this means C-Cl is the most polar, with C being the most delta positive
How does bond strength effect the halogens?
The substitution reaction involves breaking the carbon to hydrogen bond. This is strongest for the C-Cl bond and therefore is the most difficult to break.
What can be determined by electronegativity and bond strength of halogenalkanes?
They contradict eachother, for example chlorine being the most electronegative (thus most reactive) whilst it is also the carbon-chlorine bond is the strongest, meaning it is the most difficult to break.
Bond strength is normalised as overuling, so the order of the rate of hydrolysis is-
Iodo (fastest) > bromo >chloro (slowest)
How can nucleophilic substitution of halogenalkanes be detected?
Testing for the halide ion produced by adding Ag+(aq), and timing how long the precipitate takes to form.
What has to happen before testing for nucleophilic substitution?
Excess sodium hydroxide must be neutralised by adding dilute nitric acid, as NaOH would interfere with the test.
What is the equation for nucleophilic substitution?
RX + NaOH (aq) = ROH + Na+ + x- (aq)
What is the equation for adding silver nitrate (ag+) to a halide ion?
x-(aq) + Ag+(aq) = AgX(s)
How does chlorine react with silver nitrate?
White precipitate
How does bromine react with silver nitrate?
Cream precipitate
How does Iodine react with silver nitrate?
Yellow precipitate
How does chlorine react with ammmonia?
Dissolves in dilute NH3 (aq)
How does bromine react with ammonia?
Dissolves in concentrated NH3(aq)
How does iodine react with ammonia?
Does not dissolve with aqueous NH3 (aq)
What are elimination reactions and the triggers for them?
Involves the loss of a small molecule to produce a double bond
Hydrogen halides are acidic and therefore can be removed using an alkali
The alkali (such as NaOH) must be dissolved in ethanol (trigger) to avoid the substitution reaction
What can happen if the halogenalkane is insymmetrical?
More than 1 alkene can be formed, creating isomers.
The place of the double bond changes.
What are the 3 uses of halogenalkanes?
Solvents, anaesthetics and refrigerants
What are CFC’s?
Halogenalkanes containing both chlorine and fluorine
What are HFC’s?
Halogenalkanes containing fluorine as the only halogen
What is the ozone layer?
A layer surrounding the earth that contains O3 molecules.
What can happen if there’s damage to the ozone layer?
UV light can reach the earths surface, causing mutations in DNA that can lead to skin cancer.
What is the inititation stage of using CFC’s destroying the ozone?
In the upper atmosphere, UV radiation causes homolytic bond fission of the c-Cl bond in the CFC.
CCl2F2 = Cl* + CCl*F2
What is the propogation stage of using CFC’s destroying the ozone?
Cl* + O3 = ClO* +O2
ClO* + O3 = Cl* + 2O2
The chain reaction allows the decrease in many ozone molecules.
What are the alternatives to CFC’s?
Alakanes (problem of inflammibility)
HFC’s (radicals are not formed when subjected to UV radiation)
