Unit 2.6 - Halogenoalkanes Flashcards
What are halogenolkanes?
An alkane where 1 or more of the hydrogen atoms are substituted for halogen atoms (F, Cl, Br, I)
What’s the issue with many halogenoalkanes?
They’re toxic
Which halogenoalkanes in particular are toxic?
Solvents
Give an example of a toxic solvent
Dichloromethane
CH2Cl2
Describe the solubility of halogenoalkanes
Fat soluble
What’s the issue with halogenoalkanes in the body?
Accumulate in the body where they can cause damage to the liver and kidneys
What do halogenoalkanes contain?
Hydrocarbon chains with one or more halogen atoms attached in the chain in please of a hydrogen atom
How do we name halogenoalkanes?
Functional groups - F Cl Br I
Systematic name - Fluoro chloro bromo iodo
Place these systematic names in front of the alkane stem
What are the 3 types of halogenoalkanes?
Primary, secondary and tertiary
What makes a primary halogenoalkane a primary halogenoalkane?
The carbon atom that is bonded to the halogen has 1 carbon atom/ 1 R group bonded directly to it
What should you do when handling halogenoalkanes?
Wear gloves and eye protection
What are halogenoalkanes soluble in and insoluble in?
Insoluble in water
Soluble in non-polar solvents
Describe the melting and boiling points of halogenoalkanes
Greater than alkanes having similar molar masses
Why do halogenoalkanes have melting and boiling points that are greater than those of alkanes with similar molar masses?
Increase surface area d to the substitution of a halogen for hydrogen = increased Van der Waal forces = higher melting point
Which reaction is used for the preparation of halogenoalkanes?
Direct chlorination of alkanes
How do alkanes form chloroalkane?
React directly with chlorine
Under which conditions can alkanes react directly with chlorine to form a chloroalkane?
In UV light (sunlight)
What does the direct chlorination of alkanes involve the formation of?
Chlorine free radicals
What happens to which bond during direct chlorination of alkanes?
Homolytic fission of the Cl-Cl bond
What do chlorine radicals do during direct chlorination of alkanes?
Attack the alkane and a chain reaction begins
What’s the mechanism used for the direct chlorination of alkanes?
Free radical substitution
Write the equation for the formation of dichloromethane via free radical substitution
CH4 + Cl2 —> CH3Cl + HCl
CH3 + Cl2 —> CH2Cl2 + HCl
Give 5 uses of organic halogeno compounds
Solvents
CFC’s
Anaesthetics
Pesticides
Polymers
Examples of CFC’s
Refrigerants and aerosols
Example of an anaesthetic
Halothane CF3CHBrCl
Example of a pesticide
DDT
Examples of polymers
PVC, PTFE
What’s the same about ALL halogeno compounds?
All synthetic - not readily found in nature
What type of bond is the carbon-halogen bond and why?
Polar covalent bond
Carbon and halogen atoms ave inferential electronegativities and a dipole forms between the 2 atoms
What’s more electronegative - carbon or bromine - and what does this mean for it?
Bromine - draws electrons towards itself
Which part of a halogenoalkane is able to attract nucleophiles? Why?
The carbon atom bonded to the halogen
The charge distribution in the molecule has changes when bonded to a halogen
What is the carbon atom bonded to the halogen in a halogenoalkane able to attract?
Nucleophiles
Nucleophiles
A species which donates a lone pair of electrons to form a covalent bond
Give some examples of typical nucleophiles
H2O, NH3, OH-. CN-
Which part of a molecule takes part in reactions??
The lone pairs of electrons
What types of reactions do halogenoalkanes undergo?
Nucleophilic substitution
What replaces what in the nucleophilic substitution reactions in which halogenoalkanes undergo?
A nucleophile replaces a halogen
Why is it nucleophilic SUBSTITUTION which halogenoalkanes undergo?
One nucleophile replaces another (like replaces like)
Name a nucleophilic substitution reaction in which halogenoalkanes undergo?
Hydrolysis
Describe the reaction between a halogenoalkane an water on its own
Very low
How is the rate of reaction increased when reacting halogenoalkanes with water?
Refluxing the halogenoalkanes with aqueous solution of sodium hydroxide
What is refluxing a halogenoalkane with an aqueous solution of sodium hydroxide and reacting with water an example of?
Alkaline hydrolysis
What is the nucleophile in the reaction of refluxing a halogenoalkane with an aqueous solution of sodium hydroxide?
Hydroxide ion
Which 2 reactions are important to be able to distinguish between in this unit?
The hydrolysis of a halogenoalkane with……
Aqueous sodium hydroxide
Sodium hydroxide in ethanol
What type of reaction is that between sodium hydroxide in ethanol and a halogenoalakane?
An elimination reaction
What does the elimination reaction between a halogenoalkane ad sodium hydroxide in ethanol result in the formation of?
An alkene
Compare the following factors when reacting a halogenoalkane with water by adding either aqueous sodium hydroxide or sodium hydroxide in ethanol…
Reagent
Conditions
Temperature
Product
Type of reaction
Aqueous sodium hydroxide…
Sodium hydroxide
Dilute, aqueous
Reflux
Alcohol
Nucleophilic substitution
Ethanolic sodium hydroxide…
Sodium hydroxide
Concentrated, Ethanoic
Reflux
Alkene
Why is the reaction between sodium hydroxide and a halogenoalkane an elimination reaction as opposed to nucleophilic substitution when the sodium hydroxide is in ethanol as opposed to being aqueous?
Alcohol is a dehydrating agent, so it releases water during the course of the reaction
What is the reaction between a halogenoalkane and sodium hydroxide an example of?
Nucleophilic substitution
How does nucleophilic substitution occur between a halogenoalkane and sodium hydroxide?
The hydroxy group donates a lone pair of electrons to form a covalent bond with the carbon atom
The C-halogen bond is polar due to the different electronegativities of the carbon and bromine atoms
As the carbon is partially positive, it is able to attract nucleophiles
What would make a bond polar?
Different electronegativities between the atoms
When is carbon able to attract nucleophiles?
When its partially positive when the carbon-halogen bond is polar due to the differences in electronegativities
What is the nucleophilic substitution of alcohols carried out under?
Reflux
Reflux
The continuous evaporation and condensation of reactions
When is reflux used?
When reactions are slow and therefore need to be heated for a significant period of time
Describe the speed of reactions involving organic compounds and explain why this is true
Slow
Breaking strong covalent bonds
What does the process of reflux maximise?
Yield
How does the reflux process maximise yield?
The halogenoalkanes used are volatile and easily evaporate, therefore without the reflux condenser, it would evaporate straight into the air upon heating
What’s is the halogenoalkane involved in the nucleophilic substitution reaction that we need to remember specifically and what does this form with sodium hydroxide?
1-bromobutane
Butan-1-ol
Why should decanting the halogenoalkane into a round bottomed flask during a nucleophilic substitution reaction be performed in a fume cupboard?
It’s toxic
Why do we add anti-bumping granules to the halogenoalkane and sodium hydroxide during a nucleophilic substitution reaction?
2 unmixable liquids will bump, especially under heat
Draw and label reflux condenser apparatus
(See notes)
Why is a heating mantle used ideally for heating a halogenoalkane with sodium hydroxide to reflux?
Alcohol is flammable and could ignite with a naked flame
How does a reflux condenser work?
Consists of 2 glass tubes…
The inner one where vapour rises
The outer one with cold water to cool it down to condense it back
What should we NOT do to reflux condenser apparatus and why?
Seal the top
Water vapour will build up and it will blow up
Draw and label the apparatus for distillation
(See notes)
Why do we use a thermometer with distillation apparatus?
Determine purity by measuring the boiling point
Why would we need a fractioning column as part of distillation apparatus?
If we have small differences in boiling temperatures
What wld we need to include in our distillation apparatus if we have small differences in boiling temperatures?
A fractioning column
Draw and label the mechanism of the reaction between 1-bromobutane and sodium hydroxide to give butan-1-ol
(See notes)
What’s the nucleophile in the nucleophilic substitution reaction between a halogenoalkane and sodium hydroxide?
OH-
What would we need to do to obtain all available mars when showing the mechanism of a reaction such as nucleophilic substitution?
Show…
Partial charges
Lone pairs of electrons
Curley arrows correctly placed
Summarise how a nucleophilic substitution reaction occurs between a halogenoalkane and sodium hydroxide
Refluxed with aqueous sodium hydroxide for 15 minutes
Distill the product off
What does the rate of substation of elimination of halogenoalkanes depend on?
The ease with which the carbon-halogen bond can be broken
What decides the ease with which a carbon-halogen bond can be broken?
The strength of the bond, which in turn depends on the length of the bond
What happens to bond strength between a carbon and halogen down group 7?
Decreases
What happens to the bond strength between a carbon and halogen down group 7? Why?
Decreases
Gets longer and so it gets weaker
What’s the relationship between the length of a carbon-halogen bond and its strength?
The shorter the bond, the stronger it is
Describe the rate of reaction of fluoroalkanes and explain this
C-F bond is very short = it’s very strong and difficult to break
= fluooalkanes react very slowly
Which halogenoalkanes react fastest and why?
Iodoalkanes
They have the weakest bonds that are easier to break as they’re the longest
Put in order the rates of reactions of 4 halogenoalkanes
Iodoalkanes>bormoalkanes>chloroalkanes>fluoroalkanes
(React fastest). (React slowest)
What happens to the rate of reaction of halogenoalkanes as halogen atoms get larger and why?
Larger halogen atoms = carbon-halogen bond becomes longer = weaker and easier to break = the corresponding halogenoalkanes react more quickly
What can be used as solvents?
Chloroalkanes and chlorofluoroalkanes
What can chloroalkanes and chlorofluoroalkanes be used as?
Solvents (aerosols, cleaning sprays…)
What type of chlorofluoroalkanes is widely used in aerosols and fridges?
Chlorofluorocarbons
CFC’s
Chlorofluorocarbons
What are CFC’s?
Halogenoalkanes containing chlorine and fluorine atoms but NOT hydrogen atoms
Give 2 examples of CFC’s
CCl2F2
CClF3
What can CFC’s do and why?
Escape into the atmosphere as they’re small and are gases
Describe the reactivity of CFC’s
Usually unreactive
What happens to CFC’s in the upper atmosphere?
C-Cl bonds can undergo homolytic fission if exposed to ultra-violet light
Under which conditions can C-Cl bonds undergo homolytic fission?
If exposed to UV light
Why can C-Cl bonds undergo homolytic fission if exposed to ultra violet light?
Energy of UV light is greater than the energy of the carbon chlorine bond
Symbol for ozone
O3
O3
Ozone
What is ozone?
A naturally occurring substance found in the upper atmosphere
Why is ozone important? What would happen without it?
Plays an important role in absorbing ultra-violet radiation from the sun and preventing it from getting to the Earth’s surface
If there’s less ozone, more UV will reach the skin of organisms on Earth, burning it, and increasing the risk of cancer
Why do CFC’s find their way?
Into the upper atmosphere
What does UV light do to CFC’s?
Breaks them down into Cl• radicals
What do Cl• radicals come from in the atmosphere?
CFC’s
What do Cl• radicals do in the upper atmosphere?
Act as catalysts in the destruction of the ozone layer
Give the equations for the destruction of the ozone layer by chlorine free radicals
Cl• + O3 —> ClO• + O2
ClO• + O3 —> 2O2 + Cl•
Why is the reaction between Cl• radicals and ozone bad in the upper atmosphere?
It’s a self catalysing chain reaction which can repeat itself indefinitely, therefore even small quantities of chlorine radicals can significantly destroy the ozone layer
Why an even small quantities of chlorine radicals significantly destroy the ozone layer?
It’s a self-catalysing chain reaction which can repeat itself indefinitely
Why is it that a compound such as CCl2FCCl2F forms chlorine radicals and not fluorine or hydrogen radicals?
Can be explained int terms of relative bond strengths
What has the reaction between ozone in the atmosphere and Cl• free radicals resulted in?
A hole in the ozone layer
What did Chemists do to tackle the issue with CFC’s and the ozone layer?
Supported legislation to ban CFC’s completely, and they’ve been replaced in fridges and aerosols by alternative chlorine-free compounds
What’s an example of replacing CFC’s in fridges and aerosols?
Replace chlorine atoms with more fluorine’s
What is the hole in the ozone layer doing now and why?
Slowly mending itself following the legislation to ban CFC’s completely
What are the steps involved in testing for the presence of a halogen in an organic compound?
- Add aqueous sodium hydroxide to a solution of the halogenoalkane and heat the reaction to hydrolyse it
- Neutralise the excess sodium hydroxide with dilute nitric acid to avoid it interfering with the test
- Add aqueous silver nitrate to detect the presence of the halide ion
Why do we add sodium hydroxide and heat the reaction to test for the presence of a halogen in an organic compound?
Hydrolysis of an organic compound containing a halogen produces a halide ion
What is the mechanism for the reaction between aqueous sodium hydroxide to a solution of a halogenoalkane to hydrolyse it?
Nucleophilic substitution
Write an equation for the nucleophilic substitution reaction between a halogenoalkane and sodium hydroxide
RX + NaOH (aq) —> ROH + Na+ (aq) + X- (aq)
Where x = Cl-, Br- or I-
and R = the alkyl group/other organic part of the molecule
Why is it necessary for us to
Why is it important to add the sodium hydroxide when testing for the presence of a halogen in an organic compound?
Without this step, the precipitate would only form at the interface
Why do we neutralise the excess sodium hydroxide with dilute nitric acid when testing for the presence of a halogen in an organic compound?
To avoid it interfering with the next step
What will adding aqueous silver nitrate do when added to a halogenoalkane?
Yield a silver halide precipitate
Give the ionic equation for the reaction between silver nitrate and a halide ion
Ag+ (aq) + X- —> AgX (s)
Where X = Cl-, Br-, I-
Chlorine halogen with silver nitrate
White precipitate
Bromine halogen with silver nitrate
Cream precipitate
Iodine halogen with silver nitrate
Yellow precipitate
Chlorine halogen with NH3 added
Dissolves in DILUTE NH3
Bromine halogen with NH3 added
Dissolves in CONCENTRATED NH3
Iodine halogen with NH3 added
Does not dissolve in NH3
Why do we further test with ammonia when testing for the presence of a halogen in an organic compound?
The colour of the precipitate with silver nitrate can be unclear
What can we do if the precipitate between silver nitrate and a halogen isn’t clear?
Further test with ammonia to distinguish it
If we were timing the time taken for a precipitate to form, what would be the 2 practical methods that we could use to obtain the results?
- Look down through the solution with a cross on the paper underneath the flask - record the time taken for the cross to be obscured
- Colorimeter - record the time taken to reach set absorbance
WHY do carbon-halogen bonds e come easier to break as the halogen atoms become larger?
The halogen atom is further from the carbon atom, so the attraction between the shared pair of electrons and the nuclei is weaker
How does adding nitric acid stop sodium hydroxide from interfering with the silver nitrate test for halide ions?
Removed the hydroxide ions
Why wouldn’t we use hydrochloric acid in the second stage of the identification of a halogen in an organic compound process?
Hydrochloric acid contains gloried ions, which may impact the reaction with silver nitrate
What tend to react with bromine molecules?
Alkenes
If a substance reacted with 2 mol of bromine, how many double bonds does it contain? Why?
2
1 bromine molecule reacts with 1 double bond
Which properties should CFC replacements have?
Non-toxic
Non-flammable
Why should the escape or vapour be prevented when refluxing?
Complete oxidation could not occur
Vapour is flammable
Yield would be reduced
Which will react fastest - a halogenoalkane with bromine or chlorine? Explain
Bromine as the C-Br bond is weaker than the C-Cl bond (bond strength decreases with chain length)
This outweighs the effect of a greater dipole in C-Cl
How is continuous evaporation and condensation achieved with a reflux set up?
Liquid evaporates, vapour goes into condenser, cools and goes back to liquid
Why do anti-bumping granules prevent bumping?
Prevent large bubbles
What are 3 things to ensure when distilling?
Water put in the bottom of the reflux condenser —> allows better cooling
Thermometer should be opposite the opening to the condenser —> measure the temperature of the vapour
Flask receiving condensed product shouldn’t be sealed —> pressure could build up
Shoul the should the flask receiving condensed product be sealed when distilling? Why?
No
Pressure could build up
Where should the thermometer be placed when distilling and why?
Opposite the opening to the condenser
We want to measure the temperature of the vapour
Where should water be put into a reflux condenser when distilling and why?
At the bottom of the reflux condenser
Allows better cooling
How long do we reflux something for?
Long enough to establish equilibrium
What would happen if we didn’t add ethanol on the halogenoalkane reactions?
They wouldn’t mix and the precipitate would only form at the interface
Give an example of a reaction that occurs faster down group 7 and explain why this is the case
Hydrolysis
The carbon-halogen bonds get longer and weaker down the group
What does a more polar carbon-halogen bond make it?
Stronger
Why must an a,denude be immediately distilled?
To prevent further oxidation to form a carboxylic acid
Esters form a layer on the surface of water but acid and alcohol don’t. Why?
Esters —> insoluble in water (can’t form hydrogen bonds with it)
Alcohols + acids —> soluble in water (can form hydrogen bonds with it)
How would we write the reaction for the oxidation of a primary alcohol straight to an acid?
H20 as a product
2[0]
What’s the name for the reaction of adding water to an alkene to produce an alcohol?
Hydration
(NOT hydrolysis)
Which method is used to separate esters from the mixture once they’ve formed and why?
Fractional distillation
Esters have a lower boiling point than the acids and alcohols that they were formed from
(No hydrogen bonding)
How do we heat the aqueous sodium hydroxide for the first stage of testing for a halogen in an organic compound?
5 mins
Water bath
What’s the difference in how the hydroxide ion acts in ethanolic vs aqueous sodium hydroxide?
Aqueous -> as a nucleophile
Ethanolic -> as a base
