Halogenoalkanes Flashcards
What are halogenoalkanes?
Alkanes in which one or more hydrogen atom has been replaced by a halogen atom
What is the bonding in halogenoalkanes?
- covalent bonding
- the C-F, C-Cl and C-Br bonds are polar because the halogen atoms are more electronegative than carbon atoms
- the halogen attracts the bonding pair more strongly than the carbon so the halogen has a small negative charge and the carbon has a small positive charge. The bond is polar and has a permanent dipole
Why are halogenoalkanes far more reactive than alkenes?
Because they can attract other ions and molecules with polar covalent bonds
What are the two types of reaction halogenoalkanes undergo?
- substitution reactions: a halogen atom is exchanged for another atom or group of atoms in the halogenoalkane
- elimination reactions: one hydrogen atom and one halogen atom are removed from neighbouring carbon atoms in the molecule and a double bond forms between the two carbon atoms
What are nucleophiles?
Negative ions (anions) or the negative end of a polar covalent bond that has a lone pair of electrons. It will donate electrons
What are nucleophillic substitution reactions?
The nuecleophile’s lone pair of electrons forms a bond with the carbon atom as it replaces the atom or group of atoms that was bonded to the carbon
What do halogens undergo nucleophillic substitution with?
Hydroxide ions, ammonia and cyanide ions
How does nucleophillic substitution by hydroxide ions work?
• the oxygen in a hydroxide ion has three lone pairs of electrons. These are negatively charged areas of high electron density and are attracted to the small positive charge on the carbon on the polar carbon-halogen bond
• the overall reaction between 1-bromopropane and sodium hydroxide solution:
CH3CH2Br + OH- -> CH3CH2CH2OH + Br- ( 1-bromopropane -> propan-1-ol)
• the OH- nucleophile donates one of its lone pairs of electrons to form a covalent bond with the carbon atom. The C-Br bond breaks heterolytically and a bromide ion forms
Draw the mechanism for the nucleophillic substitution of halogenoalkanes with hydroxide ions
Check snap camera roll
What is used to provide the hydroxide ions in the nucleophillic substitution of halogenoalkanes?
Dilute sodium or potassium hydroxide
What are the conditions for the nucleophillic substitution of halogenoalkanes?
The reaction takes place when the reactants are refluxed in a 50/50 mixture of alcohol and water (with one of those straight condensors - snap camera roll)
What determines the reactivity between halogenoalkanes and nucleophiles?
The strength of the carbon halogen-bond. As the size of the atom increases, the bond strength decreases. Weaker bonds are broken more easily and the rate of nucleophillic substitution of the halogenoalkanes increases from chloroalkanes to iodoalkanes
How does the nucleophillic substitution by ammonia with halogenoalkanes work?
- ammonia had three bonding pairs of electrons and one lone pair and nitrogen has a small negative charge because of the electronegativity difference between N and H
- the C-Br bond breaks heterolytically and a covalent bond forms between the carbon atom and the nitrogen atom.
- Overall reaction: CH3CH2CH2Br + NH3 -> CH3CH2CH2NH2 + HBr ( 11-bromopropane-> 1-propylamine)
- the functional group NH2 is an amine
Draw the mechanism for the nucleophillic substitution by ammonia
Snap camera roll
How does the nucleophillic substitution by cyanide ions take place?
- the ion is negatively charged and has lone pairs of electrons on the carbon and nitrogen. The lone pair on car on is more important because it gained an extra electron to give cyanide its negative charge.
- The negative charge of the lone pair of electrons on the carbon in the cyanide group is attracted to the slightly positive charge of the carbon in the carbon-halogen bond
- the carbon of the CN- donates a pair of electrons to the C-Br bond and the C-Br bond breaks heterolytically
- a nitrile and bromide ion form
- overall equation: CH3CH2CH2Br + CN- -> CH3CH2CH2CN + Br- (1-bromopropane -> 1-butanenitrile)
How does the nucleophillic substitution by cyanide ions take place?
- the ion is negatively charged and has lone pairs of electrons on the carbon and nitrogen. The lone pair on car on is more important because it gained an extra electron to give cyanide its negative charge.
- The negative charge of the lone pair of electrons on the carbon in the cyanide group is attracted to the slightly positive charge of the carbon in the carbon-halogen bond
- the carbon of the CN- donates a pair of electrons to the C-Br bond and the C-Br bond breaks heterolytically
- a nitrile and bromide ion form
- overall equation: CH3CH2CH2Br + CN- -> CH3CH2CH2CN + Br- (1-bromopropane -> 1-butanenitrile)
What is used to provide the cyanide ions in the nucleophillic substitution by cyanide ions?
A solution of sodium cyanide or potassium cyanide
Why is the nucleophillic substitution of halogenoalkanes by cyanide ions particularly useful?
Because it is a way of adding to a carbon chain
What is the mechanism of nucleophillic substitution by cyanide ions?
Snap camera roll
What happens in an elimination reaction of a halogenoalkane?
A halogenoalkane forms an alkene milecule and a hydrogen halide molecule is emitted
What happens in an elimination reaction of halogenoalkanes?
- the hydroxide ions act as both a nucleophile and a base
- a lone pairof electrons on the OH- ion is attracted to the hydrogen bond on the carbon next to the carbon with the halogen bond.
- Strong bases are proton acceptors and the bond breaks heterolytically as a proton (H+) is removed. A double bond forms between the two carbon atoms and a halide ion is produced
What provides the OH- ions in an elimination reaction? And what are the conditions?
A concentrated solution of OH- ions such as potassium hydroxide or sodium hydroxide in ethanol and a high temperature
What is the mechanism for an elimination reaction with a halogenoalkane?
Snap camera roll
What are primary, secondary and tertiary halogenoalkanes?
- primary: has a halogen atom attached to a carvon atom with two hydrogen atoms
- secondary: have the halogenoalkanes attached to a carbon with one hydrogen atom
- tertiary: have no hydrogen atoms attached to the carbon with the halogen atom
What are the conditions that nucleophillic substitution and elimination favour?
• elimination: - high concentration of OH- - ethanol as a solvent - high temperature • substitution: - low concentration of OH- - water as a solvent - low temperature
With primary, secondary and tertiary halogenoalkanes which favour elimination or nucleophillic substitution?
- when primary halogenoalkanes react with a solution of hydroxide ions the products are mainly alcohols, regardless of the reaction conditions. Nucleophillic substitution is strongly favoured
- secondary halogenoalkanes can undergo both substitution and elimination reactions depending on the conditions used. Often a mixture of the appropriate alcohol and alkene is obtained
- tertiary halogenoalkanes produce mainly alkenes by elimination reactions
How is ozone formed?
In two steps • initiation: O2 (UV radiation) -> O• + O• • ozone forms when an oxygen radical collides with an oxygen atom: O• + O2 -> O3
UV radiation has enough energy to break the bonds in ozone. How does this take place?
O3 (UV radiation) -> O2 + o•
When there are no pollutants what happens with ozone being formed and broken down?
The natural rate at which ozone forms in the stratosphere equals the rate at which it is broken down and the concentration of ozone is constant
What is destroying the ozone?
As part of the natural cycle, oxygen radicals break down the ozone molecules but there are other radicals, such as chlorine and bromine radicals that also break down ozone.
How does chlorine cause ozone to break down?
• gaseous organic compounds containing chlorine and bromine atoms are made synthetically in large quantities.
• when they eventually diffuse into the stratosphere, the high-energy UV radiation breaks the C-Cl bond to produce a chlorine radical. The chlorine radicals are very reactive and can react with ozone:
Cl• + O3 -> ClO• + O2
• the chlorine oxide radical is also very reactive and reacts with another ozone molecule:
ClO• + O3 -> 2O2 + Cl•
• the chlorine radical formed now reacts with other ozone molecules and the reaction starts all over again. These are propagation steps
• overall equation: 2O3 -> 3O2
Why do chlorine radicals break down ozone 1500 times faster than oxygen radicals do?
- Because the reaction of chlorine radicals with ozone has a lower activation energy than the reaction between oxygen radicals and ozone
- 1 chlorine radical will react with about 100 000 ozone molecules before colliding with another radical and the reaction terminating
Why does the chlorine radical act as a catalyst in the breakdown of ozone?
Because it is regenerated at the end of each cycle
What are CFCs (chloroflurocarbon?)
A group of gaseous halogenoalkanes in which all of the hydrogen atoms have been replaced with chlorine and fluorine atoms
Why were CFC’s mass produced in the 1960s and 70s?
- they are very useful
- the cabon-fluorine bond has the highest bond enthalpy. And the C-Cl bond is also strong. Because they are both so strong they aren’t very reactive so CFCs are stable and stay around hundreds of years
- they have no smell or taste
- they are usually gases at room temperature
What have CFCs been used in?
- the blow-foam plastics process as they do not react with the plastics or decompose
- fridges, freezers and air-conditioning systems as refrigerants
- aerosol cans as ideal propellant gases
Why were many CFCs released into the atmosphere?
Because they were so unreactive they were considered harmless to living organisms and the environment
Most pollutants released into the atmosphere eventually break down in the troposphere by reacting with oxygen or water. What happens with CFCs?
Their stability gives them time to diffuse into the stratosphere. Here the large amounts of UV radiation break the C-Cl bonds in the CFCs to produce chlorine radicals. It’s an example of photodissociation
Why do the C-F bonds not break in the stratosphere?
Because their bond enthalpy is higher and UV radiation does not carry sufficient energy
What happens when dichlorodifluoromethane interacts with UV radiation?
Snap camera roll
What was the international agreement that called for an immediate reduction in the production and use of CFCs and other compounds that destroyed the ozone?
The montreal protocol
What have been used as replacements for CFCs?
- hydrochlorofluorocarbons (HCFCc)
* hydrofluorocarbons (HFCs)
Why are HCFCs and HFCs more environmentally sound than CFCs?
The presence of hydrogen atoms in them increases their reactivity. The C-H bonds are attacked by OH radicals in the atmosphere. The products dissolve in atomspheric water and end up in rain so don’t usually reach the stratosphere
What happens with the HCFCs that do reach the stratosphere?
The stratospheric conditions extend their lifespan to 120 years and they cause more ozone depletion than CFCs
What is the problem with HCFCs?
They are very potent greenhouse gases
