3. Halogenoalkanes Flashcards
What is a nucleophile?
A species that has a lone pair of electrons that it can donate
What are nucleophiles attracted to?
Electron deficient atoms
What are the common nucleophiles in organic chemistry?
:⁻OH, :NH₃ and :⁻CN
What is nucleophilic substitution?
A chemical reaction where a nucleophile reacts with a polar molecule, removing the functional group and taking its place
In halogenoalkanes, what is the atom that is replaced by the nucleophile during nucleophilic substitution?
The halogen
Why is the halogen atom replaced during nucleophilic substitution?
Halogens are more electronegative than carbon, resulting in a polar bond
What causes halogens to be susceptible to nucleophilic attack?
Halogens are more electronegative than carbon, and so can form polar bonds
What is :Nu⁻ used to represent?
Any nucleophile
What is a primary halogenoalkane?
One in which the halogen atom is bonded to an end carbon, and so has two hydrogens also bonded
What is a secondary halogenoalkane?
One in which the halogen atom is bonded to a carbon which is bonded to one hydrogen atom and two alky groups
What is a tertiary halogenoalkane?
One in which the halogen atom is bonded to a carbon which is bonded to no hydrogen atoms and three alky groups
What does a reaction mechanism illustrate?
The route through a reaction, which may involve intermediates
What are curly arrows used to show?
The direction of movement of an electron pair in organic reactions
What do the curly arrows specifically show during nucleophilic substitution?
- The lone pair of electrons of the nucleophile is attracted towards the slightly positive carbon in the molecule
- The electrons in the carbon-halogen bond moving towards the halogen atom, leading to the formation of a halide atom
What do halogenoalkanes react with OH⁻ to form?
- alcohol (nucleophilic substitution)
* alkene (elimination)
Where do the OH⁻ free radicals in nucleophilic substitution come from?
They will be in solution (e.g. NaOH) - you don’t just have loads of OH⁻ ions by themselves
What do halogenoalkanes react with CN⁻ to form?
A nitrile
When a nitrile is formed during nucleophilic substitution, what happens to the length of the carbon skeleton?
The carbon skeleton increases by one (e.g. eth- to prop- skeleton)
What bond is there in a CN⁻ ion?
Triple (⁻C≡N)
Is NH₃ still a nucleophile, even if it’s not an ion?
Yes; a nucleophile just needs to have a lone pair
What do halogenoalkanes react with NH₃ to form?
An amine
What happens, during nucleophilic substitution with ammonia, after the halogen has been replaced by an NH₃ molecule?
One of the hydrogen is ‘kicked off’ and bonds to another :NH₃ molecule, to form NH₄⁺
What can the amine produced in nucleophilic substitution with ammonia act as?
A nucleophile - meaning that this reaction could continue to produce a mixture of products
What are the products that could be produced if the amine produced in nucleophilic substitution with ammonia acts as a nucleophile?
2° and 3° amines, and 4° ammonium salts
Overall equation for nucleophilic substitution reaction with chloroethane and sodium hydroxide?
CH₃CH₂Cl + NaOH → CH₃CH₂OH + NaCl
Overall equation for nucleophilic substitution reaction with chloroethane and potassium cyanide?
CH₃CH₂Cl + KCN → CH₃CH₂CN + KCl
Overall equation for nucleophilic substitution reaction with bromoethane and ammonia?
CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br
Does bond polarity determine the rate of substitution?
No
What is the rate of substitution dependent on?
Bond enthalpy - the strength of the carbon-halogen bond
What is bond enthalpy?
The average enthalpy change that takes place when breaking 1 mole of a given bond in the molecule of a gaseous species under standard conditions
What happens to the carbon-halogen bond as the halogen atom becomes larger?
It becomes longer, weaker and easier to break - and the corresponding halogenoalkanes react more quickly
What type of halogenoalkanes react the most quickly?
Iodoalkanes
Why are iodoalkanes the halogenoalkanes that react the most quickly?
The C-I is the longest, weakest and easiest to break - as iodine is at the bottom of the group and the largest
What are the reagents of nucleophilic substitution for when an alcohol is produced?
NaOH₍ₐᵩ₎ or KOH₍ₐᵩ₎
What are the conditions of nucleophilic substitution for when an alcohol is produced?
Warm under reflux
What does reflux do?
Helps the reaction to go to completion
General equation for nucleophilic substitution with hydroxide?
R-X + OH⁻ → R-OH + X⁻
What are the reagents of nucleophilic substitution for when a nitrile is produced?
KCN in aqueous ethanol
What are the conditions of nucleophilic substitution for when a nitrile is produced?
Boil under reflux
General equation for nucleophilic substitution with cyanide?
R-X + CN⁻ → R-CN + X⁻
What are the reagents of nucleophilic substitution for when an amine is produced?
Excess ammonia in ethanol
What are the conditions of nucleophilic substitution for when an amine is produced?
Heat in a sealed tube
General equation for nucleophilic substitution with ammonia?
R-X + 2NH₃ → R-NH₂ + NH₄X
What is an elimination reaction?
One in which the organic molecule loses two species from adjacent carbon atoms without replacement, resulting in the formation of a double bond between the two carbon atoms
What is formed in an elimination reaction?
A double bond
What does a halogenoalkane have to react with in order for an elimination reaction to occur?
OH⁻
What do hydroxide ions act as in nucleophilic substitution and elimination respectively?
- nucleophilic substitution - act as a nucleophile
* elimination - proton acceptor (base)
What is a proton acceptor also known as?
A base
What do the curly arrows specifically show during an elimination reaction?
- The OH⁻ ion using its lone pair of electrons to form a bond with one of the hydrogen atoms on a C atom adjacent to the C involved in the C-X bond, forming a water molecule
- The electron pair from the C-H bond now becomes part of the carbon-carbon double bond, forming an alkene
- The halogen takes the pair of electrons in the C-X bond, forming a halide ion
Overall, what essentially happens in an elimination reaction?
A hydrogen atom and the halogen atom are removed from the halogenoalkane, a double bond is formed and results in an alkene
What is the reaction mechanism in elimination of haloalkanes dependent on?
- the type of halogenoalkane (1°, 2° or 3°)
2. the reaction conditions
What type of halogenoalkanes are particularly susceptible to nucleophilic substitution?
Primary
What type of halogenoalkanes are particularly susceptible to elimination?
Tertiary
What type of halogenoalkanes are particularly susceptible to both nucleophilic substitution and elimination?
Secondary
What does the reaction between a halogenoalkane and a strong base usually result in?
A mixture of substitution and elimination products
What happens if halogenoalkanes are boiled with an ethanolic solution of KOH?
Elimination will occur (OH- acts as a base)
What does ethanolic mean?
Dissolved in ethanol
What happens if halogenoalkanes are mixed with an aqueous solution of KOH at room temperature?
They will undergo nucleophilic substitution
What category of alcohols do secondary halogenoalkanes produce?
Secondary
Why can secondary and tertiary halogenoalkanes result in more than one product from elimination?
The hydrogen is always lost from a carbon atom adjacent to the carbon atom attached to the halogen
What two types of reaction could occur when 1-chlorobutane reacts with potassium hydroxide?
- nucleophilic substitution
* elimination
Symbol for ozone?
O₃
Where are small amounts of ozone present?
At ground level and just above in the troposphere
Where are large concentrations of ozone present?
In the stratosphere (upper atmosphere)
What is the appearance of ozone?
Pale blue gas
Is ozone toxic?
Yes
Equations for formation of ozone?
- O₂ → 2 O•
2. •O + O₂ → O₃
Equation for break down of ozone?
O₃ → O₂ + O•
What does ozone absorb?
Harmful UV radiation
What does the natural formation and breakdown of ozone reduce?
The concentration of UV radiation
What can UV radiation cause?
Skin cancer
What is the problem with the current rates that ozone is being broken down and formed?
- usually break down and formation occur at equal rates
* but currently it is being broken down faster than it is being formed
Why should the concentration of ozone in the atmosphere usually stay constant?
The natural formation and break down of ozone occur at equal rates
What do CFCs stand for?
Chloroflurocarbons
What can chloroalkanes and chlorofluroalkanes be used as?
Solvents
What are CFCs used in?
- coolants (in fridges)
* solvents (degreasing circuit boards, dry cleaning solvents)
What are chloroflurocarbons?
Halogenoalkanes containing chlorine and fluorine atoms but not hydrogen atoms
How do chlorine radicals affect the decomposition of ozone?
They catalyse it
Why can even small quantities of chlorine radicals significantly destroy the ozone layer?
Chlorine radicals catalyse the break down of ozone - and the process repeats itself
e.g. O₃ + •Cl → O₂ + •ClO then O₃ + •ClO → 2O₂ + •Cl
What has the decomposition of ozone due to chlorine radicals resulted in?
Formation of a hole in the ozone layer
How does the rate of breakdown of ozone molecules by chlorine radicals compare to by oxygen radicals?
Chlorine radicals break down ozone 1500 times faster
What was the result of the ozone layer depletion?
Chemists supported legislation to ban CFCs completely
What is the alternative to CFCs?
HFCs - hydroflurocarbons
Why are HFCs a good alternative to CFCs?
C-F bond stronger. So need high UV to break bonds to form free radicals
