3. Halogenoalkanes
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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