13.2 Nucleophilic substitution in halogenoalkanes Flashcards
What is a nucleophile definition
A nucleophile is a species that has a lone pair of electrons with which it can form a bond by donating its electrons to an electron deficient carbon atom.
They are reagents that attack and form bonds with positively charged carbon atoms
(phile means attracted to, and nucleus is positively charged)
So nucleophiles are attracted to positively charged carbon atoms.
What are the 3 nucleophiles i need to know
:OH-
:NH3
:CN-
These will each replace the halogen in a halogenoalkane:
In a nucleophilic substitution
What is a reaction mechanism
It describes a route from reactants to products via a series of theoretical steps.
May involve short lived intermediates
What is the general equation for nucleophilic substitution
Halogenoalkane + nucleophile —>
Alkane attached to nucleophile + halogen free radical
How are reaction mechanisms shown or drawn
With a curly arrow:
This shows how electron pairs move in organic reactions
So the lone pair of electrons are attracted to the partially positively charged carbon atom C f+
So arrow is drawn from lone pair on nucleophile, to f+ carbon
There are 2 curly arrows for this mechanism, so what does the other one do
The lower curly arrow shows the electron pair in the C-X bond (where x is the halogen), moving to the halogen atom X and making it a halide ion.
The halide ion is called the leaving group
How is rate of reaction affected by the halogen
Increases down group 7
Because fluoro-compounds are
un-reactive due to the strength of the C-F bond
So as strength of C-x bond decreases, they get more reactive
Describe the rate of reaction of sodium/potassium hydroxide OH- ion in aqueous solution as a nucleophile
with a halogenoalkane
How can you speed it up
Rate of reaction is very slow at room temp
. So warm the mixture
What solvent is used for OH- and halogenoalkane to react
What type of reaction is this
As halogenoalkanes don’t mix with water,
ETHANOL is used
However the OH- is already in aqueous solution
So the halogenoalkane and aqueous sodium hydroxide mix
This is a hydrolysis reaction
Describe the reaction of sodium hydroxide and a halogenoalkane
What is produced
Where R is the alkane
Overall reaction:
R-X + OH- —> ROH + X-
So an alcohol ROH is produced
Eg C2H5Br + OH- —> C2H5OH + :Br-
Bromoethane + OH- –>Ethanol + :Br-
In what conditions do halogenoalkanes and cyanide ions (:CN-) react
What is formed
Warm halogenoalkanes and aqueous alcoholic solution of potassium cyanide (:CN-)
Nitriles are formed
What is the reaction for a halogenoalkane and CN- ion
What arrows are there
Two arrows
Halogenoalkane with a curly arrow pointing to the f+ carbon atom next to halogen, from the lone pair on CN- ion
Another curly arrow from C-X bond pointing to the f- halogen
So the halogen is replaced by CN so the product is the halogen ion, and alkanenitrile
So the product is a nitrile, and has one extra carbon in the chain than the starting halogenoalkane because CN is added
In what conditions do halogenoalkanes react with nucleophile ammonia (:NH3)
Halogenoalkanes react in excess concentrated solution of ammonia
IN ETHANOL and under pressure
Describe reaction when halogenoalkanes react with ammonia
What is produced
Explain
It produces RNH2
With R being the alkane part
R-X + 2NH3 —> RNH2 + NH4X
- First the curly arrow from :NH3 goes to the f+ carbon atom and the second curly arrow goes from C-X bond to X which is f-
- Because ammonia is neutral and has no charge, a H+ proton must be lost to form the neutral product called an amine (NH2)
- Before the H+ ion is lost, draw it as the alkane connected to NH3 but with a curly arrow from one H+ on the nucleophile pointing towards N
And also don’t forget the :X- ion made
The H+ ion reacts with the second molecule of NH3 to form NH4+
so products are
:X- + amine + Ammonium ion
Why are nucleophilic substitutions useful
. They are a way of introducing new functional groups into organic compounds
. Halogenoalkanes can be converted to alcohols, amines and nitriles
