haloalkanes Flashcards
How do you classify a haloalkane as primary secondary or tertiary?
Remove the halogen from the carbon on which it was bonded
Look at that carbon:
If it’s bonded to 1 other Carbon it’s primary
Bonded to 2 other Carbons it’s secondary
Bonded to 3 other carbons it’s tertiary
What are the boiling points of 1-chloroalkanes in comparison to alcohols and alkanes?
Higher than alkanes, lower than alcohols
Because alkanes are non polar molecules so only have weak London forces, so lowest boiling points
Chloroalkanes are polar molecules so have permanent dipole-dipole forces which are stronger than London forces
Alcohols can form hydrogen bonds which is the strongest type of intermolecular force, so highest boiling points
Why are haloalkanes generally more reactive than alkanes?
The C-Halogen bond (except C-Flourine has a higher bond enthalpy) is weaker than a C-H bond, so breaks more easily allowing the haloalkanes to react
What is meant by the term electronegativity?
The measure of attraction of a bonded atom for the shared pair of electrons in a covalent bond
What is the consequence for the carbon-halogen bond in haloalkanes?
Because the halogens have a higher electronegativity than Carbon the C-halogen bond is polar, with the C being delta positive and halogen being delta Negative
Would an electron rich or electron poor species be attracted be attracted to the carbon in a haloalkane?
Electron rich because the carbon has a delta negative charge
What is a hydrolysis reaction?
A chemical reaction involving water or an aqueous solution of an hydroxide, that causes the breaking of a bond in a molecule
Describe a hydrolysis reaction in a haloalkane?
It’s a substitution reaction, the halogen in the haloalkane is replaced by an OH group, and a halogen hydride is formed
Describe the preparation of an alcohol by hydrolysis of a haloalkane?
Reagent: aqueos NaOH
Conditions: heat under reflux
What does reflux mean?
The continual boilin and condensing of a mixture back to the original container to ensure that the reaction takes place without the contents of the flask boiling dry
How does the apparatus used for reflux allows reflux?
As the mixture boils, the vapour rising will be condensed back down by the water cooled condenser, without escaping through the top
Why do organic reactions often require heating under reflux whereas if you were making a salt, you would just heat the reaction mixture in a beaker?
Organic compounds have simple molecular structures, so have relatively low boiling points, compared to ionic compounds such as salts
What type of mechanism is a hydrolysis reaction of alkanes?
nucleophilic substiution
What is a nucleophile?
an electron pair donor
Give 3 examples of nucleophiles?
H2O, NH3, OH-, they all have a lone pair that can be donated to form a new covalent bond
What does a curly arrow show in a reaction mechanism?
The movement of an electron pair
Where does the curly arrow start and finish?
Starts where the electron pair originates eg lone pair or covalent bond
Ends where the electron pair goes eg. to an atom to form a new covalent bond, or to an atom on breaking a bond heterolytically, resulting in a lone pair
How would you draw the hydrolysis mechanism of a halogen with a hydroxide ion, eg. Bromoethane
Draw bromomethane, Label the carbon in which the halogen is bonded as delta positive and the halogen as delta negative
Draw an OH- ion away from bromomethane as with a lone pair of electrons
Draw a curly arrow from the lone pair on the OH- to the carbon, and a curly arrow from the covalent bond in C-Br to the Br
Products formed are a bromide ion, and the ethane but with OH instead of Br
In the hydrolysis of bromoethane with an OH- ion, what is the nucleophile?
The OH- ion
In the hydrolysis of bromoethane with an OH- ion, what part of the bromoethane attracts the nucleophile?
Delta positive Carbon
Order of reactivity of Haloalkanes fastest to slowest?
iodoalkane
bromoalkane
chloroalkane
Explain the order of reactivity the haloalkanes?
The C-I bond is weaker than the C-Br bond, which is weaker than the C-Cl bond. So C-I bond will break the easiest so it’s the most reactive
Describe experiment to test the reactivity of haloalkanes?
- first reaction is nucleophilic substitution of the haloalkane using water as the nucleophile
- further reaction then takes place between the halide ion formed and the silver ions in the silver nitrate solution, to form a solid silver halide
In the experiment to test the reactivity of haloalkanes why was a hot water bath needed?
To speed up the rate of reaction
