Chapter 15 - Haloalkanes Flashcards
What are haloalkanes?
Compounds containing hydrogen, carbon and at least one halogen atom
When two or more halogens are present in a haloalkane, how do you name it?
Put the two halogens in alphabetical order
How do you name haloalkanes?
Add the suitable prefix onto the name of the longest chain
What is a nucleophile?
An electron pair donor
Why are haloalkanes polar?
The carbon-halogen bond is polar. The halogen generally has a much higher electronegativity than carbon, so the electrons are pulled slightly towards the halogen and the carbon is left with a slight positive charge
Give three examples of nucleophiles
Hydroxide ions, :OH-
Water molecules, H2O:
Ammonia molecules, :NH3
What happens when a nucleophile is attracted to a carbon?
It donated electrons to form a new covalent bond
Why are nucleophiles attracted to carbon?
The carbon-halogen bond is polar, so carbon is left with a slight positive charge
What happens when a nucleophile reacts with a haloalkane?
The nucleophile replaces the halogen in a substitution reaction
What is nucleophilic substitution?
When a nucleophile replaces a halogen in a haloalkane
What happens when a haloalkane is hydrolysed?
An alcohol is formed
Describe the nucleophilic substitution reaction to make an alcohol from a haloalkane
1) The nucleophile (OH- in this case) approaches the slightly positively charged carbon
2) The direction of the attack of this nucleophile reduces the repulsion between the negatively charged hydrogen atom and the OH- ion
3) The lone pair of electrons on the nucleophile is donated to the carbon atom
4) A new bond forms between the carbon and the oxygen of the OH- ion
5) The carbon-halogen bond breaks by heterolytic fission
6) An alcohol and halide ion are formed
Why is the reaction different when water is used as a nucleophile?
It is much slower but will eventually get the same result
Which bond in the haloalkane is broken during hydrolysis?
The carbon-halogen bond
What does the rate of hydrolysis in haloalkanes rely upon?
The strength of the carbon-halogen bond
Which haloalkanes have the weakest bonds and so react fastest?
Iodoalkanes
Which haloalkanes have the strongest bonds and so react the slowest?
Fluoroalkanes
Will primary, secondary or tertiary haloalkanes react fastest?
Tertiary
Why do tertiary haloalkanes react fastest?
The tertiary haloalkane is much more stable than the primary one and reacts with a two-step mechanism
What reaction would you conduct to measure the rate of hydrolysis of the haloalkanes?
1) Set up three test tubes, each containing 1cm^3 of ethanol and 2 drops of the haloalkane
2) Stand the tubes in a 60c water bath
3) Place a test tube containing 0.1 mol/dm3 silver nitrate in the same water bath, to allow everything to reach the same temperature
4) Add 1cm3 of the silver to each test tube
5) Start a timer and record your results
What would you observe when you hydrolyse iodoalkanes?
A yellow precipitate forms rapidly
What would you observe when you hydrolyse bromoalkanes?
A cream precipitate forms, more slowly than iodoalkanes but faster than chloroalkanes
What would you observe when you hydrolyse chloroalkanes?
A white precipitate forms very slowly
Why do we does the reaction between a haloalkane and silver nitrate produce a precipitate?
A halogen ion is formed, which forms bonds with the silver to produce silver halide, which is insoluble
