Chapter 15 Haloalkanes Flashcards
What are haloalkanes?
An alkane where at least one hydrogen has been replaced by one halogen atom.
What are the three levels of classification for haloalkanes?
Primary
Secondary
Tertiary
What is a primary haloalkane?
One Carbon atom is directly attached to the Carbon bonded with the halogen atom
What is a secondary haloalkane?
Two Carbon atoms are directly attached to the Carbon bonded with the halogen atom
What is a tertiary haloalkanes?
Three Carbon atoms are directly attached to the Carbon bonded with the halogen atom
Why are haloalkanes polar?
The Carbon to halogen bond in the molecule is polar because the halogen has a greater electronegativity compared to the Carbon atom. Therefore, the Halogen becomes slightly negative while the Carbon atom becomes slightly positive.
What are nucleophiles?
Electron pair donors
What do haloalkanes react with?
Nucleophiles because they are attracted to the slightly positive carbon atom
What mechanism does haloalkanes undertake?
Nucleophilic substitution
What nucleophiles can haloalkanes react with?
OH-
H2O
NH3
Explain the mechanism of nucleophilic substitution
The nucleophile approaches the Carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom
This direction of attack by the nucleophile minimises repulsion between the nucleophile and the slightly negative halogen atom
The lone pair of electrons of the nucleophile is attracted and donated to the slightly positive Carbon atom
A bond forms between the nucleophile and the carbon atom
The carbon- halogen bond breaks by heterolytic fission
An alcohol is formed and a halide ion
Why does the nucleophile approach from the opposite end of the haloalkanes?
Minimised repulsion between the nucleophile and the slightly negative halogen atom.
What is hydrolysis?
A chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule. Resulting in the molecule being split into two products.
How can haloalkanes be converted into alcohols?
Using aqueous sodium hydroxide heating under reflux as the reaction is slow under room temperature. This ensures that a good yield of product is obtained.
How would you compare rates of hydrolysis in haloalkanes?
-Use haloalkanes of the same chain length
-In a test tube add ethanol and the haloalkane (the ethanol is a solvent which allows reactants to mix more efficiently)
-In another test tube add aqueous silver nitrate solution and place both test tubes into a water bath at 50 degrees
-Once at the same temperature mix the contents of the test tubes
-Measure the time taken for the precipitate to form
-Repeat using the same volume of other haloalkanes
-Calculate rate using 1/time taken
-The strength of the Carbon to halogen bond determines the rate. C-Cl has the highest bond enthalpy so has the lowest rate of reaction while C-I has the weakest bond so has the greatest rate of reaction.
