Chapter 15 (approved) Flashcards
How can haloalkanes be classified?
As primary, secondary or tertiary depending on how many carbon atoms attached to the C-X carbon atom
Why is the carbon halogen bond polar in haloalkanes?
The halogen atoms are more electronegative than the carbon atoms so, the bond is closer to the halogen atom
What is the carbon atom in haloalkanes able to do due to its slight positive charge?
Attract species containing a lone pair of electrons
Nucleophile def. (1)
Electron pair donor
What happens to the nucleophile?
It is attracted to an electron deficient carbon atom, where it donates a pair of electrons to form a new covalent bond
What is hydrolysis?
A 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
Nucleophilic substitution in the hydrolysis of a haloalkane where the halogen atom is replaced by an -OH group (6)
1 The nucleophile approaches the carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom
2 This direction of attack by the -OH ion minimised repulsion between the nucleophile and the S- halogen atom
3 A lone pair of electrons on the hydroxide ion is attracted and donated to the S+ carbon atom
4 A new bond is formed between the oxygen atom of the hydroxide ion and the carbon atom
5 The carbon-halogen bond breaks by heterolytic fission
6 The new organic product is an alcohol, a halide ion is also formed
Hydrolysis of haloalkanes:
Type of reaction mechanism
Products
Reaction conditions
Nucleophilic substitution
Alcohol and halide ion
Aqueous sodium hydroxide and heating under reflux
Why must the hydrolysis of haloalkanes be done under reflux?
Because the reaction is very slow at room temperature so it must be done under reflux to obtain a good yield of product
What does the rate of hydrolysis depend on?
The strength of the carbon halogen bond in the haloalkanes
The classification of the haloalkanes (primary, secondary, tertiary)
How does the rate of hydrolysis change down the halogen group and why?
Chlorine has a very strong C-X bond due to the largest difference in electronegativity, so a lot of energy is required to break it, making it the least reactive, but down the group the Electronegativity difference decreases, leading to weaker bonds due to less energy required to overcome them and therefore more reactive compounds down the group
How do you measure the rate of hydrolysis of primary haloalkanes? (Method)
- Set up 3 test tubes, adding 1cm3 of ethanol to each, and. 2 drops of 1-chlorobutane, 1-bromobutane and 1-iodobutane to each respective test tube
- Stand the test tubes in a water bath at 60C
- Place a tests tube with 0.1 mol dm^-3 silver nitrate in the water bath and allow all tubes to reach a constant temperature
- Add 1cm^3 of the silver nitrate quickly to each of the test tubes. Immediately start a stop watch and time how long the reaction takes
- Observe and record how long it takes each precipitate (silver halide) to form
What are the expected observations from the rate of hydrolysis of primary haloalkanes reaction?
1-chlorobutane: A white precipitate forms very slowly
1-bromobutane: A cream precipitate forms slowly (faster than chloro but slower than iodo)
1-iodobutane: A yellow precipitate forms rapidly
What causes the results of the rate of hydrolysis of haloalkanes experiment? (1)
The respective bond enthalpies of the carbon-halogen bonds
How does the classification of the haloalkane affect its rate of hydrolysis?
The primary haloalkanes are hydrolysed the slowest, whilst the tertiary haloalkanes are hydrolysed the fastest
What are the two reasons that the rate of hydrolysis of tertiary alkanes is faster than for primary haloalkanes?
Tertiary carbocations are more stable, and tertiary carbocations are formed by a two step mechanism
