4.2.2 Haloalkanes

Question 1

Explain how haloalkanes are named

Answer 1

A prefix is added to the name of the longest chain to indicate the halogen.
When 2 or more halogens are present in a structure they are listed in alphabetical order

Question 2

How can haloalkanes be classified

Answer 2

Like alcohols, haloalkanes can be primary, secondary or teritary.
Dependant on the number of carbons attached to the C⁺ (follow markownikoff’s rule)

Question 3

Describe the reactive properties of haloalkanes

Answer 3

Haloalkanes are:
- volatile liquids (low b.p.)
- immiscible (don’t mix) with water

Question 4

Explain haloalkanes volatility and compare to alkanes

Answer 4

Haloalkanes have higher boiling points than alkanes because thay have permanent dipoles (𝛿⁺C - X𝛿⁻) and permanent dipole-dipole interactions are stronger than London forces. However permanent dipole-dipole interactions are still weak intermolecular forces so haloalkanes have relatively low boiling points

Question 5

Explain why haloalkanes are not miscible with water

Answer 5

Haloalkanes are immiscible with water because they do not form hydrogen bonds

Question 6

define nucleophile

Answer 6

an electron pair donor

Question 7

list some nucleophiles

Answer 7

• hydroxide ions :OH⁻
• water molecules H₂O:
• ammonia molecule :NH₃

another common nucleophile is CN⁻ (cyonide)

Question 8

define nucleophilic subsitution

Answer 8

A reaction in which a nucleophile is attracted to an electron-deficient carbon atom (C⁺), and replaces an atom or group of atoms on the carbon atom

Question 9

short notes on nucleophilic substitution of haloalkanes

Answer 9

The halogen on a haloalkane can be replaced by another atom or group of atoms e.g. ⁻OH.
There is heterolytic fission of the C-X (carbon-halogen) bond

Question 10

Define hydrolysis

Answer 10

Hydrolysis is a reaction with water that breaks a chemical compound into 2 compounds, the H and the OH in a water molecule becomes incorporated into the 2 compounds

It is an example of nucleophilic substitution

Question 11

Explain the hydrolysis of a haloalkane

Answer 11

the halogen atom is replaced by an ⁻OH group (such as from NaOH → an aqueous alkali)
The C-X bond is broken by heterolytic fission

Question 12

Outline the reaction mechanism for nucleophilic substitution

using ⁻OH

Answer 12

1. The nucleophile, ⁻OH, 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 minimises repulsion between the nucleophile and the 𝛿⁻ halogen atom.
3. A lone pair of electrons on the hydroxide ion is attracted and donated to the 𝛿+ 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 is also formed.

Question 13

Outline how haloalkanes can be converted into alcohols

Answer 13

By the use of aqueous sodium hydroxide NaOH. Using hydrolysis.
The reaction is slow at room temperature so the mixture is heated under reflux to obtain a good yeild of product.

Question 14

What is the rate of hydrolysis dependant on and what impact does this have

Answer 14

The rate of hydrolysis depends on the strength of the carbon-halogen bond in the haloalkane.
C-F bond is the strongest and the C-I bond is the weakest carbon-halogen bond.
This means that less energy is required to break the C-I bonds than other C-X bonds.
From the bond enthalpies we can predict that:
- iodoalkanes react faster than bromoalkanes
- bromoalkanes react faster than chloroalkanes
- fluoroalkanes are unreactive as a large quantity of energy is required to break the C-F bond.

Question 15

what is the general equation for the hydrolysis of primary haloalkanes with water

using CH₃(CH₂)₃X

Answer 15

CH₃(CH₂)₃X + H₂O → CH₃(CH₂)₃OH + H⁺ + X⁻

Question 16

Outline how the rate of hydrolysis of haloalkanes can be measured

Answer 16

The rate of each reaction can be followed by carrying out the reaction in the presence of aqueous silver nitrate (AgNO₃). As the reaction takes place halide ions, X⁻(aq), are produced which react with the Ag⁺(aq) ions to form a precipitate of the silver halide.
Ag⁺(aq) + X⁻(aq) → AgX(s)

Question 17

Explain how the rate of hydrolysis of haloalkanes is be measured using AgNO₃ (aq)

Answer 17

The nucleophile in the reaction is water, which is present in the aqueous silver nitrate. Haloalkanes are insoluble in water, and the reaction is carried out in the presence of an ethanol solvent. Ethanol allows the water and the haloalkane to mix and produce a single solution rather than 2 layers.

Question 18

Describe the experiment for the hydrolysis of haloalkanes

no results

Answer 18

1. Set up 3 test tubes:
   Test tube 1: Add 1cm³ of ethanol and 2 drops of 1-chlorobutane
   Test tube 2: Add 1cm³ of ethanol and 2 drops of 1-bromobutane
   Test tube 3: Add 1cm³ of ethanol and 2 drops of 1-iodobutane
2. stand the test tubes in a water bath at 60℃.
3. Place a test tube containing 0.1mol dm⁻³ silver nitrate in the water bath and allow all tubes to reach a constant temperature.
4. Add 1cm³ of silver nitrate to each of the test tubes. Immediately start a stop watch.
5. Observe the test tubes for 5 minutes and record the time taken for the precipitate to form.

Question 19

Give the results for the hydrolysis of haloalkanes experiment

Answer 19

From mixing ethanol, aqueous silver nitrate and an aqueous haloalkane.
The results:
- 1-chlorobutane produces a white precipitate very slowly.
- 1-bromobutane produces a cream precipitate that forms slower than with 1-iodobutane but faster than with 1-chorobutane.
- 1-iodobutane produces a yellow precipitate rapidly

Question 20

Explain the results from the experiment of the hydrolysis of haloalkanes

Answer 20

The results from the hydrolysis of haloalkanes experiment where chloro reacts slowest and iodo reacts fastest can be explained by bond enthalpies of the C-X bond.
- 1-Chlorobutane reacts the slowest because the C-Cl bond is the strongest
- 1-iodobutane reacts the fastest as the C-I bond is the weakest

Therefore the rate of hydrolysis increases as the strength of the carbon-halogen bond decreases.

Question 21

Why does rate of reaction differ within structural isomers of a haloalkane

Answer 21

rate can differ because of the location of the halogen-carbon bond.
tertiary haloalkanes are hydrolysed fastest whilst primary slowest.

Question 22

Why is the rate of hydrolysis faster for tertiary haloalkanes

Answer 22

because primary haloalkanes react in a one step mechanism whereas tertiary react in a two step one. The increase rate can also be attributed to the increased stability of the tertiary carbocation compared to that of the primary.

Question 23

outline the two step mechanism for tertiary haloalkanes hydrolysing

Answer 23

1. carbon-halogen bond of the tertiary haloalkane breaks by heterolytic fission, forming a tertiary carbocation and a halide ion.
2. a hydroxide ion attacks the carbocation to form the organic product.