Halogenoalkanes

Question 1

What is the mechanism called for the formation of halogenoalkanes from halogens and alkanes?

Answer 1

Free radical substitution.

Question 2

What is the mechanism for the formation of halogenoalkanes?

Answer 2

Initiation: In the presence of ultraviolet light or temperatures of over 300 degrees Celsius, halogen molecules will undergo Homolytic fission to form 2 chlorine radicals.
Propagation: A halogen molecule will react with the alkane, forming HCl and an alkyl radical.
The alkyl group then reacts with another halogen molecule, forming the halogenoalkane and another halogen radical which can restart the reaction.
Termination: A chlorine radical can react with another chlorine radical.
An alkyl radical can react with another alkyl radical.
An alkyl radical can react with a chlorine radical.

Question 3

What is a free radical?

Answer 3

A specie with an unpaired electron.

Question 4

What is a substitution reaction?

Answer 4

A reaction where an atom or a group of atoms is replaced by another atom or group of atoms.

Question 5

Why do radical substitution reactions usually have a low percentage yield?

Answer 5

Because the product formed may undergo further radical substitution reactions until all the hydrogens in the alkane have been replaced by halogen atoms.

Question 6

What are halogenoalkanes used for?

Answer 6

1. Halogenoalkenes are used to make polymers like polychloroethene (polyvinyl chloride) are used to make things like drain pipes and window frames whereas polytetrafluoroethene is usually used as coating for metal products due to the strong C-F bond, making PTFE very inert.
2. CFC (chlorofluorocarbons) are were used as refrigerants and aerosol propellants due to being non-toxic, inflammable, and being a liquid under high pressure. However, it was discovered that they reacted with ozone in the atmosphere and depleted the ozone layer which absorbs UV light from the sun.

Question 7

What is a nucleophile?

Answer 7

An atom (or a group of atoms) that is attracted to an electron-deficient atom or centre, where it donates a pair of electrons to form a new covalent bond.

Question 8

What is the mechanism for nucleophilic substitution of halogenoalkanes?

Answer 8

1. The C-X bond in a halogenoalkane is polar due to differing electronegativities between C and X. This means that the C is electron deficient and the X is electronegative.
2. The hydroxide ion is attracted to the electron deficient carbon where it donates a pair of electrons to form a new C-OH bond.
3. Simultaneously, the C-X bond breaks by heterolytic fission with the electron pair going the X, forming a -1 ion.

Question 9

How are the rates of reaction for halogenoalkanes determined?

Answer 9

1. The halogenoalkane is heated with aqueous silver nitrate.
2. Ethanol is added to act as a common solvent.
3. Water in the mixture acts as the nucleophile.
4. The silver ions in the mixture will react with the halide ion produced by the substitution reaction to form a precipitate.
5. Time how long it take for the precipitate to form for each halogenoalkane and compare.

Question 10

How might bond polarity affect rate of reaction?

Answer 10

1. C-Cl bond is the most polar, followed by C-Br bond, followed by C-I bond.
2. The C in the C-Cl bond has the biggest d+ charge, thus it most readily attracts the hydroxide ion and will give the fastest rate of reaction.

Question 11

How might bond enthalpy affect rate of reaction?

Answer 11

1. I atom is the biggest, therefore C-I bond has the lowest bond enthalpy, followed by C-Br, followed by C-Cl with the highest bond enthalpy.
2. Because the C-I bond has the lowest bond enthalpy, it will take the least amount of energy to break, thus making it the easiest to break during nucleophilic substitution and releasing a I- ion. This means that Iodoalkanes should have the fastest rate of reaction.

Question 12

What is the order for rates of reaction of halogenoalkanes?

Answer 12

Iodoalkanes have the highest rates of reaction, followed by bromoalkanes, followed by chloroalkanes. This indicates that bond enthalpies are more important factors in rate of hydrolysis.

Question 13

What conditions do hydrolysis of halogenoalkanes usually take place under?

Answer 13

The mixture of hydroxide ions and halogenoalkanes are usually heated under reflux.

Question 14

What is reflux?

Answer 14

The continual boiling and condensing of a reaction mixture to ensure that the reaction takes place without contents in the flask drying out.