2.5 HALOGENOALKANES

Question 1

What is a Homologous Series?

Answer 1

Family of compounds that share the same general formula, have similar chemical properties, show a gradation in physical properties and each consecutive member differs from the next by a CH2 unit.

Question 2

What is a functional group and what is the functional group of the Halogenoalkanes?

Answer 2

Reactive group of atoms within a compound

Functional group= Halogeno group (-Cl, -Br, -I)

Question 3

What is a Primary Halogenoalkane?

Answer 3

Halogenoalkane which has one Carbon atom directly bonded to the Carbon atom that is bonded to the Halogen atom

Question 4

What is a Secondary Halogenoalkane?

Answer 4

Halogenoalkane which has two Carbon atoms directly bonded to the Carbon atom that is bonded to the Halogen atom

Question 5

What is a Tertiary Halogenoalkane

Answer 5

Halogenoalkane which has three Carbon atoms directly bonded to the Carbon atom that is bonded to the Halogen atom

Question 6

How does electronegativity affect the mpt/bpt of Halogenoalkanes?

Answer 6

C-Cl and C-F bonds within the halogenoalkanes are polar: This causes stronger dipole-dipole attractions between halogenoalkane molecules (than van der Waal’s forces of attraction) which requires more energy to overcome hence a higher mpt/bpt

C-Br and C-I bonds within the halogenoalkanes are virtually non-polar: This causes only van der Waal’s forces of attraction between halogenoalkane molecules

Question 7

How does increasing the length of the alkyl chain of the halogenoalkane affect the mpt/bpt? (Halogeno group remains the same)

Answer 7

Increasing alkyl chain increases the mpt/bpt of Halogenoalkane
Greater Mr therefore no. electrons in molecules also increase
This creates larger dipoles between molecules hence stronger van der Waal’s forces of attraction between molecules
Requires more energy to overcome stronger van der Waal’s hence the higher mpt/bpt

Question 8

What is an elimination reaction?

Answer 8

reaction in which a small molecule is removed from a larger molecule

Question 9

Preparation of Halogenoalkanes using Phosphorous Pentachloride (PCl5)

Answer 9

PCl5 added to alcohol in fume cupboard @ RT
R-OH + PCl5 => R-Cl + PCl3O + HCl
(Phosphorous trichloride oxide = PCl3O)

Question 10

Methods for preparation of Halogenoalkanes

Answer 10

Using electrophilic substitution mechanism with HX

Using Free Radical Substitution Mechanism

Question 11

Nucleophile

Answer 11

ion or molecule with a lone pair of electrons that attacks regions of low electron density

Question 12

Substitution reaction

Answer 12

One atom or group is replaced by a different atom or group

Question 13

Reaction of Halogenoalkane and aqueous alkali (Alkaline hydrolysis)

Answer 13

General equation:

Halogenoalkane + OH- => Alcohol + Halide ion

Question 14

Reaction of Halogenoalkane and water (hydrolysis)

Answer 14

General equation

Halogenoalkane + water => alcohol + Hydrogen halide

Question 15

Why does Alkaline hydrolysis react much faster than hydrolysis

Answer 15

Water is not as good nucleophile as Hydroxide ion

-Hydroxide ion has a full -ve charge and three lone pairs.

Question 16

Hydrolysis

Answer 16

breaking up of a molecule by reaction with water

Question 17

Reaction of Halogenoalkane and Potassium cyanide

Answer 17

General equation
Halogenoalkane + CN- => Nitrile + Halide ion
(CN- = Cyanide ion)

• Refluxing halogenoalkane dissolved in ethanol with solid potassium cyanide

Question 18

Reaction of Halogenoalkane and Ammonia

Answer 18

General reaction equation
Halogenoalkane + Ammonia => R-NH2 + Hydrogen Halide
(R-NH2 = Amine)
(-NH2 = Amino group)

• Halogenoalkane is heated with conc. Ammonia in a “bomb” (sealed vessel) and forms a mixture of products called amines
• Bomb prevents an escape of gases due to the high pressure caused by heating

Halogenoalkane + ammonia => Amine + Ammonium halide
-Hydrogen halide reacts with ammonia
o HX + NH3 => NH4X

• Amine reacts with halogenoalkane
Halogenoalkane + Amine => Diamine + Hydrogen Halide
Halogenoalkane + diamine => Triamine + Hydrogen halide

Question 19

naming amines

Answer 19

naming amines
•	Alkyl group first 
•	Ends in “amine”
N.B- mixture of amines produced can be separated by fractional distillation
E.G- ethylamine or propylamine

Question 20

halogenoalkane to alkene

Answer 20

General equation
Halogenoalkane => Alkene + Hydrogen Halide
(under presence of Ethanolic Potassium Hydroxide)
Halogenoalkane + Potassium Hydroxide => Metal Halide + water + alkene

• HX would be neutralised by the alkali
o HX + OH- => X- + H2O
• Halogenoalkane is heated under reflux with potassium hydroxide dissolved in ethanol
o Hydrogen halide molecule is eliminated from the halogenoalkane forming an alkene
• Tertiary halogenoalkanes tend to undergo elimination reactions much more readily than primary halogenoalkanes

Question 21

experimental method for liquid alkene

Answer 21

1. place some (2 cm3) ethanolic potassium hydroxide in a flask add some halogenoalkane (1 cm3)
2. shake and add a few anti-bumping granules
3. reflux gently for 20 minutes
4. set up for distillation and collect the product test the product with bromine water

Question 22

experimental method for gaseous alkene

Answer 22

1. some sand is soaked with the liquid halogenoalkane, and placed in a boiling tube clamped horizontally
2. some potassium hydroxide pellets are placed in ceramic wool soaked in ethanol, and placed in the middle of the boiling tube
3. heat the halogenoalkane and the ethanolic potassium hydroxide and collect the gas produced over water
4. test the gas produced with bromine water
   N.b- Collection of a gas which was not one of the reactants confirms elimination reaction.

Question 23

Investigating rate of hydrolysis of primary halogenoalkanes

Answer 23

1. use halogenoalkanes with the same chain length and place the chloroalkane, bromoalkane and iodoalkane (approximately 1 cm3) in three separate test tubes.
2. add an equal volume of ethanol (approximately 1 cm3) to each test tube
3. add equal volumes of silver nitrate solution (approximately 1 cm3) to each test tube
4. place the test tubes in a hot water bath at 50 °C and start the stopwatch
5. Time how long it takes for each precipitate to form

Rate and amount of formation of ppt increases as Halogeno groups get heavier
• This is because the C-X bond gets weaker (hence longer) as the Halogen in the Halogeno group gets heavier
o This makes them easier to hydrolyse
• Long bonds are weak bonds

R-X + H20=> R-OH + HX
HX + AgNO3 => AgX + HNO3

Question 24

naming nitriles

Answer 24

N.B.- -CN is a nitrile group
•	Molecule named after longest carbon chain (incl. C atom of nitrile group)
•	Name ends with “nitrile”
o	Ethanenitrile
o	Propanenitrile

Question 25

why is the reaction between halogenoalkane and cyanide ion useful

Answer 25

used to increase the alkyl chain length by one carbon

Question 26

effect of increasing the number of Halogeno groups while keeping the alkyl chain length constant

Answer 26

•Adding more Halogeno groups to the halogenoalkane increases the mpt/bpt

oMore Halogeno groups added  greater the Mr of the molecules
This then means there is a greater no. electrons in molecules which creates larger dipoles between neighbouring molecules which causes an increase in mpt/bpt.
•More energy is required to overcome the stronger forces of attraction between the molecules hence a larger mpt/bpt required.

•This is effect is even more prominent if the Mr of the halogen of the Halogeno group is increased.

Question 27

effect of structural isomerism of halogenoalkanes

Answer 27

• As branching increases, the halogenoalkane molecules become more branched (i.e primary to secondary to tertiary)

oThis means that there is less contact SA between electrons of neighbouring molecules  less van der Waal’s forces of attraction between molecules
Requires less energy to overcome the weaker van der Waal’s hence the smaller mpt/bpt

Question 28

Physical properties of Halogenoalkanes

Answer 28

• Sickly sweet smell
• Immiscible in water
oDespite polarity of C-Br/Cl/F bond, halogenoalkanes cannot form Hydrogen bonds with the water molecules causing the two species to form 2 layers

Question 29

Why do Primary Halogenoalkanes favour SN2 mechanism?

Answer 29

•They would form a Primary Carbocation which is energetically unstable and much more difficult to produce
oSN2 easier
•There is no steric hindrance for the OH- ion
-direct access to the C+

Question 30

Explain, in terms of bond enthalpy and polarity why the hydrolysis of chloroethane is slower than that of bromoethane

Answer 30

bond enthalpy of C—Cl greater
C—Cl is more polar
bond enthalpy more important