Halogenoalkanes

Question 1

What is a halogenoalkane

Answer 1

An alkane with 1 or more halogens attached to it

Question 2

State the trend in boiling points of halogenoalkanes as we go down the halide group

Answer 2

Boiling point of halogenoalkanes increases down the group

Question 3

Explain the trend in boiling points of halogenoalkanes as we go down the halide group

Answer 3

Boiling point increases due to:
- Atomic size increases
- More electrons
- Greater number of electrons
- Stronger VDW forces
- More energy needed to overcome these IMF

Question 4

How do halogens affect the polarity of a bond

Answer 4

Halogens are electronegative and so pull the pair of electrons in a covalent bond towards it, making it polar

Question 5

What is a nucleophile

Answer 5

An electron pair donor

Question 6

Give 3 examples of nucleophiles

Answer 6

:CN-
:NH3
::OH-

Question 7

Why can halogenoalkanes be attacked by nucleophiles

Answer 7

They contain polar bonds

Question 8

When a halogenoalkane is attacked by a nucleophile in a substitution reaction, where is the first arrow from and where does it go to

Answer 8

From the lone pair on the nucleophile to the delta positive carbon bonded to the halogen

Question 9

What is the name of the mechanism whereby hydroxide ions react with a halogenoalkane using an aqueous solvent

Answer 9

Nucleophilic substitution

Question 10

What are the conditions required for the substitution reaction of hydroxide ions with a halogenoalkane

Answer 10

Warm aqueous NaOH (dilute)
Carried out under reflux (heat gently)

Question 11

Draw the substitution mechanism for the reaction of hydroxide ions with chloroethane using an aqueous solvent

Answer 11

Question 12

What is formed when hydroxide ions react with a halogenoalkane using an aqueous solvent

Answer 12

An alcohol

Question 13

What is the general equation for the overall reaction between a halogenoalkane and hydroxide ions using an aqueous solvent

Answer 13

RX + NaOH –> ROH + NaX
R - alkyl group (CH3)
X - Halogen

Question 14

What is formed when halogenoalkanes react with cyanide ions

Answer 14

Nitriles

Question 15

What are the conditions required for the reaction of cyanide ions with a halogenoalkane

Answer 15

Warm ethanolic potassium cyanide
Carried out under reflux

Question 16

Draw the mechanism for the reaction of cyanide ions with chloroethane

Answer 16

Question 17

What is the general equation for the overall reaction between cyanide ions and a halogenoalkane

Answer 17

RX + KCN –> RCN + KX
R - alkyl group (CH3)
X - Halogen

Question 18

What is the name of the mechanism whereby cyanide ions react with a halogenoalkane

Answer 18

Nucleophilic substitution

Question 19

What is the name of the mechanism whereby ammonia reacts with a halogenoalkane

Answer 19

Nucleophilic substitution

Question 20

What are the conditions required for the reaction between ammonia and a halogenoalkane when the desired product is a primary amine

Answer 20

Warm ethanolic ammonia
Must have excess ammonia
Heat gently under reflux

Question 21

What is produce when ammonia reacts with a halogenoalkane

Answer 21

An amine and an ammonium ion

Question 22

Why do we need excess ammonia in the reaction between ammonia and a halogenoalkane

Answer 22

As in the second phase of the reaction another molecule of NH3 acts as a base by reacting with hydrogen

Question 23

Draw the mechanism for the reaction of ammonia with a halogenoalkane

Answer 23

Question 24

How can an amine be identified

Answer 24

Fishy smell

Question 25

State the trend in reactivity of halogenoalkanes as we go down the halogen group

Answer 25

Halogenoalkanes become more reactive as we go down the group

Question 26

What determines the reactivity of a halogenoalkane

Answer 26

Bond strength/Bond enthalpy Lower bond enthalpies down the group mean weaker bond, easier to break and so more reactive

Question 27

What is the name of the mechanism where hydroxide ions react with a halogenoalkane using an ethanol solvent

Answer 27

Elimination

Question 28

What are the conditions required for the elimination reaction between hydroxide ions and a halogenoalkane

Answer 28

Hot ethanolic concentrated NaOH/KOH Carried out under reflux

Question 29

What is the main difference in the mechanism between the reaction of hydroxide ions with a halogenoalkane using an aqueous solvent vs an ethanol solvent

Answer 29

The nucleophile attacks the hydrogen on the carbon adjacent to the carbon with the halogen attached with an ethanolic solvent (elimination), rather than the carbon directly attached to the halogen with an aqueous solvent(substitution)

Question 30

Outline and name the mechanism for when hydroxide ions react with 2 chloropropane using an ethanol solvent

Answer 30

Elimination

Question 31

What is formed when hydroxide ions react with a halogenoalkane, using ethanol as a solvent

Answer 31

An alkene Water Halide ion

Question 32

What is the role of OH- in the reaction of hydroxide ions with a halogenoalkane when using ethanol as a solvent

Answer 32

OH- acts as a base

Question 33

What is the role of OH- in the reaction of hydroxide ions with a halogenoalkane when using an aqueous solvent

Answer 33

OH- acts as a nucleophile

Question 34

What is an aqueous solvent essentially

Answer 34

Water

Question 35

When reacting NaOH with a halogenoalkane, when do we form an alkene

Answer 35

When ethanol is used as the solvent

Question 36

When reacting NaOH with a halogenoalkane, when do we form an alcohol

Answer 36

When water is used as the solvent

Question 37

What is a CFC

Answer 37

Chlorofluorocarbons Molecules which have had all their hydrogens replaced by chlorine and fluorine

Question 38

What is the issue with CFC's

Answer 38

They catalyse the breakdown ozone in the atmosphere

Question 39

How do CFCs catalyse the break down ozone

Answer 39

C-Cl bonds are broken down by UV radiation in the atmosphere Radicals are then formed which catalyse the breakdown of ozone

Question 40

Why are C-Cl bonds broken by UV in CFC's and not C-F bonds

Answer 40

Easier to break as they have the lowest bond enthalpy (its a weaker bond)

Question 41

What is the formula for ozone

Answer 41

O3

Question 42

What occurs in the initiation stage of the formation of chlorine radicals which catalyse the breakdown of ozone

Answer 42

Sunlight (UV) breaks down the C-Cl bond in a CFC producing 2 radicals which react with ozone molecules

Question 43

What 2 equations occur in the propagation stage of the formation of chlorine radicals which catalyse the breakdown of ozone

Answer 43

1- Cl radical reacts with O3 to form a ClO radical intermediate and O2 2 - ClO radical reacts with more O3 to make O2 and a Cl radical (reformed so is a catalyst)

Question 44

What occurs in the termination stage of the formation of chlorine radicals which catalyse the breakdown of ozone

Answer 44

Two Cl radicals react to form Cl2

Question 45

In which stage is ozone broken down. Initiation Propagation or termination. Write down the equation for this breakdown

Answer 45

Question 46

Write the 3 stages showing the breakdown of ozone from CCl3F (can also be done by homolytic fission of Cl2 to form the Cl radicals)

Answer 46

Question 47

Why are CFCs now banned

Answer 47

The risks of ozone depletion meaning less absorption of harmful UV radiation outweigh the benefits of using them in fridges and as a propellant in deodorants

Question 48

Why is ozone depletion a concern

Answer 48

Ozone absorbs most harmful UV radiation that causes skin cancer, depletion increases the risk of this

Question 49

What do we now use as an alternative to CFCs and why

Answer 49

HFC's Do not contain any chlorine

Question 50

Does a higher temperature encourage elimination or substitution for the reaction between OH- and halogenoalkanes

Answer 50

Higher temperatures encourage elimination

Question 51

Does a higher concentration of OH- encourage elimination or substitution for the reaction between OH- and halogenoalkanes

Answer 51

Higher concentrations favour elimination.