🔴🔸2.6 -Halogenalkanes

Question 1

Define halogenoalkane

Answer 1

An alliance in which one or more hydrogen atoms have been replaced by a halogen

Question 2

Define a nucleophile

Answer 2

A species with a lone pair of electrons that can be donated to an electron deficient species

Question 3

Define a reflux

Answer 3

A process of continuous evaporation and condensation

Question 4

Define hydrolysis

Answer 4

A reaction with water to produce a new product

Question 5

What is the general formula of halogenoalkanes

Answer 5

CnH(2n+1)X (where X is a halogen)

Question 6

Why is the carbon to halogen bond in halogenoalkanes polar?

Answer 6

Halogens are more electronegative than carbon (carbon = 🔺+)

Halogen = 🔺-

Question 7

Why are halogenoalkanes susceptible to nucleophilic attack?
Where?
What does this lead to?

Answer 7

Susceptible to attack due to dipole
On the 🔺+ carbon atom
Leads to substitution

Question 8

LEARN THE NUCLEOPHILIC SUBSTITUTION MECHANISM

Answer 8

LEARN THE NUCLEOPHILIC SUBSTITUTION MECHANISM

Question 9

What can nucleotide substitution be used for?

Where is the nucleophile OH- needed for reaction provided by?

Answer 9

Preparation of alcohols from halogenoalkanes

Nucleophile provides by aqueous sodium hydroxide

Question 10

What precautions do you make to aid slow reaction in the preparation of alcohols

Answer 10

-reflux the mixture of halogenoalkane and aqueous sodium hydroxide, to stop liquid evaporation, as the reaction is slow

Question 11

Why must the tip of the container by open when liquids are refluxed?

Answer 11

Because if not the air will expand within the apparatus and blow the stopper off violently

Question 12

Describe what occurs in a reflux reaction

Answer 12

• liquids evaporate when heated
• vapour reaches condenser and condensed back into liquid form
• liquid drips back into reaction flask

Question 13

Name a benefit of reflux reactions

Answer 13

Allows continuous evaporation and condensing so liquid can be boiled for as long as needed to achieve reaction without any loss of material

Question 14

What are the two main factors affecting the rate of reaction when changing a halogen in a halogenoalkane

Answer 14

Electronegativity

Bond strength

Question 15

How does electronegativity affect rate of reaction of halogenoalkane

Answer 15

Electronegativity decreases as the size of the halogen increases
C-Cl is the most polar :: carbon most 🔺+

Question 16

How does bond strength affect rate of reaction of halogenoalkane

Answer 16

Substitution reaction involves breaking the carbon to halogen bond :: strongest for C-Cl bond most difficult to break decreasing rate of reaction.

Question 17

Name the order of rate of hydrolysis for the halogenoalkanes

Answer 17

Fastest: iodo > bromo > chloro :slowest

Question 18

How do you measure the rate of reaction of nucleophilic substitution?

Answer 18

After hydrolysis halide ion is produced in solution, can be detected by adding Ag+ (aq), usually as aqueous silver nitrate and timing how long precipitate takes to form

Question 19

Describe the test for halogens

Answer 19

1. Hydrolyse halogenoalkane with aqueous sodium hydroxide
   RX + NaOH(aq) —> ROH + Na+(aq) + X-(aq)
   2.neutralise excess sodium hydroxide with dilute nitric acid
   3.add aqueous silver nitrate
   X-(aq) + Ag+(aq) —> AgX(s)

Question 20

Colour change of halogens with Ag+

Answer 20

Chlorine = white ppt 
Bromine = cream ppt
Iodine = yellow ppt

Question 21

Results when adding NH3(aq) to precipitate formed with Ag+

Answer 21

Chlorine = dissolved in dilute NH3(aq) 
Bromine= dissolves in concentrated NH3(aq) 
Iodine= does not dissolve in NH3(aq)

Question 22

Define elimination reaction

Answer 22

A reaction that involves the loss of a small molecule to produce a double bond

Question 23

Why can halides be removed from a halogenoalkane in an elimination reaction?

Answer 23

Because they are acidic :: can be removed using an alkali

Question 24

Why does the alkali added to a halogenoalkane in an elimination reaction need to be dissolved in ethanol?

Answer 24

To avoid a substitution reaction

Question 25

LEARN ELIMINATION REACTION MECHANISM

Answer 25

LEARN ELIMINATION REACTION MECHANISM

Question 26

How many products form is the halogenoalkane is not symmetrical under an elimination reaction?

Answer 26

2

Question 27

Define a CFC

Answer 27

Halogenoalkane containing both chlorine and fluorine

Question 28

Define Ozone layer

Answer 28

A layer surrounding the earth that contains O3 molecules

Question 29

Define HFCs

Answer 29

Halogenoalkanes containing fluorine as the only halogen

Question 30

Name some uses of halogenoalkanes

Answer 30

1. Solvents
2. As anaesthetics
3. As refrigerants

Question 31

Why are halogenoalkanes good solvents

Answer 31

Polar sections (carbon halogen bond) and non polar sections (alkyl chain) :: interact with polar and non polar organic substances

Question 32

Why are halogenoalkanes used in anaesthetics

Answer 32

Many act as general anasthetics such as halothane

Question 33

Why are halogenoalkanes used as refrigerants such as CFC’s

Answer 33

Small halogenoalkanes are gases at room temperature and :: heat needed to change the liquid to a gas is removed from the fridge to cool its contents
-non flammable

Question 34

Why are the uses of halogenoalkanes now regulated?

Answer 34

Many are toxic such as polychloroalkanes and others (CFC’s) damage ozone layer, creating holes

Question 35

Why are holes in ozone layer bad?

Answer 35

Allows UV rays to reach earths surface and cause skin cancer

Question 36

Describe initiation stage of UV reaction.

Answer 36

UV radiation causes homolytic bond fission of the C-Cl bond in the CFC
Example:
CCl2F2 —> Cl• + CCl•F2

Question 37

Show propagation stage of UV reaction

Answer 37

Cl• + O3 —> ClO• + O2

Causes ozone 03 to turn to 02

Question 38

Why is a small value of chlorine radicals very damaging to ozone layer?

Answer 38

Because it’s a chain reaction :: small chlorine = decrease of many ozone molecules

Question 39

Name an alternative the CFC’s

Answer 39

HFC’s as the only halogen they contain is fluorine who’s radicals are not formed when they are exposed to UV radiation