2.6 - halogenoalkanes

Question 1

What is a halogenalkane?

Answer 1

An alkane in which one or more hydrogen atoms have been replaced by a halogen.

Question 2

Name the halogens.

Answer 2

Group 7, forms salts
Fluorine, Chlorine, Bromine, Iodine, Astatine
They are the triggers for a nucleophilic subsititution reaction

Question 3

What is the formation of a halogenalkane?

Answer 3

CnH2n+1X (where x is the halogen)
RX (shortened)

Question 4

What bond does a halogenalkane contain?

Answer 4

Carbon to halogen bond
Halogens are more electronegative than carbon, so delta positive carbon and delta negative halogen creates permant dipole interaction.
The dipole means halogenalkanes can be nucleophillically attacked on the delta positive carbon, which leads to substitution

Question 5

What is a nucleophile?

Answer 5

A species with a lone pair of electrons that can be donated to an electron deficient species (shown with a positive charge)

Question 6

Describe nucleophilic substitution.

Answer 6

The nucleophile OH- (from aqueous NaOH) comes into contact with a halogenalkane, which has a permanent dipole. The negative OH- donates electrons to +carbon, which then breaks the bond of the halogen on the end, becoming a halide ion (negative charge)

Question 7

How can alcohol be prepared from nucleophilic substitution?

Answer 7

A halogenalkane comes into contact with NaOH, and takes the OH onto the alkyl group, turning it into an alcohol.

Question 8

What are the conditions needed for nucleophilic substitution?

Answer 8

Reaction is slow so NaOH and halogenalkane must be heated for a significant amount of time
If done in a flask, evaporation would occur and yield of product would be low so it is done under reflux.

Question 9

What is meant by ‘reflux’?

Answer 9

Process of continuous evaporation and condensation.

Question 10

Why can nucleophilic subsitution be classed as hydrolysis?

Answer 10

The change can occur by using OH- in water
Using water is much slower, NaOH is faster

Question 11

What effect does changing the halogen have on haloalkanes?

Answer 11

The rate in which the halogen occurs depends on the nature of the halogen.

Question 12

What are the 2 factors when considering different halogens?

Answer 12

Electronegativity
Bond strength

Question 13

How does electronegativity effect the halogens?

Answer 13

Electronegativity decreases as the size of the halogen increases, this means C-Cl is the most polar, with C being the most delta positive

Question 14

How does bond strength effect the halogens?

Answer 14

The substitution reaction involves breaking the carbon to hydrogen bond. This is strongest for the C-Cl bond and therefore is the most difficult to break.

Question 15

What can be determined by electronegativity and bond strength of halogenalkanes?

Answer 15

They contradict eachother, for example chlorine being the most electronegative (thus most reactive) whilst it is also the carbon-chlorine bond is the strongest, meaning it is the most difficult to break.
Bond strength is normalised as overuling, so the order of the rate of hydrolysis is-
Iodo (fastest) > bromo >chloro (slowest)

Question 16

How can nucleophilic substitution of halogenalkanes be detected?

Answer 16

Testing for the halide ion produced by adding Ag+(aq), and timing how long the precipitate takes to form.

Question 17

What has to happen before testing for nucleophilic substitution?

Answer 17

Excess sodium hydroxide must be neutralised by adding dilute nitric acid, as NaOH would interfere with the test.

Question 18

What is the equation for nucleophilic substitution?

Answer 18

RX + NaOH (aq) = ROH + Na+ + x- (aq)

Question 19

What is the equation for adding silver nitrate (ag+) to a halide ion?

Answer 19

x-(aq) + Ag+(aq) = AgX(s)

Question 20

How does chlorine react with silver nitrate?

Answer 20

White precipitate

Question 21

How does bromine react with silver nitrate?

Answer 21

Cream precipitate

Question 22

How does Iodine react with silver nitrate?

Answer 22

Yellow precipitate

Question 23

How does chlorine react with ammmonia?

Answer 23

Dissolves in dilute NH3 (aq)

Question 24

How does bromine react with ammonia?

Answer 24

Dissolves in concentrated NH3(aq)

Question 25

How does iodine react with ammonia?

Answer 25

Does not dissolve with aqueous NH3 (aq)

Question 26

What are elimination reactions and the triggers for them?

Answer 26

Involves the loss of a small molecule to produce a double bond
Hydrogen halides are acidic and therefore can be removed using an alkali
The alkali (such as NaOH) must be dissolved in ethanol (trigger) to avoid the substitution reaction

Question 27

What can happen if the halogenalkane is insymmetrical?

Answer 27

More than 1 alkene can be formed, creating isomers.
The place of the double bond changes.

Question 28

What are the 3 uses of halogenalkanes?

Answer 28

Solvents, anaesthetics and refrigerants

Question 29

What are CFC’s?

Answer 29

Halogenalkanes containing both chlorine and fluorine

Question 30

What are HFC’s?

Answer 30

Halogenalkanes containing fluorine as the only halogen

Question 31

What is the ozone layer?

Answer 31

A layer surrounding the earth that contains O3 molecules.

Question 32

What can happen if there’s damage to the ozone layer?

Answer 32

UV light can reach the earths surface, causing mutations in DNA that can lead to skin cancer.

Question 33

What is the inititation stage of using CFC’s destroying the ozone?

Answer 33

In the upper atmosphere, UV radiation causes homolytic bond fission of the c-Cl bond in the CFC.
CCl2F2 = Cl* + CCl*F2

Question 34

What is the propogation stage of using CFC’s destroying the ozone?

Answer 34

Cl* + O3 = ClO* +O2
ClO* + O3 = Cl* + 2O2
The chain reaction allows the decrease in many ozone molecules.

Question 35

What are the alternatives to CFC’s?

Answer 35

Alakanes (problem of inflammibility)
HFC’s (radicals are not formed when subjected to UV radiation)