Halogenalkanes

Question 1

what is a halogenoalkane

Answer 1

an alkane in which one or more of the H atoms is replaced with a halogen

Question 2

nucleophile

Answer 2

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

Question 3

reflux

Answer 3

process of continuous evaporation or condensation

Question 4

nucleophilic substitution

Answer 4

Halogenoalkanes have a carbon to halogen bond and since halogens are more electronegative than carbon this bond is polar
dipole means halogenoalkanes are susceptible to nucleophilic attack on delta + carbon. leading to substitution

Question 5

reason reactant is heated under reflux

Answer 5

allows prolonged heating of volatile chemicals without loss.

Question 6

continuous evaporation and condensation

Answer 6

means liquids can be boiled for as long as needed to achieve a reaction without loss of material.

Question 7

effect of changing the halogen- electronegativity

Answer 7

The higher the electronegativity, the stronger the bond as electrons move closer to the halogen, the bond becomes harder to break

Question 8

Elimination reaction

Answer 8

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

Question 9

Nucleophilic substitution

Answer 9

Used to prepare alcohols from halogenoalkanes
When the vapour reaches the condenser it condenses and liquid forms. This liquid drips back to the reaction flask

Question 10

Test for halogenoalkanes

Answer 10

Aqueous sodium hydroxide often used to hydrolyze the halogenoalkane and reaction is heated
Before Aquia Silver nitrate is added, excess sodium hydroxide must be neutralised by adding dilute nitric acid because sodium hydroxide would interfere with the test

Question 11

If dilute nitric acid wasn’t added

Answer 11

If wasn’t added before Aquia Silver Nitrate, Silver Hydroxide as a brown preticipate would form

Question 12

test for halogenoalkanes results

Answer 12

Chlorine - white perticipate
Bromine - cream preticipate
Iodine - yellow

Question 13

Elimination reaction

Answer 13

One that involves a loss of a small molecule to produce a double bond

Question 14

Unsymmetrical elimination reaction

Answer 14

If the halogenoalkane is on symmetrical, more than one alkene is formed and hydrogen is removed on either side of the halogen
to make an alkene we must heat the halogenyoalkane with sodium hydroxide in pure ethanol

Question 15

Use of halogenoalkanes- solvent

Answer 15

They contain a polar section due to the presence of the carbon to halogen bond but also contain non polar section due to presence of alkyl chain
Meaning halogenalalkanes can mix with a variety of polar and nonpolar organic substances and use that solvents
Also non-flammable used in dry cleaning clothes and degreasing

Question 16

Use of halogenoalkanes- anaesthetics

Answer 16

eg. Trichloromethane CHCl3

Question 17

Use of halogenoalkanes - refrigerants

Answer 17

Presence of permanent dipole - permanent dipole attractions mean boiling temperatures are close to room temperature. they are therefore liquids and can easily be evaporated or gases that can easily be liquefied at room temp.
Eg CFS ( Chlorofluorocarbons) Since the heat needed to change the liquid to gas is removed from the fridge, cooling fridge contents
Non flammability and non toxicity of halogenoalkanes make it suitable

Question 18

Environmental effects of chlorofluorocarbons

Answer 18

Found to be toxic and can cause damage to the Earth’s ozone layer
Use have caused holes in the ozone layer especially Arctic
Ozone layer prevents harmful u v radiation reaching a surfaces so depletion of ozone layer results in penetration to earth of UV rays which could cause skin cancer
Damage process involves radical substitution reactions

Question 19

CFCS initiation stage

Answer 19

In the upper atmosphere, UV radiation causes homomoletic bond vision to the C-Cl in the CFC and produces chlorine radicals
Eg. CCl2F2 —-> Cl* + CClF2*

Question 20

CFCS propagation stage

Answer 20

Since these form a chain reaction, the presence of a small number of chlorine radicals can cause decrease of many ozone molecules
Eg.
Cl* + O3 —-> ClO* + O2
ClO* + O3 —> Cl* + 2O2

Question 21

Alternatives to chlorofluorocarbons

Answer 21

To avoid formation of chlorine radicals alternative compounds that don’t contain chlorine are being used.
HFCs only contain Flourine- Radicals are not formed when exposed to uv radiation
Insufficient energy to break the stronger C- F bonds but enough energy can break C-Cl