Halogenoalkanes

Question 1

what are halogenoalkanes?

Answer 1

organic compounds containing halogen atoms such as Fluorine, Chlorine,Bromine and Iodine.

Question 2

what is the general formula of halogenoalkanes?

Answer 2

CnH2n+1

Question 3

what are primary, secondary and tertiary halogenoalkanes?

Answer 3

primary-
contain 1 alkyl group bonded to the carbon atom that the halogen is bonded to, minor product.
secondary-
contain 2 alkyl groups bonded to the carbon atom that the halogen is bonded to, medium major product.
tertiary-
contain 3 alkyl groups bonded to the carbon atom that the halogen is bonded to, major product.

Question 4

explain the polarity of halogenoalkanes?

Answer 4

C-X bonds have different electronegativity , the C-X bond is polar. the electrons in the covalent bond are held closer to the halogen atom. this makes the halogen partially negative (δ-) and a partial positive charge (δ+) on the carbon atom, creating a dipole.
this polarity makes halogenoalkanes more reactive than alkanes.

Question 5

what forces and interactions do halogenoalkanes consist of?

Answer 5

dispersion forces and dipole-dipole interactions

Question 6

explain the trend in polarity within halogenoalkanes?

Answer 6

polarity increases as you go up the halogen group.

Question 7

explain the solubility of halogenoalkanes?

Answer 7

insoluble,
due to the non-polar R groups (alkyl groups) as they form van der waal forces

Question 8

explain how chain length affects boiling point of halogenoalkanes?

Answer 8

longer chain= higher boiling point
- because van der waal forces are increased with chain length meaning that there is more electrons present meaning it is harder to overcome these bonds and there is a larger surface area meaning that there is more points where van der waal forces can form.

Question 9

explain how branching affects boiling point of halogenoalkanes?

Answer 9

branching= reduces boiling point
- van der waal forces are weaker, less surface area of contact even if there are the same numbers of electrons.

Question 10

what affect does the halogen group have on the boiling point of halogenoalkanes?

Answer 10

increases down the group
- halogen is larger as there is more electrons, stronger van der waal forces.

Question 11

list all the halogens in order of highest bond enthalpy to weakest.

Answer 11

C-F
C-Cl
C-Br
C-I

Question 12

what does bond enthaply mean?

Answer 12

the higher the bond enthaply the harder it is to break the bond between the C-X

Question 13

what is nucleophile?

Answer 13

highly negative, electron pair donors. possess at least one lone pair of electrons. not necessary to have a negative charge. attracted to slightly positive carbon (δ+).

Question 14

what are the nucleophiles?

Answer 14

Hydroxide ion (OH⁻)
Cyanide ion (CN⁻)
Ammonia (NH₃)
Water molecule (H₂O)

Question 15

which one of those 4 is the weakest?

Answer 15

Water molecule (H₂O)

Question 16

explain the mechanism of nucleophilic substitution.

Answer 16

the nucleophile uses its lone pair to provide electrons for a new bond.
the halogen is then displaced - carbon can only have eight electrons in outer shells.

Question 17

what is elimination reaction?

Answer 17

when a halogenoalkane is heated to high temperatures under alcoholic conditions, elimination occurs.

Question 18

what base is used in elimination reaction?

Answer 18

Hydroxide ion (OH⁻), this removes the H+ ion from the halogenoalkane.

Question 19

what are the reagents needed for this mechanism?

Answer 19

potassium or sodium hydroxide.

Question 20

why must there be no water in the potassium (or sodium) hydroxide that is being dissolved in ethanol?

Answer 20

potassium reacting with water creates heat, which causes pressure which can lead to an explosion.

Question 21

explain the mechanism of an elimination reaction.

Answer 21

the base attacks the adjacent hydrogen to the carbon which has the halogen bonded to it, this then breaks the bond between the hydrogen and carbon, creating a double bond. then the halogen removes from the carbon.

Question 22

what are the byproducts of this reaction?

Answer 22

H2O and KX or NaX (where X is the halogen)

Question 23

what is formed after the reaction?

Answer 23

an alkene, a type of molecule that has a special bond called a double bond between two carbon atoms. this double bond makes alkenes different from simpler molecules like alkanes.

Question 24

what is ozone?

Answer 24

ozone is formed naturally in the upper atmosphere, is beneficial because it absorbs ultraviolet radiation.

Question 25

why is ozone beneficial?

Answer 25

it absorbs ultra violet radiation, preventing the majority of harmful UV from reaching the earth’s surface.

Question 26

what are chloroflurocarbons (CFCs) and what are their properties?

Answer 26

common halogenoalkanes containing carbon, chlorine and fluorine atoms, e.g. CCl3F.
chemically inert and non-flammable and non-toxic.

Question 27

what are hydroflurocarbons (HFCs) and what are their properties?

Answer 27

common halogenoalkanes containing carbon, hydrogen and fluorine atoms, e.g. CHF2CF2.
chemically inert and non-flammable and non-toxic.

Question 28

what happens when CFCs reach the upper atmosphere?

Answer 28

UV lighr breaks the C-Cl bond to form a Cl*. this then goes onto catalyse the decomposition of ozone and contributes to the in the ozone layer.

Question 29

why is Cl* a catalyst?

Answer 29

this is a catalyst as it is regenerated at the end of the decomposition as a catalyst is a substance that participates in a chemical reaction but remains unchanged at the end of the reaction.

Question 30

what is the overall equation for ozone decomposition?

Answer 30

2 O3(g) → 3 O2(g)

Question 31

what is propagation 1 of ozone decomposition?

Answer 31

O3 + Cl→ClO +O2
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Question 32

what is propagation 2 of ozone decomposition?

Answer 32

ClO* + O3→ 2O2 + Cl*

Question 33

why is ozone decomposition classed as a chain reaction?

Answer 33

the chlorine radicals are regenerated in the propagation steps, they act as catalysts, continuously initiating further reactions and propagating the chain reaction. this self-perpetuating cycle is characteristic of chain reactions, making the ozone depletion process catalytic and leading to the destruction of multiple ozone molecules for each chlorine radical initially formed.

Question 34

why were HFCs developed?

Answer 34

to replace CFCs as scientific research has provided sufficient evidence of the harmful effecys of CFCs, they ban the use of CFCs.