Halogenoalkanes

Question 1

What are Halogenoalkanes? (2)

Give the basic definition and general formula.

Answer 1

• Halogenoakanes are alkanes where one or more of the hydrogen atoms has been replaced by a halogen (e.g. chlorine, bromine).
• Their general formula is CnH2n+1X, where X is any halogen.

Question 2

How are Halogenoalkanes named? (3)

Explain how prefixes and position numbers are used.

Answer 2

• The prefixes -fluoro, -chloro, -bromo and -iodo indicate a halogenoalkane, and go in alphabetical order.
• Position numbers indicate the carbon on which a halogen is located.
• Prefixes such as -di, -tri and -tetra are used when there are mutiple of the same halogen on a carbon chain.
• e.g. 1,1-dibromo-2-chloropropane

Question 3

How are Halogenoalkanes Classified? (3)

Give the classifications and explain.

Answer 3

• Primary
• Secondary
• Tertiary
  This is determined by the number of C-C bonds the carbon bonded to the halogen contains.

Question 4

What is a Primary Halogenoalkane? (1)

Give the basic definition.

Answer 4

The halogen is attached to a carbon with only one C-C bond.

Question 5

What is a Secondary Halogenoalkane? (1)

Give the basic definition.

Answer 5

The halogen is attached to a carbon with two C-C bonds.

Question 6

What is a Tertiary Halogenoalkane? (1)

Give the basic definition.

Answer 6

The halogen is attached to a carbon with three C-C bonds.

Question 7

What is the Main Property of Halogenoalkanes? (3)

Give the main feature that makes halogenoalkanes reactive.

Answer 7

• In halogenoalkanes, the carbon-halogen bond is polar, due to the halogen being more electronegative.
• This makes the carbon δ+ and the halogen δ-.
• This allows them to undergo nucleophilic substitution reactions.

Question 8

Explain Melting Points of Halogenoalkanes. (3)

When do melting points increase and why?

Answer 8

• Increases with increased chain length.
• Increases down the group.
• This is because in both cases VdW forces increase, resulting in stronger attractions between molecules.

Question 9

How are Melting Points of Halogenoalkanes Different to Alkanes? (4)

Describe and explain why they are higher.

Answer 9

• As halogenoalkanes are polar, the molcules experience permanent dipoles.
• This means that halogenoalknes experience permanent dipole-dipole attractions and VdW forces, whereas alkanes only experience VdW forces,
• As a result melting and boiling points of halogenoalkanes are higher.

Question 10

Describe Solubility of Halogenoalkanes. (2)

Explain solubilities and how they change with carbon chain length.

Answer 10

• Halogenoallkanes are only slightly soluble in water, as their VdW forces and D-D forces are weaker than the hydrogen bonds between water molecules.
• Solubility of halogenoalkanes decreases as carbon chain length increases.

Question 11

Why do Carbon-Halogen Bond Enthalpies Decrease down the Group? (3)

Give three reasons.

Answer 11

• Increasing atomic radius of the halogens.
• Increasing carbon-halogen bond length.
• Decreasing electrostatic attraction between bonding electrons and nuclei - less difference in electronegativity.

Question 12

How does Rate of Reaction of Halogenoalkanes change down the Group? (3)

Describe and explain reasoning.

Answer 12

• Down the group, C-X bond enthalpy decreases due to reduced difference in electronegativity.
• As a result, energy required to break these bonds decreases. Therefore activation energy decreases.
• This means halogenoalkanes are more reactive down the group.

Question 13

Which C-X bonds are
Most Reactive and Least Reactive? (4)

Show all C-X bonds.

Answer 13

• Most reactive
• C-I
• C-Br
• C-Cl
• C-F
• Least Reactive

Question 14

How can the Reactivity of Halogenoalkanes be Measured? (3)

Describe and explain the process.

Answer 14

• Silver Nitrate test - Halogenoalkanes are dissolved in ethanol, then aqueous silver nitrate is added.
• A precipitate forms - the colour indicates type of halogen, time taken indicates rate of reaction.

Question 15

What Colour Precipitate does each Halogen Form? (3)

Give the formulas and colours.

Answer 15

• AgCl = White Precipitiate
• AgBr = Cream Precipitate
• AgI = Yellow Precipitate

Question 16

What is a Nucleophile? (1)

Give the basic definition.

Answer 16

• A nucleophile is a species which is attracted to other positive species. They donate electron pairs to form a new covalent bond.
• (nucleo = positive, phile = loving)

Question 17

Describe the Steps of a Nucleophilic Substitution Reaction. (5)

Explain how the new products are formed step by step.

Answer 17

• A nucleophile approaches the halogenoalkane which has a polar C-X bond.
• It donates its lone pair of electrons to the δ+ carbon, forming a new covalent bond.
• This causes the C-X bond to break as the halogen atom takes both the shared electrons.
• The halogen departs as a halide ion (X-), being replaced by the nucleophile.

Question 18

How is the Nucleophilic Substitution Mechanism Drawn? (2)

Describe the curly arrows.

Answer 18

• A curly arrow is drawn from the lone pair of the nucleophile (:Nuc) to the δ+ carbon.
• A second curly arrow is drawn from the C-X bond to the halogen.
• The product and halide ion are then drawn.

Question 19

What are the Three Types of Nucleophiles? (3)

Give all three nucleophiles for the mechanisms.

Answer 19

• OH- ions
• CN- ions
• NH3 (Ammonia)

Question 20

Describe Nucleophilic Substitution with OH- Ions. (2)

Name the product formed, and possible reagents.

Answer 20

• Forms alcohols from halogenoalkanes.
• Reagents include NaOH or KOH.

H2O could also be used but this is a much slower reaction.

Question 21

What are the Conditions for Nucleophilic Substitution with OH- Ions? (2)

Name the two conditions required for the reaction.

Answer 21

• High temperature - increases rate of reaction
• Aqueous conditions

Question 22

Describe Nucleophilic Substitution with CN- Ions. (2)

Name the product formed, and possible reagents.

Answer 22

• Forms nitriles from halogenoalkanes.
• Reagents include NaCN or KCN

Question 23

What are the Conditions for Nucleophilic Substitution with CN- Ions? (2)

Name the two conditions required for the reaction.

Answer 23

• Heating under reflux
• Dissolved in ethanol/ethanolic

They cannot be aqueous, as then OH- ions would be present and would form alcohols.

Question 24

Describe Nucleophilic Substitution with Ammonia. (3)

Name the product formed, explqain the mechanism.

Answer 24

• Forms amines from halogenoalkanes.
• The first step involves an ammonia molecule replacing the halogen.
• The second step involves a second ammonia molecule accepting a proton, forming the amine and an NH4+ ion.

Question 25

What are the Conditions for Nucleophilic Substitution with Ammonia? (2) | Name the two conditions required for the reaction.

Answer 25

* **Heated under pressure** * Ammonia in **excess, concentrated and ethanolic**

Question 26

What are the Classifications of Amines? (4) | Give all four types.

Answer 26

* **Primary** Amine * **Secondary** Amine * **Tertiary** Amine * **Quaternary** Ammonium Salt

Question 27

How are Different Amines Formed? (2) | Describe the process.

Answer 27

* Amines contain a nitrogen with a lone pair of electrons, allowing it to **act as an nucleophile**. * These amines **react with halogenoalkanes** to form another type of amine with an extra R group. * (*e.g. Primary ---> Secondary*)

Question 28

How is the Concentration of Different Amines Controlled? (2) | Describe the conditions which are used to control the products formed.

Answer 28

* Adding **excess ammonia** causes more **primary amines** to be formed, as more ammonia reacts with halogenoalkanes. * Adding **excess halogenoalkane** causes more **quaternary amines** to be formes, as more amines react with halogenoalkanes.

Question 29

Describe Elimination Reactions. (2) | State the product formed and the reagent required.

Answer 29

* Elimination reactions **form alkenes** from halogenoalkanes. * They **require a base** (*e.g. KOH, NaOH*) to react and undergo this reaction.

Question 30

Describe the Steps of an Elimination Reaction. (4) | Describe and explain the process of elimination, step by step.

Answer 30

* Halogenoalkanes **react with a base, usually OH- ions**. * The base **accepts a H+ ion** from the halogenoalkane. * The adjacent carbon **forms a C=C double bond**. * The movement of electrons then breaks the adjacent carbon-halogen bond, **eliminating a halide ion**.

Question 31

How is the Elimination Mechanism Drawn? (3) | Describe the curly arrows drawn.

Answer 31

* A curly arrow is drawn from the **base to a hydrogen atom**. * An arrow from this bond is then **drawn to a C-C bond**, showing the movement of electrons forming a pi bond. * Finally, an arrow is **drawn from the C-X bond to the halogen**, forming a halide ion.

Question 32

What are the Conditions Required for an Elimination Reaction? (3) | Give the three conditions required.

Answer 32

* The base must be **concentrated and ethanolic**. * Occurs in **high temperatures** and **anhydrous** conditions.

Question 33

What Types of Isomers can Form during an Elimination Reaction? (3) | Name both types of isomers, what they mean, and why they form.

Answer 33

* Structural isomers - **position of C=C bond can change** if different hydrogens are removed. * Stereoisomers - forms a double bond with **restricted rotation**, as **different hydrogens can be removed**.

Question 34

How are Halogenoalkanes Formed? (2) | Name the mechanism and what reacts.

Answer 34

* **Free Radical Substitution** Mechanism * **Halogens react with alkanes** in the presence of UV radiation.

Question 35

What are the Steps in Free Radical Substitution? (3) | Name all three stages and how many radicals form.

Answer 35

* **Initiation** (*0 -> 2 radicals*) * **Propagation** (*1 -> 1 radical*) * **Termination** (*2 -> 0 radicals*)

Question 36

What is the Initiation Step in Free Radical Substitution? (2) | Describe and state the reaction.

Answer 36

* A halogen molecule is **exposed to UV radiation**, releasing halogen radicals. * **e.g. Cl2 ---> 2Cl***

Question 37

What are the Propagation Steps in Free Radical Substitution? (3) | Describe and state the reactions.

Answer 37

* Halogen radicals **react with an alkane, forming halogenoalkanes** and regenerating radicals until reactants are used up. * **e.g. Cl* + CH4 ---> CH3* + HCl** * **CH3* + Cl2 ---> CH3Cl + Cl***

Question 38

What are the Termination Steps in Free Radical Substitution? (4) | Describe ad state all three possible reactions.

Answer 38

* **Two radicals react** to form a covalent molecule. * **e.g. CH3* + Cl* ---> CH3Cl** * **Cl* + Cl* ---> Cl2** * **CH3* + CH3* ---> C2H6**

Question 39

What Reactions Happen when each Reactant is in Excess? | Describe what reactions can happen.

Answer 39

* **Excess halogen** - more substitution reactions occur to form -di, -tri and -tetra halogenoalkanes. * **Excess alkane** - mostly single substitutions occur.

Question 40

What are CFCs? (2) | Give the basic definiton and why they are relevant.

Answer 40

* CFCs stands for **chlorofluorocarbons**, which are halogenoalkanes containing **only carbon, fluorine and chlorine**. * These were **responsible for ozone depletion** in the atmosphere, from use as refridgerants.

Question 41

What is the Function of Ozone? (1) | Explain why the ozone layer is important.

Answer 41

* Ozone reduces human exposure by **absorbing harmful UV radiation**.

Question 42

Explain the Process of Ozone Depletion. (3) | Explain how ozone depletion happens and give the overall equation.

Answer 42

* CFCs are stable molecules, however, when exposed to **UV radiation** C-X bonds can break causing **chlorine radicals to be released**. * These begin a chain reaction converting ozone into oxygen, through free radical substitution. * Overall Equation = **2O3 ---> 3O2**.

Question 43

What is the Initiation Step of Ozone Depletion? (2) | Explain how the reaction begins and give an example equation.

Answer 43

* CFCs or chlorine molecules are **exposed to high-energy UV radiation**, which breaks C-Cl bonds, releasing chlorine radicals. * **E.g. CF2Cl2 ---> CF2Cl* + Cl***

Question 44

What are the Propagation Steps in Ozone Depletion? (3) | Explain and give both propagation equations.

Answer 44

* Chlorine radicals initiate a chain reaction that depletes ozone - chlorine radicals **react with ozone**, and are then **regenerated**. * **Cl* + O3 ---> ClO* + O2** * **ClO* + O3 ---> 2O2 + Cl***

Question 45

What Alternatives to CFCs are Used? (3) | State both types of molecules and why they are used.

Answer 45

* **Hydrofluorocarbons** * **Hydrocarbons** * These molecules have high bond enthalpies and **cannot produce chlorine radicals**. This reduces ozone depletion.