Halogenoalkanes 2.5

Question 1

What are halogenoalkanes?

Answer 1

Saturated aliphatic compounds formed by replacing a hydrogen atom in an alkane by a halogen atom.

Question 2

What is the general formula for halogenoalkanes?

Answer 2

CnH2n+1X where X represents a halogen atom.

Question 3

Why are halogenoalkanes considered useful organic substrates?

Answer 3

They can readily undergo a number of useful reactions and are key intermediates in organic synthesis.

Question 4

What is a primary halogenoalkane?

Answer 4

A primary halogenoalkane has one carbon atom directly bonded to the carbon atom that is bonded to the halogen.

Exceptions are halomethanes.

Question 5

What is a secondary halogenoalkane?

Answer 5

A secondary halogenoalkane has two carbon atoms directly bonded to the carbon atom that is bonded to the halogen.

Question 6

What is a tertiary halogenoalkane?

Answer 6

A tertiary halogenoalkane has three carbon atoms directly bonded to the carbon atom that is bonded to the halogen.

Question 7

How do the boiling points of halogenoalkanes compare to alkanes with the same number of carbon atoms?

Answer 7

Halogenoalkanes have much higher boiling points than alkanes with the same number of carbon atoms.

This is due to the increased RAM of the halogen atom and stronger van der Waals’ forces.

Question 8

What type of attractions do halogenoalkanes exhibit?

Answer 8

Halogenoalkanes exhibit permanent dipole-dipole attractions due to the polar nature of the C-X bond.

Question 9

How does the boiling point vary among different halogenoalkanes with the same number of carbon atoms?

Answer 9

The boiling point increases when comparing different halogenoalkanes with the same number of carbon atoms.

Question 10

How do isomeric halogenoalkanes vary in their boiling points?

Answer 10

Isomeric halogenoalkanes can have different boiling points; for example, 2-bromo-2-methylpropane has a lower boiling point (73 °C) than 1-bromobutane (101 °C) despite having the same RMM (137).

Question 11

Why do branched halogenoalkanes have lower boiling points?

Answer 11

Increased branching in halogenoalkanes results in weaker van der Waals’ forces due to fewer points of contact, leading to lower boiling points.

Question 12

Why is the C-X bond in halogenoalkanes polar?

Answer 12

The C-X bond is polar because the halogen atom is more electronegative than the carbon atom.

Question 13

How does electronegativity change in Group VII elements?

Answer 13

Electronegativity decreases as Group VII is descended, making the C-X bond less polar.

Question 14

Are halogenoalkanes soluble in water?

Answer 14

Halogenoalkanes are usually insoluble in water.

Question 15

In what type of solvents are halogenoalkanes soluble?

Answer 15

Halogenoalkanes are soluble in organic solvents like hexane and cyclohexane.

Question 16

What is the initiation step in the reaction of halogenoalkanes?

Answer 16

CClF → •CClF + Cl•

Question 17

What is the first propagation step in the reaction of halogenoalkanes?

Answer 17

Cl• + O3 → ClO• + O2

Question 18

What is the second propagation step in the reaction of halogenoalkanes?

Answer 18

ClO• + O3 → Cl• + O2

Question 19

What is the overall reaction for halogenoalkanes?

Answer 19

2O3 → 3O2

Question 20

What is the nature of the carbon-halogen bond in halogenoalkanes?

Answer 20

The carbon-halogen bond is polar, with a slightly positive charge on the carbon atom.

Question 21

What are nucleophiles?

Answer 21

Nucleophiles are species with a lone pair of electrons or a negative charge that are attracted to areas of low electron density.

Question 22

What are the two important reactions of halogenoalkanes?

Answer 22

The two important reactions are substitution and elimination.

Question 23

What occurs during nucleophilic substitution?

Answer 23

The nucleophile is attracted to the positive charge on the carbon atom attached to the halogen, forming a bond and resulting in the halogen being removed as a halide ion.

Question 24

What happens to the electrons during nucleophilic substitution?

Answer 24

The pair of electrons in the C-X bond ends up on the halogen atom, which leaves as a halide ion.

Question 25

What is the reaction of halogenoalkanes with water called?

Answer 25

The reaction is known as hydrolysis.

Question 26

What is the general equation for the hydrolysis of halogenoalkanes?

Answer 26

C,H,X + H2O → C,H,OH + HX

Question 27

How does the rate of hydrolysis depend on the halogen in halogenoalkanes?

Answer 27

The rate of hydrolysis depends on the strength of the C-X bond; lower bond enthalpy means a weaker bond that is more easily broken.

Question 28

Which halogenoalkanes react the most slowly with water?

Answer 28

Fluoroalkanes react the most slowly despite having the most polar C-X bond.

Question 29

What is hydrolysis?

Answer 29

Hydrolysis is breaking up molecules by reaction with water.

Question 30

How can the trend of hydrolysis be investigated?

Answer 30

This trend can be investigated using silver nitrate solution.

Question 31

What reaction occurs when halide ions are produced?

Answer 31

Ag*(aq) + X'(aq) → AgX(s)

Question 32

What role does ethanol play in the reaction?

Answer 32

Ethanol is used as a solvent as it allows the halogenoalkane and silver nitrate solution to mix. Ethanol is referred to as a mutual solvent.

Question 33

Why is nitric acid often added during the reaction?

Answer 33

Nitric acid is often added to prevent precipitation of other salts.

Question 34

What is the effect of using aqueous alkali in substitution reactions?

Answer 34

Substitution of the halogen atom in a halogenoalkane by an -OH group is quicker with an aqueous solution of an alkali.

Question 35

What is commonly used as an alkali in these reactions?

Answer 35

Sodium hydroxide or potassium hydroxide.

Question 36

What is the purpose of heating the reaction under reflux?

Answer 36

The reaction is often heated under reflux to improve the yield and rate of the reaction.

Question 37

What is reflux in chemistry?

Answer 37

Reflux is repeated boiling and condensing of a reaction mixture.

Question 38

What does the cyanide ion (CN-) consist of?

Answer 38

The cyanide ion contains a carbon atom bonded to a nitrogen atom via a triple covalent bond.

Question 39

What is the result of refluxing a halogenoalkane with an ethanolic solution of potassium or sodium cyanide?

Answer 39

The product is a nitrile and contains one more carbon atom than the halogenoalkane used as the reactant.

Question 40

Provide an example of a reaction involving halogenoalkanes and cyanide.

Answer 40

CH-Br + KCN → C2H5CN + KBr

Question 41

What is the role of the ammonia molecule in substitution reactions?

Answer 41

The ammonia molecule can act as a nucleophile due to the lone pair of electrons on the nitrogen atom.

Question 42

What happens when a halogenoalkane is warmed with ammonia in ethanol?

Answer 42

Warming a halogenoalkane with a concentrated solution of ammonia in ethanol in a sealed tube produces a primary amine.

Question 43

Provide an example of a reaction involving halogenoalkane and ammonia.

Answer 43

C2H5 Br + NH3 → C2H5 NH2 + HBr ## Footnote This reaction shows the formation of a primary amine.

Question 44

What is the overall reaction when excess ammonia is used?

Answer 44

C2H5 Br + 2NH3 → C2H5 NH2 + NH4Br ## Footnote This reaction indicates the use of excess ammonia in the substitution process.

Question 45

What is the nucleophile in the nucleophilic substitution of primary halogenoalkanes?

Answer 45

The nucleophile is hydroxide ions (OH).

Question 46

What happens during the nucleophilic substitution mechanism?

Answer 46

The nucleophile (OH) is attracted to the carbon atom and uses a lone pair of electrons to form a bond with it.

Question 47

What occurs as the C-OH bond forms in the nucleophilic substitution mechanism?

Answer 47

The C-X bond breaks, with the pair of bonding electrons leaving with the halide ion (X).

Question 48

What is the theoretical mid-point in the nucleophilic substitution mechanism?

Answer 48

The theoretical mid-point is the transition state where the C-OH bond is half-formed and the C-X bond is half-broken.

Question 49

Can you provide an example of nucleophilic substitution?

Answer 49

An example is the reaction between bromoethane and hydroxide ions.

Question 50

What is represented in the mechanism of the reaction?

Answer 50

The transition state is represented in the mechanism of the reaction.

Question 51

What happens when a nucleophile attacks a primary halogenoalkane?

Answer 51

It can approach the + carbon atom from the side away from the halogen atom.

Question 52

Why can't a nucleophile approach the + carbon atom in a tertiary halogenoalkane?

Answer 52

The + carbon atom is surrounded by alkyl groups and is sterically hindered.

Question 53

What is the mechanism of nucleophilic substitution in tertiary halogenoalkanes?

Answer 53

The reaction occurs via an alternative mechanism.

Question 54

What occurs in the first stage of the reaction between 2-bromo-2-methylpropane and hydroxide ions?

Answer 54

A small proportion of the halogenoalkane ionises to give a carbocation and a bromide ion.

Question 55

How do tertiary carbocations compare to secondary or primary carbocations?

Answer 55

Tertiary carbocations are relatively stable compared with secondary or primary carbocations.

Question 56

What happens once the carbocation is formed?

Answer 56

It reacts immediately with the hydroxide ion as it is strongly attracted towards the positive carbon.

Question 57

What happens when halogenoalkanes react with hydroxide ions in an aqueous solution?

Answer 57

The hydroxide ion acts as a nucleophile and reacts with the halogenoalkane to give a substitution product, an alcohol.

Question 58

What reaction occurs when halogenoalkanes react with hydroxide ions in ethanol?

Answer 58

The hydroxide ion acts as a base, accepting a hydrogen ion from the halogenoalkane to form water, converting the halogenoalkane to an alkene in a reaction known as elimination. ## Footnote Example: CH3CHBrCH3 + KOH → CH3CH=CH2 + KBr + H2O

Question 59

What is elimination in the context of halogenoalkanes?

Answer 59

Elimination is a reaction in which a small molecule is removed from a larger molecule.

Question 60

What factors influence whether substitution or elimination occurs in halogenoalkanes?

Answer 60

The reaction conditions can be adjusted to favor one type of reaction over another, with substitution favored in primary halogenoalkanes and elimination favored in tertiary halogenoalkanes.

Question 61

Which type of halogenoalkane is more susceptible to elimination?

Answer 61

Tertiary halogenoalkanes are more susceptible to elimination than primary halogenoalkanes.

Question 62

What is the favored elimination product when using 2-bromobutane?

Answer 62

The favored product is but-2-ene as it is a more highly substituted alkene than but-1-ene.

Question 63

What are the reagents and products for nucleophilic substitution and elimination reactions?

Answer 63

Reagents include hydroxide, potassium cyanide, and ammonia. Products include alcohols, nitriles, amines, and alkenes.

Question 64

What are the two types of reactions involving hydroxide ions?

Answer 64

Substitution and elimination.

Question 65

What reaction type is favored at low temperature?

Answer 65

Substitution.

Question 66

What reaction type is favored at high temperature?

Answer 66

Elimination.

Question 67

What hydroxide concentration favors substitution?

Answer 67

Dilute.

Question 68

What hydroxide concentration favors elimination?

Answer 68

Concentrated.

Question 69

What solvent is used to favor substitution?

Answer 69

Water.

Question 70

What solvent is used to favor elimination?

Answer 70

Ethanol.

Question 71

What is the main organic product of substitution reactions?

Answer 71

Alcohol.

Question 72

What is the main organic product of elimination reactions?

Answer 72

Alkene.

Question 73

What is the reagent used for nucleophilic substitution to produce alcohol?

Answer 73

Potassium hydroxide ## Footnote Conditions: reflux, water as solvent

Question 74

What is the reagent used for nucleophilic substitution to produce nitrile?

Answer 74

Potassium cyanide ## Footnote Conditions: reflux, water as solvent for KCN, halogenoalkane in ethanol

Question 75

What is the reagent used for nucleophilic substitution to produce amine?

Answer 75

Ammonia ## Footnote Conditions: ammonia in ethanol, heat in a sealed tube

Question 76

What is the reagent used for elimination to produce alkene?

Answer 76

Potassium hydroxide ## Footnote Conditions: reflux, ethanol as solvent