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A-level Chemistry Oxford AQA > 3.3.3 Halogenoalkanes > Flashcards

3.3.3 Halogenoalkanes Flashcards

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1
Q

What is a halogenoalkane?

A

Alkanes with at least one halogen atom in place of hydrogen atom

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2
Q

Describe 3 physical properties of halogenoalkanes

A
  • Insoluble in water ∵ polar Cδ+—Xδ- bonds are not polar enough
  • Main intermolecular forces of attraction are dipole—dipole attraction and van der Waals forces
  • Halogenoalkanes mix with hydrocarbons
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3
Q

Boiling points increase with … & increases when …

A

Boiling points increase with increased chain length & increases when going down the halogen group

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4
Q

Why do boiling points increase with increased chain length or when going down the halogen group?

A

∵ larger the molecule = greater the number of electrons ∴ greater the van der Waals forces

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5
Q

Why do halogenoalkanes have higher boiling points than alkanes with similar chain lengths?

A

∵ have higher relative molecular masses & more polar

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6
Q

Why are carbon-halogen bonds polar?

A

Halogens are much more electronegative than carbon

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7
Q

What makes halogenoalkanes prone to attacks from nucleophiles?

A

δ+ charge on carbon/polar carbon-halogen bond

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8
Q

What is a nucleophile?

A

Electron-pair donor

(negatively charged ion or an atom with δ- charge)

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9
Q

What do nucleophiles have?

A

Has lone (unshared) pair of electrons

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10
Q

As you go down the halogen group, the bonds (C-X) get __ ___

A

less polar

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11
Q

What is a nucleophilic substitution reaction?

A

When a nucleophile reacts with polar molecules by replacing the functional group

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12
Q

Why are nucleophilic substitution reactions used?

A

To introduce new functional groups into organic compounds

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13
Q

Nucleophilic Substitution

Name the reactant

A

Halogenoalkane

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14
Q

Nucleophilic Substitution

Name the nucleophiles

A

:OH-, :CN- & :NH3

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15
Q

Nucleophilic Substitution

Name the condition & product for the nucleophile :OH-

A

Warm, aqueous NaOH or KOH → Alcohol

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16
Q

Nucleophilic Substitution

Name the condition & product for the nucleophile :CN-

A

Warm halogenoalkane with ethanolic KCN(aq) → Nitrile

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17
Q

Nucleophilic Substitution

Name the condition & product for the nucleophile :NH3

A

Warm halogenoalkane with excess ethanolic ammonia → Amine

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18
Q

Why is the reaction between hydroxide ions & halogenoalkanes sometimes called hydrolysis?

A

∵ in the mechanism, the exact same reaction will happen with water

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19
Q

Bromoethane + aqueous NaOH

Name the mechanism

A

Nucleophilic Substitution

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20
Q

Bromoethane + aqueous NaOH

Balanced Equation

A

C2H5Br + NaOH → C2H5OH + NaBr

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21
Q

Bromoethane + aqueous NaOH

Name of Organic Product

A

Ethanol

22
Q

Bromoethane + aqueous NaOH

Mechanism

A
23
Q

Bromoethane + NH3

Name of Mechanism

A

Nucleophilic Substitution

24
Q

Bromoethane + NH3

Balanced Equation

A
26
Q

Bromoethane + NH3

Mechanism

A
27
Q

Bromoethane + NH3

Name of Organic Compound

A

Ethylamine

29
Q

Bromoethane + KCN

Name of Mechanism

A

Nucleophilic Substitution

30
Q

Bromoethane + KCN

Mechanism

A
31
Q

Bromoethane + KCN

Balanced Equation

A

C2H5Br + KCN → C<span>2</span>H5CN + KBr

32
Q

Bromoethane + KCN

Name of Organic Compound

A

Propanenitrile

33
Q

When halogenoalkanes react with ammonia, what can the amine group in the product act as and why & what can this lead to?

A

Can acts as nucleophile∵ it has lone pair of electrons ∴ may react with halogenoalkane molecules = giving mixture of products

34
Q

Why is an excess of ammonia needed to react with a halogenoalkane to produce a high yield?

A
  1. The amine group in the product = nucleophile ∴ could react further & make secondary/tertiary amines OR
  2. Ammonia acts both initially as a nucleophile and then as a base
35
Q

What 2 factors affect the carbon-halogen bond’s reactivity?

A
  • Cδ+—Xδ- bond polarity
  • C—X bond enthalpy (i.e. strength)
36
Q

Bond Enthalpies

Bonds get ____ going down the group

A

Bonds get weaker going down the group

37
Q

Explain why fluorine has the strongest bond

A

It’s the smallest atom of halogens & shared electrons in C—F bond = strongly attracted to nucleus

38
Q

Explain why bonds get weaker going down the group

A

∵ shared electrons in C—X bond gets further away from halogen nucleus = bond becomes weaker

39
Q

What does the polarity of C—X predict & why?

A
  • C—F most reactive
    • ∵ its most polar - Cδ+ has most positive charge ∴ more easily attacked by nucleophile
  • C—I = least reactive
    • ∵ it’s least polar
40
Q

What is the main factor that decides reactivity?

A

Carbon-halogen bond enthalpy

(Experiments show that reactivity increases down the group ∵ bond enthalpy is more important factor than bond polarity)

41
Q

Why do fluoroalkanes undergo nucleophilic substitution reactions more slowly than other halogenoalkanes?

A

C-F bond is strongest = highest bond enthalpy

42
Q

Why are iodoalkanes substituted more quickly than say fluoroalkanes?

A

C-I bond has lowest bond enthalpy = easier to break

43
Q

When do you get an elimination reaction?

A

If you warm a halogenoalkane with hydroxide ions dissolved in ethanol instead of water = elimination reactions happens

44
Q

Elimination Reaction

Name the condition and product when a halogenoalkane is used as a rectant

A

Ethanolic KOH - Alkene

45
Q

2-bromopropane + ethanolic KOH

Name of Mechanism

A

Elimination Reaction

46
Q

2-bromopropane + ethanolic KOH

Balanced Equation

A

C3H7Br + KOH → C3H6 + H2O + KBr

47
Q

2-bromopropane + ethanolic KOH

Name of Organic Compound

A

Propene

48
Q

2-bromopropane + ethanolic KOH

Mechanism

A
49
Q

In an elimination reaction, why do you have to heat mixture under reflux?

A

∵ you’ll otherwise lose the volatile stuff

50
Q

In elimination reactions: small group of atoms….

A

break away from molecule

Group is NOT replaced

51
Q

When halogenoalkanes are reacted with hydroxides, they can undergo…

A

either nucleophilic substitution or elimination → 2 reactions are said to be competing

52
Q

Identify & draw an isomer of 2-bromopentane that does not react with hot, ethanolic sodium hydroxide to produce an alkene

A

1-bromo-2,2-dimethylpropane

53
Q

Suggest one change to the reaction conditions that would increase the yield of an alcohol

(Hot concentrated ethanolic potassium hydroxide reacts with halogenoalkane)

A
  • Use water as a solvent (aq conditions)
  • Lower temperature
  • Dilute KOH
54
Q

Suggest one change to the reaction conditions that would increase the yield of an alkene

A
  • Ethanolic solvent
  • High temperature
  • High concentration of KOH

A-level Chemistry Oxford AQA (34 decks)

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