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A-Level Organic Chemistry AQA (Paper 2) > Halogenoalkanes > Flashcards

Halogenoalkanes Flashcards

1
Q

What are halogens? (2)

A
  • Group 7 atoms (known as salt makers in Greek).

* Fluorine, chlorine, bromine, iodine.

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2
Q

What are halides?

A

•Group 7 ions.

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3
Q

What are halogenoalkanes/haloalkanes?

A

•They have an alkane skeleton with one or more halogen atoms in the place of hydrogen atoms.

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4
Q

What is the general formula for halogenoalkanes? (2)

A
  • CnHn+1X

* Where X is the halogen.

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5
Q

What can the general formula for halogenalkanes be shortened to?

A
  • RX

* Where R is the alkyl group (hydrocarbon chain).

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6
Q

How do you name halogenoalkanes? (5)

A
  • 1). Name the alkane.
  • 2). Use a prefix to show the halogen.
  • 3). Use a number to show where the halogen is bonded if there are isomers.
  • 4). Use prefixes (di-, tri-, tetra-) to show how many halogens are bonded.
  • 5). Halogens are listed alphabetically.
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7
Q

What are the prefixes used for halogens? (4)

A
  • Fluorine = fluoro
  • Chlorine = chloro
  • Bromine = bromo
  • Iodine = iodo
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8
Q

Draw the structure of iodo-methane.

A

•CH3I.

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9
Q

Draw the structure of 1,1-diiodo-propane. (3)

A
  • C3H6I2.
  • Two iodine atoms bonded to the first carbon.
  • Propane chain.
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10
Q

Draw the structure of dichloro-methane. (2)

A
  • CH2Cl2.

* No prefix numbers as there are no isomers.

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11
Q

What is electronegativity?

A

•The ability of an atom to attract a shared pair of electrons in a covalent bond.

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12
Q

What is a polar bond? (3)

A
  • When electrons in a bond are shared unevenly.
  • The more electronegative element has a slight negative charge (ᵟ-) in the bond.
  • The less electronegative element has a slight positive charge (ᵟ+).
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13
Q

What is the bond polarity of halogenoalkanes? (2)

A
  • C⎯⎯⎯X bond which is polar (Cᵟ+⎯⎯⎯Xᵟ-), halogens are more electronegative than carbon. (‘X’ being the halogen)
  • Bonds get less polar as you go down group 7.
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14
Q

What is the electronegativity of carbon?

A

•2.5.

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15
Q

What is the electronegativity of the halogens? (4)

A
  • Fluorine = 4.0
  • Chlorine = 3.5
  • Bromine = 2.8
  • Iodine = 2.6
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16
Q

What are the main intermolecular forces of attraction of halogenoalkanes?

A

•Dipole-dipole and van der Waal forces.

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17
Q

What is the solubility of halogenoalkanes?

A

•The polar Cᵟ+⎯⎯⎯Xᵟ- bonds are not polar enough to make the halogenoalkanes soluble in water.

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18
Q

What does the boiling point of halogenoalkanes depend on?

A

•The number of carbon atoms and halogen atoms.

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19
Q

What happens to the boiling point when the chain of halogenoalkanes increases?

A

•It increases- the larger the molecules, the greater the number of electrons and vdW forces.

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20
Q

What happens to the boiling point when you go down the halogen group?

A

•It increases- the larger the molecules, the greater the number of electrons and vdW forces.

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21
Q

When halogenoalkanes react, which bond usually breaks?

A

•C⎯⎯⎯X.

22
Q

What factors determine how readily the C⎯⎯⎯X bond reacts? (2)

A
  • The Cᵟ+⎯⎯⎯Xᵟ- bond polarity.

* The C⎯⎯⎯X bond enthalpy.

23
Q

Halogens are more electronegative than… (2)

A
  • Carbon, so the bond polarity will be Cᵟ+⎯⎯⎯Xᵟ-.
  • The carbon bonded to the halogen has a partial positive- it is electron deficient and can be attacked by reagents that are electron rich or have electron-rich areas (nucleophiles).
24
Q

What is a nucleophile?

A

•An electron pair donor.

25
Q

The polarity of the C⎯⎯⎯X bond would predict that… (2)

A
  • The C⎯⎯⎯F bond would be the most reactive, it is the most polar, the Cᵟ+ has the most positive charge and is therefore the most easily attacked by a nucleophile.
  • ∴ The C⎯⎯⎯I bond would be the least reactive because it is the least polar.
26
Q

Bond enthalpies get weaker going… (4)

A
  • Down the group.
  • Fluorine is the smallest atom of the halogens and the shared electrons in the C⎯⎯⎯F bond are strongly attracted to the fluorine nucleus. This makes a strong bond.
  • Going down the group, the shared electrons in the C⎯⎯⎯X bond get further and further away from the halogen nucleus, so the bond becomes weaker.
  • Bond enthalpies predict that iodo-compounds with the weakest bonds were the most reactive and fluoro-compounds with the strongest bonds are the least reactive.
27
Q

What have experiments confirmed to be the more important factor in reactivity of halogenoalkanes?

A

•Bond polarity as reactivity increases going down the group.

28
Q

What are nucleophiles? (3)

A
  • Reagents that attack and form bonds with positively or partially charged carbon atoms.
  • Either negatively charged ions or they have an atom with a ᵟ-.
  • They have a lone (unshared) pair of electrons which it can use to form a covalent bond.
29
Q

Where is the lone pair of a nucleophile situated?

A

•On an electronegative atom.

30
Q

Give examples of three common lone pairs. (3)

A
  • Hydroxide ion: ^-:OH.
  • Ammonia: :NH3.
  • Cyanide ion: ^-:CN.
31
Q

What are nucleophilic substitution reactions?

A

•When nucleophiles replace the halogen in a halogenoalkane.

32
Q

What are mechanisms? (2)

A
  • They describe a route from reactants to products via a series of theoretical steps.
  • These may involve short-lived intermediates.
33
Q

Draw and explain the mechanism of nucleophilic substitution. (4)

A

•The lone pair of electrons of a nucleophile is attracted towards a partially positively charged carbon atom.
•A curly arrow starts at a lone pair of electrons and moves towards Cᵟ+.
•The lower curly arrow show the electron pair in the C⎯⎯⎯X bond moving to the halogen atom, C, and making it a halide ion.
•The halide ion is called the leaving group.
(See textbook 208)

34
Q

What does the rate of nucleophilic substitution depend on? (2)

A
  • The halogen- as you go down the group the rate of reaction increases.
  • Fluoro-compounds are unreactive due to the strength of the C⎯⎯⎯F bond. (Bond enthalpies)
35
Q

What are the conditions when halogenoalkanes reacts with sodium (or potassium) hydroxide during nucleophilic substitution? (3)

A
  • Aqueous NaOH/KOH.
  • Warm/reflux
  • Halogenoalkanes do not mix with water, so ethanol is used as a solvent in which the halogenoalkane and the aqueous NaOH both mix (hydrolysis reaction, H2O has OH^-).
36
Q

What is made when halogenoalkanes react with hydroxide ions?

A

•Alcohols.

37
Q

What is made when halogenoalkanes react with cyanide ions?

A

•Nitriles are formed.

38
Q

What are the conditions when halogenoalkanes react with cyanide ions during nucleophilic substitution? (2)

A
  • Potassium cyanide solution (aqueous).

* Warm/reflux with an alcoholic solution (ethanolic).

39
Q

What is made when halogenoalkanes react with ammonia?

A

•Amines, RNH2 (a neutral product).

40
Q

Draw and explain the mechanism of nucleophilic substitution reaction with ammonia. (3)

A
  • Ammonia is a nucleophile because it has a lone pair of electrons that it can donate (although it has no negative charge) and the nitrogen atom has a delta negative charge.
  • Because ammonia is a neutral nucleophile, a proton (H^+) must be lost to form the neutral product (amine).
  • The H^+ ion reacts with a second ammonia molecule to form an NH4^+ ion.
41
Q

What are the conditions when halogenoalkanes react with ammonia during nucleophilic substitution? (3)

A
  • Excess conc NH3.
  • Dissolved in ethanol (water has OH^-).
  • Under pressure in a sealed tube and heated.
42
Q

What makes nucleophilic substitution reactions useful?

A

•They are a way of introducing new functional groups into organic compounds.

43
Q

What are elimination reactions in halogenoalkanes? (2)

A
  • When a hydrogen halide is eliminated from the molecule, leaving a double bond in its place.
  • An alkene is formed.
44
Q

What is a base?

A

•A proton (H^+) acceptor.

45
Q

What is an acid?

A

•A proton (H^+) donor.

46
Q

What are the conditions for elimination reactions when halogenoalkanes react with hydroxide ions? (3)

A
  • Hot.
  • Ethanolic.
  • Concentrated source of OH^- e.g. KOH.
47
Q

How can an OH^- ion act as a base? (2)

A
  • It removes a H^+ ion from the halogenoalkane,
  • E.g. When bromoethane reacts with potassium hydroxide, a molecule of hydrogen bromide (HBr) is eliminated then the hydrogen bromide reacts with the potassium hydroxide. The reaction produces ethene, bromide and water.
48
Q

Draw and explain the mechanism of an elimination reaction when potassium hydroxide reacts with bromoethane. (4)

A

•The OH^- ion uses its lone pairs to form a bond with one of the hydrogen atoms on the carbon next to the C-Br bond.
•These hydrogen atoms are delta positive.
•The electron pair from the C-H bond now becomes part a carbon-carbon double bond.
•The bromine takes the pair of electrons in the C-Br bond and leaves as a bromide ion (the leaving group).
(See textbook page 211).

49
Q

What are chlorofluorocarbons (CFCs)? (4)

A
  • Halogenoalkanes containing both chlorine and fluorine atoms but no hydrogen e.g. trifluoromethane (CCl3F).
  • They are very unreactive under normal conditions.
  • The short chains are gases and were used as aerosol propellants, refrigerants and blowing agents for foams like expanded polystyrene.
  • Longer chains are used as dry cleaning and de-greasing solvents.
50
Q

Explain why CF3CH3 does not lead to the depletion of the ozone in the upper atmosphere compared to other CFCs. (2)

A
  • It doesn’t contain any Cl to be released as Cl•.

* C—F bonds are strong and don’t break so no F• is released.

51
Q

Suggest why bonds in molecules such as CO2 and CF3CH3 absorb infrared radiation.

A

•Bonds vibrate/stretch/bend and are polar.

A-Level Organic Chemistry AQA (Paper 2) (17 decks)

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