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AQA A Level Chemistry > Halogenoalkanes > Flashcards

Halogenoalkanes Flashcards

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1
Q

General formula for halogenoalkanes

A

CnH2n+1X (where X is the halogen such as Br or Cl or I)

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2
Q

Primary RX

A

When the carbon atom bonded to the halogen atom is attached to one other carbon atom

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3
Q

Secondary RX

A

When the carbon atom bonded to the halogen atom is attached to two other carbon atoms

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4
Q

Tertiary RX

A

When the carbon atom bonded to the halogen atom is attached to three other carbon atoms

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5
Q

What is the reactions of halogenoalkanes called and what happens

A

Nucleophilic substitution reactions, where the halogen atom is replaced by the nucleophile

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6
Q

why is the C-X bond polar

A

Due to the difference in electronegativity. The halogen atom is more electronegative than the carbon atom

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7
Q

What attacks the electron deficient carbon atom

A

Nucleophiles such as OH-, CN-, NH3 and H20

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8
Q

What is a nucleophile

A

A species that donates a lone pair of electrons. A nucleophile is attracted to an electron deficient centre

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9
Q

What happens in the hydrolysis of halogenoalkanes by aqeuous alkali

A

When a halogenoalkane is heated under reflux with AQ NaOH or KOH, the halogen atom is replaced by the nucleophile, OH-, and an alcohol is produced

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10
Q

What happens in the hydrolysis of halogenoalkanes by water

A

Water is a poor nucleophile but it can slowly displace halide ions from a halogenoalkane

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11
Q

What does hydrolysis mean

A

the C-X bond is broken using water

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12
Q

What happens in the reaction between halogenoalkanes with alcoholic solution of potasisium cyanide (CN)

A

When a halogenoalkane is heated under reflux with KCN dissolved in ethanol, the halogen atom is replaced by the nucleophile CN-, and a nitrile is formed

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13
Q

what is the condition needed for nucleophillic substituition

A

Heat under reflux

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14
Q

What organic product is formed by nucleophilic substitution

A

Alcohol

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15
Q

What are halogenoalkanes dissolved in

A

ethanol since they are insoluble in water

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16
Q

What colour precipitate does AgCl form

A

White

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17
Q

What colour precipitate does AgBr form

A

Cream

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18
Q

What colour precipitate does AgI form

A

Yellow

19
Q

What is the condition needed for nucleophilic substitution of ammonia dissolved in ethanol

A

heat in a sealed tube

20
Q

What is ammonia used as in the nucleophilic substitution reaction

A

in the first step - as a nucleophile as it donates a lone pair of electrons

in the second step - as a base as it reacts with a H+ ion to form an NH4 + ion

21
Q

Describe the general test for a halogenoalkane

A

Add aqeuous sodium hydroxide and heat.

Then add excess dilute nitric acid followed by silver nitrate solution and note the colour of the precipitate formed.

Then add dilute aqeuous ammonia or concentrated aq ammonia to the precipitate and note the solubility of the precipitate

22
Q

When do you use dilute aq ammonia in the test for halogenoalkanes and why

A

To differentiate between white + cream precipitate or white + yellow precipitate as Cl is solube in dilute aq ammonia and Br is slightly soluble and I is insoluble

23
Q

When do you use concentrated aq ammonia in the test for halogenoalkanes and why

A

To differentiate between cream and yellow precipitates because Br is soluble in this but I is not soluble

24
Q

Why is sodium hydroxide solution added when testing for halogenoalkanes

A

To hydrolyse the halogenoalkanes and liberate the halide ion

25
Q

Why is dilute nitric acid added before the silver nitrate solution in the test for halogenoalknes

A

Nitric acid is added to neutralise the sodium hydroxide. The OH- ions would interfere with the test because they react with Ag+ to form a precipitate

26
Q

What is the trend in reactivity of halogenoalkanes

A

iodine > bromine > chlorine

27
Q

What bonds do halogenoalkanes contain

A

Polar bonds

28
Q

Why are iodoalkanes the most reactive

A

The bond enthalpy of C-I < C-Br < C-CI, so the C-I bond is the weakest.

Less energy is needed to break the C-I bond, so iodoalkanes react faster than the others

29
Q

What happens to the C-X bond as you go down group 7

A

The bond gets larger and therefore weaker as you go down G7

30
Q

What happens in elimination reactions

A

X leaves as X-

OH- removes a H+ ion from the C atom next to the C with the halogen

An alkene is formed

31
Q

What is the condition needed for an elimination reaction

A

Heat

32
Q

How to tell if the reaction is elimination or nucleophilic substitution

A

Elimination reaction uses a reagent dissolved in ethanol
Nucleophilic substitution uses an aq reagent

33
Q

Why are a mixture of alkenes formed when a secondary halogenoalkane reacts with KOH dissolved in ethanol

A

The H+ ion can be removed from a carbon atom on either side of the carbon atom that is bonded to the halogen atom

34
Q

What does OH- act as when a HX reacts with KOH dissolved in ethanol

A

a base as it removes a H+ ion

35
Q

What kind of compounds can elimination reactions never form

A

Branched compounds

36
Q

why were CFC’s used as refridgerants

A

they are unreactive ,not toxic and not corrosive

37
Q

why are CFC’S no longer used

A

They are the main source of chlorine free radicals which react with ozone and break down the Earth’s protective ozone layer

38
Q

Where is ozone formed

A

naturally in the upper atmosphere

39
Q

Why is ozone beneficial

A

It absorbs ultraviolet radiation

40
Q

What are the dangers of ultraviolet radiation entering the Earth’s atmosphere

A

causes skin cancer and eye cataracts

41
Q

Why are HFC’s used instead of CFC’s now

A

HFC’s are not broken down by ultraviolet radiation as the C-F bonds are very strong and no chlorine free radicals can be formed

42
Q

Equations which show how chlorine atoms catalyse decomposition of ozone

A

Cl* + O3 → ClO* + O2

ClO* + O3 → 2O2+ Cl*

43
Q

What is the overall reaction for the depletion of ozone

A

2O3 —–> 3O2 and chlorine free radicals are the catalyst

44
Q

Why might excess ammonia be used

A

To prevent further substitution

AQA A Level Chemistry (27 decks)

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