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Chemistry AS AQA > Haloalkanes > Flashcards

Haloalkanes Flashcards

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1
Q

General formula of haloalkanes

A

Cn H2n+1 X

or R-X

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2
Q

Primary haloalkane

A

The carbon that the halogen is bonded to is itself only bonded to one other carbon

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3
Q

Secondary haloalkane

A

The carbon that the halogen is bonded to is itself bonded to two other carbons

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4
Q

Tertiary haloalkane

A

The carbon that the halogen is bonded to is itself bonded to three other carbons

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5
Q

Why are haloalkanes not soluble in water?

A

Because the C-X bond is not polar enough.

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6
Q

What mechanism forms haloalkanes?

A

Free radical substitution

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7
Q

What are the three steps in free radical substitution?

A

Initiation, propagation, termination.

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8
Q

Describe how free radical substitution happens

A

Initiation- UV light breaks the bond in the halogen molecule creating two halogen atoms each with a lone electron (free radical). This is homolytic fission. Propagation- Halogen radical reacts with hydrogen atom from an alkane making a methyl free radical and hydrogen halide. The methyl free radical reacts with a halogen molecule creating a haloalkane and another halogen free radical. Termination. Free radicals are removed by reacting with other free radicals.

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9
Q

Where is ozone naturally formed?

A

In the stratosphere and troposphere

upper atmosphere

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10
Q

Why is ozone beneficial?

A

It blocks most of the UV radiation from the sun which is harmful to life.

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11
Q

How do chlorine atoms form in the upper atmosphere?

A

CFCs from refrigerators end up in the stratosphere. The UV radiation is absorbed by them and it breaks the C-Cl bond. One of the shared electrons goes to the released chlorine atom forming a chlorine free radical.

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12
Q

Chlorine and ozone equations

A 
CCl3F -> *CCl2F + Cl
*Cl + O3 -> *ClO + O2
*ClO + O3 -> *Cl + 2O2
2O3 -> 3O2
Equation 3 shows how chlorine free radicals can be regenerated so continue to catalyse the decomposition of the ozone layer.
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13
Q

What alternatives to CFCs have been made?

A

Alkanes and hydrofluorocarbons (chlorine free)

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14
Q

What is the role of the chlorine atom in the decomposition of ozone?

A

To find an alternative route or lower activation energy.

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15
Q

Define nucleophile

A

A negative ion or molecule that is able to donate a pair of electrons (its lone pair) to an electron deficient area of another reactant.

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16
Q

Mechanism for nucleophilic substitution with hydroxide ions

A

Arrow from lone pair on oxygen atom to the delta positive carbon atom. Arrow from C-X bond to the delta negative halogen. OH is where X was and X is the leaving group with a lone pair and full negative charge. 50:50 ethanol:water. Can happen at room temperature but slowly. Forms an alcohol.

17
Q

Mechanism for nucleophilic substitution with cyanide ions

A

Arrow from lone pair on the carbon atom (cyanide) to the delta positive carbon atom (haloalkane). Arrow from C-X bond to the delta negative halogen. CN is where X was and X is the leaving group with a lone pair and full negative charge. The solution is aqueous and alcoholic and warmed. The product is a nitrile with one more carbon than the original haloalkane.

18
Q

Mechanism for nucleophilic substitution with ammonia

A

Arrow from lone pair of nitrogen atom to the delta positive carbon. Arrow from C-X bond to the delta negative halogen. NH3 is where X was and X is leaving group with a lone pair and full negative charge. Arrow from N+-H bond to N. Arrow from lone pair of nitrogen atom from other ammonia molecule to same H. Forms amine (with lone pair on N) and ammonium (+ on N). Needs excess concentrated ammonia solution in ethanol carried out under pressure.

19
Q

Define hydrolysis

A

Splitting molecules using water

20
Q

Experiment to compare the rates of hydrolysis of haloalkanes

A

Add 1cm^3 of each to separate test tubes. Pipette 1cm^3 of silver nitrate solution into one and time how long it takes until a precipitate is first seen. Do the same with the other test tubes under the same temperature. Shorter time means a higher rate of reaction,

21
Q

Mechanism for elimination with hydroxide ions

A

Arrow from lone pair of oxygen atom to hydrogen bonded to carbon next to the carbon the halogen is bonded to. Arrow from C-H bond (same H) to C-C bond. Arrow from C-X bond to halogen. X is leaving group with a lone pair and full negative charge. Forms an alkene and water. Needs ethanol as solvent with no water and is heated.

22
Q

Which reaction will each type of haloalkane tend to do?

A

Primary - substitution
Secondary - both
Tertiary- elimination

Chemistry AS AQA (14 decks)

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