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AQA A-Level Organic Chemistry > 3.4 Alkenes > Flashcards

3.4 Alkenes Flashcards

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1
Q

What are some properties about alkenes?

A 
Unsaturated hydrocarbons 
Homologous series
Functional group of a double bond
General formula CnH2n (only applies to one double bond) 
Insoluble in the water
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2
Q

Are alkenes more or less reactive than alkanes? Why?

A

More

They have a double bond

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3
Q

Why are alkenes insoluble in water?

A

Water uses hydrogen bonding and alkenes use van der vaals

Therefore they have incompatible intermolecular forces

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4
Q

What is the trend in boiling points relating to carbon chain length? Why?

A

As the carbon chain length increases the boiling point increases

It has a larger surface area therefore stronger intermolecular forces are which are harder to overcome

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5
Q

What does the bonding in alkenes include?

A

Sigma bond

Pi bond

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6
Q

What is stereoisomerism?

A

When molecules have the same molecular and structural formulae but a different arrangement of the atoms in space

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7
Q

Why does stereoisomerism occur in alkenes?

A

Because the double bond in alkenes does not allow rotation

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8
Q

What type of isomerism is stereoisomerism?

A

Geometrical isomerism or E-Z isomerism

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9
Q

In E-Z isomerism how do you determine which molecule is E or Z?

A

Z - the one with the larger atomic mass on one side

E - the molecule with a smaller difference in masses on each side

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10
Q

What are the reactions of alkenes?

A

Combustion
Electrophilic addition
Electrophilic addition reaction with concentrated sulphuric acid
Reaction with water

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11
Q

What are some points about combustion of alkenes?

A

They don’t burn very well often incomplete forming carbon

Alkenes are made by cracking making them valuable and expensive for plastics

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12
Q

In electrophilic reactions what is the reagent?

A

Electrophile

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13
Q

What is an electrophile?

A

Attracted to negative charge (electrons)

Therefore the less EN charged atom in the molecule (the positive delta one) acts as the electrophile

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14
Q

What are some examples of electrophiles?

A 
HCl
HBr
HI
Cl2
Br2
I2
H2SO4 
H2O
H2
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15
Q

What is an addition reaction?

A

Where 2 or more molecules join together by breaking the π bond to form a single product

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16
Q

What is the first part of the mechanism for electrophilic addition reactions?

A

Put the electrophile closest to the alkene in the vertical molecule

An arrow from the double bond to the electrophile
An arrow from the middle of the bond of the electrophile to the more EN atom attached (the delta negative one)

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17
Q

What is the second part of the mechanism for electrophilic addition reactions?

A

A temporary intermediate is formed called a carbocation

An arrow goes from the free radical of the negatively charged atom to the carbon on the carbocation

The carbon with only 3 bonds has a positive charge on it

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18
Q

What is the third part of the mechanism for electrophilic addition reactions?

A

Draw the (probably halogenoalkane) by putting the free radical in the gap on the carbon

19
Q

What takes place in the first part of electrophilic addition reactions?

A

Heterolytic fission

Meaning an atom takes both electrons as opposed to one to each

20
Q

What is the carbocation scale?

A

Tertiary - secondary - primary

21
Q

Explain the carbocations?

A

Tertiary - 3 carbons attached to the carbon with only 3 bonds

Secondary - 2 carbons attached to the carbon with only 3 bonds

Primary - 1 carbon attached to the carbon with only 3 bonds

22
Q

What is important about the stability of carbocations?

A

When the alkene double bond breaks it has two possible outcomes

Therefore you get more of the product with a more stable intermediate/carbocation

23
Q

What is the stability of the carbocations?

A

The inductive effect of the alyl groups as they cancel out the positive charge

The electrons are being pulled towards the positive charge

24
Q

How can Br2 Cl2 etc… be used as electrophiles in electrophilic addition reactions?

A

Because of fluctuating electron density
This causes a temporary induced dipole giving rise to Van der Waal forces

The delta postive end will be attracted to the double bond making a permanent dipole as the double bonds electrons are available

25
Q

What is the display formula of cH2SO4 (Sulphuric acid)?

A 
H +
          |
         O -
          |
    O=S=O
          |
         O
          |
         H
26
Q

What is different about electrophilic addition reaction with sulphuric acid?

A

Nothing it’s the exact same mechanism

27
Q

What can be created after electrophilic addition of sulphuric acid?

A

Alcohols
Where H2O is added
And H2SO4 (sulphuric acid) or H3PO4 (phosphoric acid) can act as a catalyst

28
Q

What are the conditions for making alcohols from alkenes?

A

High heat

High pressure

29
Q

What is the test for alkenes?

A

Add bromine water

Because an alkene is unsaturated it will discolour the bromine water

30
Q

What is a polymer?

A

Many repeating subunits joining to form a larger molecule

31
Q

What is polymerisation?

A

Joining many monomers together into a long chain

32
Q

What are some problems with plastic?

A

Non-biodegradable
Addition polymers are un reactive and can’t be broken down by bacteria etc
Therefore landfill sites are becoming full

33
Q

What type of polymers do we know?

A

Addition polymers

34
Q

How do you show polymerisation?

A

Draw the monomer with n on the left
Draw an arrow with pressure above it and catalyst below it
Draw the single bond monomer inside square brackets with the bonds going out the square brackets
And a small n on the outside of the brackets

35
Q

How do we name polymers?

A

Add poly in-front of the monomer

36
Q

What is high density polythene?

A

Straight chains = chains are closer together leading to more effective Van der Waals forces
Higher surface area of contact
Larger SA:V ratio
Higher melting point

37
Q

Conditions for forming high density polymers?

A

Low temperature and pressure

Using a Ziegler-Natta catalyst

38
Q

What is low density polythene?

A

Branched chains = chains further apart leading to weaker Van der Waals forces

39
Q

Conditions for forming low density polymers?

A

High temperature and pressure

40
Q

What is cross linking?

A

Where polymer chains are joined by covalent bonds and not intermolecular forces
Therefore the polymer molecules can’t slide over each other easily and become rigid

41
Q

What is a thermosoftening polymer?

A

Consists of individual tangled polymer chains
When heated intermolecular forces become weak and it softens
It can harden when cool
Therefore it can be remolded

43
Q

What are plasticers?

A

Additives that change the properties of plastics
Small molecules eg esters that fit between the polymer chains allowing them to slide over each other easily (behaving like molecular lubricants)
This makes plastic more flexible

44
Q

How can we recycle plastics?

A

Mechanical recycling - sort, wash and grind into pellets then melt to be remoulded
But the quality of the polymer reduces

Feedstock recycling - heat to produce new monomers to make new polymers

46
Q

What is a thermosetting polymer?

A

Consists of polymer chains with cross links - covalent bonds between chains
When heated it doesn’t melt or soften as the bonds are strong and rigid

You can’t recycle it

AQA A-Level Organic Chemistry (14 decks)

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